Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1¯100], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at K points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV.

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Few-layer epitaxial graphenes grown on vicinal 6H–SiC (0001) were characterized by confocal Raman imaging.
In the beginning of the growth, the surface of SiC substrate was covered with monolayer graphene. Next,
few-layer graphenes started to grow toward directions perpendicular to [11–20] of the SiC substrate. The
shift in the G-peak was not straightforward with the increase in number of graphene layers. This result can
be interpreted that the in-plane compressive stress from the substrate depends on the domain size of graphene.
The 2D-peak frequency shifted to higher frequency side due to strong compressive strain from the
substrate with increasing of the growth times.

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

Formation and electronic states of graphene nanoribbons with arm-chair edges (AGNR) are studied on the SiC(0001) vicinal surfaces toward the [$1\bar{1}00$] direction. The surface step and terrace structures of both 4H and 6H-SiC substrates are used as the growth templates of one-dimensional arrays of AGNRs, which are prepared using the carbon molecular beam epitaxy followed by hydrogen intercalation. A band gap is observed above the π-band maximum by angle-resolved photoelectron spectroscopy (ARPES) for the both samples. The average widths of the AGNRs are 6 and 10 nm, and the estimated average band gaps are 0.40 and 0.28 eV for the 4H- and 6H- substrates, respectively. A simple and phenomenological inverse relation between the energy gap and AGNR width works in the analyses of the ARPES data.

DOI： 10.1088/1361-648x/aced30

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Other Link： https://iopscience.iop.org/article/10.1088/1361-648X/aced30/pdf

Language：English   Publishing type：Research paper (scientific journal)  

Formation and electronic states of graphene nanoribbons with arm-chair edges (AGNR) are studied on the SiC(0001) vicinal surfaces toward the [1 ̄100] direction. The surface step and terrace structures of both 4H and 6H-SiC substrates are used as the growth templates of one-dimensional arrays of AGNRs, which are prepared using the carbon molecular beam epitaxy followed by hydrogen intercalation. A band gap is observed above the π-band maximum by angle-resolved photoelectron spectroscopy (ARPES) for the both samples. The average widths of the AGNRs are 6 and 10 nm, and the estimated average band gaps are 0.40 and 0.28 eV for the 4H- and 6H- substrates, respectively. A simple and phenomenological inverse relation between the energy gap and AGNR width works in the analyses of the ARPES data.

Language：English   Publishing type：Research paper (scientific journal)  

The carrier dynamics in various types of epitaxial graphene layers on SiC substrates was investigated by means of time- and angle-resolved photoemission spectroscopy (TARPES). Layer-dependent electron doping was observed in the Dirac bands of quasicrystalline bilayer graphene after optical pumping, leading to generation of transient voltage between the upper and lower layers. The amount of photoinduced carrier transport depends on the distance from the substrate. A comparison of the TARPES results of single-layer graphene between flat and stepped SiC substrates experimentally indicates that a source of the doping carriers likely originates from interface step states. The dynamic model is described based on the electronic structure, calculated using density functional theory.

DOI： 10.1103/PhysRevB.105.115304

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

The present article proposes a data analysis method for experimentally-derived measurements, which consists of an auto-optimization procedure and a sensitivity analysis. The method was applied to the results of a total-reflection high-energy positron diffraction (TRHEPD) experiment, a novel technique of determining surface structures or the position of the atoms near the material surface. This method solves numerically the partial differential equation in the fully-dynamical quantum diffraction theory with many trial surface structures. In the sensitivity analysis, we focused on the experimental uncertainties and the variation over individual fitting parameters, which was analyzed by solving the eigenvalue problem of the variance-covariance matrix. A modern massively parallel supercomputer was used to complete the analysis within a moderate computational time. The sensitivity analysis provides a basis for the choice of variables in the data analysis for practical reliability. The effectiveness of the present analysis method was demonstrated in the structure determination of a Si4O5N3/6H-SiC(0001)-(root 3x root 3) R30 degrees surface. Furthermore, this analysis method is applicable to many experiments other than TRHEPD. (C) 2021 The Author(s). Published by Elsevier B.V.

DOI： 10.1016/j.cpc.2021.108186

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The electronic structure of 3 degrees-twisted bilayer graphene (TBG) is studied by angle-resolved photoelectron spectroscopy (ARPES). Sub-mm-sized TBG prepared by direct bonding in a high vacuum enabled us to use conventional ARPES band mapping with synchrotron light. The results indicate that strong interlayer coupling makes a moire potential for the Dirac electrons and significantly modifies the graphene bands around the (K) over bar points such as the band splitting and electron velocity reduction. The observed electronic structure is consistently reproduced by tight-binding calculations combined with a band unfolding method.

DOI： 10.1103/PhysRevMaterials.5.L051001

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Twisted bilayer graphene (TBG), in which two monolayer graphene are stacked with an in-plane rotation angle, has recently become a hot topic due to unique electronic structures. TBG is normally produced in air by the tear-and-stack method of mechanical exfoliation and transferring graphene flakes, by which a sizable, millimeter-order area, and importantly clean interface between layers are hard to obtain. In this study, we resolved these problems by directly transferring the easy-to-exfoliate CVD-grown graphene on SiC substrate to graphene in a high vacuum without using any transfer assisting medium and observed electronic band modulations due to the strong interlayer coupling.

DOI： 10.35848/1882-0786/ab99d1

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Ultrafast carrier dynamics in a graphene system are very important in terms of optoelectronic devices. Recently, a twisted bilayer graphene has been discovered that possesses interesting electronic properties owing to strong modifications in interlayer couplings. Thus, a better understanding of ultrafast carrier dynamics in a twisted bilayer graphene is highly desired. Here, we reveal the unbalanced electron distributions in a quasicrystalline 30° twisted bilayer graphene (QCTBG), using time- A nd angle-resolved photoemission spectroscopy on the femtosecond time scale. We distinguish time-dependent electronic behavior between the upper- A nd lower-layer Dirac cones and gain insight into the dynamical properties of replica bands, which show characteristic signatures due to Umklapp scatterings. The experimental results are reproduced by solving a set of rate equations among the graphene layers and substrate. We find that the substrate buffer layer plays a key role in initial carrier injections to the upper and lower layers. Our results demonstrate that QCTBG can be a promising element for future devices.

DOI： 10.1021/acsnano.9b06091

Language：English   Publishing type：Research paper (scientific journal)  

Ultrafast carrier dynamics in a graphene system are very important in terms of optoelectronic devices. Recently, a twisted bilayer graphene has been discovered that possesses interesting electronic properties owing to strong modifications in interlayer couplings. Thus, a better understanding of ultrafast carrier dynamics in a twisted bilayer graphene is highly desired. Here, we reveal the unbalanced electron distributions in a quasicrystalline 30 degrees twisted bilayer graphene (QCTBG), using time- and angle-resolved photoemission spectroscopy on the femtosecond time scale. We distinguish time-dependent electronic behavior between the upper- and lower-layer Dirac cones and gain insight into the dynamical properties of replica bands, which show characteristic signatures due to Umklapp scatterings. The experimental results are reproduced by solving a set of rate equations among the graphene layers and substrate. We find that the substrate buffer layer plays a key role in initial carrier injections to the upper and lower layers. Our results demonstrate that QCTBG can be a promising element for future devices.

DOI： 10.1021/acsnano.9b06091

Language：English   Publishing type：Research paper (scientific journal)  

SiC(0001)-(1×1)-H consisting of monohydride is a preferred starting surface structure for synthesis of the two-dimensional materials on SiC(0001). Here we report preparation of the SiC(0001)-(1×1)-H by atomic hydrogen exposure and structure determination of the SiC(0001)-(1×1)-H by a quantitative LEED analysis. Our data show that the SiC(0001)-(1×1)-H is indeed a bulk terminated unreconstructed SiC(0001) surface. The sample morphology was also investigated using AFM. The dominance of one of two possible inequivalent surface terminations of 4H polytype of SiC crystal was confirmed.

DOI： 10.1103/PhysRevB.99.235434

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SiC(0001)-(1 x 1)-H consisting of monohydride is a preferred starting surface structure for synthesis of the two-dimensional materials on SiC(0001). Here we report preparation of the SiC(0001)-(1 x 1)-H by atomic hydrogen exposure and structure determination of the SiC(0001)-(1 x 1)-H by a quantitative LEED analysis. Our data show that the SiC(0001)-(1 x 1)-H is indeed a bulk terminated unreconstructed SiC(0001) surface. The sample morphology was also investigated using AFM. The dominance of one of two possible inequivalent surface terminations of 4H polytype of SiC crystal was confirmed.

DOI： 10.1103/PhysRevB.99.235434

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The symmetry of a surface or interface plays an important role in determining the spin splitting and texture of a two-dimensional band. Spin-polarized bands of a triangular lattice atomic layer (TLAL) consisting of Sn on a SiC(0001) substrate is investigated by spin- and angle-resolved photoelectron spectroscopy. Surprisingly, both Zeeman- and Rashba-type spin-split bands, without and with spin degeneracy, respectively, coexist at a K point of the Sn TLAL. The K point has a threefold symmetry without inversion symmetry according to the crystal structure including the SiC periodicity, meaning that the Zeeman-type is consistent with the symmetry of the lattice while the Rashba-type is inconsistent. Our density functional calculations reveal that the charge density distribution of the Rashba-type (Zeeman-type) band shows (no) inversion symmetry at the K point. Therefore, the symmetry of the charge density distribution agrees with both types of the spin splitting.

DOI： 10.1103/PhysRevLett.122.126403

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The symmetry of a surface or interface plays an important role in determining the spin splitting and texture of a two-dimensional band. Spin-polarized bands of a triangular lattice atomic layer (TLAL) consisting of Sn on a SiC(0001) substrate is investigated by spin- and angle-resolved photoelectron spectroscopy. Surprisingly, both Zeeman- and Rashba-type spin-split bands, without and with spin degeneracy, respectively, coexist at a K point of the Sn TLAL. The K point has a threefold symmetry without inversion symmetry according to the crystal structure including the SiC periodicity, meaning that the Zeeman-type is consistent with the symmetry of the lattice while the Rashba-type is inconsistent. Our density functional calculations reveal that the charge density distribution of the Rashba-type (Zeeman-type) band shows (no) inversion symmetry at the K point. Therefore, the symmetry of the charge density distribution agrees with both types of the spin splitting.

DOI： 10.1103/PhysRevLett.122.126403

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Single wall carbon nanotube (SWCNT) is considered as an ideal candidate for next-generation nanoelectronics owing to its unusual properties. Here we have performed an in-depth theoretical analysis of the effect of vacancy defects and curvature on the phonon properties of (10; 0)and (10; 10)SWCNTs using the forced vibrational method. We report that Raman active E_2g mode softens towards the low-frequency region with increasing vacancies and curvature in both types of CNTs. Vacancy induces some new peaks at low-frequency region of the phonon density of states. Phonon localization properties are also manifested. Our calculated mode pattern and localization length show that optical phonon at Raman D-band frequency is strongly localized in vacancy defected and large curved CNTs. Our findings will be helpful in explaining the thermal conductivity, specific heat capacity, and Raman spectra in vacancy type disordered CNTs, as well as electron transport properties of CNT-based nanoelectronic devices.

DOI： 10.7567/JJAP.57.02CB08

Language：English   Publishing type：Research paper (scientific journal)  

Single wall carbon nanotube (SWCNT) is considered as an ideal candidate for next-generation nanoelectronics owing to its unusual properties. Here we have performed an in-depth theoretical analysis of the effect of vacancy defects and curvature on the phonon properties of (10; 0)and (10; 10)SWCNTs using the forced vibrational method. We report that Raman active E_2g mode softens towards the low-frequency region with increasing vacancies and curvature in both types of CNTs. Vacancy induces some new peaks at low-frequency region of the phonon density of states. Phonon localization properties are also manifested. Our calculated mode pattern and localization length show that optical phonon at Raman D-band frequency is strongly localized in vacancy defected and large curved CNTs. Our findings will be helpful in explaining the thermal conductivity, specific heat capacity, and Raman spectra in vacancy type disordered CNTs, as well as electron transport properties of CNT-based nanoelectronic devices.

DOI： 10.7567/JJAP.57.02CB08

Language：English   Publishing type：Research paper (scientific journal)  

We report the details of the effects of the ¹⁰B isotope and those of B and N vacancies combined with the isotope on the phonon modes of two-dimensional hexagonal boron nitride (h-BN). The phonon density of states and localization problems are solved using the forced vibrational method, which is suitable for an intricate and disordered system. We observe an upward shift of Raman-active E_2g-mode optical phonons (32 cm^-1) for a 100% ¹⁰B isotope, which matches well with the experiment and simple harmonic oscillator model. However, a downward shift of E_2g-mode phonons is observed for B or N vacancies and the combination of the isotope and vacancy-type disordered BN. Strong localized eigenmodes are found for all types of defects, and a typical localization length is on the order of ∼7 nm for naturally occurring BN samples. These results are very important for understanding the heat dissipation and electron transport properties of BN-based nanoelectronics.

DOI： 10.7567/JJAP.57.02CB04

Language：English   Publishing type：Research paper (scientific journal)  

Sn atomic layers attract considerable interest owing to their spin-related physical properties caused by their strong spin-orbit interactions. We performed Sn intercalation into the graphene/SiC(0001) interface and found a new type of Sn atomic layer. Sn atoms occupy on-top sites of Si-terminated SiC(0001) with in-plane Sn-Sn bondings, resulting in a triangular lattice. Angle-resolved photoemission spectroscopy revealed characteristic dispersions at and points, which agreed well with density functional theory calculations. The Sn triangular lattice atomic layer at the interface showed no oxidation upon exposure to air, which is useful for characterization and device fabrication ex situ.

DOI： 10.7567/APEX.11.015202

Language：English  

Vacancy and curvature effects on the phonon properties of single wall carbon nanotube

DOI： 10.7567/JJAP.57.02CB08

Language：English   Publishing type：Research paper (scientific journal)  

Sn atomic layers attract considerable interest owing to their spin-related physical properties caused by their strong spin-orbit interactions. We performed Sn intercalation into the graphene/SiC(0001) interface and found a new type of Sn atomic layer. Sn atoms occupy on-top sites of Si-terminated SiC(0001) with in-plane Sn-Sn bondings, resulting in a triangular lattice. Angle-resolved photoemission spectroscopy revealed characteristic dispersions at (K) over bar and (M) over bar points, which agreed well with density functional theory calculations. The Sn triangular lattice atomic layer at the interface showed no oxidation upon exposure to air, which is useful for characterization and device fabrication ex situ. (c) 2018 The Japan Society of Applied Physics

DOI： 10.7567/APEX.11.015202

Language：English   Publishing type：Research paper (scientific journal)  

We report the details of the effects of the 10B isotope and those of B and N vacancies combined with the isotope on the phonon modes of two- dimensional hexagonal boron nitride (h-BN). The phonon density of states and localization problems are solved using the forced vibrational method, which is suitable for an intricate and disordered system. We observe an upward shift of Raman-active E2g-mode optical phonons (32 cm%1) for a 100% 10B isotope, which matches well with the experiment and simple harmonic oscillator model. However, a downward shift of E2g-mode phonons is observed for B or N vacancies and the combination of the isotope and vacancy-type disordered BN. Strong localized eigenmodes are found for all types of defects, and a typical localization length is on the order of >7 nm for naturally occurring BN samples. These results are very important for understanding the heat dissipation and electron transport properties of BN-based nanoelectronics.

Language：English   Publishing type：Research paper (scientific journal)  

Local electron−phonon coupling of a one-dimen-sionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic
structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron−phonon coupling to the out-of-plane phonon at the K̅ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron−phonon coupling in the surface graphene.

Language：English   Publishing type：Research paper (scientific journal)  

Local electronphonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electronphonon coupling to the out-of-plane phonon at the (K) over bar points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electronphonon coupling in the surface graphene.

DOI： 10.1021/acs.nanolett.7b00606

Language：English   Publishing type：Research paper (scientific journal)  

Local electron-phonon coupling of a one-dimensionally nanorippled graphene is studied on a SiC(0001) vicinal substrate. We have characterized local atomic and electronic structures of a periodically nanorippled graphene (3.4 nm period) prepared on a macrofacet of the 6H-SiC crystal using scanning tunneling microscopy/spectroscopy (STM/STS) and angle-resolved photoelectron spectroscopy (ARPES). The rippled graphene on the macrofacets distributes homogeneously over the 6H-SiC substrate in a millimeter scale, and thus replica bands are detected by the macroscopic ARPES. The STM/STS results indicate the strength of electron-phonon coupling to the out-of-plane phonon at the K̄ points of graphene is periodically modified in accordance with the ripple structure. We propose an interface carbon nanostructure with graphene nanoribbons between the surface rippled graphene and the substrate SiC that periodically modifies the electron-phonon coupling in the surface graphene.

DOI： 10.1021/acs.nanolett.7b00606

Language：English   Publishing type：Research paper (scientific journal)  

Graphene and AlN are promising materials, interesting to combine together. In this study, we will present first results for direct growth of graphene on bulk AlN and on AlN templates using chemical vapor deposition, including the annealing of these substrates at high temperatures. Atomic force microscopy (AFM) enabled us to study the evolution of the AlN surface morphology after annealing and growth. Few-layer graphene deposition is demonstrated on the basis of X-ray photoemission and Raman spectroscopy. AFM view of graphene wrinkles on the N-face of bulk AlN.

DOI： 10.1002/pssa.201600436

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial graphene growth on SiC surfaces is considered advantageous in terms of device application. However, the first graphitic layer on SiC transforms to a buffer layer because of strong coupling with the substrate. The properties of several subsequent layers are also significantly degraded. One method to decouple graphene from the substrate is Si intercalation. In the present work, we report observation and analysis of interface structures formed by Si intercalation in between the graphene layer and the SiC(0001) surface depending on Si coverage and influence of these interfaces on graphene electronic structure by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and theoretical first-principles calculations. The STM appearance of observed periodic interface structures strongly resembles previously known Si-rich phases on the SiC(0001) surface. Based on the observed range of interface structures we discuss the mechanism of graphene layer decoupling and differences in stability of the Si-rich phases on clean SiC(0001) and in the graphene/SiC(0001) interface region. We also discuss a possibility to tune graphene electronic properties by interface engineering.

DOI： 10.1103/PhysRevB.94.245421

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial graphene growth on SiC surfaces is considered advantageous in terms of device application. However, the first graphitic layer on SiC transforms to a buffer layer because of strong coupling with the substrate. The properties of several subsequent layers are also significantly degraded. One method to decouple graphene from the substrate is Si intercalation. In the present work, we report observation and analysis of interface structures formed by Si intercalation in between the graphene layer and the SiC(0001) surface depending on Si coverage and influence of these interfaces on graphene electronic structure by means of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and theoretical first-principles calculations. The STM appearance of observed periodic interface structures strongly resembles previously known Si-rich phases on the SiC(0001) surface. Based on the observed range of interface structures we discuss the mechanism of graphene layer decoupling and differences in stability of the Si-rich phases on clean SiC(0001) and in the graphene/SiC(0001) interface region. We also discuss a possibility to tune graphene electronic properties by interface engineering.

DOI： 10.1103/PhysRevB.94.245421

Language：English   Publishing type：Research paper (scientific journal)  

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core levelspectrashowasignaturefortheexistenceofPb Sichemicalbondsattheinterface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

Language：English   Publishing type：Research paper (scientific journal)  

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

DOI： 10.1002/smll.201600666

Language：Others   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/smll.201670142

Language：English   Publishing type：Research paper (scientific journal)  

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moire superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing p-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of Pb Si chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 degrees C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

DOI： 10.1002/smll.201600666

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/smll.201670142

Language：English   Publishing type：Research paper (scientific journal)  

Due to the extreme chemical inertness of silicon carbide (SiC), in-situ thermal desorption is commonly utilized as a means to remove surface contamination prior to initiating critical semiconductor processing steps such as epitaxy, gate dielectric formation, and contact metallization. In-situ thermal desorption and silicon sublimation has also recently become a popular method for epitaxial growth of mono and few layer graphene. Accordingly, numerous thermal desorption experiments of various processed silicon carbide surfaces have been performed, but have ignored the presence of hydrogen, which is ubiquitous throughout semiconductor processing. In this regard, the authors have performed a combined temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) investigation of the desorption of molecular hydrogen (H₂) and various other oxygen, carbon, and fluorine related species from ex-situ aqueous hydrogen fluoride (HF) and in-situ remote hydrogen plasma cleaned 6H-SiC (0001) surfaces. Using XPS, the authors observed that temperatures on the order of 700-1000 °C are needed to fully desorb C-H, C-O and Si-O species from these surfaces. However, using TPD, the authors observed H₂ desorption at both lower temperatures (200-550 °C) as well as higher temperatures (>700 °C). The low temperature H₂ desorption was deconvoluted into multiple desorption states that, based on similarities to H₂ desorption from Si (111), were attributed to silicon mono, di, and trihydride surface species as well as hydrogen trapped by subsurface defects, steps, or dopants. The higher temperature H₂ desorption was similarly attributed to H₂ evolved from surface O-H groups at ∼750 °C as well as the liberation of H₂ during Si-O desorption at temperatures >800 °C. These results indicate that while ex-situ aqueous HF processed 6H-SiC (0001) surfaces annealed at <700 °C remain terminated by some surface C-O and Si-O bonding, they may still exhibit significant chemical reactivity due to the creation of surface dangling bonds resulting from H₂ desorption from previously undetected silicon hydride and surface hydroxide species.

DOI： 10.1116/1.4921526

Language：English   Publishing type：Research paper (scientific journal)  

Due to the extreme chemical inertness of silicon carbide (SiC), in-situ thermal desorption is commonly utilized as a means to remove surface contamination prior to initiating critical semiconductor processing steps such as epitaxy, gate dielectric formation, and contact metallization. In-situ thermal desorption and silicon sublimation has also recently become a popular method for epitaxial growth of mono and few layer graphene. Accordingly, numerous thermal desorption experiments of various processed silicon carbide surfaces have been performed, but have ignored the presence of hydrogen, which is ubiquitous throughout semiconductor processing. In this regard, the authors have performed a combined temperature programmed desorption (TPD) and x-ray photoelectron spectroscopy (XPS) investigation of the desorption of molecular hydrogen (H-2) and various other oxygen, carbon, and fluorine related species from ex-situ aqueous hydrogen fluoride (HF) and in-situ remote hydrogen plasma cleaned 6H-SiC (0001) surfaces. Using XPS, the authors observed that temperatures on the order of 700-1000 degrees C are needed to fully desorb C-H, C-O and Si-O species from these surfaces. However, using TPD, the authors observed H-2 desorption at both lower temperatures (200-550 degrees C) as well as higher temperatures (>700 degrees C). The low temperature H-2 desorption was deconvoluted into multiple desorption states that, based on similarities to H-2 desorption from Si (111), were attributed to silicon mono, di, and trihydride surface species as well as hydrogen trapped by subsurface defects, steps, or dopants. The higher temperature H-2 desorption was similarly attributed to H2 evolved from surface O-H groups at similar to 750 degrees C as well as the liberation of H-2 during Si-O desorption at temperatures >800 degrees C. These results indicate that while ex-situ aqueous HF processed 6H-SiC (0001) surfaces annealed at <700 degrees C remain terminated by some surface C-O and Si-O bonding, they may still exhibit significant chemical reactivity due to the creation of surface dangling bonds resulting from H-2 desorption from previously undetected silicon hydride and surface hydroxide species. (C) 2015 American Vacuum Society.

