Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Harada Yuichi Last modified date:2021.07.07

Professor / Global Innovation Center / Global Innovation Center


Papers
1. Sameh Okashaa, Yoshiaki Sekine, Satoshi Sasaki, Yuichi Haradaa, Atomic layer deposition of aluminum (111) thin film by dimethylethylaminealane precursor, Thin Solid Films, https://doi.org/10.1016/j.tsf.2021.138784, 732, 138784, 2021.06, We report the growth of aluminum (111) thin film by atomic layer deposition (ALD) technique with dimethy- lethylaminealane (DMEAA) as a precursor. It is found that the metallic underlayer is essential to grow uniform aluminum films by DMEAA precursor. As a titanium thin film is used as the underlayer, grown aluminum thin film shows (111) orientation irrespective of substrates. The lattice constant and superconducting transition temperature of the aluminum thin films are the same as the bulk one. These findings suggest that ALD technique provides high quality of the aluminum thin films and have potential for the applications of superconducting devices. We discuss ALD technique with DMEAA precursor is the promising method for fabricating vertical small Josephson tunnel junctions, which can be used as the superconducting quantum bits..
2. Nobumitsu Hirose, Yuuichi Harada, Shigeru Yoshimori, Mitsuo Kawamura, Microcontact Josephson triode fabricated by self-alignment process technique, Electronics and Communications in Japan, Part II: Electronics (English translation of Denshi Tsushin Gakkai Ronbunshi), 73, 1, 1-7, 1990.01, A Josephson triode is a three-terminal superconductive device integrated with three Josephson devices. One of the three devices is used as a local oscillator and the triode can be used as a heterodyne detector. In such an application, since a Josephson device is used as a local oscillator, the frequency of the oscillator can be varied continuously, and the thermal noise is extremely low. However, there is no experimental report regarding the Josephson triode. In this study, microcontact Josephson triodes were fabricated by the self-alignment process technique using electron beam exposure. This device showed a negative resistance in the current-voltage characteristic of the converter junction..
3. Yuichi Harada, Nobumitsu Hirose, Yoshinori Uzawa, Shigeru Yoshimori, Matsuo Sekine, Mitsuo Kawamura, Analysis of quasiparticle injection three-terminal device, Infrared Physics, 10.1016/0020-0891(91)90103-M, 32, C, 129-137, 1991, The QPI (quasiparticle injection) devices have the most transistor-like characteristics in many superconducting three-terminal devices. In particular, Quiteron which operates with the principle of suppression of the energy gap only to modulate the conductance of the output junction has a promising feature, such as a large gain, non-latching operation, and binary inversion. In this paper, we have investigated the behavior of the energy gaps to analyze the characteristics of the QPI devices. Current-voltage characteristics of QPI device is calculated by using these values of the energy gaps. First we have introduced the nonequilibrium GL equation. Then we have derived a set of equations for QPI devices by using this equation. It is found that the energy gap of middle layer has the singularity under quasiparticle injection and the QPI device may have a large gain for suitable junction resistances..
4. Y. Harada, N. Hirose, Y. Uzawa, M. Sekine, S. Yoshimori, M. Kawamura, Properties of all-Nb thin film microbridges fabricated by nanometer process, Physica C: Superconductivity and its applications, 10.1016/0921-4534(91)91401-O, 185-189, PART 4, 2555-2556, 1991.12, We have fabricated all-Nb thin film microbridges by using newly developed nanometer process. These microbridges show the high sensitivity of the radiation detection. We have investigated the properties of microbridges by changing the bridge-region parameters..
5. Kazuo Yamaguchi, Hiroyuki Ozaki, Akira Hirao, Nobumitsu Hirose, Yuuichi Harada, Yoshinori Uzawa, Matsuo Sekine, Shigeru Yoshimori, Mitsuo Kawamura, New Positive EB Resist with Strong Resistance to Plasma Damage, Journal of the Electrochemical Society, 10.1149/1.2069330, 139, 3, L33-L34, 1992.03, Copolymers of methyl methacrylate (MMA) and 3–triethoxysilylpropyl methacrylate (ESPMA) were synthesized and employed as an electron beam (EB) resist. The copolymer with a content of 10 mol% ESPMA worked as a positive EB resist and had 4–10 times as strong a resistance as poly(methylmethacrylate) (PMMA) to CBrF3 plasma etching. The resolution and sensitivity were almost the same as those of PMMA. From experimental results concerning the insolubility of the plasma–irradiated copolymer and measurement of IR spectra, the enhanced resistance to plasma etching appears to be caused by formation of a cross–linking structure based on the Si–O–Si linkage..
6. Yoshinori Uzawa, Etsuo Kawate, Nobumitsu Hirose, Isao Motohashi, Yuichi Harada, Matsuo Sekine, Yukinobu Miki, Masahiro Okaji, Ac Josephson effect in superconducting all-Nb thin-film nanobridges up to 693 GHz, Applied Physics Letters, 10.1063/1.108472, 61, 8, 967-969, 1992.12, All-niobium thin-film nanobridges were fabricated with bridge sizes of 40-nm length, 50-nm width, and 100-nm thickness. We investigated the performance of the nanobridges by monitoring Shapiro steps induced on the current-voltage curves by an optically pumped far-infrared laser. These steps have been observed up to 693 GHz (433 μm). This is a twofold increase over that previously reported on thin-film nanobridges..
