Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Takeaki Yajima Last modified date:2023.09.28

Associate Professor / Electronic Devices / Department of Electronics / Faculty of Information Science and Electrical Engineering

1. Zero-dimensionality of a scaled-down VO2 metal-insulator transition via high-resolution electrostatic gating.
2. CMOS-based area-and-power-efficient neuron and synapse circuits for time-domain analog spiking neural networks.
3. Hang Yin, Takeaki Yajima, Self-synchronized Rectifier with Phase Information Extracted from Vibration Energy Harvester, Sensors and Materials, 10.18494/SAM3863, 34, 5, 1899-1907, 2022.05.
4. Ultra-low-power switching circuits based on a binary pattern generator with spiking neurons.
5. Low Power IoT Circuit Technology Using Spiking Neuron Circuits.
6. Temperature dependence of resistivity increases induced by thiols adsorption in gold nanosheets.
7. T. Tanaka, T. Yajima, K. Uchida, Impact of defects in self-assembled monolayer on humidity sensing by molecular functionalized transistors, Jpn. J. Appl. Phys., 2020.03.
8. X. Lu, T. Nishimura, T. Yajima, A. Toriumi, Understanding of Fermi level pinning at metal/germanium interface based on semiconductor structure, APEX, 13, 031003, 2020.03.
9. M. Xie, T. Nishimura, T. Yajima, A. Toriumi, Reaction of GeO2 with Ge and crystallization of GeO2 on Ge, J. Appl. Phys., 127, 024101, 2020.02.
10. J. Chen, W. Mao, L. Gao, F. Yan, T. Yajima, N. Chen, Z. Chen, H. Dong, B. Ge, P. Zhang, X. Cao, M. Silde, Y. Jiang, T. Terai, J. Shi, Electron‐Doping Mottronics in Strongly Correlated Perovskite, Adv. Mater., 32, 6, 1905060, 2019.12.
11. B. Kim, Y. Hikita, T. Yajima, H. Hwang, Heteroepitaxial vertical perovskite hot-electron transistors down to the monolayer limit., Nature Commun., 10, 5312, 2019.12.
12. T. Yajima, T. Tanaka, Y. Samata, K. Uchida, A. Toriumi, High-speed low-energy heat signal processing via digital-compatible binary switch with metal-insulator transitions., International Electron Devices Meeting (IEDM), 10.1109/IEDM19573.2019.8993502, 903-906, 2019.12.
13. Tomonori Nishimura, Xuan Luo, Soshi Matsumoto, Takeaki Yajima, Akira Toriumi, Almost pinning-free bismuth/Ge and/Si interfaces, AIP ADVANCES, 10.1063/1.5115535, 9, 9, 2019.09, In this work, we investigated the band alignment at bismuth (Bi)/germanium (Ge) and Bi/silicon (Si) interfaces to understand the mechanism of strong Fermi level pinning (FLP) at element metal/Ge and/Si interfaces. Bi/Ge and/Si interfaces exhibit almost ideal alignment deviating from the trend of strong FLP at element metal/Ge and/Si interfaces. This result suggests that the strong FLP at element metal/Ge and/Si interfaces is mainly caused by the metal-induced gap states (MIGS) in case of the free electron density of metal, and that the weak FLP at direct metal/Ge and/Si interfaces including germanide/Ge and silicide/Si interfaces is comprehensively understandable from the MIGS in case of low electron density. Furthermore, we also discuss impacts of interface structures on the band alignment at the MIGS-weakened interface..
14. Delta-temperatural electronic transportation achieved in metastable perovskite rare-earth nickelate thin films.
15. Overcoming synthetic metastabilities and revealing metal-to-insulator transition & thermistor bi-functionalities for d-band correlation perovskite nickelates.
16. T. Yajima, M. Minohara, C. Bell, H. Y. Hwang, Y. Hikita, Inhomogeneous barrier heights at dipole-controlled SrRuO3/Nb:SrTiO3 Schottky junctions, Applied Physics Letters, 10.1063/1.5052712, 11, 221603, 2018.11.
17. T. Yajima, G. Oike, S. Yamaguchi, S. Miyoshi, T. Nishimura, A. Toriumi, Hydrogenation of the wide-gap oxide semiconductor as a room-temperature and 3D-compatible electron doping technique., AIP Advances, 10.1063/1.5055302, 8, 115133, 2018.09.
18. T. Yajima, T. Nishimura, and A, Toriumi, Analog Spike Processing with High Scalability and Low Energy Consumption Using Thermal Degree of Freedom in Phase Transition Materials., VLSI (Technology), 10.1109/VLSIT.2018.8510649, 18, 27-28, 2018.07.
19. Y Noma, W Song, T Nishimura, T Yajima, A Toriumi, Anomalous Spectral Shape Evolution of Ge Raman Shift in Oxidation of SiGe., IEEE Electron Devices Technology and Manufacturing (EDTM), 10.1109/EDTM.2018.8421461, 2018.03.