DOI： 10.1116/1.4921526

Language：English   Publishing type：Research paper (other academic)  

Terahertz field-induced transmission enhancement in monolayer epitaxial graphene is observed with increasing terahertz field. Photoexcitation leads to further transmission enhancement that is found to be less for the higher terahertz field amplitudes.

Language：Japanese  

Fabrication and characterization of graphene lateral superlattices on SiC facets (1)

DOI： 10.11470/jsapmeeting.2015.2.0_3439

Language：Japanese  

Fabrication and characterization of graphene lateral superlattices on SiC facets (2)

DOI： 10.11470/jsapmeeting.2015.2.0_3440

Language：English   Publishing type：Research paper (scientific journal)  

Structural and optical properties of AlN grown on AlN(0001) by the solid source solution growth (3SG) method were investigated. Transmission electron microscopy (TEM) analysis revealed that the geometrical relationship between the growth directions and slip planes influenced the dislocation propagation behaviors and annihilation mechanisms. Panchromatic and monochromatic images in the cathodoluminescence (CL) spectrum further revealed that C impurities were segregated near the surface, while Al vacancies were widely distributed in the AlN/AlN(0001) grown using the 3SG method.

DOI： 10.7567/JJAP.54.085501

Language：English   Publishing type：Research paper (other academic)  

Terahertz field-induced transmission enhancement in monolayer epitaxial graphene is observed with increasing terahertz field. Photoexcitation leads to further transmission enhancement that is found to be less for the higher terahertz field amplitudes.

DOI： 10.1364/CLEO_AT.2015.JW2A.63

Language：English   Publishing type：Research paper (scientific journal)  

We report highly anisotropic appearance of the quantum Hall effect (QHE) in epitaxial single-layer graphene grown on a vicinal SiC(0001) substrate. Well-developed QHE with zero resistance manifests itself for the current along the steps, whereas the QHE is obscured by pronounced positive magnetoresistance with quadratic magnetic-field dependence for the current across the steps. The latter, as well as the small slope of the Hall resistance, implies the presence of parallel conduction due to remnant carriers in the SiC substrate, albeit with seeming inconsistency with the zero resistance observed for the former current direction. We interpret the anisotropic behavior by assuming that the parallel conduction is sizable along the steps but is virtually prohibited across the steps.

DOI： 10.1007/s10909-015-1277-y

Language：English   Publishing type：Research paper (scientific journal)  

We report highly anisotropic appearance of the quantum Hall effect (QHE) in epitaxial single-layer graphene grown on a vicinal SiC(0001) substrate. Well-developed QHE with zero resistance manifests itself for the current along the steps, whereas the QHE is obscured by pronounced positive magnetoresistance with quadratic magnetic-field dependence for the current across the steps. The latter, as well as the small slope of the Hall resistance, implies the presence of parallel conduction due to remnant carriers in the SiC substrate, albeit with seeming inconsistency with the zero resistance observed for the former current direction. We interpret the anisotropic behavior by assuming that the parallel conduction is sizable along the steps but is virtually prohibited across the steps.

DOI： 10.1007/s10909-015-1277-y

Language：English   Publishing type：Research paper (scientific journal)  

We report the fabrication of conical shape micropore structures on silicon nitride/silicon (Si₃N₄/Si) substrate to function as a fluidic channel as well as a supporting structure for sensing membrane. Excellent shape of conical structure with large diameter of top hole size and small diameter of bottom hole size (1-2 μm) was successfully fabricated by utilizing a combination of grinding process, fine polishing and multistep focused ion beam (FIB) milling. A so-called re-deposition phenomena seems to be unavoidable in the multistep FIB milling resulted to the formation of tip-like structure at every milling step. Low surface roughness is effective in improving the shape of circumference of the milling area. The possibility to integrate such conical shape structure with graphene was also demonstrated. This result opens up the breakthrough towards new generation of analytical platform for biosensor application.

DOI： 10.1016/j.mee.2014.11.011

Language：English   Publishing type：Research paper (scientific journal)  

We report nonlinear terahertz (THz) field transmission through photoexcited monolayer epitaxial graphene via differential transmission measurements enabled by optical-pump/intense-terahertz-probe (OPITP) spectroscopy. After photoexcitation of graphene, a transmission enhancement, defined by a positive differential transmission of the intense terahertz probe pulse, is observed. This is due to suppression of the graphene photoconductivity arising from an increased carrier scattering rate due to the increase in the carrier density and the extra energy from the photoexcited hot carriers. Thus, the transient enhancement in transmission increases as the optical pump fluence increased. Most interestingly, we observe that the transmission enhancement after photoexcitation decreases as the THz field strength is increased, which we attribute to the combined effects of the intense THz electric field and the optical pump fluence on the carrier scattering rate. We model the carrier dynamics in the graphene using the length gauge interaction Hamiltonian with the inclusion of short-range scattering by neutral impurities and the interaction of the carriers with optical phonons. Comparing the experimental and simulated transmission results, we extract the nonequilibrium effective lattice temperature of graphene as a function of the optical pump fluence and THz field strength.

DOI： 10.1103/PhysRevB.91.035422

Language：English   Publishing type：Research paper (scientific journal)  

We report nonlinear terahertz (THz) field transmission through photoexcited monolayer epitaxial graphene via differential transmission measurements enabled by optical-pump/intense-terahertz-probe (OPITP) spectroscopy. After photoexcitation of graphene, a transmission enhancement, defined by a positive differential transmission of the intense terahertz probe pulse, is observed. This is due to suppression of the graphene photoconductivity arising from an increased carrier scattering rate due to the increase in the carrier density and the extra energy from the photoexcited hot carriers. Thus, the transient enhancement in transmission increases as the optical pump fluence increased. Most interestingly, we observe that the transmission enhancement after photoexcitation decreases as the THz field strength is increased, which we attribute to the combined effects of the intense THz electric field and the optical pump fluence on the carrier scattering rate. We model the carrier dynamics in the graphene using the length gauge interaction Hamiltonian with the inclusion of short-range scattering by neutral impurities and the interaction of the carriers with optical phonons. Comparing the experimental and simulated transmission results, we extract the nonequilibrium effective lattice temperature of graphene as a function of the optical pump fluence and THz field strength.

DOI： 10.1103/PhysRevB.91.035422

Language：English   Publishing type：Research paper (scientific journal)  

This letter performed polarized microscopic laser Raman scattering spectroscopy on the curved edges of transferred epitaxial graphene on SiO₂/Si. The intensity ratio between the parallel and perpendicular polarized D band is evolved, providing a spectroscopy-based technique to probe the atomic-scale edge structures in graphene. A detailed analysis procedure for non-ideal disordered curved edges of graphene is developed combining the atomic-scale zigzag and armchair edge structures along with some point defects. These results could provide valuable information of the realistic edges of graphene at the atomic-scale that can strongly influence the performance of graphene-based nanodevices.

DOI： 10.1063/1.4904469

Language：English   Publishing type：Research paper (scientific journal)  

We present a systematic theoretical investigation of the vibrational properties of C-doped single layer hexagonal boron nitride (h-BN). Our studies have been carried out by the forced vibrational method, which is based on the idea of mechanical resonance and efficient for very complex and large system. We have estimated the phonon density of states (PDOSs) of h-BCN network with random and regular distribution of C atoms. It is found that the PDOS greatly depends on the C distribution and coverage. For randomly distributed C atoms, we observe that the longitudinal and the transverse optical (LO) and (TO) phonon branches for in-plane motion are nondegenerate at the Γ-point of the Brillouin zone. We determine a critical value of C concentration for the onset of this C-induced vibrational transition. We have found that C concentrations of about 10% and higher, the E_2g peak of h-BN has been reduced into a shoulder or it has completely disappeared. For h-BCN network with regular domains of C, the PDOSs changes more abruptly. With the increase of C concentration, the high frequency optical phonons peaks above the 1400 cm^-1 increase linearly while the h-BN peaks below the 1400 cm^-1 are broadened and distorted. The disorder causes the phonon modes to be localized in the real space. Phonon localization in the hybrid BCN network is studied and the extent of localization is quantified by the typical mode pattern and the localization length. Spatial analyses of the eigenvectors using typical mode patterns show that Γ-point of the LO and TO phonon modes is strongly localized and show random behavior within a region of several nanometers in the BCN structure. In particular at 1400 cm^-1, a typical localization length is on the order of ≈4 nm for randomly distributed C atoms and ≈8.5 nm for the regular domains of C of 20% concentration, while at 1590 cm^-1, these values are ≈2 nm and ≈4 nm for randomly distributed C atoms and regular domains of C, respectively. These results are expected to stimulate further studies aimed at better understanding of the phenomena allied with vibrational properties such as thermal conductivity, specific heat capacity, and electron-phonon interaction of h-BN and BCN networks.

DOI： 10.1016/j.commatsci.2014.04.047

Language：English   Publishing type：Research paper (scientific journal)  

We report nonlinear terahertz (THz) effects in monolayer graphene, giving rise to transmission enhancement of a single-cycle THz pulse when the incident THz peak electric field is increased. This transmission enhancement is attributed to reduced photoconductivity, due to saturation effects in the field-induced current and increased intraband scattering rates arising from transient heating of electrons. We have developed a tight-binding model of the response using the length gauge interaction Hamiltonian that provides good qualitative agreement. The model fully accounts for the nonlinear response arising from the linear dispersion energy spectrum in graphene. The results reveal a strong dependence of the scattering time on the THz field, which is at the heart of the observed nonlinear response.

DOI： 10.1063/1.4902096

Language：English   Publishing type：Research paper (other academic)  

We report nonlinear terahertz transmission through photoexcited graphene using optical-pump/terahertz-probe technique. Transient enhancement in the terahertz transmission of the graphene sample is observed when the sample is photoexcited. This transmission enhancement is found to decrease nonlinearly with the terahertz electric field.

DOI： 10.1109/IRMMW-THz.2014.6956170

Language：English   Publishing type：Research paper (scientific journal)  

Boron and nitrogen doping in graphene has important implications in graphene-based devices. We investigate systematically the vibrational properties of B- and N-doped graphene with vacancies using forced vibrational method. We have calculated the phonon density of states (PDOSs), typical mode patterns and phonon localization length for different concentration of B, N and vacancies. We find that the interference between native point defects and B or N dopant break down the phonon degeneracy at the Cyrillic capital letter GHE point of the LO and TO modes, distort and shift down the PDOSs significantly. We observe a broadening and softening of the Raman active E2_g phonon mode with an increase of B and N atoms. The PDOS peaks for the mixture of vacancies and B or N atoms show the remarkable increase in the low-frequency region induced by their defect formations. Our computer experiments demonstrate that the disordered graphene show the spatially localized vibrations due to the resonant vibrations of the impurity atoms relative to the main C atoms. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the phonon is localized strongly within a region of several nanometers in the random disordered graphene structures. In particular, a typical localization length is on the order of ≈9.5 nm for B- and N-doping, ≈9 nm for mixture of B-doping and vacancy, and ≈8.5 nm for mixture of N-doping and vacancy concentrations of 20%. This study provides a useful basis for the understanding of a wide variety of physical properties such as thermal conductivity, specific heat capacity, and electron-phonon interaction, as well as in the experiments of infrared, Raman, and neutron-diffraction spectra of doped-graphene.

DOI： 10.1016/j.commatsci.2014.01.040

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Polarized microscopic laser Raman scattering spectroscopy accompanied with simulation model has been used to systematically analyze the details of the edge structures of the transferred epitaxial graphene. The evolution of the intensity ratio between the parallel polarized D band (VV) and the perpendicular polarized D band (VH) is determined, providing a spectroscopy-based method to probe the atomic scale edge structures in graphene. Based on the experimental Raman results, we develop a rather precise atomic scale edge model combining the armchair and zigzag edges along with some point defects. The calculated polarization dependent VH/VV ratios of the developed model are perfectly matched with the D band intensity ratios of the realistic graphene edges. The developed edge structures are also supported by the atomic force microscopy (AFM) measurements. Moreover, we calculate the vibrational eigenmodes of graphene with some realistic edge and disordered configurations. We find that the typical mode patterns for in-plane optical phonon modes (iTO) at the K point are localized near the armchair edges or the defects, which are conceptually well agreed with the high intensity D peak in the Raman spectra come from the iTO phonon near the K point activating by the armchair edges or the defects. (C) 2014 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.carbon.2014.06.023

Language：English   Publishing type：Research paper (scientific journal)  

Polarized microscopic laser Raman scattering spectroscopy accompanied with simulation model has been used to systematically analyze the details of the edge structures of the transferred epitaxial graphene. The evolution of the intensity ratio between the parallel polarized D band (VV) and the perpendicular polarized D band (VH) is determined, providing a spectroscopy-based method to probe the atomic scale edge structures in graphene. Based on the experimental Raman results, we develop a rather precise atomic scale edge model combining the armchair and zigzag edges along with some point defects. The calculated polarization dependent VH/VV ratios of the developed model are perfectly matched with the D band intensity ratios of the realistic graphene edges. The developed edge structures are also supported by the atomic force microscopy (AFM) measurements. Moreover, we calculate the vibrational eigenmodes of graphene with some realistic edge and disordered configurations. We find that the typical mode patterns for in-plane optical phonon modes (iTO) at the K point are localized near the armchair edges or the defects, which are conceptually well agreed with the high intensity D peak in the Raman spectra come from the iTO phonon near the K point activating by the armchair edges or the defects.

DOI： 10.1016/j.carbon.2014.06.023

Language：English   Publishing type：Research paper (scientific journal)  

The film and interface structures of epitaxial silicon oxynitride (SiON) thin film grown on a SiC(0001) surface were investigated by photoelectron diffraction. Forward focusing peaks (FFPs) corresponding to the directions from the photoelectron emitter atom to the surrounding atoms appeared in the photoelectron intensity angular distribution (PIAD). By comparing N 1s PIAD with those of Si 2p and C 1s, we confirmed that the nitrogen atoms at SiON/SiC interface replace carbon atoms at stacking fault sites. Two kinds of oxygen atom sites exist in the previously proposed model [T. Shirasawa et al.: Phys. Rev. Lett. 98, 136105 (2007)]. FFP corresponding to Si-O-Si perpendicular bonds was observed in the O 1s PIAD, while diffraction rings were observed in the KLL Auger electron intensity angular distribution (AIAD), which were attributed to the diffraction patterns from outermost oxygen sites. Furthermore, O Kedge X-ray absorption spectra combined with AIAD were analyzed. An electronic structure specific to each oxygen atom site was successfully separated.

DOI： 10.7566/JPSJ.83.044604

Language：English   Publishing type：Research paper (scientific journal)  

The film and interface structures of epitaxial silicon oxynitride (SiON) thin film grown on a SiC(0001) surface were investigated by photoelectron diffraction. Forward focusing peaks (FFPs) corresponding to the directions from the photoelectron emitter atom to the surrounding atoms appeared in the photoelectron intensity angular distribution (PIAD). By comparing N 1s PIAD with those of Si 2p and C 1s, we confirmed that the nitrogen atoms at SiON/SiC interface replace carbon atoms at stacking fault sites. Two kinds of oxygen atom sites exist in the previously proposed model [T. Shirasawa et al.: Phys. Rev. Lett. 98, 136105 (2007)]. FFP corresponding to Si-O-Si perpendicular bonds was observed in the O 1s PIAD, while diffraction rings were observed in the KLL Auger electron intensity angular distribution (AIAD), which were attributed to the diffraction patterns from outermost oxygen sites. Furthermore, O K-edge X-ray absorption spectra combined with AIAD were analyzed. An electronic structure specific to each oxygen atom site was successfully separated.

DOI： 10.7566/JPSJ.83.044604

Language：English   Publishing type：Research paper (scientific journal)  

An epitaxial silicon-oxide monolayer of chemical composition of Si2O3 (the Si2O3 layer) formed on hexagonal SiC(000 (1) over bar) surfaces has been observed by scanning tunneling microscopy (STM). Filled-and empty-state STM images with atomic resolution support the previously reported model. Typical structural defects in the Si2O3 layer are found to be missing SiOn (n = 1, 2, 3) molecules. The band gap of the Si2O3 layer obtained by point tunneling spectroscopy is 5.5 +/- 0.5 eV, exhibiting considerable narrowing from that of bulk SiO2, 8.9 eV. It is proposed that the Si2O3 layer is suitable as a relevant interface material for formation of SiC-based metal-oxide-semiconductor devices. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4863753

Language：English   Publishing type：Research paper (scientific journal)  

The phonon properties of hydrogen-passivated armchair graphene nanoribbons (AGNRs) with different vacancy concentrations are investigated theoretically. We calculate the change in the phonon density of states (PDOSs) due to a broad range of vacancies and hydrogen passivation effects using forced vibrational method. A large downshift of prominent Raman active U point LO mode phonons with an increase of vacancy concentration or decrease of ribbon widths are observed. We find an increasing peak intensities for the C-H stretching mode with the decrease of ribbon width or the increase of defect density. An inserted vacancy concentration of 10% and higher induce the broadening and distorting of the PDOS peaks significantly. The localization properties of phonon due to defects were also studied. The typical mode pattern of K point iTO mode phonons show the spatial localized vibrations persuaded by armchair edges or vacancies, which are in conceptually good agreement with the large D band of the Raman spectra comes from the armchair-edges or the imperfections of crystal. The typical displacement pattern for C-H stretching mode shows a random displacement of H atoms in contrast to C atoms. Our simulation results show the significant impact of vacancy defects on the vibrational properties of GNRs.

DOI： 10.1016/j.carbon.2014.08.049

Language：English   Publishing type：Research paper (other academic)  

We report temperature dependence and thermal hysteresis behavior of terahertz transmission through photoexcited graphene. We vary the temperature between room temperature and 180° C, and use the optical-pump/terahertz-probe differential transmission technique.

Language：English   Publishing type：Research paper (other academic)  

We report temperature dependence and thermal hysteresis behavior of terahertz transmission through photoexcited graphene. We vary the temperature between room temperature and 180° C, and use the optical-pump/terahertz-probe differential transmission technique.

Language：English   Publishing type：Research paper (other academic)  

We report temperature dependence and thermal hysteresis behavior of terahertz transmission through photoexcited graphene. We vary the temperature between room temperature and 1800 C, and use the optical-pump/terahertz-probe differential transmission technique.

Language：English   Publishing type：Research paper (scientific journal)  

An epitaxial silicon-oxide monolayer of chemical composition of Si ₂O₃ (the Si₂O₃ layer) formed on hexagonal SiC(0001̄) surfaces has been observed by scanning tunneling microscopy (STM). Filled- and empty-state STM images with atomic resolution support the previously reported model. Typical structural defects in the Si ₂O₃ layer are found to be missing SiO_n (n=1, 2, 3) molecules. The band gap of the Si₂O₃ layer obtained by point tunneling spectroscopy is 5.5±0.5eV, exhibiting considerable narrowing from that of bulk SiO₂, 8.9eV. It is proposed that the Si₂O₃ layer is suitable as a relevant interface material for formation of SiC-based metal-oxide-semiconductor devices.

DOI： 10.1063/1.4863753

Language：English   Publishing type：Research paper (other academic)  

We report nonlinear terahertz effects in monolayer graphene using intense few-cycle THz pulses. We observe enhancement in the THz transmission through graphene when we increase the THz electric field. Following photo-excitation, we observed reduction in the THz transmission. This photo-induced reduction in the THz transmission is found to decrease when we increase the THz electric field.

DOI： 10.1109/IRMMW-THz.2013.6665850

Language：English   Publishing type：Research paper (scientific journal)  

We have studied phonon properties of graphene theoretically with different concentrations of ¹³C isotope and vacancy-type defects. The forced vibrational method, which is based on the mechanical resonance to extract the pure vibrational eigenmodes by numerical simulation, has been employed to compute the phonon density of states (PDOSs) and mode pattern of isotope-disordered graphene as well as a combined isotope and vacancy-type defective graphene structure. We observe a linear reduction of the E _2g mode frequencies with an increase in ¹³C concentration due to the reduced mass variation of the isotope mixture. We find a downshift of the E_2g mode of 65 cm^{- 1}, which is a very good agreement with the experimental results, and the phonon frequencies described by the simple harmonic oscillator model. The vacancy-type defects break down the phonon degeneracy at the ETH.

DOI： 10.1016/j.diamond.2013.10.013

Language：English   Publishing type：Research paper (scientific journal)  

We have studied phonon properties of graphene theoretically with different concentrations of 13C isotope and vacancy-type defects. The forced vibrational method, which is based on the mechanical resonance to extract
the pure vibrational eigenmodes by numerical simulation, has been employed to compute the phonon density of states (PDOSs) and mode pattern of isotope-disordered graphene as well as a combined isotope and vacancy-type defective graphene structure. We observe a linear reduction of the E2g mode frequencies with an increase in 13C concentration due to the reduced mass variation of the isotope mixture.We find a downshift of the E2g mode of 65 cm−1, which is a very good agreement with the experimental results, and the phonon frequencies described by the simple harmonic oscillator model. The vacancy-type defects break down the phonon degeneracy at the Г point of the LO and TOmodes, distort and shift down the phonon density of states significantly. The PDOS peaks for the combined isotope and vacancy-type defects showthe remarkable increase in the low frequency region induced by their defect formations. Due to phonon scattering by 13C isotope or vacancies, some graphene phonon wave functions become localized in the real space. Our numerical experiments reveal that the lattice vibrations in the defective graphene show the remarkably different properties such as spatial localization of lattice vibrations due to their random structures from those in the perfect graphene. The calculated typical mode patterns for in-plane K point optical phonon modes indicate that the features of strongly localized state depend on the defect density, and the phonon is localized stronglywithin a region of several nanometers in the random percolation network structures. In particular, for in-plane K point optical phonon modes, a typical localization length is on the order of ≈7 nm for isotope impurities, ≈5 nm for vacancy-type defects and ≈6nm for mixed-type defects at high defect concentrations of 30%. Our findings can be useful for the interpretation
of experiments on infrared, Raman, and neutron-diffraction spectra of defective graphene, aswell as in the study of a wide variety of other physical properties such as thermal conductivity, specific heat capacity, and electron–phonon interaction.

Language：English   Publishing type：Research paper (scientific journal)  

The effect of vacancy-type defects on the vibrational properties of graphene nanoribbons has been
discussed numerically. We have computed the phonon density of states and mode pattern over a broad
range of vacancies using the forced vibrational method which is based on the mechanical response to
extract the pure vibrational eigenmodes by numerical simulation. We find that the armchair-edge and
the vacancy-type defects break down the phonon degeneracy at the U point of the LO and TO mode,
distort and shift down the phonon density of states significantly. The phonon density of states in the
armchair graphene nanoribbons with vacancy-type defects show the remarkable increase in the low
frequency region induced by their defect formations. The mode patterns obtained by our numerical
experiments reveal that the in-plane optical phonon modes in the K point are localized near the armchair-
edges which are in good agreement with the high intensity D peak in the Raman spectra originate
from the armchair-edge. The simulation results also demonstrate that the lattice vibrations in the defective graphene nanoribbons show the remarkably different properties such as spatial localizations of lattice vibrations due to their random structures from those in the perfect graphene nanoribbons. These differences manifest themselves in the predicted temperature behavior of the constant-volume specific heat capacity of both structures.