7. Nobumitsu Hirose, Yuichi Harada, Matsuo Sekine, Shigeru Yoshimori, Mitsuo Kawamura, All-Nb thin film microbridge-type Josephson junction for submillimeter-wave detection, Infrared Physics, 10.1016/0020-0891(93)90078-L, 34, 5, 445-455, 1993.10, A copolymer of methyl methacrylate (MMA) and 3-triethoxysilylpropyl methacrylate (ESPMA) was proposed as a positive-working electron beam (EB) resist of a new type. It was found that it had 4-10 times the resistance of polymethylmethacrylate (PMMA) against CBrF3 plasma damage. The mechanism of etching Nb in reactive-ion-etching (RIE) was deciphered. Using the new EB resist and nanometer process technology, an all-Nb thin film microbridge was fabricated. It shows the a.c. Josephson effect, i.e. the Shapiro steps upto the 11th were observed under millimeter-wave (70 GHz) radiation. In addition the coherently working performance of the series array of these thin-film microbridges were observed definitively..
8. C. D. Chen, P. Delsing, D. B. Haviland, Yuichi Harada, T. Claeson, Vortex mobility in two-dimensional arrays of small Josephson junctions, Proceedings of the 20th International Conference on Low Temperature Physics Physica B: Condensed Matter, 10.1016/0921-4526(94)90824-9, 194-96, pt 1, 989-990, 1994.01, We have measured the temperature and magnetic field dependencies of the zero bias resistance for several 2D arrays of small aluminum Josephson junctions. The normal state resistances RN of the junctions vary between 2.2 and 7.5 kΩ whereas the ratio of Josephson coupling energy EJ to the charging energy Ec ranges between 4.3 and 0.6, where Ec=e2/2C, C being the junction capacitance. The vortex mobility is deduced from the frustration (i.e. the number of flux quantum per unit cell) dependence of zero bias resistance. The mobility decreases when the temperature is lowered, resulting in a decrease of resistance. Fitting the data to a simple exponential form, we find the barrier for the vortex hopping to be -aEJ, with a≈0.3. For all arrays, there exists a crossover temperature Tcr which separates the regime of thermally assisted hopping from that of quantum creep of vortices. For our samples, Tcr is close to the theoretically predicted value of ℏω-p$//2π, where ℏω-p$/=(8EJEc) 1/2 ..
9. D. B. Haviland, Y. Harada, P. Delsing, C. D. Chen, T. Claeson, Observation of the resonant tunneling of cooper pairs, Physical Review Letters, 10.1103/PhysRevLett.73.1541, 73, 11, 1541-1544, 1994.01, We demonstrate how the Coulomb blockade of tunneling and the coherent Josephson tunneling of electron pairs can coexist in the same experiment. Measurements were made on a circuit consisting of two dc SQUIDs connected in series, with a gate capacitively coupled to the center electrode. Peaks in the current as a function of bias voltage shifted position with gate voltage in agreement with the theory of resonant Cooper pair tunneling..
10. P. Delsing, C. D. Chen, D. B. Haviland, Yuichi Harada, T. Claeson, Charge solitons and quantum fluctuations in two-dimensional arrays of small Josephson junctions, Physical Review B, 10.1103/PhysRevB.50.3959, 50, 6, 3959-3971, 1994.01, We have measured the current-voltage (IV) characteristics of several two-dimensional arrays of small Josephson junctions as a function of temperature, T and magnetic field B. The junctions have relatively large charging energies EC1 K, and normal-state resistances RN in the range of 4-150 kΩ. From the IV characteristics we can deduce the zero-bias resistance R0 and the threshold voltage Vt which reveal important information about the dynamics and statics of charge solitons in the array. R0(T) increases with decreasing temperature and may be described by thermal activation of charge solitons, characterized by an activation energy Ea. When the electrodes are in the normal state, Ea is close to 1/4EC. At low T, the thermal activation behavior breaks down, and R0(T) levels off to a value that can be attributed to the quantum fluctuations in the array. This interpretation places limitations on the observability of the charge unbinding, Kosterlitz-Thouless-Berezinskii transition for single electrons. When the electrodes are superconducting, Ea is much larger and dependent on B. In several samples, both Ea and Vt oscillate with B, having a period corresponding to one flux quantum per unit cell. For increasing magnetic fields, Vt increases until B250-450 G where it starts to decrease rapidly. We interpret the B dependence of Ea and Vt as a result of competition between Cooper-pair solitons and single-electron solitons..
11. C. D. Chen, P. Delsing, D. B. Haviland, Y. Harada, T. Claeson, Vortex mobility in two-dimensional arrays of small Josephson junctions, Physica B: Physics of Condensed Matter, 10.1016/0921-4526(94)90824-9, 194-196, PART 1, 989-990, 1994.02, We have measured the temperature and magnetic field dependencies of the zero bias resistance for several 2D arrays of small aluminum Josephson junctions. The normal state resistances RN of the junctions vary between 2.2 and 7.5 kω whereas the ratio of Josephson coupling energy EJ to the charging energy Ec ranges between 4.3 and 0.6, where Ec= e2 2C, C being the junction capacitance. The vortex mobility is deduced from the frustration (i.e. the number of flux quantum per unit cell) dependence of zero bias resistance. The mobility decreases when the temperature is lowered, resulting in a decrease of resistance. Fitting the data to a simple exponential form, we find the barrier for the vortex hopping to be -aEJ, with a≈0.3. For all arrays, there exists a crossover temperature Tcr which separates the regime of thermally assisted hopping from that of quantum creep of vortices. For our samples, Tcr is close to the theoretically predicted value of ωp 2π, where ωp=(8EJEc) 1 2..