20. Xuan Tian, Shigehisa Shibayama, Tomonori Nishimura, Takeaki Yajima, Shinji Migita, Akira Toriumi, Evolution of ferroelectric HfO2 in ultrathin region down to 3 nm, Applied Physics Letters, 10.1063/1.5017094, 112, 10, 2018.03, The ferroelectric properties of ultrathin Y-doped HfO2 films were investigated. Ferroelectricity was demonstrated experimentally in 3 nm-thick Y-doped HfO2 via direct detection of displacement currents during polarization switching. The dependence on the HfO2 thickness within the 30 to 3 nm range revealed that the ferroelectric properties decrease rapidly below a critical thickness. In the ultrathin HfO2 region, methods such as higher Y doping or metal capping annealing were required to further stabilize the ferroelectric phase. These methods could be used to enhance the switchable polarization (Psw) to 35 μC/cm2 in 5 nm- and 10 μC/cm2 in 3 nm-thick Y-doped HfO2. This paper indicates that HfO2 ferroelectricity is scalable even in the ultrathin region..
21. X Luo, T Nishimura, T Yajima, A Toriumi, Potential Influence of Surface Atomic Disorder on Fermi-level Pinning at Metal/SiGe Interface, Electron Devices Technology and Manufacturing (EDTM), 10.1109/EDTM.2018.8421410, 2018.03.
22. Tomonori Nishimura, Xiaoyu Tang, Takeaki Yajima, Akira Toriumi, Rigidity enhancement of GeO2by Y doping for reliable Ge gate stacks, Electron Devices Technology and Manufacturing Conference (EDTM), 10.1109/EDTM.2018.8421529, 2018.03.
23. Di Lu, Yasuyuki Hikita, David J. Baek, Tyler A. Merz, Hiroki Sato, Bongju Kim, Takeaki Yajima, Christopher Bell, Arturas Vailionis, Lena F. Kourkoutis, Harold Y. Hwang, Strain Tuning in Complex Oxide Epitaxial Films Using an Ultrathin Strontium Aluminate Buffer Layer, Physica Status Solidi - Rapid Research Letters, 10.1002/pssr.201700339, 12, 3, 2018.03, A reliable method to apply biaxial strain over a wide range of values with minimal dislocation generation is critical for the study of strain dependent physical properties in oxide thin films and heterostructures. In this work, we systematically controlled the strain state in a perovskite manganite thin film by as much as 1% using a new ultrathin strain-releasing buffer layer Sr3Al2O6, and observed signatures of accompanying magnetic and metal–insulator transitions. The near-zero strain state is achieved within five nanometers of buffer layer thickness, substantially thinner than any oxide epitaxial buffer layers that can continuously tune the film strain states. Furthermore, the majority of misfit dislocations were confined to the Sr3Al2O6 layer, structurally decoupling defects in the film from the substrate..
24. X. Tian, L. Xu, S. Shibayama, T. Nishimura, T. Yajima, S. Migita, A. Toriumi, Sub-nm EOT ferroelectric HfO2 on p+Ge with highly reliable field cycling properties, Technical Digest - International Electron Devices Meeting, IEDM, 10.1109/IEDM.2017.8268508, 17, 37.1.1-37.1.4, 2018.01, 5-nm-Thick ferroelectric Y-doped HfO2 was intensively studied. The thickness dependence of ferroelectric properties indicates that stable ferroelectric characteristics are maintained down to 5-nm-thick by taking care of doping and capping effects. Furthermore, the cycling performance shows no wake-up behavior, no obvious degradation after 108 cycles. These results not only enable us to use ferroelectric HfO2 for practical application, but also point out intrinsic properties in ultrathin ferroelectric HfO2 film from materials science point of view..
25. Jikun Chen, Xinyou Ke, Jiaou Wang, Takeaki Yajima, Haijie Qian, Song Sun, Dipole-correlated carrier transportation and orbital reconfiguration in strain-distorted SrNbxTi1-xO3/KTaO3, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 10.1039/c7cp06495k, 19, 44, 29913-29917, 2017.11, Strong electron-correlations can result in un-conventional transportation behaviour, such as metalinsulator transitions, high temperature superconductivity and bad metal conduction. Here we report a distinct transportation characteristic achieved by actively coupling the carriers with randomly distributed lattice-dipoles for strain-distorted SrNbxTi1-xO3. The strong electron correlations split the conduction band, and lead to a distinguished thermal-emitted carrier transportation with an activation energy of similar to 10(-2) eV. Further consistency was demonstrated by the respective changes in orbital configurations observed in near edge X-ray absorption fine structures. The present investigation demonstrates new mechanisms for regulating the carrier transportation using polaronic electron correlations..
26. Nishimura Tomonori, Yajima Takeaki, Toriumi Akira, Reconsideration of Metal Work Function at Metal/Semiconductor Interface, SEMICONDUCTOR PROCESS INTEGRATION 10, 10.1149/08004.0107ecst, 80, 4, 107-112, 2017.10.