Language：English   Publishing type：Research paper (scientific journal)  

The effect of vacancy-type defects on the vibrational properties of graphene nanoribbons has been discussed numerically. We have computed the phonon density of states and mode pattern over a broad range of vacancies using the forced vibrational method which is based on the mechanical response to extract the pure vibrational eigenmodes by numerical simulation. We find that the armchair-edge and the vacancy-type defects break down the phonon degeneracy at the ⌈ point of the LO and TO mode, distort and shift down the phonon density of states significantly. The phonon density of states in the armchair graphene nanoribbons with vacancy-type defects show the remarkable increase in the low frequency region induced by their defect formations. The mode patterns obtained by our numerical experiments reveal that the in-plane optical phonon modes in the K point are localized near the armchair-edges which are in good agreement with the high intensity D peak in the Raman spectra originate from the armchair-edge. The simulation results also demonstrate that the lattice vibrations in the defective graphene nanoribbons show the remarkably different properties such as spatial localizations of lattice vibrations due to their random structures from those in the perfect graphene nanoribbons. These differences manifest themselves in the predicted temperature behavior of the constant-volume specific heat capacity of both structures.

DOI： 10.1016/j.commatsci.2013.06.047

Language：English   Publishing type：Research paper (scientific journal)  

Graphene nanoribbons (GNRs) were grown on n-type vicinal 6H-SiC substrates as a template, consisting of periodic nanosurface, by chemical vapor deposition (CVD). Selective growth was achieved and resulted in narrow (5-10nm width) and millimeter-long GNRs. The GNRs contained randomly rotated domains, however, initial nuclei indicated single domain features, possibly aligned to step edges. The resistivity measurement on GNRs grown on semi-insulating SiC substrate indicated transport properties only along GNRs but no current flow across GNRs, indicating growth of electrically isolated bunches of GNRs.

DOI： 10.7567/APEX.6.055102

Language：English   Publishing type：Research paper (scientific journal)  

Graphene nanoribbons (GNRs) were grown on n-type vicinal 6H-SiC substrates as a template, consisting of periodic nanosurface, by chemical vapor deposition (CVD). Selective growth was achieved and resulted in narrow (5-10 nm width) and millimeter-long GNRs. The GNRs contained randomly rotated domains, however, initial nuclei indicated single domain features, possibly aligned to step edges. The resistivity measurement on GNRs grown on semi-insulating SiC substrate indicated transport properties only along GNRs but no current flow across GNRs, indicating growth of electrically isolated bunches of GNRs. (c) 2013 The Japan Society of Applied Physics

DOI： 10.7567/APEX.6.055102

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevB.87.159907

Language：English  

DOI： 10.1103/PhysRevB.87.159907

Language：English   Publishing type：Research paper (scientific journal)  

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [11̄00], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at K points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV.

DOI： 10.1103/PhysRevB.87.121407

Language：English   Publishing type：Research paper (scientific journal)  

We present a method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consists of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1 (1) over bar 00], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high-density GNRs, enable us to observe the electronic structure at K points by angle-resolved photoemission spectroscopy, showing a clear band-gap opening of at least 0.14 eV. DOI: 10.1103/PhysRevB. 87.121407

DOI： 10.1103/PhysRevB.87.121407

Language：English   Publishing type：Research paper (scientific journal)  

Experimental results of Au/Ni Schottky contacts formed on three kinds of p-GaN layers, which were grown at different turn-off temperatures (300, 600, and 900 °C) of NH₃ gas supply (T_NH3) upon cooling at the end of growth to control the surface stoichiometry are reported. In the internal photoemission results, the Schottky barrier heights of all the samples were as large as around 2.2 eV, and a slight increase of 0.1 eV was observed when T _NH3 decreased from 900 to 300 °C. At the same time, carrier capture and emission from acceptor-like midgap-level defects decreased as T _NH3 decreased. The N-rich cooling condition tends to passivate the acceptor-type defects or create donor-type defects for compensation, and the pinning position at the interface might be moved slightly toward the conduction band edge.

DOI： 10.7567/JJAP.52.01AF05

Language：English   Publishing type：Research paper (scientific journal)  

Thermal decomposition of vicinal SiC substrates with self-organized periodic nanofacets is a promising method to produce large graphene sheets toward the commercial exploitation of graphenes superior electronic properties. The epitaxial graphene films grown on vicinal SiC comprise two distinct regions of terrace and step; and typically exhibit anisotropic electron transport behavior, although limited areas in the graphene film showed ballistic transport. To evaluate the role of terraces and steps in electron transport properties, we compared graphene samples with terrace and step regions grown on 4H-SiC(0001). Arrays of field effect transistors were fabricated on comparable graphene samples with their channels parallel or perpendicular to the nanofacets to identify the source of measured reduced mobility. Minimum conductivity and electron mobility increased with the larger proportional terrace region area; therefore, the terrace region has superior transport properties to step regions. The measured electron mobility in the terrace region, 1000 cm2/Vs, is 10 times larger than that in the step region, 100 cm2/Vs.We conclusively determine that parasitic effects originate in regions of graphene that grow over step edges in 4H-SiC(0001).

DOI： 10.1063/1.3679400

Language：English   Publishing type：Research paper (scientific journal)  

Few-layer epitaxial graphenes grown on vicinal 6H-SiC (0001) were characterized by confocal Raman imaging. In the beginning of the growth, the surface of SiC substrate was covered with monolayer graphene. Next, few-layer graphenes started to grow toward directions perpendicular to [11-20] of the SiC substrate. The shift in the G-peak was not straightforward with the increase in number of graphene layers. This result can be interpreted that the in-plane compressive stress from the substrate depends on the domain size of graphene. The 2D-peak frequency shifted to higher frequency side due to strong compressive strain from the substrate with increasing of the growth times.

DOI： 10.1016/j.diamond.2012.02.017

Language：English   Publishing type：Research paper (scientific journal)  

Uniaxial deformation of the conical conduction band of single-layer graphene is investigated on a SiC(0001) substrate vicinal to the [1 (1) over bar 00] direction using angle-resolved photoemission spectroscopy. The substrate surface consists of highly anisotropic terraces with sub-10 nm width in the [1 (1) over bar 00] direction as confirmed by scanning tunneling microscopy and atomic force microscopy. Graphene covers all the substrate surface including the substrate steps. The substrate provides a unidirectional potential for the graphene on that. The group velocities of the conduction band at K points are reduced in the direction parallel to the strong potential modulation while they are unchanged in the perpendicular direction.

DOI： 10.1103/PhysRevB.85.195416

Language：English   Publishing type：Research paper (scientific journal)  

Few-layer epitaxial graphenes grown on vicinal 6H-SiC (0001) were characterized by confocal Raman imaging. In the beginning of the growth, the surface of SiC substrate was covered with monolayer graphene. Next, few-layer graphenes started to grow toward directions perpendicular to [11-20] of the SiC substrate. The shift in the G-peak was not straightforward with the increase in number of graphene layers. This result can be interpreted that the in-plane compressive stress from the substrate depends on the domain size of graphene. The 2D-peak frequency shifted to higher frequency side due to strong compressive strain from the substrate with increasing of the growth times. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.diamond.2012.02.017

Language：English   Publishing type：Research paper (scientific journal)  

Uniaxial deformation of the conical conduction band of single-layer graphene is investigated on a SiC(0001) substrate vicinal to the [11̄00] direction using angle-resolved photoemission spectroscopy. The substrate surface consists of highly anisotropic terraces with sub-10 nm width in the [11̄00] direction as confirmed by scanning tunneling microscopy and atomic force microscopy. Graphene covers all the substrate surface including the substrate steps. The substrate provides a unidirectional potential for the graphene on that. The group velocities of the conduction band at K points are reduced in the direction parallel to the strong potential modulation while they are unchanged in the perpendicular direction.

DOI： 10.1103/PhysRevB.85.195416

Publishing type：Research paper (scientific journal)  

Thermal decomposition of vicinal SiC substrates with self-organized periodic
nanofacets is a promising method to produce large graphene sheets toward the
commercial exploitation of graphene’s superior electronic properties. The epitaxial
graphene films grown on vicinal SiC comprise two distinct regions of terrace and
step; and typically exhibit anisotropic electron transport behavior, although limited
areas in the graphene film showed ballistic transport. To evaluate the role of terraces
and steps in electron transport properties, we compared graphene samples with
terrace and step regions grown on 4H-SiC(0001). Arrays of field effect transistors
were fabricated on comparable graphene samples with their channels parallel or perpendicular
to the nanofacets to identify the source of measured reduced mobility.
Minimum conductivity and electron mobility increased with the larger proportional
terrace region area; therefore, the terrace region has superior transport properties to
step regions. The measured electron mobility in the terrace region, ∼1000 cm2/Vs, is
10 times larger than that in the step region, ∼100 cm2/Vs.We conclusively determine
that parasitic effects originate in regions of graphene that grow over step edges in
4H-SiC(0001).

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial 3C-SiC (β-SiC) thin layers are grown on Si (001) substrates by carbonization in moderate-pressure microwave plasmas, typically used for diamond film deposition. The substrate temperature and the CH₄ gas concentration diluted in H₂ gas are varied from 1000 to 1200°C and from 2 to 8vol.%, respectively. The grown layers are characterized by scanning electron microscopy, Raman spectroscopy, x-ray diffraction, Fourier transform infrared spectroscopy, reflection high energy electron diffraction, and energy-dispersive x-ray spectroscopy. For 2% CH₄, epitaxial 3C-SiC about 10nm thick is grown only when the temperature is increased to 1200°C, while polycrystalline 3C-SiC is grown for temperatures less than 1200°C. For 8% CH₄, epitaxial 3C-SiC is grown even for temperatures less than 1200°C, but the thickness of the SiC layer is reduced. Some amounts of amorphous carbon and diamond phases are found to grow on the SiC layers.

DOI： 10.1016/j.surfcoat.2011.04.021

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial 3C-SiC (beta-SiC) thin layers are grown on Si (001) substrates by carbonization in moderate-pressure microwave plasmas, typically used for diamond film deposition. The substrate temperature and the CH4 gas concentration diluted in H-2 gas are varied from 1000 to 1200 degrees C and from 2 to 8 vol.%, respectively. The grown layers are characterized by scanning electron microscopy, Raman spectroscopy, x-ray diffraction, Fourier transform infrared spectroscopy, reflection high energy electron diffraction, and energy-dispersive x-ray spectroscopy. For 2% CH4, epitaxial 3C-SiC about 10 nm thick is grown only when the temperature is increased to 1200 degrees C, while polycrystalline 3C-SiC is grown for temperatures less than 1200 degrees C. For 8% CH4, epitaxial 3C-SiC is grown even for temperatures less than 1200 degrees C, but the thickness of the SiC layer is reduced. Some amounts of amorphous carbon and diamond phases are found to grow on the SiC layers. (C) 2011 Elsevier By. All rights reserved.

DOI： 10.1016/j.surfcoat.2011.04.021

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial 3C-SiC (β-SiC) thin layers are grown on Si (001) substrates by carbonization in moderate-pressure
microwave plasmas, typically used for diamond film deposition. The substrate temperature and the CH4 gas
concentration diluted in H2 gas are varied from 1000 to 1200 °C and from 2 to 8 vol.%, respectively. The grown
layers are characterized by scanning electron microscopy, Raman spectroscopy, x-ray diffraction, Fourier
transform infrared spectroscopy, reflection high energy electron diffraction, and energy-dispersive x-ray
spectroscopy. For 2% CH4, epitaxial 3C-SiC about 10 nm thick is grown only when the temperature is increased
to 1200 °C, while polycrystalline 3C-SiC is grown for temperatures less than 1200 °C. For 8% CH4, epitaxial
3C-SiC is grown even for temperatures less than 1200 °C, but the thickness of the SiC layer is reduced. Some
amounts of amorphous carbon and diamond phases are found to grow on the SiC layers.

Publishing type：Research paper (scientific journal)  

The 2-steradian (full hemisphere) Si 2p and C 1s photoelectron intensity angular distributions (PIADs) of the 6HSiC(0001) surface 4 off towards the [1100] direction were measured. In a bulk crystal, pairs of mirrored local atomicsites with respect to the {1100} planes exist. Thus, a sixfold symmetry is expected for PIADs from the bulk. However, all the measured PIADs showed a threefold symmetry owing to the preferential appearance of terraces with one type of local atomic site caused by anisotropic step bunching along the [1120] direction. Taking the finite inelastic mean free path of photoelectrons into account, PIADs for one kind of Si and C atomic sites were successfully derived by solving an inverse matrix. Three strong forward focusing peaks due to nearby Si and C atoms have been separated from those formed by farther atoms. They showed a circular dichroism of rotational shift around the incident-light axis, which corresponds to the parallax in stereo viewing.

Language：English   Publishing type：Research paper (other academic)  

The epitaxial silicon oxynitride (SiON) layer grown on a 6H-SiC(0001) surface is studied with core level photoemission spectroscopy. Si 2p spectra show three spectral components other than the bulk one. Chemical shifts and emission angle dependence of these components are well explained within a framework of a determined structure model of the SiON layer.

DOI： 10.4028/www.scientific.net/MSF.675-677.15

Language：English   Publishing type：Research paper (scientific journal)  

The 2 pi-steradian (full hemisphere) Si 2p and C 1s photoelectron intensity angular distributions (PIADs) of the 6H-SiC(0001) surface 4 degrees off towards the [1 (1) over bar 00] direction were measured. In a bulk crystal, pairs of mirrored local atomic sites with respect to the {1 (1) over bar 00} planes exist. Thus, a sixfold symmetry is expected for PIADs from the bulk. However, all the measured PIADs showed a threefold symmetry owing to the preferential appearance of terraces with one type of local atomic site caused by anisotropic step bunching along the [11 (2) over bar0] direction. Taking the finite inelastic mean free path of photoelectrons into account, PIADs for one kind of Si and C atomic sites were successfully derived by solving an inverse matrix. Three strong forward focusing peaks due to nearby Si and C atoms have been separated from those formed by farther atoms. They showed a circular dichroism of rotational shift around the incident-light axis, which corresponds to the parallax in stereo viewing.

DOI： 10.1143/JPSJ.80.013601

Language：English   Publishing type：Research paper (scientific journal)  

The 2π-steradian (full hemisphere) Si 2p and C 1s photoelectron intensity angular distributions (PIADs) of the 6H-SiC(0001) surface 4° off towards the [11̄00] direction were measured. In a bulk crystal, pairs of mirrored local atomic sites with respect to the {11̄00} planes exist. Thus, a sixfold symmetry is expected for PIADs from the bulk. However, all the measured PIADs showed a threefold symmetry owing to the preferential appearance of terraces with one type of local atomic site caused by anisotropic step bunching along the [112̄0] direction. Taking the finite inelastic mean free path of photoelectrons into account, PIADs for one kind of Si and C atomic sites were successfully derived by solving an inverse matrix. Three strong forward focusing peaks due to nearby Si and C atoms have been separated from those formed by farther atoms. They showed a circular dichroism of rotational shift around the incident-light axis, which corresponds to the parallax in stereo viewing.

DOI： 10.1143/JPSJ.80.013601

Language：English   Publishing type：Research paper (scientific journal)  

Few layer epitaxial graphenes (1.8-3.0 layers) grown on vicinal 6H-SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers.

DOI： 10.1063/1.3466150

Language：English   Publishing type：Research paper (scientific journal)  

Massless pi bands of graphene grown on a SiC(0001) substrate can be affected by the scattering at the boundaries and the interface superstructure. We investigated the pi band structure and width of the single-and double-layer graphenes grown on a vicinal SiC(0001) substrate using angle-resolved photoemission spectroscopy. The pi electron scattering at the substrate steps makes the spectrum width anisotropic but no difference occurs in the pi band shape. Quasi-2x2 replicas of the pi band due to the interface 6 root 3x6 root 3R30 degrees superstructure were observed in the single-layer graphene while they were absent in the double-layer graphene.

DOI： 10.1103/PhysRevB.82.045428

Language：English   Publishing type：Research paper (scientific journal)  

Few layer epitaxial graphenes (1.8-3.0 layers) grown on vicinal 6H-SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3466150]

DOI： 10.1063/1.3466150

Language：English   Publishing type：Research paper (scientific journal)  

Massless π bands of graphene grown on a SiC(0001) substrate can be affected by the scattering at the boundaries and the interface superstructure. We investigated the π band structure and width of the single- and double-layer graphenes grown on a vicinal SiC(0001) substrate using angle-resolved photoemission spectroscopy. The π electron scattering at the substrate steps makes the spectrum width anisotropic but no difference occurs in the π band shape. Quasi- 2×2 replicas of the π band due to the interface 6√3×6√3R30°superstructure were observed in the single-layer graphene while they were absent in the double-layer graphene.

DOI： 10.1103/PhysRevB.82.045428

Language：English   Publishing type：Research paper (scientific journal)  

Anisotropic transport in graphene field-effect transistors fabricated on a vicinal SiC substrate with a self-organized periodic nanofacet structure is investigated. Graphene thermally grown on a vicinal substrate contains two following regions: atomically flat terraces and nanofacets (atomically stepped slopes). The graphene film at a nanofacet is continuously connected between two neighboring terrace films. Anisotropic transport properties are clearly observed, indicating a difference in the graphene properties of the two regions. The observed anisotropic properties are discussed in terms of the effects of nanofacet structures on conductivity and electron mobility.

DOI： 10.1063/1.3309701

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial graphene is formed on vicinal SiC (0001) surfaces via high temperature annealing in vacuum. Steps act as a significant "kicker" of graphene nucleation to feed C atoms. At elevated temperatures, graphene growth is controlled by the decomposition of Si-C bonds at step edges, Si desorption, and C diffusion on the surface. The limited Si desorption is due to the dependence of the growth rate on the thickness of graphene layers. The fabricated graphene layer(s) acts as a Si-diffusion barrier, which in turn induces local thermal equilibrium between the graphene layer and the SiC surface. C atoms preferentially diffuse along the steps, resulting in anisotropic layer-by-layer growth, which is characteristic in this system.

DOI： 10.1103/PhysRevB.81.041406

Language：English   Publishing type：Research paper (other academic)  

We report microscopic Raman scattering studies of epitaxial graphene grown on SiC substrates using a deep-ultraviolet (UV) laser excitation at 266 nm to elucidate the interaction between the graphene layer and the substrate. The samples were grown on the Si-face of vicinal 6H-SiC (0001) substrates by sublimation of Si from SiC. The G band of the epitaxial graphene layer was clearly observed without any data manipulation. Increasing the number of graphene layers, the peak frequency of the G-band decreases linearly, while the peak width and the intensity increase. The G-band frequency of the graphene layers on SiC is higher than those of exfoliated graphene, which has been ascribed to compression from the substrate.

DOI： 10.4028/www.scientific.net/MSF.645-648.611

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial graphene is formed on vicinal SiC(0001) surfaces via high temperature annealing in vacuum. Steps act as a significant "kicker" of graphene nucleation to feed C atoms. At elevated temperatures, graphene growth is controlled by the decomposition of Si-C bonds at step edges, Si desorption, and C diffusion on the surface. The limited Si desorption is due to the dependence of the growth rate on the thickness of graphene layers. The fabricated graphene layer(s) acts as a Si-diffusion barrier, which in turn induces local thermal equilibrium between the graphene layer and the SiC surface. C atoms preferentially diffuse along the steps, resulting in anisotropic layer-by-layer growth, which is characteristic in this system.

DOI： 10.1103/PhysRevB.81.041406

Language：English   Publishing type：Research paper (scientific journal)  

Electronic structures of a silicon-oxynitride (SiON) layer (∼0.6 nm in thickness) epitaxially grown on 6H-SiC (0001) were investigated on atomic-layer scale using soft x-ray absorption spectroscopy and x-ray emission spectroscopy (XAS and XES) and first-principles calculations. The SiON layer has a hetero-double-layered structure: an interfacial silicon nitride layer and a silicon oxide overlayer. The element-specific XAS and XES measurements revealed layer-resolved energy-band profiles. Measured gap sizes are 6.3±0.6 eV at the nitride layer and 8.3±0.8 eV at the oxide layer. The nitride and oxide layers have almost the same energy of conduction-band minimum (CBM) being ∼3 eV higher than CBM of the SiC substrate. The energy-band profiles of the SiON layer are qualitatively reproduced by the calculations. The calculations show that broadening of bandgap of the substrate occurs only at an interfacial SiC bilayer.

DOI： 10.1103/PhysRevB.79.241301

Language：English   Publishing type：Research paper (scientific journal)  

Electronic structures of a silicon-oxynitride (SiON) layer (similar to 0.6 nm in thickness) epitaxially grown on 6H-SiC(0001) were investigated on atomic-layer scale using soft x-ray absorption spectroscopy and x-ray emission spectroscopy (XAS and XES) and first-principles calculations. The SiON layer has a hetero-double-layered structure: an interfacial silicon nitride layer and a silicon oxide overlayer. The element-specific XAS and XES measurements revealed layer-resolved energy-band profiles. Measured gap sizes are 6.3 +/- 0.6 eV at the nitride layer and 8.3 +/- 0.8 eV at the oxide layer. The nitride and oxide layers have almost the same energy of conduction-band minimum (CBM) being similar to 3 eV higher than CBM of the SiC substrate. The energy-band profiles of the SiON layer are qualitatively reproduced by the calculations. The calculations show that broadening of bandgap of the substrate occurs only at an interfacial SiC bilayer.

DOI： 10.1103/PhysRevB.79.241301

Language：English   Publishing type：Research paper (scientific journal)  

We have first observed C₆₀ nuclei formation induced by electron-beam-irradiation during van der Waals epitaxy of solid C₆₀ layers on a graphene sheets. The experimental results strongly indicate that the positional control of the C₆₀ nuclei on the graphene sheet is expected to fabricate new nano-structure by the C₆₀ clusters on the graphene sheet and it will lead us to new principle devices such as the solid Q-bit clusters of N@C_60..

DOI： 10.1016/j.diamond.2008.11.005

Language：English   Publishing type：Research paper (scientific journal)  

Ordered nanofacet structures on vicinal 6H-SiC(0001) surfaces, consisting of pairs of a (0001) basal plane and a (11 (2) over barn) facet, are investigated in terms of stable surface stacking of the (0001) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the (11 (2) over barn) facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.susc.2008.12.025

Language：English   Publishing type：Research paper (scientific journal)  

Ordered nanofacet structures on vicinal 6H-SiC(0 0 0 1) surfaces, consisting of pairs of a (0 0 0 1) basal plane and a (1 1 over(2, ̄) n) facet, are investigated in terms of stable surface stacking of the (0 0 0 1) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the (1 1 over(2, ̄) n) facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed.