12. P. Delsing, C. D. Chen, D. B. Haviland, Y. Harada, T. Claeson, Thermal activation and injection of charge solitons in 2D-arrays of small Josephson junctions., Physica B: Physics of Condensed Matter, 10.1016/0921-4526(94)90826-5, 194-196, PART 1, 993-994, 1994.02, We have measured the zero bias resistance, R0, and the threshold voltage, Vt, of 2D arrays of small Josephson junctions as functions of temperature and magnetic field. At low temperature, the Coulomb blockade dominates due to the relatively large charging energy EC= e2 2C (C being the junction capacitance). We find that the zero bias resistance may be described by thermal activation of charge solitons in most cases, i.e., R0≈k exp( Ea kBT). In the normal state, the activation energy Ea is close to 0.25 EC. The measured activation energy at low magnetic field is less than 0.25EC+Δ (where Δ is the superconducting gap), but larger than EC for all arrays. In a few samples, where the Josephson coupling energy EJ is relatively large, Ea oscillates with the magnetic field. The period of the oscillation corresponds to one flux quantum per unit cell and the amplitude is roughly EJ. In these samples the threshold voltage also oscillates at low magnetic fields. Such behavior of both Ea and Vt is a clear indication that also Cooper pair solitons contribute to the charge transport..
13. Y. Harada, D. B. Haviland, C. D. Chen, P. Delsing, T. Claeson, Magnetic flux and gate voltage modulation of the current in a superconducting loop of ultra-small tunnel junctions, Physica B: Physics of Condensed Matter, 10.1016/0921-4526(94)90837-0, 194-196, PART 1, 1015-1016, 1994.02, We have measured the current-voltage (I-V) characteristics of a loop of four superconducting tunnel junctions at low temperatures (T≈50mK). In each branch of the loop, tunneling occurs through a small capacitance island (C≈1fF). In this geometry we can vary the magnetic flux Φ in the loop and the voltage Vg on gate electrodes, which have capacitance Cg to the small islands. We find that both Φ and Vg modulate the I-V curve..
14. Y. Harada, D. B. Haviland, P. Delsing, C. D. Chen, T. Claeson, Fabrication and measurement of a Nb based superconducting single electron transistor, Applied Physics Letters, 10.1063/1.112255, 65, 5, 636-638, 1994.12, A new four-layer resist system was developed to fabricate small capacitance niobium tunnel junctions. Nb/AlOx/Al junctions were used to make a superconducting single electron transistor composed of two dc SQUIDs in series. The current-voltage characteristics were measured and two types of resonant tunneling of Cooper pairs were observed at different magnetic fields..
15. C. D. Chen, P. Delsing, D. B. Haviland, Yuichi Harada, T. Claeson, Scaling behavior of the magnetic-field-tuned superconductor-insulator transition in two-dimensional Josephson-junction arrays, Physical Review B, 10.1103/PhysRevB.51.15645, 51, 21, 15645-15648, 1995.01, We have studied the superconductor-insulator (SI) phase transition for two-dimensional (2D) arrays of small Josephson junctions in a weak magnetic field. The data were analyzed within the context of the theory of the magnetic-field-tuned SI transition in 2D superconductors. We show resistance scaling curves over several orders of magnitude for the 2D arrays. The critical exponent zB is determined to be 1.05, in good agreement with the theory. Moreover, the transverse (Hall) resistance at the critical field is found to be very small in comparison to the longitudinal resistance..
16. Yuichi Harada, Hideaki Takayanagi, Arkadi A. Odintsov, Coherent cooper-pair tunneling in a superconducting single electron transistor, Japanese Journal of Applied Physics, 10.1143/JJAP.34.4572, 34, 8, 4572-4574, 1995.08, We investigate how coherent Cooper-pair tunneling depends on the competition between Josephson coupling energy Ej and charging energy Eq. Measurements were performed on a circuit consisting of two dc-SQUIDs in series, with a gate capacitively coupled to the central island. As the Ej/Ec ratio decreased, coherent Cooper pairs tended to tunnel incoherently. Measured data show good agreement with theoretical calculations in strong and weak coupling limits. This experiment implies the manifestation of Heisenberg's uncertainty principle in a superconductor..
17. C. Chen, P. Delsing, D. Haviland, Y. Harada, T. Claeson, Flux flow and vortex tunneling in two-dimensional arrays of small Josephson junctions, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.54.9449, 54, 13, 9449-9457, 1996.01, We have measured the temperature dependence and magnetic field dependence of the zero-bias resistance ((Formula presented)) as well as the current-voltage (I-V) characteristics for several two-dimensional arrays of small aluminum Josephson junctions. (Formula presented)(T) decreases with decreasing temperature, which can be described in terms of two types of vortex motion: flux, flow, and vortex tunneling. At temperatures higher than the Kosterlitz-Thouless transition temperature (T>(Formula presented)) or at a bias current greater than the current corresponding to the onset of the nonlinear I-V characteristics (I>(Formula presented)), the effective damping resistance which characterizes flux-flow motion is found to be approximately equal to the junction normal-state resistance (Formula presented). At low temperatures and at small bias current, (Formula presented) is temperature independent and remains finite down to our minimum attainable temperature. This finite resistance is found to be dependent on the array size as well as the junction parameters..
18. Yuichi Harada, Hideaki Takayanagi, Arkadi A. Odintsov, Cooper-pair tunneling in small junctions with tunable Josephson coupling, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.54.6608, 54, 9, 6608-6613, 1996.01, We investigate Cooper-pair tunneling in a circuit consisting of two dc-superconducting quantum interference devices in series, with a gate capacitively coupled to the central island. Measurements cover a wide range of values of the ratio between Josephson coupling energy (Formula presented) and charging energy (Formula presented). The (Formula presented) ratio dependence of the supercurrent is well described by the orthodox theory provided that strong fluctuations of the Josephson phase due to the electromagnetic environment are taken into account. Our data can be interpreted in terms of squeezing of the charge fluctuations with decreasing (Formula presented) ratio..