27. Lun Xu, Tomonori Nishimura, Shigehisa Shibayama, Takeaki Yajima, Shinji Migita, Akira Toriumi, Kinetic pathway of the ferroelectric phase formation in doped HfO2 films, JOURNAL OF APPLIED PHYSICS, 10.1063/1.5003918, 122, 12, 2017.09, The dopant-induced ferroelectric HfO2 formation has been systematically investigated by using cation (Sc, Y, Nb, Al, Si, Ge, and Zr) and anion (N) dopants. Both differences and similarities are discussed among various dopants by focusing on two major factors, the oxygen vacancy (Vo) and the dopant ionic size. First, the doping concentration dependence of the remanent polarization in 27 (+/-2) nm HfO2 films is quantitatively estimated. Then, by comparing the polarization result with the structural transformation in doped HfO2, the pathway of the dopant-induced HfO2 phase transition is discussed among monoclinic, ferroelectric orthorhombic, tetragonal, and cubic phases. Finally, it is addressed that a dopant species independent phase transition route may exist in HfO2 owing to the same kinetic transition process, in which the ferroelectric phase seems to be at an intermediate state between tetragonal and monoclinic phases. Published by AIP Publishing..
28. X. Wang, T. Nishimura, T. Yajima, A. Toriumi, Thermal oxidation kinetics of germanium, APPLIED PHYSICS LETTERS, 10.1063/1.4997298, 111, 5, 052101, 2017.07, Thermal oxidation kinetics of Ge was investigated by the O-18 tracing study and re-oxidation experiments of the SiO2/GeO2 stacked oxide-layer. The results suggest that Ge oxidation kinetics is completely different from that expected from the Deal-Grove model and that Ge is oxidized by GeO2 on Ge instead of O-2 at the interface. This oxidation process forms large amounts of oxygen vacancies in GeO2, which facilitate the diffusion of oxygen atoms in GeO2. This means that oxygen atoms diffuse through GeO2 with an exchange type of process. Based on experimental results, a possible kinetics for Ge oxidation is discussed. Published by AIP Publishing..
29. Tony Chi Liu, Shoichi Kabuyanagi, Tomonori Nishimura, Takeaki Yajima, Akira Toriumi, n(+)Si/pGe Heterojunctions Fabricated by Low Temperature Ribbon Bonding With Passivating Interlayer, IEEE ELECTRON DEVICE LETTERS, 10.1109/LED.2017.2699658, 38, 6, 716-719, 2017.06, A bonding technique via passivating interlayer formation is proposed for bulkmaterial heterojunction fabrication. n(+)Si/pGe heterojunctions were fabricated by a ribbon bonding with interfaces passivated by an amorphous interlayer. With a highest process temperature as low as 150. degrees C, the bonded junctions exhibited rectifying characteristics with a turn-on voltage of 0.3 V as an ideal Si/Ge heterojunction and an ideality factor of 2.15. This technique shows a great potential for bulk material heterojunction formation, especially when ultimately abrupt junctions and low temperature processes are needed..
30. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Akira Toriumi, Thermodynamic understanding and analytical modeling of interfacial SiO2 scavenging in HfO2 gate stacks on Si, SiGe, and SiC, APPLIED PHYSICS LETTERS, 10.1063/1.4979711, 110, 14, 2017.04, This work thermodynamically and experimentally generalizes the interfacial SiO2 scavenging in HfO2 gate stacks from on Si to on other channel materials including SiGe and SiC and proposes a generalized formulation for this process. By paying attention to the Si chemical potential in the SiO2 interfacial layer (SiO2-IL) significantly affected by the substrate, it clarifies that Si in the substrate is indispensable to trigger the scavenging process. Thanks to this understanding, we demonstrate that the scavenging is extendable to next generation of channel materials containing Si such as SiGe and SiC with well-controlled high-k gate stacks. In addition, via formulating the diffusion-reaction-diffusion kinetics, an analytical relation like the Deal-Grove model is obtained for SiO2-IL scavenging in high-k gate stacks. Published by AIP Publishing..
31. Jikun Chen, Hongyi Chen, Feng Hao, Xinyou Ke, Nuofu Chen, Takeaki Yajima, Yong Jiang, Xun Shi, Kexiong Zhou, Max Dobeli, Tiansong Zhang, Binghui Ge, Hongliang Dong, Huarong Zeng, Wenwang Wu, Lidong Chen, Ultrahigh Thermoelectric Performance in SrNb0.2Ti0.8O3 Oxide Films at a Submicrometer-Scale Thickness, ACS ENERGY LETTERS, 10.1021/acsenergylett.7b00197, 2, 4, 915-921, 2017.04, Localized refrigeration and power generation via thermoelectric technology rely on efficient thermoelectric materials with high performance at room temperature. Although the two-dimensional electron gas (2DEG)-related materials exhibit ultrahigh thermoelectric performance near room temperature, such performance is only preserved at thicknesses within subnanometer scales, limited by the requirement of two-dimensional size confinements. Here we report ultrahigh thermoelectric performance similar to 2DEG-related materials but achieved in SrNb0.2Ti0.8O3 oxide films with a submicrometer-scale thickness by regulating strain induced lattice polarizations and interfacial polarizations. A large figure of merit, zT, and power factor (similar to 10(2)-10(3) mu W cm(-1) K-2) were achieved near room temperature, and the maximum zT is estimated to be similar to 1.6 for a 49 nm thick film. These performances exceed those of the existing n-type thermoelectric materials for room-temperature uses and the reported best oxide materials beyond subnanometer scales. The earth-abundant elemental composition of the oxide film paves the way toward potential applications in thermoelectric thin film devices with a microscale thickness..