DOI： 10.1016/j.susc.2008.12.025

Language：English   Publishing type：Research paper (scientific journal)  

We have investigated on the van der Waals epitaxy of a solid C₆₀ layer on a graphene sheet and have first successfully achieved it. It is expected that the hetero-structures of the solid C₆₀ layer and the graphene sheet opens new opportunities in the carbon nano-electronics.

DOI： 10.1016/j.diamond.2008.03.011

Language：English   Publishing type：Research paper (scientific journal)  

We have investigated on the van der Waals epitaxy of a solid C-60 layer on a graphene sheet and have first successfully achieved it. It is expected that the hetero-structures of the solid C60 layer and the graphene sheet opens new opportunities in the carbon nano-electronics. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.diamond.2008.03.011

Language：English   Publishing type：Research paper (other academic)  

We demonstrate three - dimensional (3D) structuring of materials by femtosecond laser irradiation. The self-organized ordered formation of ripples by a laser raster-scanning on the surface of 4H-SiC is demonstrated. This method is scalable up to areas with sum-millimeter cross-sections. The role of ripple-like structures in the case of the surface and in-bulk micro-structuring of different materials is discussed. The field enhancement effects at the nano-structured surfaces and their role in ripple formation are estimated.

DOI： 10.1117/12.751889

Language：English   Publishing type：Research paper (scientific journal)  

After high-temperature H2 etching, vicinal SiC(0001) surfaces showed periodically ordered nanofacet structures consisting of pairs of (0001) and (112̄n). Here, we found that the characteristic ordering distance of ∼10nm is independent of the vicinal angle (4°-8°off). However, fluctuation in the ordering distance is dependent on the vicinal angle. The 5.7°off surface showed superior periodicity. The classical elastic theory of a surface predicted the characteristic (constant) ordering distance but not the fluctuation in ordering periodicity. By introducing "quantized step bunching" due to periodic surface energy, which is unique to polymorphic SiC, the fluctuation is described.

DOI： 10.1103/PhysRevLett.99.016102

Language：English   Publishing type：Research paper (scientific journal)  

After high-temperature H-2 etching, vicinal SiC(0001) surfaces showed periodically ordered nanofacet structures consisting of pairs of (0001) and (11 (2) over barn). Here, we found that the characteristic ordering distance of similar to 10 nm is independent of the vicinal angle (4 degrees-8 degrees off). However, fluctuation in the ordering distance is dependent on the vicinal angle. The 5.7 degrees off surface showed superior periodicity. The classical elastic theory of a surface predicted the characteristic (constant) ordering distance but not the fluctuation in ordering periodicity. By introducing "quantized step bunching" due to periodic surface energy, which is unique to polymorphic SiC, the fluctuation is described.

DOI： 10.1103/PhysRevLett.99.016102

Language：English   Publishing type：Research paper (scientific journal)  

Nucleation and growth mode of GaN during molecular beam epitaxy on vicinal SiC surfaces, consisting of a pair of self-ordered periodic nanofacets, (0001) and (112̄n), are investigated. Well-defined surface nanostructures on SiC enable us to understand growth physics. Here, the Ga-adsorption process on SiC is noticed in particular, and its effects on initial GaN growth stages are examined using reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). With the presence of a Ga-adlayer on a SiC surface, GaN nucleation occurs at step sites on (112̄n) facets, followed by step-flow growth. In contrast, without a Ga-adlayer, GaN nucleation is predominantly observed on (0001) terraces. Interestingly, the crystal structure of the resultant film differs in each case from a typical wultzite (2H) to a 6H-polytype, without and with a Ga-adlayer, respectively.

DOI： 10.1143/JJAP.46.L348

Language：English   Publishing type：Research paper (scientific journal)  

Nucleation and growth mode of GaN during molecular beam epitaxy on vicinal SiC surfaces, consisting of a pair of self-ordered periodic nanofacets, (0001) and (112n), are investigated. Well-defined surface nanostructures on SiC enable us to understand growth physics. Here, the Ga-adsorption process on SiC is noticed in particular, and its effects on initial GaN growth stages are examined using reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). With the presence of a Ga-adlayer on a SiC surface, GaN nucleation occurs at step sites on (112n) facets, followed by stepflow growth. In contrast, without a Ga-adlayer, GaN nucleation is predominantly observed on (0001) terraces. Interestingly, the crystal structure of the resultant film differs in each case from a typical wultzite (2H) to a 6H-polytype, without and with a Ga-adlayer, respectively.

DOI： 10.1143/JJAP.46.L348

Language：English   Publishing type：Research paper (scientific journal)  

Hydrogen-gas etching of a 6H-SiC(0001) surface and subsequent annealing in nitrogen atmosphere leads to the formation of a silicon oxynitride (SiON) epitaxial layer. A quantitative low-energy electron diffraction analysis revealed that the SiON layer has a hetero-double-layer structure: a silicate monolayer on a silicon nitride monolayer via Si-O-Si bridge bonds. There are no dangling bonds in the unit cell, which explains the fact that the structure is robust against air exposure. Scanning tunneling spectroscopy measured on the SiON layer shows a bulk SiO2-like band gap of ∼9eV. Great potential of this new epitaxial layer for device applications is described.

DOI： 10.1103/PhysRevLett.98.136105

Language：English   Publishing type：Research paper (scientific journal)  

Hydrogen-gas etching of a 6H-SiC(0001) surface and subsequent annealing in nitrogen atmosphere leads to the formation of a silicon oxynitride (SiON) epitaxial layer. A quantitative low-energy electron diffraction analysis revealed that the SiON layer has a hetero-double-layer structure: a silicate monolayer on a silicon nitride monolayer via Si-O-Si bridge bonds. There are no dangling bonds in the unit cell, which explains the fact that the structure is robust against air exposure. Scanning tunneling spectroscopy measured on the SiON layer shows a bulk SiO2-like band gap of similar to 9 eV. Great potential of this new epitaxial layer for device applications is described.

DOI： 10.1103/PhysRevLett.98.136105

Language：English   Publishing type：Research paper (scientific journal)  

Extremely high pressures (∼10TPa) and temperatures (5×105K) have been produced using a single laser pulse (100nJ, 800 nm, 200 fs) focused inside a sapphire crystal. The laser pulse creates an intensity over 1014W/cm2 converting material within the absorbing volume of ∼0.2μm3 into plasma in a few fs. A pressure of ∼10TPa, far exceeding the strength of any material, is created generating strong shock and rarefaction waves. This results in the formation of a nanovoid surrounded by a shell of shock-affected material inside undamaged crystal. Analysis of the size of the void and the shock-affected zone versus the deposited energy shows that the experimental results can be understood on the basis of conservation laws and be modeled by plasma hydrodynamics. Matter subjected to record heating and cooling rates of 1018K/s can, thus, be studied in a well-controlled laboratory environment.

DOI： 10.1103/PhysRevLett.96.166101

Language：English   Publishing type：Research paper (scientific journal)  

Nucleation stages of carbon nanotubes (CNTs) on a SiC(0001) Si-face (4° off toward [112̄0]) by surface decomposition are investigated by means of in-situ scanning tunneling microscopy (STM). After a relatively short heating time at 1600°C, moiré patterns and small nuclei appear on the surface due to the evaporation of Si atoms and, as a result of this, surface graphitization. Modelling of the moiré pattern indicates the formation of a single graphene sheet on top of the SiC surface. Local swelling (∼1 nm in diameter) of such a graphene sheet, possibly resulting from the compressive stress at the graphene/ SiC interface, gives rise to a CNT nucleus. In the stages that follow, the nucleus is continuously and unidirectionally prolonged and rolled-up, which may result in a final form such as a CNT.

DOI： 10.1143/JJAP.44.L803

Language：English   Publishing type：Research paper (scientific journal)  

On-axis and vicinal 4H- and 6H-SiC(0001) surfaces have been investigated using atomic force microscopy and cross-sectional high-resolution transmission electron microscopy. We observed one-dimensionally ordered SiC surface nanostructures, which were energetically induced by self-ordering of nano-facets on any surfaces, regardless of polytypes and vicinal angles, after gas etching at high temperature. Two facet planes were typically revealed; (0001) and high index (112̄n) that are formed by equilibrium surface phase separation. A (112̄n) surface may have a free energy minimum due to attractive step-step interactions.

Language：English   Publishing type：Research paper (scientific journal)  

We present the growth of AlN films on vicinal substrates of SiC(0001), The layers were grown by plasma-assisted molecular beam epitaxy (PAMBE) and examined in-situ using reflection high-energy electron diffraction (RHEED). Morphological and structural properties were characterized by atomic force microscopy (AFM) and High Resolution X-Ray Diffraction (HRXRD), respectively. The obtained results by AFM show two main differences with AlN films grown on nominally flat SiC substrates: a) the absence of spiral growth hillocks, and b) the formation of predominantly straight steps. HRXRD measurements on misoriented substrate rocking curves have revealed good structural quality of AlN films (tilt and twist), as well as a disorientation of the c-planes of the AlN with respect to the substrate.

DOI： 10.1002/pssb.200303268

Language：English   Publishing type：Research paper (scientific journal)  

The fabrication and evaluation of a UV light-emitting diode (LED) incorporating GaN quantum dots as the active layer is demonstrated. The GaN quantum dots were fabricated on an Al_xGa_1-xN (x ∼ 0.1) surface using Si as an antisurfactant. Exposing the Al_xGa_1-xN surface to the Si antisurfactant prior to GaN growth enabled the formation of quantum dots on a surface where growth by the Stranski-Krastanov mode would not be possible. A fairly high density of dots (10¹⁰-10¹¹ cm^-2) with controllable dot sizes was achieved. Room temperature luminescence at 360 nm was clearly observed during current injection (cw) into an LED structure including the GaN quantum dots. The origin of the electroluminescence is discussed by comparing it to photoluminescence measurements.

DOI： 10.1143/jjap.42.l885

Language：English   Publishing type：Research paper (scientific journal)  

A study was performed on the structural anisotropy in gallium nitride (GaN) films. The films were investigated in terms of nucleation, coalescence and growth front evolution. The effects of silicon carbide (SiC) surface configuration on GaN film growth physics were examined. The results showed an anisotropic characteristic for the GaN film deposited on the vicinal stepped SiC surfaces.

DOI： 10.1063/1.1605791

Language：English   Publishing type：Research paper (scientific journal)  

A novel dislocation reducing mechanism was successfully introduced into the conventional 2-step metalorganic vapor phase epitaxial (MOVPE) growth method of GaN/sapphire wafer by inserting an additional intermediate-temperature (IT)-GaN buffer between the low-temperature (LT)-GaN buffer and the main high-temperature (HT)-GaN layer. During the growth of the IT-GaN buffer, high-density islands with faceted slopes were formed. Vertically propagating dislocations from the GaN/sapphire interface were bent at the faceted slopes so as to gather at the island/island boundaries, where dislocation loops were formed efficiently. Due to such an island induced dislocation control mechanism, the dislocation density was reduced markedly from the 10⁹ cm^-2 range to the mid-10⁸ cm^-2 range during the growth of the IT-GaN buffer. As a result, the GaN/sapphire wafers with low dislocation density of 3 × 10⁸ cm^-2 were successfully grown by such a 3-step MOVPE method, whereas high-density dislocations (1 × 10⁹ cm^-2) remained in the final GaN/sapphire wafers grown by the conventional 2-step growth method without dislocation control.

Language：English   Publishing type：Research paper (scientific journal)  

The linear alignment of self-assembled GaN quantum dots (QD) grown by molecular beam epitaxy on AlN using vicinal SiC substrates was demonstrated. It was shown that stepped AlN layers can be grown on such SiC substrates depending on growth parameters. The possibility of controlling the island spatial distribution was also demonstrated using atomic force microscopy.

DOI： 10.1063/1.1538334

Language：English   Publishing type：Research paper (scientific journal)  

II-VI quantum dots (QDs) have been under debate on their ripening properties for some time, and a unified understanding of this phenomenon will be given in this paper. Improvement of QD size uniformity is the main concern for practical applications. Scaling of the dot size distributions on some II-VI QDs will be discussed for the better understanding of the size distributions and for the improvement of the dot size uniformity. QD size distributions also tend to smear out the special features of QDs which are expected to modulate electron-photon interactions in optical microcavities coupled with QDs. Study of energy relaxation processes in QDs reveals the possibility of selectively exciting QDs which are in resonance with longitudinal optical (LO)-phonon emission processes. This scheme will be applied to the study of the strong coupling regime of optical microcavities coupled with QDs. CdS/ZnS QDs embedded in pyramidal three-dimensional microcavities are also examined to study the capability to observe enhanced spontaneous emissions by the coupling with cavity modes in pyramidal microcavities.

DOI： 10.1016/S0022-0248(02)01862-6

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II-VI quantum dots (QDs) have been Linder debate on their ripening properties for some time, and a unified understanding of this phenomenon will be given in this paper. Improvement of QD size uniformity is the main concern for practical applications. Scaling of the dot size distributions on some II-VI QDs will be discussed for the better understanding of the size distributions and for the improvement of the dot size uniformity. QD size distributions also tend to smear out the special features of QDs which are expected to modulate electron-photon interactions in optical microcavities coupled with QDs. Study of energy relaxation processes in QDs reveals the possibility of selectively exciting QDs which are in resonance with longitudinal optical (LO)-phonon emission processes. This scheme will be applied to the study of the strong coupling regime of optical microcavities coupled with QDs. CdS/ZnS QDs embedded in pyramidal three-dimensional microcavities are also examined to study the capability to observe enhanced spontaneous emissions by the coupling with cavity modes in pyramidal microcavities. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(02)01862-6

Language：English   Publishing type：Research paper (scientific journal)  

Vicinal [Formula presented] and [Formula presented] surfaces have been investigated using atomic force microscopy and cross-sectional high-resolution transmission electron microscopy. We observed the characteristic self-ordering of nanofacets on any surface, regardless of polytypes and vicinal angles, after gas etching at high temperature. Two facet planes are typically revealed: (0001) and high index [Formula presented] that are induced by equilibrium surface phase separation. A [Formula presented] plane may have a free energy minimum due to attractive step-step interactions. The differing ordering distances in [Formula presented] and [Formula presented] polytypes imply the existence of SiC polytypic dependence on nanofaceting. Thus, it should be possible to control SiC surface nanostructures by selecting a polytype, a vicinal angle, and an etching temperature.

DOI： 10.1103/PhysRevLett.91.226107

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The fabrication and evaluation of a UV light-emitting diode (LED) incorporating GaN quantum dots as the active layer is demonstrated. The GaN quantum dots were fabricated on an Al_xGa_1-xN (x∼0.1) surface using Si as an antisurfactant. Exposing the Al_xGa _1-xN surface to the Si antisurfactant prior to GaN growth enabled the formation of quantum dots on a surface where growth by the Stranski-Krastanov mode would not be possible. A fairly high density of dots (10 ¹⁰-10¹¹ cm^-2) with controllable dot sizes was achieved. Room temperature luminescence at 360 nm was clearly observed during current injection (cw) into an LED structure including the GaN quantum dots. The origin of the electroluminescence is discussed by comparing it to photoluminescence measurements.

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High quality epitaxial films of Al _xGa _1-xN, grown on SiC substrates, were investigated using spatially resolved cathodoluminescence (CL), scanning electron microscopy, and atomic force microscopy. A variation in the observed peak energy position of the CL was related to alloy fluctuations. CL was used to reveal relative alloy fluctuations of approximately 1% on a sub-micrometer scale, with a precision difficult to surpass with other available techniques. By correlating data from the different techniques, a model was derived. The main feature of it is an alloy fluctuation on the micrometer scale, seeded during the initial growth and extending through the epitaxial film. These alloy fluctuations seems to be related to terrace steps (≈5 nm in height), formed preferentially at scratches on the SiC surface. This investigation indicates that the initial growth of epitaxial films is critical and structures formed at the beginning of the growth tend to persist throughout the growth. Further, a strain gradient from the SiC interface extending towards the surface, was observed.

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The organisation of GaN quantum dots on vicinal (0001) AlN surfaces has been investigated. Such surfaces were obtained using (0001)-misoriented SiC substrates. It is shown that AlN growth leads to a step bunching instability that can be kinetically controlled by temperature. The study of the GaN growth mechanism reveals a preferential alignment along AlN step edges and facets. Finally, the possibility of obtaining an efficient control of the island spatial distribution via step density and periodicity is demonstrated.

DOI： 10.1002/1521-3951(200212)234:3<939::AID-PSSB939>3.0.CO;2-L

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Nitrogen (N) doping in ZnO is studied to realize reproducible p-type conductivity. Undoped ZnO layers prepared on a-face of sapphire substrates with H₂O vapor-assisted growth showed n-type conductivity. However, N-doped ZnO (ZnO:N) layers grown in the similar manner showed the type conversion to p-type conductivity. As-grown p-type ZnO:N layers showed low net acceptor concentrations (N_A-N_D) of ∼ 10¹⁴ cm^-3, but thermal annealing of the N-doped ZnO samples as well as the optimization of growth parameters increased the N_A-N_D up to ∼ 5 × 10¹⁶ cm^-3. Photoluminescence measurements showed consistent spectra with the electrical properties by a clear conversion from neutral donor-bound exciton emission in n-ZnO to neutral acceptor-bound exciton emission in the p-ZnO layers.

DOI： 10.1143/jjap.41.l1281

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Nitrogen (N) doping in ZnO is studied to realize reproducible p-type conductivity. Undoped ZnO layers prepared on a-face of sapphire substrates with H2O vapor-assisted growth showed n-type conductivity. However, N-doped ZnO (ZnO:N) layers grown in the similar manner showed the type conversion to p-type conductivity. As-grown p-type ZnO:N layers showed low net acceptor concentrations (N-A-N-D) of similar to10(14) cm(-3), but thermal annealing of the N-doped ZnO samples as well as the optimization of growth parameters increased the N-A-N-D up to similar to5 x 10(16) cm(-3). Photoluminescence measurements showed consistent spectra with the electrical properties by a clear conversion from neutral donor-bound exciton emission in n-ZnO to neutral acceptor-bound exciton emission in the p-ZnO layers.

DOI： 10.1143/JJAP.41.L1281

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Achieving p-type conductivity in the wide-bandgap semiconductors is still a major research challenge. The AlNSiC alloys have the potential for p- and n-type conductivity based on the properties inherited from the easily-to-dope SiC alloy. However, the growth of high quality AlN-SiC alloys was not achieved yet. In this work we propose a new approach for the growth of AlN-SiC alloys by sequential supply epitaxy (SSE) in a low-pressure metalorganic chemical vapor deposition reactor. The alternated supply of trimethyl aluminum and tetraethyl silane during a first sequence followed by the supply of ammonia and ethylene during a second sequence is used to reduce down to almost eliminating the gas-phase reactions. Also, the SSE technique makes it possible to grow the AlN-SiC solid solutions at temperatures rather low, in the range 1200-1300degreesC. Solid solutions with a rich content of AlN were investigated. The electron spectroscopy for chemical analysis (ESCA) measurements show that SiC is incorporated in AlN and solid solution is actually achieved. The alloy compositions as revealed by ESCA were in agreement with those estimated from X-ray diffraction. The full width at half maximum of the AlN-SiC diffraction peak is in the range 200-300 s. The surface of the epilayers was mirror-like and had a roughness with a RMS value < 5 nm. (C) 2002 Eslevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(01)01703-1

Language：English   Publishing type：Research paper (scientific journal)  

Achieving p-type conductivity in the wide-bandgap semiconductors is still a major research challenge. The AlNSiC alloys have the potential for p- and n-type conductivity based on the properties inherited from the easily-to-dope SiC alloy. However, the growth of high quality AlN-SiC alloys was not achieved yet. In this work we propose a new approach for the growth of AlN-SiC alloys by sequential supply epitaxy (SSE) in a low-pressure metalorganic chemical vapor deposition reactor. The alternated supply of trimethyl aluminum and tetraethyl silane during a first sequence followed by the supply of ammonia and ethylene during a second sequence is used to reduce down to almost eliminating the gas-phase reactions. Also, the SSE technique makes it possible to grow the AlN-SiC solid solutions at temperatures rather low, in the range 1200-1300°C. Solid solutions with a rich content of AlN were investigated. The electron spectroscopy for chemical analysis (ESCA) measurements show that SiC is incorporated in AlN and solid solution is actually achieved. The alloy compositions as revealed by ESCA were in agreement with those estimated from X-ray diffraction. The full width at half maximum of the AlN-SiC diffraction peak is in the range 200-300 s. The surface of the epilayers was mirror-like and had a roughness with a RMS value <5nm.

DOI： 10.1016/S0022-0248(01)01703-1

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Nitrogen and aluminum near K-edge absorption measurements of wurtzite AlN, GaN and InN, and their ternary compounds (AlGaN, InGaN and InAlN) at various molar fractions have been performed using synchrotron radiation. Using the linear polarization of synchrotron radiation, absorption measurements with different incident light angles were also performed. The spectral distribution of the nitrogen K absorption spectra clearly depends on both the incident light angles and the molar fractions of the samples. That of the aluminum K absorption spectra also shows the clear angle dependence, but it does not show the drastic molar dependence. Spectral shape comparisons among the various molar fractions, different incident angles and between the two ion sites are discussed. The numerical component analysis of the K absorption spectra is also presented.

DOI： 10.1002/1521-3951(200111)228:2<461::AID-PSSB461>3.0.CO;2-Q

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Self-assembled GaN quantum dots (QDs) grown on Al^0.15Ga^0.85N using Si as anti-surfactant were investigated by resonant Raman scattering. Phonons of GaN QDs of different sizes were probed selectively by using laser excitation energies of 3.53 and 5.08 eV. Phonon confinement effects were evidenced in GaN QDs of 2-3 nm height. Resonant Raman scattering on GaN grown on Al^0.23Ga^0.77N after the deposition of an increasing amount of Si anti-surfactant, i.e., the morphological transition from a GaN quantum well (2D) to GaN quantum dots (0D), was also investigated.

DOI： 10.1002/1521-3951(200111)228:1<195::AID-PSSB195>3.0.CO;2-B

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Nitrogen and aluminum near K-edge absorption measurements of wurtzite AIN, GaN and InN, and their ternary compounds (AlGaN, InGaN and InAIN) at various molar fractions have been performed using synchrotron radiation. Using the linear polarization of synchrotron radiation, absorption measurements with different incident light angles were also performed. The spectral distribution of the nitrogen K absorption spectra clearly depends on both the incident light angles and the molar fractions of the samples. That of the aluminum K absorption spectra also shows the clear angle dependence, but it does not show the drastic molar dependence. Spectral shape comparisons among the various molar fractions, different incident angles and between the two ion sites are discussed. The numerical component analysis of the K absorption spectra is also presented.

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The role of anti-surfactant atoms in the form of an atomic nano-mask is discussed with reference to specific examples of quantum dot formation on the surface of a substrate and the termination of dislocations in the growth of GaN. Such examples demonstrate the significant role of additional (doped) atoms for crystal growth. (C) 2001 Published by Elsevier Science B.V.