19. Ken'ichi Tanaka, Yoshiro Arikawa, Matsuo Sekine, Motoichi Ohtsu, Yuichi Harada, Martin Danerud, Highly sensitive and wideband optical detection in patterned YBa2Cu3O7-δ thin films, Applied Physics Letters, 10.1063/1.115815, 68, 22, 3174-3176, 1996.12, We have measured the optical responses up to 18 GHz in patterned YBa2Cu3O7-δ thin films at various wavelengths by optical heterodyne mixing. The responsivity of the detectors is higher than 50 V/W below 1 Hz at various wavelengths and the responsivity is 20 mV/W at 780 nm and 150 mV/W at 1.55 μm wavelengths in the regime of the modulation frequency from 3 GHz to 18 GHz..
20. Ken'ichi Tanaka, Yuichi Harada, Martin Danerud, Matsuo Sekine, Motoichi Ohtsu, Highly sensitive and wideband optical detector in patterned YBa2Cu3O7-d thin film, Photodetectors: Materials and Devices II Proceedings of SPIE - The International Society for Optical Engineering, 316-325, 1997.12, We have demonstrated an optical detector based on patterned YBa2Cu3O7-δ thin films. The responses up to 18 GHz were observed using two wavelength lasers. The amplitude of the light was modulated up to 18 GHz by an optical heterodyne mixing system. The responsivity of the detectors is over 50 V/W for different wavelength lasers below 1 Hz and the responsivity is 4 mV/W at the 780 nm wavelength and 30 mV/W at 1.5 μm in the region from 3 GHz to 18 GHz. Our detector is one of the promising candidates for highly sensitive and wideband optical receiver..
21. Hideaki Takayanagi, Tatsushi Akazaki, Minoru Kawamura, Yuichi Harada, Junsaku Nitta, Superconducting junctions using AlGaAs/GaAs heterostructures with high Hc2 NbN electrodes, 14th International Conference on the Physica E: Low-Dimensional Systems and Nanostructures, 10.1016/S1386-9477(01)00410-6, 12, 1-4, 922-926, 2002.01, We investigated a superconductor-semiconductor-superconductor junction formed by two superconducting NbN electrodes and a two-dimensional electron gas (2DEG) in an AlGaAs/GaAs heterostructure. We obtained a good ohmic contact between NbN/AuGeNi electrodes and 2DEG by annealing them at 450 °C for 1 min in an N2 atmosphere. We observed a decrease in the resistance caused by Andreev reflection (AR) within the superconducting energy gap voltage in a zero magnetic field in this structure. We found that the peculiar features of the magnetoresistance in the transition region can be qualitatively explained by considering the existence of the AR in high magnetic fields..
22. Yuichi Harada, S. Jensen, T. Akazaki, H. Takayanagi, Anomalous magnetic flux periodicity of supercurrent in mesoscopic SNS Josephson junctions, Physica C: Superconductivity and its Applications, 10.1016/S0921-4534(01)01023-1, 367, 1-4, 229-233, 2002.02, The anomalous flux periodicity of supercurrent in mesoscopic SNS Josephson junctions was measured. The flux focusing factor was examined in order to obtain the precise magnetic flux in the normal region. Results showed that the magnetic flux periodicity became twice, when the junction lengths was shorter than 400 nm..
23. A. Richter, K. Matsuda, Y. Harada, H. Tamura, T. Akazaki, Y. Hirayama, H. Takayanagi, A field-induced semiconductor quantum dot defined by a single metallic front-gate, 2nd International Conference on Semiconductor Quantum Dots, QD 2002 Physica Status Solidi C: Conferences, 10.1002/pssc.200303080, 4, 1317-1320, 2003.12, We present studies on the electronic transport in a quantum dot based on a back-gated undoped GaAs/AlGaAs heterostructure. A high-quality two-dimensional electron gas can be induced in the structure by applying a back-gate voltage. The quantum dot is well defined by means of a single metallic frontgate. We observe clear regions of Coulomb blockade and pronounced tunneling peaks in a broad range of source-drain and front-gate voltages. The formation of the quantum dot and energetic evolution of the associated tunneling peaks are studied as a function of the applied front-gate voltage. Excellent agreement to the calculated Coulomb diamond pattern using the 'constant interaction model' is achieved, if a front-gate voltage dependent change of the dot capacitance is taken into account..
24. Yun Sok Shin, Roland Brunner, Akihiro Shibatomi, Toshiaki Obata, Tomohiro Otsuka, Jun Yoneda, Yasuhiro Shiraki, Kentarou Sawano, Yasuhiro Tokura, Yuichi Harada, Koji Ishibashi, Seigo Tarucha, Aluminum oxide for an effective gate in Si/SiGe two-dimensional electron gas systems, Semiconductor Science and Technology, 10.1088/0268-1242/26/5/055004, 26, 5, 2011.04, We investigate the gating properties of Si/SiGe two-dimensional electron gas systems with various gate materials and fabricate a lateral Si/SiGe quantum dot by gating through an Al2O3 film. In comparison to the conventional surface Schottky gates, gating through a thin Al2O 3 layer provides a strong suppression of leakage current. The fabricated quantum dot shows a periodic current oscillation or Coulomb oscillation with negligible gate leakage, indicating that the gating employed may be good for implementing Si/SiGe-based spin qubit devices with quantum dots..