32. Takeaki Yajima, Tomonori Nishimura, Akira Toriumi, Identifying the Collective Length in VO2 Metal-Insulator Transitions, SMALL, 10.1002/smll.201603113, 13, 12, 2017.03.
33. L. Xu, S. Shibayama, K. Izukashi, T. Nishimura, T. Yajima, S. Migita, A. Toriumi, General relationship for cation and anion doping effects on ferroelectric HfO2 formation, Technical Digest - International Electron Devices Meeting, IEDM, 10.1109/IEDM.2016.7838477, 25.2.1-25.2.4, 2017.01, This work discusses the general relationship for cation and anion doping effects on the HfO2 para-/ferroelectric transition, which will provide us a helpful instruction for precise HfO2 ferroelectricity design. In addition, ferroelectric N-doped HfO2 has been demonstrated as a gate dielectric film on an oxide semiconductor for ferroelectric field-effect transistors (FeFETs)..
34. T. Yajima, T. Nishimura, A. Toriumi, Functional Passive Material VO2 for Analogue Signal Processing with High-Speed, Low Power, and Robust Performance, Technical Digest - International Electron Devices Meeting, IEDM, 10.1109/IEDM.2016.7838540, 2016.12.
35. L. Xu, S. Shibayama, K. Izukashi, T. Nishimura, T. Yajima, S. Migita, A. Toriumi, General relationship for cation and anion doping effects on ferroelectric HfO2 formation, Technical Digest - International Electron Devices Meeting, IEDM, 10.1109/IEDM.2016.7838477, 2016.12.
36. Lun Xu, Tomonori Nishimura, Shigehisa Shibayama, Takeaki Yajima, Shinji Migita, Akira Toriumi, Ferroelectric phase stabilization of HfO2 by nitrogen doping, APPLIED PHYSICS EXPRESS, 10.7567/APEX.9.091501, 9, 9, 2016.09, We report that nitrogen (N) doping can drive the ferroelectricity of HfO2. It was found that N doping can cause the transition from a monoclinic phase to a highly symmetric phase. The role of N doping is discussed from the viewpoints of charge balance and bond-constraining effects. The former is responsible for the structural transformation from a paraelectric phase to a ferroelectric phase by forming an oxygen vacancy. In addition, Hf-N and N-O bonds with covalent characteristics have strong effects on HfO2 structural and electrical properties, and thus contribute to a marked HfO2 para-/ferroelectric transition. (C) 2016 The Japan Society of Applied Physics.
37. Lun Xu, Takeaki Yajima, Tomonori Nishimura, Akira Toriumi, Anomalous electrical properties of Au/SrTiO3 interface, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.55.08PB04, 55, 8, 2016.08, Metal/dielectric interface properties of Au/SrTiO3 (STO) and SrRuO3/SrTiO3 (SRO/STO) interfaces were investigated using metal/STO/heavily Nb-doped STO (0.5 wt% Nb:STO) capacitors. The observed interfacial capacitance at SRO/STO accords with results predicted theoretically, whereas that at the Au/STO interface is strongly suppressed, suggesting an intrinsic low-k (dielectric constant) interfacial layer formation at the Au/STO interface owing to in situ evaporated Au after STO film deposition. Furthermore, metal/0.01 wt% Nb:STO junctions were also analyzed. It was found that the SRO/Nb: STO junction forms an ideal Schottky dipole, whereas the Au/Nb: STO junction exhibits anomalous electrical properties. (C) 2016 The Japan Society of Applied Physics.
38. Tomonori Nishimura, Lun Xu, Shigehisa Shibayama, Takeaki Yajima, Shinji Migita, Akira Toriumi, Ferroelectricity of nondoped thin HfO2 films in TiN/HfO2/TiN stacks, JAPANESE JOURNAL OF APPLIED PHYSICS, 10.7567/JJAP.55.08PB01, 55, 8, 2016.08, We report on the impact of TiN interfaces on the ferroelectricity of nondoped HfO2. Ferroelectric properties of nondoped HfO2 in TiN/HfO2/TiN stacks are shown in capacitance-voltage and polarization-voltage characteristics. The Curie temperature is also estimated to be around 500 degrees C. The ferroelectricity of nondoped HfO2 clearly appears by thinning HfO2 film down to > 35 nm. We directly revealed in thermal treatments that the ferroelectric HfO2 film on TiN was maintained by covering the top surface of HfO2 with TiN, while it was followed by a phase transition to the paraelectric phase in the case of the open surface of HfO2. Thus, it is concluded that the ferroelectricity in nondoped HfO2 in this study was mainly driven by both of top and bottom TiN interfaces. (C) 2016 The Japan Society of Applied Physics.