DOI： 10.1016/S1386-9477(01)00181-3

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Nucleation and growth kinetics of AlN films on atomically smooth 6H-SiC (0001) surfaces, which were obtained by HCl etching at elevated temperatures prior to growth, were investigated. The surface morphology and the defect density of AlN films on such surfaces were significantly improved compared to those on as-received SiC surfaces. This is due to enhanced diffusion length and reduced incoherent boundaries at the coalescence regions of the AlN islands. AlN nuclei on the as-received SiC surface were crystallographically misaligned and thus induced incoherent boundaries at the coalescence stage, resulting in the delay of the two-dimensional growth mode transition and defect formation in AlN films.

DOI： 10.1063/1.1377309

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Self-assembled GaN quantum dots grown on Al_0.15Ga_0.85N using Si as antisurfactant have been investigated by resonant Raman scattering. Phonons of GaN quantum dots of different sizes and the Al_0.15Ga_0.85N barrier layer were probed selectively by varying the laser excitation energy from 3.53 to 5.08 eV. Phonon confinement effects were studied on GaN quantum dots of 2-3 nm height. We show that although grown using Si (a common donor for GaN) as an antisurfactant, only a small electron concentration is present in the GaN quantum dots. Implications on the role of Si for the formation of the GaN quantum dots will be discussed.

DOI： 10.1063/1.1347386

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A discussion on the role of anti-surfactant atoms in the form of an atomic nano-mask was presented. This was discussed with reference to specific examples of termination of dislocations in growth of GaN and quantum dot formation on the surface of substrate. The role of additional (doped) atoms for crystal growth was demonstrated.

DOI： 10.1016/S1386-9477(01)00181-3

Language：English   Publishing type：Research paper (scientific journal)  

Monolithic II-VI semiconductor microcavities for the blue-green region grown by metal-organic vapor-phase epitaxy have been demonstrated. ZnSe/MgS-superlattice (ZnSe/MgS-SL) layers were used for the distributed Bragg reflectors (DBRs). The DBR with only 5 periods showed the high reflectivity of 92% at the wavelength of 510 nm due to the large difference of refractive indices between ZnSe acid MgS layers. In a monolithic II-VI microcavity structure based on these DBRs, a clear cavity resonance mode was observed in the blue-green region for the first time. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0022-0248(00)00803-4

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A new approach toward epitaxial growth of group III nitrides using an `anti-surfactant' is presented. Two unique phenomena, quantum dot formation and dislocation termination, were recognized using this approach. The presence of Si atoms as an anti-surfactant on (Al)GaN surfaces modified the nitride epitaxial growth kinetics. These phenomena appeared to be independent; however, the growth mechanisms indicated a common surface event, which included the formation of a monolayer thick Si-N mask (nano-mask) within the fractional coverage on the surface. The SiN nano-mask influenced the morphology of the deposited GaN surface, i.e. quantum structures, and also contributed to the termination of threading dislocations in GaN films.

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Step bunching on 6H-SiC (0001)-vicinal face etched by HCl at 1300-1500 °C is investigated by atomic force microscopy. When the substrate has the inclination toward near 〈0110〉 or even 〈1120〉, continuous parallel and periodic microsteps with six-bilayer height are laid perpendicular to the off direction, although those perpendicular to 〈1120〉 are apt to decompose into three bilayer or less. Formation mechanism of unit-cell-height steps is discussed based on consideration of bond configuration at step edges.

DOI： 10.1063/1.126663

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Main factors which determine the size, the standard deviations which show the degree of the size fluctuations for the average dot height and diameter, and density in ZnSe self-organized quantum dots (QDs) grown on ZnS layers were studied. By lowering the growth temperature the QDs average size and its standard deviation decreased and the density increased due to the slower surface migration. With the application of the scaling theory, it was revealed that the normalized size distributions were uniquely determined by the nucleation process although the apparent standard deviations of the QD sizes were dependent on the growth temperature. The influence of surface roughness of the underneath layer on the formation of the relations of the dot height and diameter was also examined. It was shown that the fluctuation of the surface potential contributes significantly to the apparent standard deviations of ZnSe self-organized QDs sizes.

DOI： 10.1007/s11664-000-0037-0

Language：English   Publishing type：Research paper (scientific journal)  

Main factors which determine the size, the standard deviations which show the degree of the size fluctuations for the average dot height and diameter, and density in ZnSe self-organized quantum dots (QDs) grown on ZnS layers were studied. By lowering the growth temperature the QDs average size and its standard deviation decreased and the density increased due to the slower surface migration. With the application of the scaling theory, it was revealed that the normalized size distributions were uniquely determined by the nucleation process although the apparent standard deviations of the QD sizes were dependent on the growth temperature. The influence of surface roughness of the underneath layer on the formation of the relations of the dot height and diameter was also examined. It was shown that the fluctuation of the surface potential contributes significantly to the apparent standard deviations of ZnSe self-organized QDs sizes.

DOI： 10.1007/s11664-000-0037-0

Language：English   Publishing type：Research paper (scientific journal)  

We report on an investigation of the optical properties of GaN quantum dots (QDs) grown by means of metalorganic vapor phase epitaxy. The growth regime for GaN on Al_xGa_1-xN was observed to change from two- to three-dimensional, forming GaN QDs, when Si was deposited on the Al_xGa_1-xN surface prior to the GaN growth. These QDs showed a redshift of the photo luminescence (PL) energy from the increased Coulomb energy induced by a compression of the exciton Bohr radius. Furthermore, a diminishing temperature-dependent shift of the PL energy with decreasing QD size caused by a reduction of the longitudinal-optical phonon coupling was found. We also show that the size of the QDs is a critical parameter for the optical nonlinearities. For large dots, the dominant nonlinearity in the PL is the bandgap renormalization but when the size of the dots was reduced below the critical size of 10 nm thick and 30 nm diameter, the state-filling effect became dominant.

DOI： 10.1063/1.372429

Language：English   Publishing type：Research paper (scientific journal)  

Optical and electrical properties of GaN films grown on α-Al₂O₃ (0 0 0 1) substrates by GSMBE using In-doping method were investigated. It was found that both of them were improved, compared to those of a nondoped GaN one. μ-PL results at 20 K indicated that In-doped films emit luminescence more uniformly than that of a nondoped one. Furthermore, Hall effect measurements at 300 K showed higher electron mobility of In-doped samples than that of a nondoped one. It is suggested that the presence of In during the growth of GaN films plays a role in reducing the number of structural imperfections to improve the optical and electrical properties.

DOI： 10.1016/S0022-0248(99)00578-3

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ZnSe/ZnMgS distributed Bragg reflectors (DBRs) with a high refractive-index contrast were grown on GaAs (100) substrates by metalorganic vapor-phase epitaxy. The difference of the refractive indices between ZnSe and Zn_0.27Mg_0.73S was estimated to be about 0.52 at 510 nm, which is very large compared with previous II-VI DBRs. The maximum reflectivity of the grown ZnSe/Zn_0.27Mg_0.73S DBRs (with only 5-periods) was measured to be 93% at 510 nm at room temperature. DBRs with a high refractive-index contrast can reduce the penetration depth of light into the DBR and have the potential to increase the strength of the exciton-photon coupling in a microcavity.

DOI： 10.1016/S0022-0248(00)00259-1

Language：English   Publishing type：Research paper (scientific journal)  

Monolithic II-VI semiconductor microcavities for the blue-green region grown by metal-organic vapor-phase epitaxy have been demonstrated. ZnSe/MgS-superlattice (ZnSe/MgS-SL) layers were used for the distributed Bragg reflectors (DBRs). The DBR with only 5 periods showed the high reflectivity of 92% at the wavelength of 510 nm due to the large difference of refractive indices between ZnSe and MgS layers. In a monolithic II-VI microcavity structure based on these DBRs, a clear cavity resonance mode was observed in the blue-green region for the first time.

DOI： 10.1016/S0022-0248(00)00803-4

Language：English   Publishing type：Research paper (scientific journal)  

Epitaxial films of AlGaN grown on SiC have been investigated using cathodoluminescence, photoluminescence, scanning electron microscope and energy dispersive X-ray emission (EDX). It was found that the films were smooth and homogeneous except for occasional pits, which display characteristic luminescence. EDX measurements reveal that the central part of the pits are Ga rich. A model for the structure of the pits and their formation mechanism is presented.

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We report on the first artificial fabrication of self-assembling AlGaN quantum dots (QDs) on AIGaN surfaces using metal organic chemical vapor deposition (MOCVD). The AlGaN QDs are fabricated using a growth mode change from 2-dimensional step-flow growth to 3-dimensional island formation by modifying the AlGaN surface energy with Si anti-surfactant. The average lateral size and the thickness of fabricated AlGaN QDs, as determined by AFM, are approximately 20 nm and 6nm, respectively. The dot density was found to be controlled from 5xl010 cm2 down to 2xl09 cm2 by increasing the dose of Si anti-surfactant. We obtained the photoluminescence (PL) from AlGaN QDs embedded with Alo.ssGao.eaN capping layers. The Al incorporation in AlGaN QDs was controllable within the range of 1-5 %.

Language：English   Publishing type：Research paper (scientific journal)  

We report on the first artificial fabrication of self-assembling AlGaN quantum dots (QDs) on AlGaN surfaces using metal organic chemical vapor deposition (MOCVD). The AlGaN QDs are fabricated using a growth mode change from 2-dimensional step-flow growth to 3-dimensional island formation by modifying the AlGaN surface energy with Si anti-surfactant. The average lateral size and the thickness of fabricated AlGaN QDs, as determined by AFM, are approximately 20 nm and 6nm, respectively. The dot density was found to be controlled from 5×10¹⁰ cm^-2 down to 2×10⁹ cm ^-2 by increasing the dose of Si anti-surfactant. We obtained the photoluminescence (PL) from AlGaN QDs embedded with Al_0.38Ga _0.62N capping layers. The Al incorporation in AlGaN QDs was controllable within the range of 1-5%.

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We have investigated the effects of In doping on the optical properties of GaN films grown by gas-source molecular-beam epitaxy. Time-resolved photoluminescence was carried out to study the transient optical properties of the epitaxial films. In comparison to the undoped GaN film, the spontaneous emission lifetime was prolonged from below 20 to 70 ps by doping with In. Under high-excitation density, stimulated emission was observed from both samples. The threshold excitation density was found to be reduced in the In-doped sample. These significant improvements of the optical properties are attributed to the effective suppression of the formation of the nonradiative recombination centers caused by a change of the growth kinetics induced by a small amount of In supplied during growth of the GaN films.

DOI： 10.1063/1.125178

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We report on an investigation of the coupling between excitons and longitudinal optical phonons as a function of GaN quantum dot size. Photoluminescence measurements of the quantum dots from cryogenic temperatures up to above room temperature are presented. The experiments were performed on ensembles of AlN capped GaN quantum dots grown on an Al_0.15Ga_0.85N surface by means of metalorganic vapor phase epitaxy. The results are analyzed on the basis of a Bose-Einstein-type expression describing the exciton to longitudinal optical phonon coupling of the dots as a function of the lattice temperature. A reduction of the exciton to LO-phonon coupling with decreasing quantum dot size was found.

DOI： 10.1063/1.124876

Language：English   Publishing type：Research paper (scientific journal)  

The relation of the size and luminescence of self-organized ZnSe quantum dots (QDs) was studied. At the growth temperature of 350 °C, the average dot size is 2.0 nm high and 11 nm wide and the density is 1×10¹⁰ cm^-2. The zinc-blende structure of this dot is clear. The measured photoluminescence peaks from these samples show a blueshift and are in good agreement with the calculated quantum size effect estimated by atomic force microscopy.

DOI： 10.1063/1.124333

Language：English   Publishing type：Research paper (scientific journal)  

Uncapped GaN dots on AIGaN barrier material, grown by metal organic chemical vapor deposition on 6H-SiC substrates, were studied. Cathodoluminescence (CL) microscopy and scanning electron microscopy (SEM) were used to investigate both luminescence and structure of individual GaN dots. The correlation between the luminescence and the actual position of self-assembled dots was demonstrated. The position of a dot was established with high resolution SEM and a CL image was used to display the corresponding luminescence. The spectrum from a single dot was obtained by positioning the electron beam on one particular dot. The luminescence from dots with a lateral size of 100 nm and a height of 40 nm was determined to be 3.47 eV.

DOI： 10.1063/1.124147

Language：English   Publishing type：Research paper (scientific journal)  

We demonstrate InGaN and AlGaN quantum dots (QDs) formation on AlGaN surfaces vir metal organic chemical vapor deposition (MOCVD). Si anti-surfactant was used in order to modify surface energy balance for changing growth mode from 2-dimensional step-now growth to 3-dimensional nano-scale island formation. The average lateral size and thickness of the InGaN and AlGaN QDs are 10-20 nm and 5 nm, respectively. Intense photoluminescence (PL) was observed from InGaN QDs even in room temperature. In and Al incorporation in InGaN and AlGaN QDs were estimated to be 22-52% and 1-5 %, respectively, from the PL spectrum.

Language：English   Publishing type：Research paper (scientific journal)  

We demonstrate InGaN and AlGaN quantum dots (QDs) formation on AlGaN surfaces vir metal organic chemical vapor deposition (MOCVD). Si anti-surfactant was used in order to modify surface energy balance for changing growth mode from 2-dimensional step-flow growth to 3-dimensional nano-scale island formation. The average lateral size and thickness of the InGaN and AlGaN QDs are 10-20 nm and 5 nm, respectively. Intense photoluminescence (PL) was observed from InGaN QDs even in room temperature. In and Al incorporation in InGaN and AlGaN QDs were estimated to be 22-52% and 1-5%, respectively, from the PL spectrum.

DOI： 10.1016/S0167-9317(99)00207-5

Language：English   Publishing type：Research paper (scientific journal)  

The enhancement effect of surface decomposition using supersonic beam was found for the first time, in the fabrication of GaN quantum dots on AlGaN/6H-SiC(0 0 0 1) surfaces by gas-source molecular beam epitaxy. GaN quantum dots were successfully fabricated using Si which was supplied by supersonic beam of CH₃SiH₃, while the GaN dots could not be formed using usual beam of CH₃SiH₃. The optical properties of the fabricated GaN quantum dots were investigated by photoluminescence measurements. The advantage by using supersonic beam technique in the enhancement of surface reaction was demonstrated.

DOI： 10.1016/S0022-0248(98)01362-1

Language：English   Publishing type：Research paper (scientific journal)  

High quality GaN films of low etch pit density (10⁷ cm^-2) have been prepared by metal organic chemical vapour deposition (MOCVD) on 6H-SiC substrate with Aln buffer layer. The buffer layer was pretreated by alternating pulsative supply (APS) of trimethyl gallium (TMG) and NH₃ gases. Nitrogen atom is expected to incorporate into the gallium cluster formed by APS process. The sizes of GaN grains at the initial stage have increased by the treatment. The photoluminescence spectrum of the resulting GaN films at 13 K shows sharp near band-edge emission peak at 3.47 eV with 12 meV of FWHM value. The PL intensity of donor-acceptor transition peak at 3.26 eV was reduced with decrease of defect density.

DOI： 10.1016/S0022-0248(98)00893-8

Language：English   Publishing type：Research paper (scientific journal)  

Semiconductor photonic dots were fabricated by selectively growing ZnS dots on a GaAs substrate. The optical properties of the dots were examined by optical reflection spectroscopy and the results were compared with computed optical resonance modes. The photonic dots performed as visible wavelength-size optical resonators with each resonance exhibiting Q values on the order of 160-300.

DOI： 10.1063/1.123714

Language：English   Publishing type：Research paper (scientific journal)  

We demonstrate InGaN and AlGaN quantum dots (QDs) formation on AlGaN surfaces vir metal organic chemical vapor deposition (MOCVD). Si anti-surfactant was used in order to modify surface energy balance for changing growth mode from 2-dimensional step-flow growth to 3-dimensional nano-scale island formation. The average lateral size and thickness of the InGaN and AlGaN QDs are 10-20 nm and 5 nm, respectively. Intense photoluminescence (PL) was observed from InGaN QDs even in room temperature. In and Al incorporation in InGaN and AlGaN QDs were estimated to be 22-52% and 1-5%, respectively, from the PL spectrum.

DOI： 10.1016/S0167-9317(99)00448-7

Language：English   Publishing type：Research paper (scientific journal)  

We studied experimentally the influence of the size of GaN quantum dots on the exciton photoluminescence at 10 K under high optical excitation for which the spatial confinement was found to be a critical parameter for optical nonlinearities such as band-gap renormalization (BGR) and state-filling effects. As the dot size decreases, the BGR characteristic of bulk like dots becomes dominated by the state-filling nonlinearity. In addition, three-dimensional confinement is demonstrated by a comparison with a quantum well.

DOI： 10.1016/S0038-1098(98)00574-2

Language：English   Publishing type：Research paper (scientific journal)  

The correspondence between the E2 level (∼E_c-0.55 eV) in n-type GaN undergoing thermoionization and photoionizution was established. The optical cross section in the vicinity of the threshold for photoionization of this level was measured by means of capacitance transient spectroscopy. Analysis using the formulation of Chantre yielded the optical activation energy, E^o=0.85 eV, and the Franck-Condon parameter, d_FC=0.30 eV at 90 K.

DOI： 10.1063/1.123180

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GaN quantum structures with various fractional dimensions were fabricated on Al_xGa_1-xN surfaces. The Al_xGa_1-xN surface was treated with a Si antisurfactant prior to the GaN deposition. This treatment was found to be effective in modifying the structural dimensions of the thin GaN layer. Without Si a GaN quantum well structure having a dimensionality of two was achieved in step flow growth mode. As the deposited amount of Si was increased, a morphological transition from quantum well (2D) to quantum dot (0D-like) occurred. At some Si doses the resulting structures possessed fractional dimensions. We observed that GaN quantum structures with various fractional dimensions could be controllably fabricated solely by varying the total amount of the deposited Si antisurfactant. A model concerning masking by Si-N bondings is introduced to explain the morphological transitions.

DOI： 10.1002/(SICI)1521-3951(199911)216:1<431::AID-PSSB431>3.0.CO;2-3

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We demonstrate photoluminescence (PL) from self-assembling InGaN quantum dots (QDs), which are artificially fabricated on AlGaN surfaces via metalorganic chemical vapor deposition. InGaN QDs are successfully fabricated by the growth mode transition to three-dimensional nanoscale island formation by using "antisurfactant" silicon on AlGaN surface. The diameter and height of the fabricated InGaN QDs are estimated to be ∼10nm and ∼5nm, respectively, by an atomic-force microscope (AFM). Indium mole fraction of In_xGa_1-xN QDs is controlled from x=∼0.22 to ∼0.52 by varying the growth temperature of QDs. Intense photoluminescence is observed even at room temperature from InGaN QDs embedded with the GaN capping layers. In addition, from the temperature dependence of the PL-peak energy, we convincingly show that the PL emission actually comes from the InGaN QDs.

DOI： 10.1063/1.121168

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Nanoscale GaN dots were successfully formed on Al_xGa_1-xN/6H-SiC(0001) surfaces by gas-source molecular beam epitaxy. It was found that the growth mode can be changed by introducing Si before GaN growth, where the Si is believed to play an important role in the change of the Al_xGa_1-xN surface free energy. Without introducing Si, the GaN growth mode was two dimensional and (1×3) reconstruction was observed. The growth mode of GaN was changed from two-dimensional to three-dimensional by introducing Si on the Al_xGa_1-xN surface. In situ reflection high-energy electron diffraction and atomic force microscopy observations were used to monitor and characterize the growth processes and surface morphology.

DOI： 10.1063/1.120731

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The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size. This effect was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius. From this observation, absolute values of the exciton binding energy as a function of quantum dot size are determined.

DOI： 10.1063/1.122098

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Solid solutions of aluminum nitride (AlN) and silicon carbide (SiC) have been grown at 900-1300°C on vicinal α(6H)-SiC(0001) substrates by plasma-assisted, gas-source molecular beam epitaxy. Under specific processing conditions, films of (AlN)_x(SiC)_1-x with 0.2 ≤ x ≤ 0.8, as determined by Auger electron spectrometry (AES), were deposited. Reflection high-energy electron diffraction (RHEED) was used to determine the crystalline quality, surface character, and epilayer polytype. Analysis of the resulting surfaces was also performed by scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) revealed that monocrystalline films with x ≥ 0.25 had the wurtzite (2H) crystal structure; however, films with x < 0.25 had the zincblende (3C) crystal structure.

DOI： 10.1557/JMR.1998.0257

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Solid solutions of aluminum nitride (AlN) and silicon carbide (SiC) have been grown at 900-1300 degrees C on vicinal alpha(6H)-SiC(0001) substrates by plasma-assisted, gas-source molecular beam epitaxy, Under specific processing conditions, films of (AlN)(x)(SiC)(1-x) with 0.2 less than or equal to x less than or equal to 0.8, as determined by Auger electron spectrometry (AES), were deposited. Reflection high-energy electron diffraction (RHEED) was used to determine the crystalline quality, surface character, and epilayer polytype. Analysis of the resulting surfaces was also performed by scanning electron microscopy (SEM). High-resolution transmission electron microscopy (HRTEM) revealed that monocrystalline films with x greater than or equal to 0.25 had the wurtzite (2H) crystal structure; however, films with x < 0.25 had the zincblende (3C) crystal structure.

Language：English   Publishing type：Research paper (scientific journal)  

GaN films were successfully grown on Al₂O₃(0 0 0 1) and 6H-SiC(0 0 0 1) substrates by chemical beam epitaxy (CBE) using triethylgallium (TEG) and ammonia (NH₃) as group III and group V sources. Reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM) and X-ray diffraction (XRD) were used to characterize the growth processes, surface morphologies and the film qualities of GaN. It was found that the GaN growth mode was two-dimensional and that the GaN quality was improved by increasing the NH₃ supply. XRD measurements show that the quality of GaN films grown on 6H-SiC(0 0 0 1) substrates is better than that grown on Al₂O₃(0 0 0 1) substrates. AFM characterizations illustrate that the surface morphologies of GaN are greatly influenced by the NH₃ flow rates.

DOI： 10.1016/S0022-0248(98)00057-8

Language：English   Publishing type：Research paper (scientific journal)  

This paper describes the fabrication of quantum dots of GaN, a typical III-V nitride group semiconductor. On investigating GaN growth on the AlGaN mixed crystal surface to be used as a optical and electron confinement layer for device applications, it is found that two-dimensional step flow growth mode can be changed to three-dimensional growth mode by Si supply. This phenomenon is proposed to be effective for fabrication of GaN quantum dots. From the growth kinetics or the energy point of view, Si functions to prevent GaN step flow growth on the AlGaN surface. Furthermore, the free energy on the AlGaN surface is varied by additions of Si. The optical characteristics of GaN quantum dots are evaluated by the photoluminescence method. The dependence of the emission energy from GaN quantum dots on the dot size is confirmed. A laser structure was fabricated and stimulated emission was observed by optical pumping. (C) 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(6): 20-26, 1998.

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In situ reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) observations were performed to monitor and characterize the growth processes and surface morphology of GaN on Al_xGa_1-xN surface in gas-source molecular beam epitaxy. It was found that the growth mode can be changed by introducing Si before GaN growth, where the Si is believed to play an important role in the change of the Al_xGa_1-xN surface free energy. Without introducing Si, the GaN growth mode was two-dimensional and (1 × 3) reconstruction was observed. The growth mode of GaN was changed from two-dimensional to three-dimensional by introducing Si on the Al_xGa_1-xN surface. Nanoscale GaN dots were successfully formed on Al_xGa_1-xN/6H-SiC(0 0 0 1) surfaces. Furthermore, the density of the GaN dots was found to be dependent on the amount of Si dose and the growth temperature.