25. Kyoichi Suzuki, Yuichi Harada, Fumihiko Maeda, Koji Onomitsu, Toru Yamaguchi, Koji Muraki, Gate operation of InAs/AlGaSb heterostructures with an atomic-layer- deposited insulating layer, Applied Physics Express, 10.1143/APEX.4.125702, 4, 12, 2011.12, Surface treatment of the GaSb cap layer is investigated for the gate operation of InAs/AlGaSb heterostructures with an Al 2O 3 insulating layer grown by atomic layer deposition. We show that dilute HCl treatment for only 10 s effectively removes the electron trap states at the GaSb/Al 2O 3 interface and greatly improves the gate operation. In contrast, HCl treatment followed by water rinse results in adverse effects. Using an equivalent circuit model, we deduce the interface trap state density for the no-treatment case to be D it ∼ 10 13cm -2eV -1, which is reduced to well below 10 12 cm -2eV -1 by the HCl treatment. Our data also show that deep donor states in the AlGaSb barrier impede the gate operation and cause gate hysteresis..
26. Hiroshi Irie, Yuichi Harada, Hiroki Sugiyama, Tatsushi Akazaki, In 0:75Ga0:25as quantum point contacts utilizing wrap-gate geometry, Applied Physics Express, 10.1143/APEX.5.024001, 5, 2, 2012.02, A fabrication technique for quantum point contacts (QPCs) in an InGaAs/InAlAs two-dimensional electron gas heterostructure is reported. The key challenge in realizing a well-behaved QPC is efficient electrostatic control of a one-dimensional channel using the gate electrode. The fabricated QPCs employ a 100-nm-wide mesa constriction and a gate electrode that wraps around the constriction for three-dimensional electricfield gating. In addition, conformal aluminum-oxide growth by atomic layer deposition is employed to suppress gate leakage while minimizing the interface state density. The wrap-gate QPCs show clear conductance steps, demonstrating well-defined quantized transverse modes in the InGaAs-based one-dimensional channel..
27. Kazuyuki Hirama, Hisashi Sato, Yuichi Harada, Hideki Yamamoto, Makoto Kasu, Thermally stable operation of h-terminated diamond FETs by NO 2 adsorption and Al 2O 3 passivation, IEEE Electron Device Letters, 10.1109/LED.2012.2200230, 33, 8, 1111-1113, 2012.07, Using the NO 2 adsorption and Al 2O 3 passivation technique, we improved the thermal stability of hydrogen-terminated diamond field-effect transistors (FETs) and then demonstrated stable operation at 200 °C in a vacuum for the first time. At 200 °C, the drain current I DS of a passivated diamond FET remained constant for at least more than 2 h. No degradation of FET characteristics was observed after the 200 °C heating cycle. Furthermore, a passivated diamond FET with a gate length of 0.2 μm showed high maximum I DS of-1000 mA/mm and an RF output power density of 2 W/mm..
28. Narihiko Maeda, Masanobu Hiroki, Satoshi Sasaki, Yuichi Harada, High-temperature characteristics in normally off AlGaN/GaN heterostructure field-effect transistors with recessed-gate enhanced-barrier structures, Applied Physics Express, 10.1143/APEX.5.084201, 5, 8, 2012.08, High-temperature characteristics in normally off AlGaN/GaN heterostructure field-effect transistors (HFETs) with recessed-gate enhanced-barrier structures were examined up to 300 °C, where a high threshold voltage (V th) of +3.0 V and a high drain current density (I d) of 610mA/mm were obtained at room temperature (RT). Interestingly, Id did not degrade significantly up to 300 °C with a small positive shift in Vth from +3.0 to +3.5 V. A model has been proposed that channel electrons should experience a potential step when they pass the nonrecessed/recessed boundary region in recessed-gate structures, which should be related to the observed high-temperature characteristics..
29. Kazuyuki Hirama, Hisashi Sato, Yuichi Harada, Hideki Yamamoto, Makoto Kasu, Diamond field-effect transistors with 1.3A/mm drain current density by Al 2O 3 passivation layer, Japanese journal of applied physics, 10.1143/JJAP.51.090112, 51, 9, 2012.09, Using nitrogen-dioxide (NO 2) adsorption treatment and Al 2O 3 passivation technique, we improved drain current (IDS) of hydrogen-terminated (Hterminated) diamond field-effect transistors (FETs). The Al 2O 3 passivation layer also serves as a gate-insulator in a gate region. Maximum IDS (I DSmax) of -1:35A/mm was obtained for the diamond FETs with NO 2 adsorption and the Al 2O 3 passivation layer. This I DSmax is the highest ever reported for diamond FETs and indicates that the Al 2O 3 passivation layer can stabilize adsorbed NO 2, which increases the hole carrier concentration on the H-terminated diamond surface. In RF small-signal characteristics, the diamond FETs with NO 2 adsorption and the Al 2O 3 passivation layer showed high cutoff-frequency (fT) and maximum frequency of oscillation (f max) in a wide gate-source voltage (VGS) range (>10 V). This is because the Al 2O 3 gate insulator with a high potential barrier against hole carriers can confine and control the high concentration of hole carriers and then high forward-bias voltage can be applied without noticeable gate leakage current..
30. Shinichi Tanabe, Makoto Takamura, Yuichi Harada, Hiroyuki Kageshima, Hiroki Hibino, Quantum hall effect and carrier scattering in quasi-free-standing monolayer graphene, Applied Physics Express, 10.1143/APEX.5.125101, 5, 12, 2012.12, The quantum Hall effect has been observed in quasi-free-standing monolayer graphene on SiC for the first time. This was achieved by decreasing the carrier density while applying gate voltage in top-gated devices. The charge neutrality point was also clearly observed, which has not been reported in top-gated structures. The mobilities at constant carrier densities did not show apparent temperature dependence up to 300 K, and conductivity was linearly dependent on carrier density. These results indicate that Coulomb scattering induced by charged impurities limits the mobility of quasi-free-standing monolayer graphene up to 300 K..