39. Takeaki Yajima, Go Oike, Tomonori Nishimura, Akira Toriumi, Independent control of phases and defects in TiO2 thin films for functional transistor channels, PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, 10.1002/pssa.201600006, 213, 8, 2196-2202, 2016.08.
40. Tomonori Nishimura, Takeaki Yajima, Akira Toriumi, Reexamination of Fermi level pinning for controlling Schottky barrier height at metal/Ge interface, APPLIED PHYSICS EXPRESS, 10.7567/APEX.9.081201, 9, 8, 2016.08, The element metal/germanium (Ge) interface exhibits a strong Fermi level pinning (FLP), which is usually characterized on the basis of Ge side semiconductor properties. In this work, we demonstrate that metal properties significantly affect the Schottky barrier height (SBH) on Ge. Metallic germanides show both FLP alleviation and a clear substrate orientation dependence of SBH on Ge, despite the nearly perfect FLP and very slight orientation dependence in the element metal case. As a result, ohmic characteristics are observed at germanide/n-Ge (111) junctions. The metal properties required to alleviate the FLP on Ge are also discussed. (C) 2016 The Japan Society of Applied Physics.
41. Tomonori Nishimura, Choong Hyun Lee, Toshimitsu Nakamura, Takeaki Yajima, Kosuke Nagashio, Koji Kita, Akira Toriumi, Recent Progress of Junction Technology for Germanium CMOS, 2015 15TH INTERNATIONAL WORKSHOP ON JUNCTION TECHNOLOGY (IWJT), 10.1109/IWJT.2015.7467098, 65-68, 2016.05, To realize scaled Ge CMOS device, there are several challenges of junction formation in nMOSFET. (Fermi level pinning, activation and diffusion of n-type impurities and junction leakage, etc.) In this paper, we report recent progresses of controllability of band alignment at metal/Ge interface, understanding of n-type impurity activation in Ge, and impact of oxygen in Ge on n(+)/p junction leakage..
42. Shoichi Kabuyanagi, Tomonori Nishimura, Takeaki Yajima, Akira Toriumi, Experimental evidence of zone-center optical phonon softening by accumulating holes in thin Ge, AIP ADVANCES, 10.1063/1.4941072, 6, 1, 2016.01, We discuss the impact of free carriers on the zone-center optical phonon frequency in germanium (Ge). By taking advantage of the Ge-on-insulator structure, we measured the Raman spectroscopy by applying back-gate bias. Phonon softening by accumulating holes in Ge film was clearly observed. This fact strongly suggests that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than the dopant atom counterpart. Furthermore, we propose that the free carrier effect on phonon softening is simply understandable from the viewpoint of covalent bonding modification by free carriers. (C) 2016 Author(s)..
43. Takeaki Yajima, Tomonori Nishimura, Akira Toriumi, Positive-bias gate-controlled metal-insulator transition in ultrathin VO2 channels with TiO2 gate dielectrics, NATURE COMMUNICATIONS, 10.1038/ncomms10104, 6, 2015.12.
44. Shoichi Kabuyanagi, Tomonori Nishimura, Takeaki Yajima, Akira Toriumi, Gate-bias dependent phonon softening observed in Ge MOSFETs, ECS Transactions, 10.1149/06910.0075ecst, 69, 10, 75-80, 2015.10, We investigate impacts of free carriers on phonon frequency of zone-center optical phonon in Germanium (Ge). By taking advantage of Ge-on-insulator structure, Raman measurement with applying the back-gate bias was performed. The phonon softening was clearly observed by increasing hole density. It is also pointed out that the phonon softening in heavily-doped Ge is mainly attributed to the free carrier effect rather than dopant atom one. Furthermore, we show that the free carrier effect on phonon softening can be simply and reasonably interpreted from the view point of modification of the covalent bonding by free carriers..
45. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Akira Toriumi, Significance of Kinetic-linkage of Oxygen Vacancy with SiO2/Si Interface for SiO2-IL Scavenging in HfO2 Gate Stacks, 2015 SILICON NANOELECTRONICS WORKSHOP (SNW), 2015.09, This work clarifies roles of substrate-Si in SiO2-IL scavenging in HfO2/SiO2/Si gate stack experimentally, and points out a coupling effect of oxygen vacancy (V-O) with SiO2/Si interface from thermodynamic viewpoint..
46. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Akira Toriumi, Study of Si kinetics in interfacial SiO2 scavenging in HfO2 gate stacks, APPLIED PHYSICS EXPRESS, 10.7567/APEX.8.061304, 8, 6, 2015.06, In this study, what occurs at the SiO2/substrate interface and how Si atoms in SiO2 behave in interfacial SiO2 scavenging in a HfO2 gate stack have been investigated. Since the same scavenging occurs on both Si and SiC substrates, the SiC substrate is used to study the role of the substrate. The characterizations of the SiO2/SiC interface show no Si growth on the substrate and no consumption of the substrate in the SiO2 scavenging. Isotope tracing experiments for a HfO2/SiO2/Si stack further demonstrate that atomic Si is generated in scavenging and diffuses out through the HfO2 layer. On the basis of these findings, the reaction at the SiO2/substrate interface is thermodynamically discussed. (C) 2015 The Japan Society of Applied Physics.
47. Yajima, Takeaki, Hikita, Yasuyuki, Minohara, Makoto, Bell, Christopher, Mundy, Julia A., Kourkoutis, Lena F., Muller, David A., Kumigashira, Hiroshi, Oshima, Masaharu, Hwang, Harold Y, Controlling band alignments by artificial interface dipoles at perovskite heterointerfaces, Nature Communications, 10.1038/ncomms7759, 2015.06.
48. Shoichi Kabuyanagi, Tomonori Nishimura, Takeaki Yajima, Akira Toriumi, Effect of Free Carrier Accumulation or Depletion on Zone-center Vibrational Mode in Ge, 2015 SILICON NANOELECTRONICS WORKSHOP (SNW), 2015.06.
49. Takeaki Yajima, Yuma Ninomiya, Tomonori Nishimura, Akira Toriumi, Drastic change in electronic domain structures via strong elastic coupling in VO2 films, PHYSICAL REVIEW B, 10.1103/PhysRevB.91.205102, 91, 20, 2015.05.
50. Shoichi Kabuyanagi, Tomonori Nishiumura, Takeaki Yajima, Akira Toriumi, Nondestructive characterization of oxide/germanium interface by direct-gap photoluminescence analysis, APPLIED PHYSICS EXPRESS, 10.7567/APEX.8.051301, 8, 5, 2015.05, We demonstrate that direct-gap photoluminescence (PL) analysis is useful for the nondestructive, fast, and in-line characterization of the oxide/germanium (Ge) interface without making actual devices. The fact that Ge has a quasi-direct gap enables us to utilize direct-gap PL intensity as a good indicator for interface quality estimation. We experimentally confirm the validity of the present analysis, by comparing PL spectra with capacitance-voltage characteristics. The impact of the band bending at the interface on PL intensity is also discussed. Furthermore, the effect of forming gas annealing at oxide/Ge interfaces is discussed by taking advantage of the present method. (C) 2015 The Japan Society of Applied Physics.
51. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Kosuke Nagashio, Akira Toriumi, Analytical Formulation of SiO2-IL scavenging in HfO2/SiO2/Si gate stacks - A key is the SiO2/Si interface reaction, Technical Digest - International Electron Devices Meeting, IEDM, 10.1109/IEDM.2014.7047094, 2015-, February, 21.2.1-21.2.4, 2015.02, The scavenging kinetics of ultra-thin-SiO&lt
interface layer (IL) in HfO&lt
/Si stacks is investigated by focusing on SiO&lt
/Si interface reaction in addition to both O and Si atom kinetics. SiO&lt
/Si interface serves as a stage that the oxygen vacancy (VO) is converted to Si release from SiO&lt
with the help of Si substrate. Based on both diffusion kinetics and possible reaction, an analytical model for two-stage SiO&lt
-IL scavenging in high-k gate stack is proposed..
52. Yajima, Takeaki, Minohara, Makoto, Bell, Christopher, Kumigashira, Hiroshi, Oshima, Masaharu, Hwang, Harold Y., Hikita, Yasuyuki, Enhanced Electrical Transparency by Ultrathin LaAlO3 Insertion at Oxide Metal/Semiconductor Heterointerfaces, Nano Letters, 10.1021/nl504169m, 2015.02.
53. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Kosuke Nagashio, Akira Toriumi, Effect of Si substrate on interfacial SiO2 scavenging in HfO2/SiO2/Si stacks, APPLIED PHYSICS LETTERS, 10.1063/1.4901172, 105, 18, 2014.11, The scavenging kinetics of an ultra-thin SiO2 interface layer (SiO2-IL) in an HfO2/SiO2/Si stack is discussed by focusing on the substrate effect in addition to oxygen diffusion. O-18 tracing experiments demonstrate that the O-atom moves from the SiO2-IL to the HfO2 layer during scavenging. SiO2-IL scavenging with various substrates (Si, SiC, and sapphire) has been found to be significantly different, which suggests that the Si in the substrate is also necessary to continuously cause the scavenging. Based on these findings and thermodynamic considerations, a kinetic model where oxygen vacancy (VO) transferred from the HfO2 reacts with the SiO2, which is in contact with the Si-substrate, is proposed for the SiO2-IL scavenging. (C) 2014 AIP Publishing LLC..