DOI： 10.1016/S0022-0248(98)00196-1

Language：English   Publishing type：Research paper (scientific journal)  

GaN growth was performed on 6H-SiC (0001) substrates by gas-source molecular beam epitaxy (GSMBE), using ammonia (NH₃) and solid Ga as V and III sources. Two kinds of reflection high-energy electron diffraction (RHEED) patterns, namely (1 × 1) and (2 × 2), were observed during the GaN growth depending on the growth conditions, which correspond to nitrogen-rich and Ga-rich surfaces, respectively. Phase diagram of the two surface states via growth conditions was also obtained, indicating the surface V/III ratio change during the growth. It was found that the GaN film quality was greatly improved under nitrogen-rich growth conditions by X-ray diffraction (XRD) and photoluminescence (PL) characterizations.

Language：English   Publishing type：Research paper (scientific journal)  

This paper describes the fabrication of quantum dots of GaN, a typical III-V nitride group semiconductor. On investigating GaN growth on the AlGaN mixed crystal surface to be used as a optical and electron confinement layer for device applications, it is found that two-dimensional step flow growth mode can be changed to three-dimensional growth mode by Si supply. This phenomenon is proposed to be effective for fabrication of GaN quantum dots. From the growth kinetics or the energy point of view, Si functions to prevent GaN step flow growth on the AlGaN surface. Furthermore, the free energy on the AlGaN surface is varied by additions of Si. The optical characteristics of GaN quantum dots are evaluated by the photoluminescence method. The dependence of the emission energy from GaN quantum dots on the dot size is confirmed. A laser structure was fabricated and stimulated emission was observed by optical pumping.

DOI： 10.1002/(SICI)1520-6432(199806)81:6<20::AID-ECJB3<3.0.CO;2-7

Language：English   Publishing type：Research paper (scientific journal)  

Thin films of silicon carbide (SiC) have been grown at 1000-1500°C on vicinal and on-axis α(6H)-SiC(0 0 0 1) substrates by gas-source molecular-beam epitaxy (GSMBE). Growth on on-axis and off-axis 6H-SiC(0 0 0 1) substrates using the SiH₄-C₂H₄ system resulted in 3C-SiC(1 1 1) epilayers under all conditions of reactant gas flow and temperature. By adding H₂ to the SiH₄-C₂H₄ system, films of 6H-SiC(0 0 0 1) were deposited on vicinal 6H-SiC substrates at deposition temperature ≥1350°C. Kinetic analysis of the deposition of 3C-SiC films with respect to reactant inputs and growth temperature is presented. From the data, the deposition of 3C-SiC appears to be surface-reaction-controlled. Reflection high-energy electron diffraction (RHEED) and high-resolution transmission electron microscopy (HRTEM) was used to determine the crystalline quality, surface character and epilayer polytype.

DOI： 10.1016/S0022-0248(97)00471-5

Language：English   Publishing type：Research paper (scientific journal)  

Thin films of silicon carbide (SiC) have been grown at 1000-1500 degrees C on vicinal and on-axis alpha(6H)-SiC(0 0 0 1) substrates by gas-source molecular-beam epitaxy (GSMBE). Growth on on-axis and off-axis 6H-SiC(0 0 0 1) substrates using the SiH4-C2H4 system resulted in 3C-SiC(1 1 1) epilayers under all conditions of reactant gas flow and temperature. By adding H-2 to the SiH4-C2H4 system, films of 6H-SiC(0 0 0 1) were deposited on vicinal 6H-SiC substrates at deposition temperature greater than or equal to 1350 degrees C. Kinetic analysis of the deposition of 3C-SiC films with respect to reactant inputs and growth temperature is presented. From the data, the deposition of 3C-SiC appears to be surface-reaction-controlled. Reflection high-energy electron diffraction (RHEED) and high-resolution transmission electron microscopy (HRTEM) was used to determine the crystalline quality, surface character and epilayer polytype. (C) 1998 Elsevier Science B.V. All rights reserved.

Language：English   Publishing type：Research paper (scientific journal)  

This study presents quantitative microanalysis of the Ti-Al-Cr system using analytical electron microscopy. The absorption correction is made using the ζ-factor method developed recently, which does not require knowledge of thickness nor density at the point of analysis. A brief summary of the method is given to illustrate the process for simultaneous determination of the composition and thickness at the point of analysis. It is demonstrated that the ζ-factor method is applicable to the ternary Ti-Al-Cr system.

DOI： 10.1093/oxfordjournals.jmicro.a023564

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GaN quantum dots on an AlGaN surface can be fabricated by the use of Si as a surface modifier for the formation of the quantum dots. The details of this mechanism is not yet fully understood. The scope of this paper is to investigate how the growth temperature is affecting the formation of these types of structures. By the use of cathodoluminescence it was possible to visualize the spatial distribution of the luminescence from the quantum dots, the barrier and luminescence tentatively attributed to deep level impurities induced by the Si treatment.

DOI： 10.4028/www.scientific.net/msf.264-268.1335

Language：English   Publishing type：Research paper (scientific journal)  

GaN growth was perform on 6H-SiC (0001) substrates by gas-source molecular beam epitaxy (GSMBE), using ammonia (NH₃) as a nitrogen source. Two kinds of reflection high-energy electron diffraction (RHEED) patterns, named (1×1) and (2×2), were observed during the GaN growth depending on the growth conditions. By careful RHEED study, it was verified that the (1×1) pattern was corresponded to a H₂-related nitrogen-rich surface, while (2×2) pattern was resulted from a Ga-rich surface. By x-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) characterizations, it was found that the GaN quality changed drastically grown under different RHEED patterns. GaN film grown under the (1×1) RHEED pattern showed much better qualities than that grown under the (2×2) one.

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

We demonstrate photoluminescence from self-assembling InGaN quantum dots (QDs), which are artificially fabricated on AlGaN surfaces via metal-organic chemical vapor deposition. InGaN QDs are successfully fabricated by the growth mode transition from step-flow to three dimensional island formation by using anti-surfactant silicon on AlGaN surface. The diameter and height of the fabricated InGaN QDs are estimated to be approximately 10 nm and approximately 5 nm, respectively, by an atomic-force-microscope (AFM). Indium mole fraction of In_xGa_1-xN QDs is controlled from x = approximately 0.22 to approximately 0.52 by varying the growth temperature of QDs. Intense photoluminescence is observed even at room temperature from InGaN QDs embedded with the GaN capping layers. In addition, the temperature-dependent energy shift of the photoluminescence peak-energy shows a localization behavior.

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Stimulated emission was observed from optically pumped GaN quantum dots in an Al_xGa_1-x N separate confinement heterostructure fabricated on 6H-SiC(0001) substrate by metal organic chemical vapor deposition. Nanostructural GaN quantum dots, with an average size of ∼10 nm width, ∼1-2 nm height, and density of ∼10¹¹Cm^-2, were self-assembled on the Al_xGa_1-xN cladding layer surface. The stimulated emission peak was observed at ∼3.48 eV, which is ∼50 meV lower than that of spontaneous emission. The excitation power dependence on the emission intensity clearly indicates threshold pump power density of 0.75 MW/cm² for the onset of stimulated emission.

DOI： 10.1063/1.119877

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Gas-source molecular beam epitaxy has been employed to grown thin films of SiC and AlN on vicinal and on-axis 6H-SiC(0001). Growth using the SiH₄-C₂H4 system resulted in 3C-SiC(111) epilayers under all conditions of reactant gas flow and temperatures. Films of 6H-SiC(OOOl) were deposited on vicinal 6H-SiC(0001) substrates using the SiH₄-C₂H₄-H₂ system at deposition temperatures ≥ 1350°C. In situ doping was achieved by intentional introduction of nitrogen and aluminum into the growing crystal. Monocrystalline AlN was deposited using evaporated Al and ECR plasma derived N or NH3. Films <50Å grown on the vicinal substrates had higher defect densities compared to those on the on-axis substrates due to the higher density of inversion boundaries forming at most SiC steps in the former material. Metal/AlN/6H-SiC (0001) thin film heterostructures which had a density of trapped charges as low as of 1 × 10¹¹ cm^-2 at room temperature were prepared without post growth treatment. Superior single crystal AlN/SiC heterostructures were achieved when very thin AlN was deposited on the on-axis substrates. Single phase monocrystalline solid solutions of (AlN)_x(SiC)_1-x were deposited between 0.2≤×≤0.8. A transition from the zincblende to the wurtzite structure was observed at x≈0.25.

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Gas-source molecular beam epitaxy has been employed to grown thin films of SiC and AlN on vicinal and on-axis 6H-SiC(0001). Growth using the SiH4-C2H4 system resulted In 3C-SiC(111) epilayers under all conditions of reactant gas flow and temperatures. Films of 6H-SiC(0001) were deposited on vicinal 6H-SiC(001) substrates using the SiH4-C2H4-H-2 system at deposition temperatures greater than or equal to 1350 degrees C. In situ doping was achieved by intentional introduction of nitrogen and aluminum into the growing crystal,
Monocrystalline AlN was deposited using evaporated Al and ECR plasma derived N of NH3. Films <50 Angstrom grown on the vicinal substrates had higher defect densities compared to those on the on-axis substrates due to the higher density of inversion boundaries forming at most SiC steps in the former material, Metal/AlN/6H-SiC(0001) thin film heterostructures which had a density of trapped charges as low as of 1 x 10(11) cm(-2) at room temperature were prepared without post growth treatment. Superior single crystal AlN/SiC heterostructures were achieved when very thin AIN was deposited on the on-axis substrates. Single phase monocrystalline solid solutions of (AlN)(x)(SiC)(1-x) were deposited between 0.2 less than or equal to x less than or equal to 0.8. A transition from the zinchlende to the wurtzite structure was observed at x approximate to 0.25, (C) 1997 Elsevier Science S.A.

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The homoepitaxial growth of SiC thin films by solid-and gas-source molecular beam epitaxy is reviewed and discussed. Our recent results regarding the homoepitaxial growth of single crystal 3C-SiC(111) and 6H-SiC(0001) thin films are also presented. The 3C-SiC(111) films were grown on both vicinal and on-axis 6H-SiC(0001) substrates at temperatures between 1000 and 1500 °C using SiH₄ and C₂H₄. They contained double positioning boundaries and stacking faults and the surface morphology and growth rate depended strongly on temperature. Films of GH-SiC(0001) with low defect densities were deposited at high growth rates on vicinal 6H-SiC(0001) substrates by adding H₂ to the reactant mixture at temperatures between 1350 and 1500 °C. At temperatures below 1350 °C. only the cubic phase was formed. A kinetic analysis of the SiC deposition process is also presented. The SiC films were resistive with an n-type character and a lower N concentration than the p-type CVD-grown epilayers of the substrate. Undoped 6H-SiC films with the lowest atomic nitrogen and electron concentration had a mobility of 434 cm² V^-1 s^-1, the highest room temperature value ever reported for this polytype. Both the 6H-SiC(0001) and the 3C-SiC(111) epilayers were controllably doped using a NH₃/H₂ mixture (for lighly n-doped films), pure N₂ (for heavily n-doped SiC epilayers) and Al evaporated from a standard effusion cell (for p-type doping).

DOI： 10.1002/1521-3951(199707)202:1<379::AID-PSSB379>3.0.CO;2-2

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The homoepitaxial growth of SiC thin films by solid- and gas-source molecular beam epitaxy is reviewed and discussed. Our recent results regarding the homoepitaxial growth th of single crystal 3C-SiC(111) and 6H-SiC(0001) thin films are also presented. The 3C-SiC(111) films were grown on both vicinal and on-axis 6H-SiC(0001) substrates at temperatures between 1000 and 1500 degrees C using SiH4 and C2H4. They contained double positioning boundaries and stacking faults and the surface morphology and growth rate depended strongly on temperature. Films of 6H-SiC(0001) with low defect densities were deposited at high growth rates on vicinal 6H-Sic(0001) substrates by adding H-2 to the reactant mixture at temperatures between 1350 and 1500 degrees C. At temperatures below 1350 degrees C, only the cubic phase was formed. A kinetic analysis of the SiC deposition process is also presented. The SiC films were resistive with an n-type character and a lower N concentration than the p-type CVD-grown epilayers of the substrate. Undoped 6H-SiC films with the lowest atomic nitrogen and electron concentration had a mobility of 434 cm(2) V-1 s(-1): the highest room temperature value ever reported for this polytype. Both the 6H-SiC(0001) and the 3C-SiC(111) epilayers were controllably doped using a NH3/H-2 mixture (for lighly n-doped films), pure Nz (for heavily n-doped SiC epilayers) and Al evaporated from a standard effusion cell (for p-type doping).

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Amorphous, hexagonal and cubic phases of BN were grown via ion beam assisted deposition on Si(1 0 0) substrates. Gas-source molecular beam epitaxy of the III-V nitrides is reviewed. Sapphire(0 0 0 1) is the most commonly employed substrate with 6H-SiC(0 0 0 1), ZnO(1 1 1) and Si(1 1 1) also being used primarily for the growth of wurtzite GaN(0 0 0 1) in tandem with previously deposited GaN(0 0 0 1) or AlN(0 0 0 1) buffer layers. Silicon(0 0 1), GaAs(0 0 1), GaP(0 0 1) and 3C-SiC(0 0 1) have been employed for growth of cubic (zincblende) β-GaN(0 0 1). The precursor materials are evaporated metals and reactive N species produced either via ECR or RF plasma decomposition of N₂ or from ammonia. However, point defect damage from the plasma-derived species has resulted in a steady increase in the number of investigators now using ammonia. The growth temperatures for wurtzite GaN have increased from 650 ± 50°C to 800 ± 50°C to enhance the surface mobility of the reactants and, in turn, the efficiency of decomposition of ammonia and the microstructure and the growth rate of the films. Doping has been achieved primarily with Si (donor) and Mg (acceptor); the latter has been activated without post-growth annealing. Simple heterostructures, a p-n junction LED and a modulation-doped field-effect transistor have been achieved using GSMBE-grown material.

DOI： 10.1016/S0022-0248(97)00077-8

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Thin films of AlN and GaN are deposited primarily via the common forms of organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE). Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN and GaN primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces inversion domain boundaries in the AlN at the SiC steps; this sequence problem may discourage the nucleation of GaN. Films of AlN and GaN grown by MBE at 650°C are textured; monocrystalline films are achieved at 1050°C by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed.

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Nanoscale GaN quantum dots were fabricated in Al_xGa_1-xN confined layer structures via metallorganic chemical vapor deposition (MOCVD)), by using a 'surfactant' which can modify the GaN growth mode on AlGaN surfaces. A two dimensional growth mode (step-flow-like) of GaN films on Al_xGa_1-xN (x = 0 ≈ 0.2) surfaces, that is energetically commenced under the conventional growth conditions, was intentionally changed into a three dimensional mode by adding tetraethyl-silane (TESi) used as a surfactant onto the AlGaN substrate surface prior to the GaN deposition. The surfactant is believed to inhibit the GaN film from wetting the AlGaN surface due to the change in surface free energy. The resulting morphological structures of GaN dots were found to be sensitive to; the doping rate of TESi, the Al content (x) of the Al_xGa_1-xN layer, and the growth temperature. A very intense photoluminescence (PL) emission was observed from the GaN dots embedded in the AlGaN layers. The quantum size effect in terms of the blue-shift in a PL peak position was verified using the GaN dot samples having different dot sizes.

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A low dislocation density of ∼ 10^7-8 cm^-2 in GaN thin films on 6H-SiC(0001) substrates grown by metalorganic chemical vapor deposition was achieved. By considering possible origins of dislocations in the GaN/AlN/SiC structure, two major dislocation reduction routes are proposed; ultra-thin AlN buffer layers and smooth AlN surfaces in an atomic scale. Experimentally, the effects of the surface roughness and structural perfection of the AlN buffer layer on GaN film quality were extensively investigated as a function of AlN film thickness. The reduced dislocation density was realized by using ultra-thin AlN buffer layers having a thickness of ∼ 1.5 nm, which is below the critical value for misfit dislocation generation. The smoother surface morphology and enhanced structural quality of ultra-thin AlN buffer layers were found to be the main parameters in reducing the defect density in the GaN film.

DOI： 10.1016/S0022-0248(96)00611-2

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Nanoscale GaN quantum dots were fabricated on Al_xGa_1-xN layer surfaces via metalorganic chemical vapor deposition. In order to achieve a self-assembling dot structure, a two-dimensional growth mode (step flow) of GaN films on Al_xGa_1-xN (x = 0-0.2) surfaces that is energetically commenced under the conventional growth conditions was intentionally modified into a three-dimensional mode by using a "surfactant." The surfactant is believed to inhibit the GaN film from wetting the AlGaN surface due to the change in surface free energy. The resulting morphological structures of GaN dots were found to be sensitive to the doping rate of tetraethyl silane used as a surfactant, the Al content (x) of the Al_xGa_1-xN layer, and the growth temperature. A very intense photoluminescence emission was observed from the GaN dots embedded in the AlGaN layers.

DOI： 10.1063/1.117830

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Metal/AlN/n-type 6H-SiC(0001) heterostructures have been prepared by growing wurtzite AlN layers on vicinal 6H-SiC(0001) using gas-source molecular beam epitaxy. High-resolution transmission electron microscopy results show that the interface between the AlN layer and the Si-terminated 6H-SiC substrate is microstructurally abrupt, but contains defects originating at step sites on the 6H-SiC surface. The interface is found to have a low density of trapped charges of 1 × 10¹¹ cm^-2 at room temperature without any postgrowth treatment. This value is comparable to those reported for thermally grown and deposited oxides on n-type 6H-SiC(0001), and indicates the formation of a high quality interface.

DOI： 10.1063/1.117347

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Silicon carbide (SiC) and aluminum nitride (AlN) thin films have been grown on 6H-SiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 1050°C. Step flow, step bunching and the deposition of 6H-SiC occurred at the outset of the exposure of the (1 X 1) vicinal substrate surface to C₂H₄/Si₂H₆ gas flow ratios of 1, 2 and 10. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3C-SiC islands using the gas flow ratio of one or the ethylene-rich ratios, respectively. The (3 X 3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms, which in turn promotes step flow growth. Essentially atomically flat monocrystalline AlN surfaces were obtained using on-axis substrates. Island-like features were observed on the vicinal surface. The coalescence of the latter features at steps gave rise to inversion domain boundaries (IDBs) as a result of the misalignment of the Si/C bilayer steps with the AlN bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of IDBs. Undoped, highly resistive (10² Ω · cm) and Mg-doped, p-type (0.3 Ω · cm) monocrystalline GaN films having a thickness of 0.4-0.5 μm have also been grown via the same technique on AlN buffer layers without post-processing annealing.

DOI： 10.1016/0022-0248(95)01023-8

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Surfaces of 6H-SiC(0001) homoepitaxial layers deposited on vicinal (∼3.5° off (0001) towards [1120]) and on-axis 6H-SiC wafers by chemical vapour deposition have been investigated using ultra-high vacuum scanning tunneling microscopy. Undulating step configurations were observed on both the on-axis and the vicinal surfaces. The former surface possessed wider terraces than the latter. Step heights on both surfaces were ∼0.25nm corresponding to single bilayers containing one Si and one C layer. After annealing at T>1100°C for 3-5 min in UHV, selected terraces contained honeycomb-like regions caused by the transformation to a graphitic surface as a result of Si sublimation. A model of the observed step configuration has been proposed based on the observation of the [2̄110] or [12̄10] orientations of the steps and energetic considerations. Additional deposition of very thin (∼2 nm) SiC films on the above samples by gas source molecular beam epitaxy was performed to observe the evolution of the surface structure. Step bunching and growth of 6H-SiC layers and formation of 3C-SiC islands were observed on the vicinal and the on-axis surfaces, respectively, and controlled by the diffusion lengths of the adatoms.

DOI： 10.1016/0039-6028(95)01105-6

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Defect formation mechanisms in AlN thin films grown on 6H-SiC(001) substrates have been investigated as a function of film thickness to 180 nm using conventional and high-resolution transmission electron microscopy (HRTEM). The presence of particular Si-terminated steps on the vicinal 6H-SiC surface caused the introduction of planar defects at the initial stage of growth. By contrast, micrographs of films deposited on the on-axis substrate showed a much lower density of defects. Atomistic structural models of the interface and deposited AlN are proposed based on the HRTEM images and the results of companion scanning tunneling microscopy (STM) studies of the vicinal 6H-SiC(001) surface. The introduction of additional defects, which were primarily threading dislocations, was observed with increasing thickness of the AlN films.

Language：English   Publishing type：Research paper (scientific journal)  

Defect formation mechanisms in AlN thin films grown on 6H-SiC(0001) substrates have been investigated as a function of film thickness to 180 nm using conventional and high-resolution transmission electron microscopy (HRTEM). The presence of particular Si-terminated steps on the vicinal 6H-SiC surface caused the introduction of planar defects at the initial stage of growth. By contrast, micrographs of films deposited on the on-axis substrate showed a much lower density of defects. Atomistic structural models of the interface and deposited AlN are proposed based on the HRTEM images and the results of companion scanning tunneling microscopy (STM) studies of the vicinal 6H-SiC(0001) surface. The introduction of additional defects, which were primarily threading dislocations, was observed with increasing thickness of the AlN films.

Language：English   Publishing type：Research paper (scientific journal)  

The effect of gas flow ratios (C₂H₄/Si₂H₆ = 1,2,10) on the growth mode of SiC thin films on vicinal α(6H)-SiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 950-1150°C has been investigated. Step flow, step bunching and the deposition of 6H-SiC occurred at the outset of the exposure of the (1 × 1) surface to the reactants using any flow ratio. Subsequent deposition resulted in either step flow and continued growth of 6H films using C₂H₄/Si₂H₆ = 1 or nucleation and coalescence of 3C-SiC islands on the 6H terraces using C₂H₄/Si₂H₆ = 2 and 10. The initial stage of A1N film growth on these substrates and the occurrence of defects has also been investigated. Essentially atomically flat A1N surfaces, indicative of two-dimensional growth, were obtained using on-axis substrates. Island-like features were observed on the vicinal surfaces. The coalescence of the latter features gave rise to double positioning boundaries as a result of the misalignment of the Si/C bilayer steps with the A1/N bilayers in the growing films. The quality of the thicker A1N films was strongly influenced by the concentration of these boundaries. The following sections describe the procedures used to deposit and analyze these two materials as well as detail the results and conclusions of this research.