31. J. Burnett, T. Lindström, M. Oxborrow, Y. Harada, Y. Sekine, P. Meeson, A. Ya Tzalenchuk, Slow noise processes in superconducting resonators, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.87.140501, 87, 14, 2013.04, Slow noise processes, with characteristic timescales ∼1 s, have been studied in planar superconducting resonators. A frequency-locked loop is employed to track deviations of the resonator center frequency with high precision and bandwidth. Comparative measurements are made in varying microwave drive and temperature, and between bare resonators and those with an additional dielectric layer. All resonators are found to exhibit flicker frequency noise which increases with decreasing microwave drive. We also show that an increase in temperature results in a saturation of flicker noise in resonators with an additional dielectric layer, while bare resonators stop exhibiting flicker noise, instead showing a random frequency walk process..
32. Kyoichi Suzuki, Yuichi Harada, Koji Onomitsu, Koji Muraki, Edge channel transport in the InAs/GaSb topological insulating phase, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.87.235311, 87, 23, 2013.06, Transport in InAs/GaSb heterostructures with different InAs layer thicknesses is studied using a six-terminal Hall bar geometry with a 2-μm edge channel length. For a sample with a 12-nm-thick InAs layer, nonlocal resistance measurements with various current/voltage contact configurations reveal that the transport is dominated by edge channels with a negligible bulk contribution. Systematic nonlocal measurements allow us to extract the resistance of individual edge channels, revealing sharp resistance fluctuations indicative of inelastic scattering. Our results show that the InAs/GaSb system can be tailored to have conducting edge channels while keeping a gap in the bulk region and provide a way of studying two-dimensional topological insulators even when quantized transport is absent..
33. Narihiko Maeda, Masanobu Hiroki, Satoshi Sasaki, Yuichi Harada, High-temperature characteristics in recessed-gate AlGaN/GaN enhancement-mode heterostructure field effect transistors with enhanced-barrier structures, Japanese journal of applied physics, 10.7567/JJAP.52.08JN18, 52, 8 PART 2, 2013.08, Recessed-gate AlGaN/GaN enhancement-mode (E-mode) heterostructure field-effect transistors (HFETs) with enhanced-barrier structures were fabricated, and their high-temperature characteristics were examined. Owing to enhanced-barrier structures, where a thin AlGaN layer with a higher Al composition is inserted into the AlGaN barrier, excellent E-mode operation with threshold voltage (Vth) higher than +3 V and drain current density (Id) higher than 600mA/mm was obtained at room temperature. Interestingly, Id did not decrease much at high temperatures, i.e., Id decreased from 610 to 590mA upon a temperature raise from RT to 300 °C. Open-gate devices with and without recessed-gate structures were fabricated and their two-terminal characteristics were comparatively examined, revealing that the observed peculiar characteristics are ascribed to recessed-gate structures. A model for explaining the observed high-temperature characteristics is proposed..
34. Yuichi Nagahisa, Yuichi Harada, Eisuke Tokumitsu, Unipolar behavior in graphene-channel field-effect-transistors with n-type doped SiC source/drain regions, Applied Physics Letters, 10.1063/1.4833755, 103, 22, 2013.11, To realize graphene-channel field-effect-transistors (GFETs) with unipolar behavior and high on/off current ratios, we fabricated and characterized top-gate GFETs with n-type doped SiC (n-SiC) source/drain (S/D) regions on 4H-SiC(0001) substrates. 0-2 mono-layers (MLs) of graphene were grown on a monoatomic interfacial layer called zero-layer (ZL) by vacuum annealing. The 0-2 graphene MLs on the ZL were converted into 1-3 MLs of graphene without a ZL by annealing in H2. The GFETs with n-SiC S/D regions and 1-3 MLs of graphene without a ZL showed unipolar behavior with a high on/off current ratio of 2.7 × 103..
35. Satoshi Sasaki, Kouta Tateno, Guoqiang Zhang, Henri Suominen, Yuichi Harada, Shiro Saito, Akira Fujiwara, Tetsuomi Sogawa, Koji Muraki, Encapsulated gate-all-around InAs nanowire field-effect transistors, Applied Physics Letters, 10.1063/1.4832058, 103, 21, 2013.11, We report the fabrication of lateral gate-all-around InAs nanowire field-effect transistors whose gate overlaps the source and drain electrodes and thus fully encapsulates the nanowire channel. They feature large drive current and transconductance that surpass those of conventional non-gate-overlap devices. The improved device characteristics can be attributed to the elimination of access resistance associated with ungated segments between the gate and source/drain electrodes. Our data also reveal a correlation between the normalized transconductance and the threshold voltage, which points to a beneficial effect of our wet-etching procedure performed prior to the atomic-layer-deposition of the gate dielectric..