54. Tomonori Nishimura, ChoongHyun Lee, Takeaki Yajima, Kosuke Nagashio, Akira Toriumi, Atomic-scale Planarization of Ge (111), (110) and (100) Surfaces, 2014 7TH INTERNATIONAL SILICON-GERMANIUM TECHNOLOGY AND DEVICE MEETING (ISTDM), 10.1109/ISTDM.2014.6874698, 127-128, 2014.08.
55. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Kosuke Nagashio, Akira Toriumi, Kinetic Model for Scavenging of SiO2 Interface Layer in HfO2 Gate Stacks, 2014 IEEE SILICON NANOELECTRONICS WORKSHOP (SNW), 10.1109/SNW.2014.7348532, 2014.06, This work discusses effects of the substrate on the scavenging of SiO2 interface layer (SiO2-IL) in HfO2 gate stacks. We propose a model that SiO2-IL scavenging occurs through the reaction of the oxygen vacancy (V-O) transferred from HfO2 to SiO2 with oxygen formed at SiO2/substrate interface..
56. Tomonori Nishimura, Shoichi Kabuyanagi, Wenfeng Zhang, Choong Hyun Lee, Takeaki Yajima, Kosuke Nagashio, Akira Toriumi, Atomically flat planarization of Ge(100), (110), and (111) surfaces in H-2 annealing, APPLIED PHYSICS EXPRESS, 10.7567/APEX.7.051301, 7, 5, 2014.05, We demonstrate that Ge(100), (110), and (111) Ge surfaces are planarized with atomic level step and terrace structures in H-2 annealing. The temperature required for such planarization is different among the three orientations. The step edge structure on the Ge(100) surface is composed of alternate smooth and rough steps (S-a + S-b steps) owing to the (2 x 1) reconstruction on that surface. It is also shown that the terrace widths on the Ge(110) and (111) surfaces are on average controlled by adjusting the off-angle from the respective surface orientation. (C) 2014 The Japan Society of Applied Physics.
57. Xiuyan Li, Takeaki Yajima, Tomonori Nishimura, Kosuke Nagashio, Akira Toriumi, HfO2-assisted SiO2 reduction in HfO2/SiO2/Si stacks, THIN SOLID FILMS, 10.1016/j.tsf.2013.10.142, 557, 272-275, 2014.04, The ultra-thin SiO2 interface layer (SiO2-IL) in the HfO2/SiO2/Si stack was found to be reduced during the ultra-high vacuum (UHV) annealing. Along with thin SiO2-IL reduction in UHV, SiO desorption and silicide formation also took place. The thermal desorption spectroscopy and X-ray photoelectron spectroscopy results showed that the SiO2-IL reduction occurred at temperatures lower than the temperature showing SiO desorption and silicidation by accurately controlling the annealing conditions. The mechanism of SiO2-IL reduction can be explained by the oxygen diffusion through oxygen vacancies in HfO2 generated in the gate stack formation and thermal treatment. It is also understandable that the silicidation process is associated with the inhomogeneous SiO desorption. (c) 2013 Elsevier B. V. All rights reserved..
58. Julia A. Mundy, Yasuyuki Hikita, Takeaki Hidaka, Takeaki Yajima, Takuya Higuchi, Harold Y. Hwang, David A. Muller, Lena F. Kourkoutis, Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition, NATURE COMMUNICATIONS, 10.1038/ncomms4464, 5, 2014.03, Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases..
59. Go Oike, Takeaki Yajima, Tomonori Nishimura, Kosuke Nagashio, Akira Toriumi, High electron mobility (16 cm2/Vsec) FETs with high on/off ratio (10^6) and highly conductive films (10^2 S/cm) by chemical doping in very thin (∼20 nm) TiO2 films on thermally grown SiO2, Technical Digest - International Electron Devices Meeting, IEDM, 10.1109/IEDM.2013.6724610, 2013.12.
60. T. Nishimura, S. Kabuyanagi, C. H. Lee, T. Yajima, K. Nagashio, A. Toriumi, Atomically Flat Germanium (111) Surface by Hydrogen Annealing, ULSI PROCESS INTEGRATION 8, 10.1149/05809.0201ecst, 58, 9, 201-206, 2013.10, In germanium (Ge) MOSFET technology, surface planarization is a serious concern for mobility enhancement at high carrier density, reliability improvement of gate dielectrics and morphology control for non-planar FETs. In this work, (111)-oriented Ge substrates were annealed at 350-750 degrees C in pure H-2 atmosphere, and the surface structure and morphology were analyzed with atomic force microscopy. A step and terrace structure was observed on the surface after the H-2 annealing above 500 degrees C. The terrace width is controllable by the off-angle of the initial surface within 0.3 degrees at least. The roughness root mean square (RMS) at 100 x 100 nm on single terrace is similar to 0.05 nm which is almost our detection limit, which implies that the single terrace on H-2 annealed Ge (111) is atomically flat. Furthermore, even though the initial surface roughness RMS is intentionally increased up to 0.6 nm, atomically flat terrace structure could be obtained by the H-2 annealing..