DOI： 10.1016/0022-0248(95)01052-1

Language：English   Publishing type：Research paper (scientific journal)  

Organometallic vapor phase epitaxy (OMVPE) and molecular beam epitaxy (MBE) are the most common methods for the growth of thin films of AlN and GaN. Sapphire is the most common substrate; however, a host of materials have been used with varying degrees of success. Both growth techniques have been employed by the authors to grow AlN, GaN and Al_xGa_1-xN thin films primarily on 6H-SiC(0001). The mismatch in atomic layer stacking sequences along the growth direction produces double positioning boundaries in AlN and the alloys at the SiC steps; this sequence problem appears to discourage the two-dimensional nucleation of GaN. Films of these materials grown by MBE at 650°C are textured; monocrystalline films are achieved between 850°C (pure GaN) and 1050°C (pure AlN) by this technique and OMVPE. Donor and acceptor doping of GaN has been achieved via MBE without post growth annealing. Acceptor doping in CVD material requires annealing to displace the H from the Mg and eventually remove it from the material. High brightness light emitting diodes are commercially available; however, numerous concerns regarding metal and nitrogen sources, heteroepitaxial nucleation, the role of buffer layers, surface migration rates as a function of temperature, substantial defect densities and their effect on film and device properties, ohmic and rectifying contacts, wet and dry etching and suitable gate and field insulators must and are being addressed. Selected issues surrounding the growth of these materials with particular examples drawn from the authors' research are presented herein.

Language：English   Publishing type：Research paper (scientific journal)  

The initial stage of AlN film growth on 6H-SiC(0001) substrates by plasma-assisted, gas source molecular beam epitaxy (PAGSMBE) has been investigated in terms of growth mode and interface defects. Cross-sectional high resolution transmission electron microscopy (HRTEM) was used to observe the microstructure of the deposited films and the AlN/SiC interfaces. Surface morphologies and interface atomic structures were compared between films grown on vicinal and on-axis surfaces. Essentially atomically flat AlN surfaces were obtained using on-axis substrates. This is indicative of two-dimensional growth to a thickness of ∼15 Å. Islandlike features were observed on the vicinal surface. The coalescence of these features at steps gave rise to double positioning boundaries (DPBs) as a result of the misalignment of the Si/C bilayer steps with the Al/N bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of DPBs formed at the outset of growth.

DOI： 10.1063/1.114173

Language：English   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

Silicon carbide thin films have been grown on vicinal 6H-SiC(0001) substrates by gas-source molecular beam epitaxy at 1050°C. The effect of gas flow ratios (C₂H₄Si₂H₆=1,2,10) on the growth mode was examined via cross-sectional high resolution transmission electron microscopy and in situ reflection high energy electron diffraction. Step flow, step bunching, and the deposition of 6H-SiC occurred at the outset of the exposure of the (1×1) surface to the reactants using any flow ratio. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3C-SiC islands using the gas flow ratio of one of the ethylene-rich ratios, respectively. The (3×3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms which, in turn, promotes step flow growth.

DOI： 10.1063/1.112513

Language：English   Publishing type：Research paper (scientific journal)  

The morphology and interface chemistry occurring during the initial deposition of BN, AlN and GaN films via metal evaporation and N₂ decomposition under UHV conditions have been determined. FTIR spectroscopy and TEM revealed the consecutive deposition of an initial 20angstrom layer of a-BN, 20-60angstrom of oriented h-BN, and a final layer of polycrystalline c-BN. This sequence is attributed primarily to increasing intrinsic compressive stress in the films. XPS analysis revealed the growth of GaN on sapphire to occur via the Stranski-Krastanov mode; growth on SiC showed characteristics of three-dimensional growth. AlN grew layer-by-layer on both substrates. Vicinal 6H-SiC(0001) substrate surfaces contain closely spaced, single bilayer steps. During deposition of Si and C at 1050°C, 6H layers initially form and step bunching occurs. The latter phenomenon results in more widely spaced steps, the nucleation of 3C-SiC both on the new terraces and at the larger steps and formation of double position boundaries. The C/Si ratio in the gaseous reactants also affects the occurrence of these three phenomena.

DOI： 10.1557/proc-339-351

Language：English   Publishing type：Research paper (scientific journal)  

Pseudomorphic structures containing β(3C)-SiC and 2H-AlN have been grown on vicinal α(6H)-SiC(0001) at 1050°C by plasma-assisted, gas-source molecular beam epitaxy. Reflection-high energy electron diffraction and cross-sectional high-resolution transmission electron microscopy showed all layers to be monocrystalline. The AlN layers were uniform in thickness. Defects in these layers initiated at steps on the 6H-SiC film. The 3C-SiC layers contained a high density of stacking faults and microtwins caused primarily by the interfacial stresses generated by the mismatch in lattice parameters between AlN and β-SiC coupled with the very low stacking fault energy of SiC. This is the first report of the deposition of single crystal SiC/AlN/SiC thin film heterostructures on any substrate as well as the first report of the epitaxial growth of single crystal layers of binary materials with three different crystal structures.

DOI： 10.1063/1.109062

Language：English   Publishing type：Research paper (scientific journal)  

Monocrystalline AlN(0001) films with few defects were deposited on vicinal α(6H)-SiC(0001) wafers via plasma-assisted, gas-source molecular beam epitaxy within the temperature range of 1050–1200 °C The Al was thermally evaporated from an effusion cell. An electron cyclotron resonance plasma source was used to produce activated nitrogen species. Growth on vicinal Si(100) at 900–1050 °C resulted in smooth, highly oriented A1N(0001) films.

DOI： 10.1557/JMR.1993.2310

Language：English   Publishing type：Research paper (scientific journal)  

Solid solutions of aluminum nitride (AIN) and silicon carbide (SiC), the only intermediate phases in their respective binary systems, have been grown at 1050 °C on a(6H)-SiC(0001) substrates cut 3-4° off-axis toward [1120] using plasma-assisted, gas-source molecular beam epitaxy. A film having the approximate composition of (AlN)0.3(SiC)0.7 as determined by Auger spectrometry, was selected for additional study and is the focus of this note. High resolution transmission electron microscopy (HRTEM) revealed that the film was monocrystalline with the wurtzite (2H) crystal structure.

DOI： 10.1557/JMR.1993.1477

Language：English   Publishing type：Research paper (scientific journal)  

A novel reactor for layer-by-layer deposition of compound semiconductors has been designed and commissioned for the deposition of SiC. The substrates rested on a heated, rotating platform. They encountered individual fluxes of Si₂H₆ and C₂H₄ and subsequently paused beneath a hot filament. The filament was used to encourage the surface reaction between silicon adatoms and carbon precursors. Heteroepitaxial films were grown between 850 and 980 °C on Si(100) substrates oriented 3° off-axis toward 〈011〉. They were analyzed for composition, crystallinity, growth per cycle, and morphology using depth profiling Auger spectroscopy, reflection high-energy electron diffraction, ellipsometry and transmission electron microscopy. Growth, as measured by ellipsometry and transmission electron microscopy, corresponded to approximately one monolayer per cycle. Monocrystalline films were achieved. Initial growth and characterization results of representative films are presented and discussed.

DOI： 10.1016/0040-6090(93)90158-L

Language：English   Publishing type：Research paper (scientific journal)  

Single-crystal epitaxial films of cubic /3(3C)-SiC(lll) have been deposited on hexagonal a(6H)-SiC(0001) substrates oriented 3-4° toward [1120] at 1050–1250 °C via gas-source molecular beam epitaxy using disilane (Si2H6 and ethylene (C2H4). High-resolution transmission electron microscopy revealed that the nucleation and growth of the β(3C)-SiC regions occurred primarily on terraces between closely spaced steps because of reduced rates of surface migration at the low growth temperatures. Double positioning boundaries were observed at the intersections of these regions.

DOI： 10.1557/JMR.1993.2753

Event date： 2014.7

Language：Japanese   Presentation type：Oral presentation (invited, special)  

Venue：滋賀   Country：Japan  

Event date： 2014.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東海大学   Country：Japan  

Event date： 2011.10

Presentation type：Oral presentation (general)  

Venue：早稲田   Country：Japan  

Event date： 2011.9 - 2011.5

Presentation type：Oral presentation (general)  

Venue：名古屋   Country：Japan  

Event date： 2011.6 - 2011.7

Presentation type：Oral presentation (invited, special)  

Venue：滋賀   Country：Japan  

Event date： 2011.5

Presentation type：Oral presentation (general)  

Venue：Suwon   Country：Korea, Republic of  

Event date： 2010.11

Presentation type：Oral presentation (general)  

Venue：大阪   Country：Japan  

Event date： 2010.10

Presentation type：Oral presentation (general)  

Venue：仙台   Country：Japan  

Event date： 2010.6

Presentation type：Oral presentation (general)  

Venue：Strasbourg   Country：France  

Event date： 2010.3

Presentation type：Oral presentation (general)  

Venue：岡山   Country：Japan  

Event date： 2009.11 - 2009.12

Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2009.11 - 2009.12

Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2009.10

Presentation type：Oral presentation (general)  

Venue：Nurnberg   Country：Germany  

Event date： 2009.10

Presentation type：Oral presentation (general)  

Venue：Nurnberg   Country：Germany  

Event date： 2009.10

Presentation type：Oral presentation (general)  

Venue：Nurnberg   Country：Germany  

Event date： 2009.10

Presentation type：Oral presentation (general)  

Venue：仙台   Country：Japan  

Event date： 2009.9

Presentation type：Oral presentation (general)  

Venue：福岡   Country：Japan  

Event date： 2009.9

Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

Event date： 2009.2

Presentation type：Oral presentation (general)  

Venue：岡崎   Country：Japan  

Event date： 2008.12 - 2009.12

Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2008.11

Presentation type：Oral presentation (general)  

Venue：Aizu   Country：Japan  

Event date： 2008.11

Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

Event date： 2008.11

Presentation type：Oral presentation (general)  

Venue：仙台   Country：Japan  

Event date： 2008.7

Presentation type：Oral presentation (general)  

Venue：守山市   Country：Japan  

Event date： 2007.11

Presentation type：Oral presentation (general)  

Venue：Boston   Country：United States  

Event date： 2007.11

Presentation type：Oral presentation (general)  

Venue：Boston   Country：United States  

Event date： 2007.9

Presentation type：Oral presentation (general)  

Venue：Berlin   Country：Germany  

Event date： 2007.9

Presentation type：Oral presentation (general)  

Venue：Berlin   Country：Germany  

Event date： 2007.9

Presentation type：Oral presentation (general)  

Venue：Wien   Country：Austria  

Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2024.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東京   Country：Japan  

SiC 基板上へのエピタキシャルグラフェンの成長には表面熱分解法や CVD 法が用いられている. いずれの方法においても，SiC 上にまずエピタキシャルなカーボンバッファー層((6√3×6√ 3)R30°構造)(以下 6R3 層)が形成される.この 6R3 層と SiC 界面に水素をインターカレーション することにより，6R3 層はフリースタンディングな単層グラフェンへと構造遷移し[1]，高移動度 を示すことが知られている.水素インターカレーションは比較的低温(〜600°C)の水素雰囲気で行 われるが，グラフェンへの構造遷移過程はあまり調べられていない.我々は水素インターカレー ション温度や処理時間を変化させた場合に，グラフェンの歪み分布や歪み量が変化する現象を観 察し，遷移過程において準安定な中間状態がある可能性を示唆した[2].本報告では，そのような 中間状態の構造および電子状態を DFT+タイトバインディング計算により明らかにすることを目的 とした

Event date： 2024.3

Language：Japanese  

Country：Japan  

Event date： 2024.3

Language：Japanese  

Country：Japan  

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

グラフェンの格子歪み(勾配)により誘起される擬磁場は，実磁場では実現出来ない強 度[1]や局所性[2]から電子物性や伝導特性に大きな影響を与える可能性があり，非常に興 味深い.我々はグラフェンに周期擬磁場を付与する試みとして，SiC 表面に注目した研究 を行っている.SiC のユニットセルは c 軸方向に長い周期を有する(ポリタイプ)ことか ら，表面には様々な周期構造が現れ，グラフェンを転写することにより周期擬磁場が実現 できる可能性がある.本研究では、Si(0001)微傾斜面において発現するステップバンチン グや周期的ナノファセット構造[3]に着目し，周期歪みの付与を試みた.

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

SiC(0001)上にグラフェンがエピタキシャル成長する際は、まず(6√3 × 6√3)𝑅30°構造のバ ッファー層(以下 6R3 バッファー層)が形成する。その後、SiC との界面に新たな 6R3 バ ッファー層が成長することで、6R3 バッファー層がグラフェン化する。SiC(0001)上のエピ タキシャルグラフェンは、6R3 バッファー層によるフォノン散によって、電子移動度の低下 や内在的なドーピングが報告されている。一方で、ツイスト二層グラフェン(TBG)では、 構造緩和によりナノメートルオーダーで局所歪みが生じ、その結果擬磁場が誘起されるこ とが理論的に示唆されている(1)。更に、6R3 バッファー層上の上層と下層の非等価なドープ と構造緩和により、バンド変調が起こることが今村らにより報告されている(2)。6R3 バッフ ァー層はグラフェンと類似なハニカム構造を有することから、6R3 バッファー層上に単層 グラフェンをツイスト転写することで、構造緩和や疑磁電場が発生し、グラフェンの物性が 変化する可能性がある。本研究では、6R3 バッファー層上にグラフェンをツイスト転写し、 構造及びラマン特性の変化を評価した。

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

グラフェンの成長には SiC 表面熱分解法や金属基板上の CVD 法が用いられている。SiC 熱分解 法では、大面積で高品質なエピタキシャルグラフェンが成長できるが、層厚制御が困難であると いう問題がある。空間的に均一な層数のグラフェンは、転写グラフェンに必須である。そこで 我々は C2H4 ガスを原料とする CVD 法で均一なエピタキシャルバッファー層(以下 6R3 層)を成 長し、更に水素インターカレーションを施すことによって、単層のフリースタンディング(FS)グ ラフェンを得ている。しかしながら、完全に剥離させることが難しいことから、水素インターカ レーションに問題があると考え、そのメカニズムを明らかにすることを本研究の目的とした。
まず、4H-SiC(0001)面(Si 面)を高温水素エッチングすることによりステップ・テラス構造を 形成した。その後、1400°C、1 atm で C2H4(26.8 ppm)を供給し、SiC(0001)面上に 6R3 層を成長 した。更に、600~1000°C、1 atm において水素(1 slm)でインターカレーションを行った。インタ ーカレーション時間と温度をパラメータとして、顕微ラマン分光および AFM 等により評価を行 った。

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

近年、グラフェンに格子歪みを加えることで擬似的な磁場(擬磁場)が発現することで電子状態が変 調することが報告されており、グラフェンの歪み状態の制御は重要な課題である。2層ツイストグラ フェン(TBG)では、モアレ構造に起因した局所構造緩和(歪み)により電子状態が変化することが知ら れている[1]。このような TBG のアナロジーとして，我々は 格子不整合を加味した系−即ち SiC(0001)上 の単層グラフェン−を考えている.SiC(0001)基板へ単層グラフェンを直接転写することにより、SiC と グラフェンの格子不整合+転写回転角度(モアレ)を取り入れることにより，新たな電子状態の創製を 目指している。

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

グラフェンは、格子歪みによってベクトルポテンシャルが生じ、擬磁場が誘起されること により電子状態が変化することが示されている[1]。また、銅上に形成された1次元リップル グラフェン(RG)において無磁場でランダウ量子化が確認されており[2]、周期擬磁場によって 従来と異なる量子化が起こることが示されている。しかし、ナノスケールの1次元 RG の周 期や歪み量を実験的に制御することは難しい。本研究では、RG の周期を構造的に制御し、 歪み量および擬磁場との相関を明らかにすることを目的とする。周期 RG 構造の形成には， AFM カンチレバーを用いた SiC 表面へのナノインデンテーションとグラフェン転写を用い る。

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

グラフェンに格子歪みを加えるとベクトルポテンシャルが導入され、電子状態が変調される。この 時導入される場は擬磁場(Pseudo-Magnetic Field)と呼ばれ、近年注目を集めている。擬磁場を導入する 格子歪みの一つにグラフェンのリップル化が挙げられ、理論的にはリップルグラフェンには周期的に 符号が反転する擬磁場が形成されると予測されている[1]。しかしながら、均一で周期の揃ったサンプ ルを作製することは困難であり、このことがリップルグラフェンの構造・電子状態の実験的観察を妨 げている。そこで、我々は SiC-m面に着目した。SiC-m 面は周期的な凹凸構造を有しており、これをテ ンプレートとしてグラフェンを転写することにより均一なリップルグラフェンの形成を図った[今村、 田中、本講演会]。

Event date： 2023.9 - 2024.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：熊本   Country：Japan  

近年、SiC-MOSFET の微細化のため (0001)面をトレンチ型に加工した MOSFET が注 目されている。このトレンチ型 MOSFET では、m 面(1-100)をチャネルとするため従来 のプレーナー型に比べ電子移動度が向上することが期待されている。しかしながら、SiC-m 面や酸化物層の構造は未解明であり、構造の理解のためには更なる検討が必要である。そこ で本研究では 4H-SiC m 面基板の水素エッチング、真空アニール処理を行い、m 面の表面 構造観察を試みた。

Event date： 2023.9

Language：English  

Country：Japan  

Event date： 2023.3

Language：Japanese  

Country：Japan  

Event date： 2022.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北大学   Country：Japan  

Event date： 2020.9

Language：Japanese   Presentation type：Symposium, workshop panel (public)  

Venue：オンライン   Country：Japan  

Event date： 2020.9 - 2022.9

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Online   Country：Japan  

Event date： 2019.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Chemnitz   Country：Germany  

Event date： 2019.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Chemnitz   Country：Germany  

Event date： 2019.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Chemnitz   Country：Germany  

Event date： 2018.3

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：筑波   Country：Japan  

Event date： 2017.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

Graphene is expected to be used for the next generation ultra-fast switching devices. For this purpose the bandgap opening is the central issue and thus graphene nanoribbons1 (GNRs) have been investigated by many researchers. We reported GNR growth on periodic nanofacet SiC surfaces by molecular beam epitaxy (MBE) and showed the bandgap opening at K-point by angle-resolved photoemission spectroscopy (ARPES)2. We also found the periodically rippled graphene on the facets on vicinal SiC substrates via surface decomposition. As is illustrated in Fig. 1 GNR-like layer is possibly present at the interface3. To investigate the “real” feature of the interface layer by STM/STS, Raman spectroscopy, and LEED we tried to remove the top rippled graphene.

Event date： 2017.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

Graphene growth by thermal decomposition of SiC(0001) surface is more advantageous than other methods because of epitaxial nature on wafer scale. However, the electrical properties are somehow limited by the presence of surface steps and excess bilayer, which causes the carrier scattering and thus degrade the mobility. To solve the problems exfoliation and transferring of epitaxial graphene grown SiC has recently been paid attention. However, the transfer of monolayer graphene directly from a SiC surface is difficult because its relatively strong interaction with a buffer layer ((6√3 ́6√3)R30° structure (6R3)). J. Kim et al. reported a layer resolved graphene transfer technique from SiC surface using Ni stressor layer1. They demonstrated that the electronic properties of graphene are remarkably improved by deleting the bilayer region and transferring it to a flat surface2. In addition, transferring monolayer graphene with defined crystal orientation to various substrates enables to realize controllable moiré superstructures, which shows fractal quantum Hall effects3.

Event date： 2017.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

Researches on atomic layer materials of group IV elements attracts great attention. Among them,
stanene, an atomic layer of Sn, is expected to exhibit such unique characteristics as a two dimensional topological insulator [1] and quantum anomalous Hall effect [2]. The growth of stanene on Bi2Te3 has been achieved [3] but no experimental evidence on such characteristics is yet reported. On the other hand, since Sn shows a large spin orbit coupling (SOC), the spin and electronic states in terms of Rashba splitting, antiferromagnetic order and Mott insulator transition etc. are investigated. Adatom Sn (√3×√3) R30° (R3) structure on semiconductor substrates (Ge, SiC) has been reported [4, 5]. Thus, we have been aiming at forming stanene on SiC(0001) surfaces. Here, we tried Sn intercalation at graphene/SiC interfaces, which are advantageous to any ex-situ measurements of a Sn interlayer.

Event date： 2017.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

The research of 2D Dirac materials has been one of the main topics in recent material science, because of wide possible device application and fascinating physics. Most studies, however, are concentrated on graphene-like honeycomb lattice materials (graphene, silicene, germanene, stanene, etc.)1,2, though other lattice types may possess similar electronic structure features. Dense triangular lattice atomic layers (TLAL) of heavy group IV elements (Sn, Pb) on SiC(0001) surface and at graphene/SiC(0001) interface could be such materials (Fig. 1a,b). Additionally, heavy group IV elements exhibit strong spin-orbit coupling (SOC) which may significantly contribute to electronic structure of 2D layers and result in spin-polarized states. Previous studies did consider sparse triangular systems of group IV adatoms on SiC surface in terms of frustrating magnetic order, spin liquids, and Mott-type insulator state3. However, dense atomic layers where Sn (or Pb) atoms directly interconnected by in-plane bonds are unexplored.

Event date： 2017.1 - 2018.1

Language：Japanese   Presentation type：Oral presentation (general)  

Country：Japan  

Graphene を始めとした原子層物質の研究において,同じ IV 族元素である Sn の原子層 (stanene)の形成が試みられている[1].Stanene は 2 次元トポロジカル絶縁性[2]やスピン量 子ホール効果(SQHE)などの物性が期待されている.そこで我々は SiC 上への stanene 形成を 目的として,SiC 上の Sn の吸着—成長に関する研究を行っている.SiC は 6√3 バッファー層 を介してグラフェンのエピタキシャル成長が生じる.また,化学的安定性やワイドギャップ 半導体であることから,2次元物質の成長及び物性測定に適した基板材料である.また,SiC 上の Sn はモット絶縁体の状態を調べる研究対象にもなっており,吸着 Sn 同士の大きな相互 作用に関しても注目されている[3].
本研究では SiC(0001)-adatom Si に現れる様々な表面構造に着目し,Sn 吸着状態や成長モ ードへの影響を調べた.過去の研究において Si(111)7×7 表面の Sn は,再表層の Si ダング リングボンドを終端し,Si との混成がないことが報告されている[4].今回,Si-(√3×√3), (2√3×2√13)構造上及びグラフェン/SiC 界面への Sn 成長を行い,形成された構造を XPS・ ARPES・LEED 等を用いた解析を行った.

Event date： 2017.1 - 2018.1

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：仙台   Country：Japan  

Event date： 2016.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

Graphene nanoribbons (GNRs) are of importance for switching device applications because of possible bandgap opening at K-points. Many approaches have been proposed to tailor narrow ribbons, however, so far no effective or practical method is achieved.
SiC is known to be a good template for graphene epitaxial growth via surface decomposition. There have been many researches on polar SiC(0001) surfaces, on which an uniform thick graphene layer is grown. This structure is beneficial to investigate Dirac physics in terms of crystal quality. In order to modify electronic structure at K-points of graphene on such SiC substrates, e.g. bandgap creation, it is necessary to consider alternating growth ideas. We here demonstrate graphene growth on nonpolar SiC(1-100) surfaces [1, 2] showing lower crystal symmetry, that may induce anisotropic graphene growth, and thus GNR formation.