36. Toichiro Goto, Yuichi Harada, David Cox, Koji Sumitomo, Fabrication of a ring structure at the aperture of a hole for the efficient suspension of a lipid bilayer, Japanese Journal of Applied Physics, 10.7567/JJAP.53.096503, 53, 9, 2014.01, We propose a hydrophilic-raised-ring structure at the aperture of a hole for the efficient suspension of a lipid bilayer. This ring structure can be fabricated by using a combination consisting of a focused ion beam (FIB) and a hydrogen silsesquioxane (HSQ) negative electron beam (EB) resist layer on a substrate. Negative EB resist at the aperture of a hole can be selectively reacted with a scattered gallium ion beam during the FIB milling process, and a ring structure can be obtained by employing a development process. Assessments of the cross section of the ring revealed a two-step structure. The suspension of lipid bilayers on the ring structures was assessed by observing the green fluorescence of calcein dye inside the holes. Our observation of fluorescent images showed that the lipid bilayers on the ring structures sealed the calcein solutions. This result indicates that our fabrication processes can be used to produce nanobio devices that need lipid bilayer suspensions..
37. Shinichi Tanabe, Makoto Takamura, Yuichi Harada, Hiroyuki Kageshima, Hiroki Hibino, Effects of hydrogen intercalation on transport properties of quasi-free-standing monolayer graphene, Japanese Journal of Applied Physics, 10.7567/JJAP.53.04EN01, 53, 4 SPEC. ISSUE, 2014.01, We report that mobility in quasi-free-standing monolayer graphene grown on SiC(0001), when compared at the same carrier density, depends on the annealing temperature used for hydrogen intercalation. This was verified by measuring mobility in top-gated devices using quasi-freestanding monolayer graphene obtained by annealing at different temperatures. The density of charged impurities varies with annealing temperature, and it influences transport properties. Our systematic investigation shows that annealing temperatures between 700 and 800 °C are optimum for obtaining high-mobility quasi-free-standing monolayer graphene with the lowest number of charged impurities..
38. Nahoko Kasai, Rick Lu, Touichiro Goto, Aya Tanaka, Shingo Tsukada, Yuichi Harada, Koji Sumitomo, Observation of neuronal death in vitro by SEM and optical microscopy, e-Journal of Surface Science and Nanotechnology, 10.1380/ejssnt.2014.179, 12, 179-184, 2014.04, We observed neurons cultivated on an indium tin oxide (ITO) substrate using scanning electron microscopy (SEM) whose high spatial resolution allows us to observe neuronal morphological details. First, we optimized the fixation condition of cultivated neurons for SEM observation. The first fixation with paraformaldehyde and glutaraldehyde, and the second fixation with OsO4 were both necessary to avoid cell removal during the preparation before SEM observation. After optimization, we examined the morphological changes of neurons under an apoptotic condition, induced by staurosporine, by using both SEM and an immunochemical technique. Interestingly, the addition of staurosporine induced both apoptosis and a necrotic phenomenon. Immunostaining analysis revealed late-stage apoptosis after early-stage apoptosis, which was observed induced in cortical neurons by staurosporine for the first time. We confirmed that the SEM imaging of neurons is very useful as regards observing the apoptotic process. It is also a promising tool for understanding such neuronal activities as synaptic formation and axonal growth. Further examination of the interaction between neurons and substrates will contribute to the implementation of the artificial neurological devices..
39. M. Ono, E. Kuramochi, G. Zhang, H. Sumikura, Y. Harada, D. Cox, M. Notomi, Emission enhancement in nanowire-nanoantenna system fabricated by nanomanipulation, 27th IEEE Photonics Conference, IPC 2014 2014 IEEE Photonics Conference, IPC 2014, 10.1109/IPCon.2014.6995485, 532-533, 2014.12, We fabricated a unique system in which a semiconductor nanowire and a gold bowtie nanoantenna are combined. InP nanowire is precisely placed in the nanogap of the nanoantenna with a nanomanipulator installed in a focused ion beam system. By measuring the intensity mapping of the photoluminescence from the nanowire, we observed a significantly large enhancement at the antenna gap. We also calculated the electric field numerically, and the simulated results showed that the large enhancement that is obtained was caused by plasmonic resonance and was affected by the breakdown of the field suppression effect in the subwavelength nanowire..
40. D. Y. Hori, R. Kou, T. Tsuchizawa, Y. Kobayashi, Yuichi Harada, H. Hibino, T. Yamamoto, K. Yamada, H. Nakajima, Graphene optical modulator on silicon waveguide controlled by fine metal-Top gate, 13th IEEE International Conference on Group IV Photonics, GFP 2016 2016 IEEE 13th International Conference on Group IV Photonics, GFP 2016, 10.1109/GROUP4.2016.7739064, 90-91, 2016.11, We proposed a fine-metal gated graphene optical modulator on a CMOS compatible silicon photonic platform. A maximum extinction ratio of 1.2dB is realized by using a 25-nm thick Al
2
O
3
gate capacitor. Optimized device structure and initial Fermi energy dependences are discussed..
41. Rai Kou, Yosuke Hori, Tai Tsuchizawa, Kaori Warabi, Yuzuki Kobayashi, Yuichi Harada, Hiroki Hibino, Tsuyoshi Yamamoto, Hirochika Nakajima, Koji Yamada, Ultra-fine metal gate operated graphene optical intensity modulator, Applied Physics Letters, 10.1063/1.4972306, 109, 25, 2016.12, A graphene based top-gate optical modulator on a standard silicon photonic platform is proposed for the future optical telecommunication networks. On the basis of the device simulation, we proposed that an electro-absorption light modulation can be realized by an ultra-narrow metal top-gate electrode (width less than 400 nm) directly located on the top of a silicon wire waveguide. The designed structure also provides excellent features such as carrier doping and waveguide-planarization free fabrication processes. In terms of the fabrication, we established transferring of a CVD-grown mono-layer graphene sheet onto a CMOS compatible silicon photonic sample followed by a 25-nm thick ALD-grown Al2O3 deposition and Source-Gate-Drain electrodes formation. In addition, a pair of low-loss spot-size converter for the input and output area is integrated for the efficient light source coupling. The maximum modulation depth of over 30% (1.2 dB) is observed at a device length of 50 μm, and a metal width of 300 nm. The influence of the initial Fermi energy obtained by experiment on the modulation performance is discussed with simulation results..