61. S. Harashima, C. Bell, M. Kim, T. Yajima, Y. Hikita, H. Y. Hwang, Coexistence of two-dimensional and three-dimensional Shubnikov–de Haas oscillations in Ar+-irradiated KTaO3, Phys. Rev. B, 10.1103/PhysRevB.88.085102, 88, 8, 085102, 2013.08.
62. Kumari Gaurav Rana, Takeaki Yajima, Subir Parui, Alexander F. Kemper, Thomas P. Devereaux, Yasuyuki Hikita, Harold Y. Hwang, Tamalika Banerjee, Hot electron transport in a strongly correlated transition-metal oxide, SCIENTIFIC REPORTS, 10.1038/srep01274, 3, 2013.02, Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La0.7Sr0.3MnO3 (LSMO)- Nb-doped SrTiO3 (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 +/- 0.10 unit cells (u.c.) at -1.9 V, increasing to 2.02 +/- 0.16 u.c. at -1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges..
63. Design of interfacial dipole in perovskite oxides and its application to transistors.
64. Bongju Kim, Daeyoung Kwon, Takeaki Yajima, Christopher Bell, Yasuyuki Hikita, Bog G. Kim, Harold Y. Hwang, Reentrant insulating state in ultrathin manganite films, APPLIED PHYSICS LETTERS, 10.1063/1.3628659, 99, 9, 2011.08, The transport and magnetic properties of La0.7Sr0.3MnO3 thin-films grown by pulsed laser deposition on (LaAlO3)(0.3)(SrAl0.5Ta0.5O3)(0.7) single crystal substrates have been investigated. A systematic series with various thicknesses of La0.7Sr0.3MnO3 was used to establish a phase diagram which showed a clear difference compared to films grown on SrTiO3 substrates, highlighting the importance of film thickness, and substrate strain. At 8 unit cells, the boundary between the metallic and insulating groundstates, a second abrupt metal-insulator transition was observed at low temperatures, which could be tuned by magnetic field, and was interpreted as a signature of electronic phase separation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3628659].
65. Takeaki Yajima, Yasuyuki Hikita, Harold Y. Hwang, A heteroepitaxial perovskite metal-base transistor, NATURE MATERIALS, 10.1038/NMAT2946, 10, 3, 198-201, 2011.03.
66. Yanwu Xie, Christopher Bell, Takeaki Yajima, Yasuyuki Hikita, Harold Y. Hwang, Charge Writing at the LaAlO3/SrTiO3 Surface, NANO LETTERS, 10.1021/nl1012695, 10, 7, 2588-2591, 2010.07, Biased conducting-up atomic force microscopy (AFM) has been shown to write and erase nanoscale metallic lines at the LaAlO3/SrTiO3 interface Using various AFM modes, we show the mechanism of conductivity switching is the writing of surface charge These charges are stably deposited on a wide range of LaAlO3 thicknesses, including bulk crystals. A strong asymmetry with writing polarity was found for 1 and 2 unit cells of LaAlO3, providing experimental evidence for a theoretically predicted built-in potential.
67. T. Higuchi, T. Yajima, L. Fitting Kourkoutis, Y. Hikita, N. Nakagawa, D. A. Muller, H. Y. Hwang, Mn3O4 precipitates in laser-ablated manganite films, APPLIED PHYSICS LETTERS, 10.1063/1.3193667, 95, 4, 043112, 2009.07, Precipitates formed during the growth of manganite thin films by pulsed laser deposition have been an obstacle for fabricating high quality devices incorporating these ferromagnetic metals. In order to analyze the nature of these precipitates, we have investigated their spectroscopic and structural properties by scanning transmission electron microscopy. For LaMnO3 films, crystallites of Mn3O4 are found to segregate out from stoichiometric films to accommodate a net cation off-stoichiometry during growth. By tuning the laser spot conditions, these precipitates can be eliminated..
68. Yasuyuki Hikita, Mitsuru Nishikawa, Takeaki Yajima, Harold Y. Hwang, Termination control of the interface dipole in La0.7Sr0.3MnO3/Nb:SrTiO3 (001) Schottky junctions, PHYSICAL REVIEW B, 10.1103/PhysRevB.79.073101, 79, 7, 2009.02, In order to investigate the interface termination dependence of perovskite band alignments, we have studied the Schottky barrier height at La0.7Sr0.3MnO3/Nb:SrTiO3 (001) heterointerfaces. As the Nb:SrTiO3 semiconductor was varied from TiO2 termination to SrO termination by variable insertion of a SrMnO3 layer, a large systematic increase in the Schottky barrier height was observed. This can be ascribed to the evolution of the interface dipole induced to screen the polar discontinuity at the interface, which gives a large internal degree of freedom for tuning band diagrams in oxides..