Event date： 2016.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

Graphene is an important 2D material with extremely useful properties for device applications. Growth of graphene on substrates faces the problem of interface influence on graphene's properties. In particular, graphene forms buffer layer when growing on SiC(0001) which does not possess characteristic graphene band dispersion and affect, often negatively, subsequent graphene layers. The interface between graphene and SiC can be modified by different atoms intercalation. In the present work, we consider one of the simplest case of Si intercalation. Various amounts of Si atoms were intercalated into graphene/SiC(0001) interface and studied by means of electron diffraction (LEED, RHEED), scanning tunneling microscopy (STM), angle-resolved photoemission (ARPES) and computational methods. We have shown that interface structures are similar to those formed by Si adsorption on clean SiC(0001) surface. The formation of these interface structures decouples graphene layer from substrate and restore it's linear electronic band dispersion.

Event date： 2015.11 - 2015.12

Language：English   Presentation type：Oral presentation (general)  

Venue：Boston   Country：United States  

Event date： 2015.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Giardini Naxos   Country：Italy  

Event date： 2015.7

Language：English   Presentation type：Oral presentation (general)  

Venue：仙台   Country：Japan  

Event date： 2015.6 - 2015.7

Language：English   Presentation type：Oral presentation (general)  

Venue：Nagoya   Country：Japan  

Event date： 2014.8

Language：English   Presentation type：Oral presentation (general)  

Venue：Fukuoka   Country：Japan  

Event date： 2013.9 - 2013.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Miyazaki   Country：Japan  

Event date： 2013.9 - 2013.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Miyazaki   Country：Japan  

Event date： 2013.9 - 2013.10

Language：English   Presentation type：Oral presentation (general)  

Venue：Miyazaki   Country：Japan  

Event date： 2013.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Yokohama   Country：United States  

Event date： 2013.4

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：Yokohama   Country：Japan  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2012.11

Language：English   Presentation type：Oral presentation (general)  

Venue：Boston, MA   Country：United States  

Event date： 2012.9

Language：English   Presentation type：Oral presentation (general)  

Venue：Yokohama   Country：Japan  

Event date： 2011.11 - 2011.12

Presentation type：Symposium, workshop panel (public)  

Venue：Boston, MA   Country：United States  

Language：English  

Country：Japan  

Language：English  

Country：Japan  

Ion-assisted reactive evaporation (IARE) using low-energy (100 eV) nitrogen ions was employed to deposit TiN films onto the vicinal Si-terminated (0001) face of alpha(6H)-SiC single crystals at 350-degrees-C in an ultrahigh vacuum chamber operated at a working pressure of 2 X 10(-4) Torr. The initial exposure of the SiC surface to nitrogen ions for a 2-min period resulted in Si-N bonding. This exposure was also an important step in attaining ohmic contact properties at low temperature, and the formation of a thin (almost-equal-to 5-15 angstrom) amorphous layer at the interface. The disorder was maintained after the deposition of 5 angstrom of TiN. The TiN contacts were ohmic after deposition; they showed little change in microstructural or electrical properties after annealing at 450, 550, and 600-degrees-C for 15-30 min. The results of this research indicate that a metal-insulator-semiconductor structure was responsible for obtaining the contact properties.

Language：English  

Country：Japan  

A novel reactor for layer-by-layer deposition of compound semiconductors has been designed and commissioned for the deposition of SiC. The substrates rested on a heated, rotating platform. They encountered individual fluxes of Si2H6 and C2H4 and subsequently paused beneath a hot filament. The filament was used to encourage the surface reaction between silicon adatoms and carbon precursors. Heteroepitaxial films were grown between 850 and 980-degrees-C on Si(100) substrates oriented 3-degrees off-axis toward [011]. They were analyzed for composition, crystallinity, growth per cycle, and morphology using depth profiling Auger spectroscopy, reflection high-energy electron diffraction, ellipsometry and transmission electron microscopy. Growth, as measured by ellipsometry and transmission electron microscopy, corresponded to approximately one monolayer per cycle. Monocrystalline films were achieved. Initial growth and characterization results of representative films are presented and discussed.

Language：English  

Country：Japan  

Pseudomorphic structures containing beta(3C)-SiC and 2H-AlN have been grown on vicinal alpha(6H)-SiC(0001) at 1050-degrees-C by plasma-assisted, gas-source molecular beam epitaxy. Reflection-high energy electron diffraction and cross-sectional high-resolution transmission electron microscopy showed all layers to be monocrystalline. The AlN layers were uniform in thickness. Defects in these layers initiated at steps on the 6H-SiC film. The 3C-SiC layers contained a high density of stacking faults and microtwins caused primarily by the interfacial stresses generated by the mismatch in lattice parameters between AlN and beta-SiC coupled with the very low stacking fault energy of SiC. This is the first report of the deposition of single crystal SiC/AlN/SiC thin film heterostructures on any substrate as well as the first report of the epitaxial growth of single crystal layers of binary materials with three different crystal structures.

Language：English  

Country：Japan  

Solid solutions of aluminum nitride (AlN) and silicon carbide (SiC), the only intermediate phases in their respective binary systems, have been grown at 1050-degrees-C on alpha(6H)-SiC(0001) substrates cut 3-4-degrees off-axis toward [1120BAR] using plasma-assisted, gas-source molecular beam epitaxy. A film having the approximate composition of (AlN)0.3(SiC)0.7, as determined by Auger spectrometry, was selected for additional study and is the focus of this note. High resolution transmission electron microscopy (HRTEM) revealed that the film was monocrystalline with the wurtzite (2H) crystal structure.

Language：English  

Country：Japan  

Monocrystalline AlN(0001) films with few defects were deposited on vicinal alpha(6H)-SiC(0001) wafers via plasma-assisted, gas-source molecular beam epitaxy within the temperature range of 1050-1200-degrees-C. The Al was thermally evaporated from an effusion cell. An electron cyclotron resonance plasma source was used to produce activated nitrogen species. Growth on vicinal Si(100) at 900-1050-degrees-C resulted in smooth, highly oriented AlN(0001) films.

Language：English  

Country：Japan  

Single-crystal epitaxial films of cubic beta(3C)-SiC(111) have been deposited on hexagonal alpha(6H)-SiC(0001) substrates oriented 3-4-degrees toward [1120]BAR at 1050-1250-degrees-C via gas-source molecular beam epitaxy using disilane (Si2H6) and ethylene (C2H4). High-resolution transmission electron microscopy revealed that the nucleation and growth of the beta(3C)-SiC regions occurred primarily on terraces between closely spaced steps because of reduced rates of surface migration at the low growth temperatures. Double positioning boundaries were observed at the intersections of these regions.

Language：English  

Country：Japan  

Language：English  

Country：Japan  

Surfaces of 6H-SiC(0001) homoepitaxial layers deposited on vicinal (similar to 3.5 degrees off (0001) towards [<11(2)over bar 0>]) and on-axis 6H-SiC wafers by chemical vapour deposition have been investigated using ultra-high vacuum scanning tunneling microscopy. Undulating step configurations were observed on both the on-axis and the vicinal surfaces. The former surface possessed wider terraces than the latter. Step heights on both surfaces were similar to 0.25 nm corresponding to single bilayers containing one Si and one C layer. After annealing at T > 1100 degrees C for 3-5 min in UHV, selected terraces contained honeycomb-like regions caused by the transformation to a graphitic surface as a result of Si sublimation. A model of the observed step configuration has been proposed based on the observation of the <(2)over bar 110>] or [<1(2)over bar 10>] orientations of the steps and energetic considerations. Additional deposition of very thin (similar to 2 nm SiC films on the above samples by gas source molecular beam epitaxy was performed to observe the evolution of the surface structure. Step bunching and growth of 6H-SiC layers and formation of 3C-SiC islands were observed on the vicinal and the on-axis surfaces, respectively, and controlled by the diffusion lengths of the adatoms.

Language：English  

Country：Japan  

The effect of gas flow ratios (C2H4/Si2H6 = 1,2,10) on the growth mode of SiC thin films on vicinal alpha(6H)-SiC(0001) substrates by gas source molecular beam epitaxy (GSMBE) at 950-1150 degrees C has been investigated. Step flow, step bunching and the deposition of 6H-SiC occurred at the outset of the exposure of the (1 x 1) surface to the reactants using any flow ratio. Subsequent deposition resulted in either step flow and continued growth of 6H films using C2H4/Si2H6 = 1 or nucleation and coalescence of 3C-SiC islands on the 6H terraces using C2H4/Si2H6 = 2 and 10. The initial stage of AlN film growth on these substrates and the occurrence of defects has also been investigated. Essentially atomically flat AlN surfaces, indicative of two-dimensional growth, were obtained using on-axis substrates. Island-like features were observed on the vicinal surfaces. The coalescence of the latter features gave rise to double positioning boundaries as a result of the misalignment of the Si/C bilayer steps with the Al/N bilayers in the growing films. The quality of the thicker AlN films was strongly influenced by the concentration of these boundaries. The following sections describe the procedures used to deposit and analyze these two materials as well as detail the results and conclusions of this research.

Language：English  

Country：Japan  

Silicon carbide (SiC) and aluminum nitride (ALN) thin films have been grown on 6H-SiC(0001) substrates by gas-source molecular beam epitaxy (GSMBE) at 1050 degrees C, Step flow, step bunching and the deposition of 6H-SiC occurred at the outset of the exposure of the (1x1) vicinal substrate surface to C2H4/Si2H6 gas flow ratios of 1, 2 and 10. Subsequent deposition resulted in step flow and continued growth of 6H films or formation and coalescence of 3C-SiC islands using the gas flow ratio of one or the ethylene-rich ratios, respectively. The (3 x 3) surface reconstruction observed using the former ratio is believed to enhance the diffusion lengths of the adatoms, which in turn promotes step flow growth, Essentially atomically flat monocrystalline AIN surfaces were obtained using on-axis substrates. Island-like features were observed on the vicinal surface. The coalescence of the latter features at steps gave rise to inversion domain boundaries (IDBs) as a result of the misalignment of the Si/C bilayer steps with the AIN bilayers in the growing film. The quality of thicker AIN films is strongly influenced by the concentration of IDBs. Undoped, highly resistive (10(2) Omega . cm) and Mg-doped, p-type (0.3 Omega . cm) monocrystalline GaN films having a thickness of 0.4-0.5 mu m have also been grown via the same technique on ALN buffer layers without post-processing annealing.

Language：English  

Country：Japan  

Metal/A1N/n-type 6H-SiC(0001) heterostructures have been prepared by growing wurtzite AIN layers on vicinal 6H-SiC(0001) using gas-source molecular beam epitaxy, High-resolution transmission electron microscopy results show that the interface between the AlN layer and the Si-terminated 6H-SiC substrate is microstructurally abrupt, but contains defects originating at step sites on the 6H-SiC surface. The interface is found to have a low density of trapped charges of -2 at room temperature without any postgrowth treatment. This value is comparable to 1x10(11) cm(-2) those reported for thermally grown and deposited oxides on n-type 6H-SiC(0001), and indicates the formation of a high quality interface. (C) 1996 American Institute of Physics.

Language：English  

Country：Japan  

Nanoscale GaN quantum dots were fabricated on AlxGa1-xN layer surfaces via metalorganic chemical vapor deposition. In order to achieve a self-assembling dot structure, a two-dimensional growth mode (step flow) of GaN films on AlxGa1-xN (x=0-0.2) surfaces that is energetically commenced under the conventional growth conditions was intentionally modified into a three-dimensional mode by using a ''surfactant.'' The surfactant is believed to inhibit the GaN film from wetting the AlGaN surface due to the change in surface free energy. The resulting morphological structures of GaN dots were found to be sensitive to the doping rate of tetraethyl silane used as a surfactant, the Al content (x) of the AlxGa1-xN layer, and the growth temperature. A very intense photoluminescence emission was observed from the GaN dots embedded in the AlGaN layers. (C) 1996 American Institute of Physics.

Language：English  

Country：Japan  

Amorphous, hexagonal and cubic phases of BN were grown via ion beam assisted deposition on Si(100) substrates. Gas-source molecular beam epitaxy of the III-V nitrides is reviewed. Sapphire(0001) is the most commonly employed substrate with 6H-SiC(0001), ZnO(111) and Si(111) also being used primarily for the growth of wurtzite GaN(0001) in tandem with previously deposited GaN(0001) or AlN(0001) buffer layers. Silicon(001), GaAs(001), GaP(001) and 3C-SiC(001) have been employed for growth of cubic (zincblende) beta-GaN(001). The precursor materials are evaporated metals and reactive N species produced either via ECR or RF plasma decomposition of N-2 or from ammonia. However, point defect damage from the plasma-derived species has resulted in a steady increase in the number of investigators now using ammonia. The growth temperatures for wurtzite GaN have increased from 650 +/- 50 degrees C to 800 +/- 50 degrees C to enhance the surface mobility of the reactants and, in turn, the efficiency of decomposition of ammonia and the microstructure and the growth rate of the films. Doping has been achieved primarily with Si (donor) and Mg (acceptor); the latter has been activated without post-growth annealing. Simple heterostructures, a p-n junction LED and a modulation-doped field-effect transistor have been achieved using GSMBE-grown material.

Language：English  

Country：Japan  

Stimulated emission was observed from optically pumped GaN quantum dots in an AlxGa1-xN separate confinement heterostructure fabricated on 6H-SiC(0001) substrate by metal organic chemical vapor deposition. Nanostructural GaN quantum dots, with an average size of similar to 10 nm width, similar to 1-2 nm height, and density of similar to 10(11) cm(-2), were self-assembled on the AlxGa1-xN cladding layer surface. The stimulated emission peak was observed at similar to 3.48 eV, which is similar to 50 meV lower than that of spontaneous emission. The excitation power dependence on the emission intensity clearly indicates threshold pump power density of 0.75 MW/cm(2) for the onset of stimulated emission. (C) 1997 American Institute of Physics.

Language：English  

Country：Japan  

Nanoscale GaN dots were successfully formed on AlxGa1-xN/6H-SiC(0001) surfaces by gas-source molecular beam epitaxy. It was found that the growth mode can be changed by introducing Si before GaN growth, where the Si is believed to play an important role in the change of the AlxGa1-xN surface free energy, Without introducing Si, the GaN growth mode was two dimensional and (1 x 3) reconstruction was observed, The growth mode of GaN was changed from two-dimensional to three-dimensional by introducing Si on the AlxGa1-xN surface. In situ reflection high-energy electron diffraction and atomic force microscopy observations were used to monitor and characterize the growth processes and surface morphology. (C) 1998 American Institute of Physics.

Language：English  

Country：Japan  

We demonstrate photoluminescence (PL) from self-assembling InGaN quantum dots (QDs), which are artificially fabricated on AlGaN surfaces via metalorganic chemical vapor deposition. InGaN QDs are successfully fabricated by the growth mode transition to three-dimensional nanoscale island formation by using "antisurfactant" silicon on AlGaN surface. The diameter and height of the fabricated InGaN QDs an estimated to be similar to 10 nm and similar to 5 nm, respectively, by an atomic-force microscope (AFM). Indium mole fraction of InxGa1-xN QDs is controlled from x = similar to 0.22 to similar to 0.52 by varying the growth temperature of QDs. Intense photoluminescence is observed even at room temperature from InGaN QDs embedded with the GaN capping layers. In addition, from the temperature dependence of the PL-peak energy, we convincingly show that the PL emission actually comes from the InGaN QDs. (C) 1998 American Institute of Physics. [S0003-6951(98)01814-2].

Language：English  

Country：Japan  

In situ reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM) observations were performed to monitor and characterize the growth processes and surface morphology of GaN on Al(x)Ga(1-x)N surface in gas-source molecular beam epitaxy. It was found that the growth mode can be changed by introducing Si before GaN growth, where the Si is believed to play an important role in the change of the Al(x)Ga(1-x)N surface free energy. Without introducing Si: the GaN growth mode was two-dimensional and (1 x 3) reconstruction was observed. The growth mode of GaN was changed from two-dimensional to three-dimensional by introducing Si on the Al(x)Ga(1-x)N surface. Nanoscale GaN dots were successfully formed on Al(x)Ga(1-x)N/6H-SIC(0 0 0 1) surfaces. Furthermore, the density of the GaN dots was found to be dependent on the amount of Si dose and the growth temperature. (C) 1998 Elsevier Science B.V. All rights reserved.

Language：English  

Country：Japan  

GaN growth was performed on 6H-SiC (0001) substrates by gas-source molecular beam epitaxy (GSMBE), using ammonia (NH3) and solid Ga as V and Ill sources. Two kinds of reflection high-energy electron diffraction (RHEED) patterns, namely (1x1) and (2x2), were observed during the GaN growth depending on the growth conditions, which correspond to nitrogen-rich and Ga-rich surfaces, respectively. Phase diagram of the two surface states via growth conditions was also obtained, indicating the surface V/III. ratio change during the growth. It was found that the GaN film quality was greatly improved under nitrogen-rich growth conditions by X-ray diffraction (XRD) and photoluminescence (PL) characterizations.

Language：English  

Country：Japan  

The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size. This effect was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius. From this observation, absolute values of the exciton binding energy as a function of quantum dot size are determined. (C) 1998 American Institute of Physics. [S0003-6951(98)03134-9]

Language：English  

Country：Japan  

The correspondence between the E2 level (similar to E-c-0.55 eV) in n-type GaN undergoing thermoionization and photoionization was established. The optical cross section in the vicinity of the threshold for photoionization of this level was measured by means of capacitance transient spectroscopy. Analysis using the formulation of Chantre yielded the optical activation energy, E-0=0.85 eV, and the Franck-Condon parameter, d(FC)=0.30 eV at 90 K. (C) 1999 American Institute of Physics. [S0003-6951(99)03004-1].

Language：English  

Country：Japan  

Here we describe a strategy toward constructing semiconductor photonic dots in the ultraviolet to blue region. An array of ZnS dots was grown on a GaAs substrate with a selective growth method. The ZnS dots have a pyramidal structure with the base plane of 800 nm square and the height of 300 nm. The {034} crystallographic planes form the sidewalls of the pyramids. Therefore, the size of the pyramidal dots is uniquely determined by the mask patterning. The optical reflection spectra showed clear resonance peaks which are reasonably assigned by the calculation of the resonance modes. Each resonance showed the Q values on the order of 160-300, a reasonable value to observe the modification of the total spontaneous emission rate in this kind of photonic dots. (C) 1999 American Institute of Physics. [S0003-6951(99)03614-1].

Language：English  

Country：Japan  

Uncapped GaN dots on AlGaN barrier material, grown by metal organic chemical vapor deposition on 6H-SiC substrates, were studied. Cathodoluminescence (CL) microscopy and scanning electron microscopy (SEM) were used to investigate both luminescence and structure of individual GaN dots. The correlation between the luminescence and the actual position of self-assembled dots was demonstrated. The position of a dot was established with high resolution SEM and a CL image was used to display the corresponding luminescence. The spectrum from a single dot was obtained by positioning the electron beam on one particular dot. The luminescence from dots with a lateral size of 100 nm and a height of 40 nm was determined to be 3.47 eV. (C) 1999 American Institute of Physics. [S0003-6951(99)05123-2].

Language：English  

Country：Japan  

Self-organized ZnSe quantum dots (QDs) were grown on (100) ZnS/GaAs surfaces to study the relation of the size dispersion and luminescence. The exact dot sizes were obtained by measurements of atomic force microscope with its tip calibration and transmission electron microscope. The average dot size was 2.0 nm high and 11 nm in its diameter and the density was 1 x 10(10) cm(-2). Transition energies of ZnSe QDs were calculated using these measured dot sizes. These calculated peaks were in reasonable agreement with measured photoluminescence (PL) peaks. It was also revealed that the broadening of the PL spectra from ZnSe QDs were consistently explained by the dot size distribution. (C) 1999 American Institute of Physics. [S0003-6951(99)03928-5].

Language：English  

Country：Japan  

We demonstrate InGaN and AlGaN quantum dots (QDs) formation on AlGaN surfaces vir metal organic chemical vapor deposition (MOCVD). Si anti-surfactant was used in order to modify surface energy balance for changing growth mode from 2-dimensional step-flow growth to 3-dimensional nano-scale island formation. The average lateral size and thickness of the InGaN and AlGaN QDs are 10-20 nm and 5 nm, respectively. Intense photoluminescence (PL) was observed from InGaN QDs even in room temperature. In and Al incorporation in InGaN and AlGaN QDs were estimated to be 22-52% and 1-5%, respectively, from the PL spectrum.

Language：English  

Country：Japan  

We report on an investigation of the optical properties of GaN quantum dots (QDs) grown by means of metalorganic vapor phase epitaxy. The growth regime for GaN on AlxGa1-xN was observed to change from two- to three-dimensional, forming GaN QDs, when Si was deposited on the AlxGa1-xN surface prior to the GaN growth. These QDs showed a redshift of the photo luminescence (PL) energy from the increased Coulomb energy induced by a compression of the exciton Bohr radius. Furthermore, a diminishing temperature-dependent shift of the PL energy with decreasing QD size caused by a reduction of the longitudinal-optical phonon coupling was found. We also show that the size of the QDs is a critical parameter for the optical nonlinearities. For large dots, the dominant nonlinearity in the PL is the bandgap renormalization but when the size of the dots was reduced below the critical size of 10 nm thick and 30 nm diameter, the state-filling effect became dominant. (C) 2000 American Institute of Physics. [S0021-8979(00)04708-3].

Language：English  

Country：Japan  

ZnSe/ZnMgS distributed Bragg reflectors (DBRs) with a high refractive-index contrast were grown on GaAs (1 0 0) substrates by metalorganic vapor-phase epitaxy. The difference of the refractive indices between ZnSe and Zn0.27Mg0.73S was estimated to be about 0.52 at 510 nm, which is very large compared with previous II-VI DBRs. The maximum reflectivity of the grown ZnSe/Zn0.27Mg0.73S DBRs (with only 5-periods) was measured to be 93 % at 510 nm at room temperature. DBRs with a high refractive-index contrast can reduce the penetration depth of light into the DBR and have the potential to increase the strength of the exciton-photon coupling in a microcavity. (C) 2000 Elsevier Science B.V. All rights reserved.

Language：Japanese  

Country：Japan  

アンチサーファクタントによるGaN層の低転位化

Language：English  

Country：Japan  

Vicinal 4H and 6H-SiC(0001) surfaces have been investigated using atomic force microscopy and cross-sectional high-resolution transmission electron microscopy. We observed the characteristic self-ordering of nanofacets on any surface, regardless of polytypes and vicinal angles, after gas etching at high temperature. Two facet planes are typically revealed: (0001) and high index (11 (2) over barn) that are induced by equilibrium surface phase separation. A (11 (2) over barn) plane may have a free energy minimum due to attractive step-step interactions. The differing ordering distances in 4H and 6H polytypes imply the existence of SiC polytypic dependence on nanofaceting. Thus, it should be possible to control SiC surface nanostructures by selecting a polytype, a vicinal angle, and an etching temperature.

Presentation type：Oral presentation (general)  

Venue：群馬県高崎市   Country：Japan  

Epitaxial SiO/SiN superstructures on SiC surfaces

Type：Scientific advice/Review 

Type：Scientific advice/Review 

Type：Competition, symposium, etc. 

Number of participants：800

Type：Competition, symposium, etc. 

Type：Competition, symposium, etc. 

Type：Scientific advice/Review 

Type：Competition, symposium, etc. 

Number of participants：500

Type：Competition, symposium, etc. 

Number of participants：800

Type：Scientific advice/Review