42. Hiroshi Irie, Clements Todt, Norio Kumada, Harada, Yuichi, Hiroki Sugiyama, Tatsushi Akazaki, Koji Muraki, Andreev reflection and bound state formation in a ballistic two-dimensional electron gas probed by a quantum point contact, Physical Review B, 10.1103/PhysRevB.94.155305, 94, 155305-6, 2016.10, We study coherent transport and bound state formation of Bogoliubov quasiparticles in a high-mobility In0.75Ga0.25As two-dimensional electron gas (2DEG) coupled to a superconducting Nb electrode by means of a quantum point contact (QPC) as a tunable single-mode probe. Below the superconducting critical temperature of Nb, the QPC shows a single-channel conductance greater than the conductance quantum 2e2/h at zero bias, which indicates the presence of Andreev-reflected quasiparticles, time-reversed states of the injected electron, returning back through the QPC. The marked sensitivity of the conductance enhancement to voltage bias and perpendicular magnetic field suggests a mechanism analogous to reflectionless tunneling—a hallmark of phase-coherent transport, with the boundary of the 2DEG cavity playing the role of scatterers. When the QPC transmission is reduced to the tunneling regime, the differential conductance vs bias voltage probes the single-particle density of states in the proximity area. Measured conductance spectra show a double peak within the superconducting gap of Nb, demonstrating the formation of Andreev bound states in the 2DEG. Both of these results, obtained in the open and closed geometries, underpin the coherent nature of quasiparticles, i.e., phase-coherent Andreev reflection at the InGaAs/Nb interface and coherent propagation in the ballistic 2DEG..
43. Harada, Yuichi, Nanowire-nanoantenna coupled system fabricated by nanomanipulation, OPTICS EXPRESS, 10.1364/OE.24.008647, 24, 8, 8647-8659, 2016.04.
44. Harada, Yuichi, Detection of Coherent Terahertz Radiation from a High-Temperature Superconductor Josephson Junction by a Semiconductor Quantum-Dot Detector, PHYSICAL REVIEW APPLIED, 10.1103/PhysRevApplied.5.024010, 5, 2, 2016.02.
45. Harada, Yuichi, Gate-controlled semimetal-topological insulator transition in an InAs/GaSb heterostructure, PHYSICAL REVIEW B, 10.1103/PhysRevB.91.245309, 91, 24, 2015.06.
46. Harada, Yuichi, Self-aligned gate-all-around InAs/InP core-shell nanowire field-effect transistors, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.54.04DN04, 54, 4, 2015.04.
47. Harada, Yuichi, Iriel, Hiroshi, Sugiyama, Hiroki, Akazaki, Tatsushi, Josephson coupling through one-dimensional ballistic channel in semiconductor-superconductor hybrid quantum point contacts, PHYSICAL REVIEW B, 10.1103/PhysRevB.89.165415, 89, 16, 2014.04, We study a superconducting quantum point contact made of a narrow In0.75Ga0.25As channel with Nb proximity electrodes. The narrow channel is formed in a gate-fitted constriction of InGaAs/InAlAs/InP heterostructure hosting a two-dimensional electron gas.When the channel opening is varied with the gate, the Josephson critical current exhibits a discretized variation that arises from the quantization of the transverse momentum in the channel. The quantization of Josephson critical current persists down to the single-channel regime, providing an unambiguous demonstration of a semiconductor-superconductor hybrid Josephson junction involving only a single ballistic channel..
48. Yuichi Harada, N. Hirose, Y. Uzawa, M. Sekine, Characteristics of all-Nb thin film microbridges by nanometer fabrication, 23rd International Conference on Solid State Devices and Materials - SSDM '91 Conference on Solid State Devices and Materials, 520-522, 1991, We have fabricated the all-Nb thin film microbridges by our developed nanometer process. These microbridges have high sensitivity of radiation and reliability. It is observed that the properties of devices change by varying the sizes of bridge-region. We have also fabricated the series arrays and observed that they operate coherently..
49. Nobumitsu Hirose, Yuichi Harada, Shigeru Yoshimori, Mitsuo Kawamura, FABRICATION OF SELF-ALIGNMENT MICRO-CONTACT JOSEPHSON JUNCTIONS., Transactions of the Institute of Electronics and Communication Engineers of Japan. Section E, E69, 4, 425-426, 1986.04, We fabricated micro-contact Josephson junctions by self-alignment process, using double-layer-resist electron beam exposure and RIE. We chose NB as superconductive materials. The junctions show the Josephson effect under millimeter wave (70 GHz) radiation less than 100 mu w and the 7th Shapiro step..
50. Yuichi Harada, Nobumitsu Hirose, Yoshinori Uzawa, Properties of the nb thin-film nanobridges prepared by nanometer fabrication process, Japanese Journal of Applied Physics, Part 1: Regular Papers & Short Notes, 10.1143/JJAP.30.3933, 30, 12, 3933-3937, 1991, We describe the properties of Nb thin-film nanobridges fabricated by means of our original nanometer-scale process. Our original nanofabrication techniques include the synthesis of a new electron beam (EB) resist and the use of CBrF3 as an etchant for reactive ion etching (RIE). Fabricated nanobridges have high sensitivity of radiation and high reliability. It is observed that the properties of nanobridges can be changed by varying the bridge length and its thickness. We have also fabricated the series arrays and observed that they operate coherently..