Article
Physics, Applied
Hironori Okumura
Summary: The critical layer thickness for (AlGa) (2) O-3 heteroepitaxial growth on beta-Ga2O3 (010) substrates via plasma-assisted molecular beam epitaxy was reported, along with the electrical properties of heavily tin-doped (AlGa) (2) O-3 layers. The aluminum composition in the (AlGa) (2) O-3 layers was reproducibly controlled within 19% by changing aluminum fluxes. Pseudomorphic growth of (Al0.12Ga0.88) (2) O-3 and (Al0.15Ga0.85) (2) O-3 layers of certain thicknesses on beta-Ga2O3 (010) substrates was achieved.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2021)
Article
Materials Science, Coatings & Films
Fikadu Alema, Takeki Itoh, William Brand, Marko Tadjer, Andrei Osinsky, James S. Speck
Summary: This study reports on the MOCVD growth of smooth (010) (AlxGa1-x)(2)O-3 and (100) (AlyGa1-y)(2)O-3 epitaxial films on beta-Ga2O3 substrates using N2O for oxidation. The Al incorporation efficiency depends on process conditions, and higher Al incorporation is achieved at lower reactor pressure, substrate temperature, and higher gas phase Al concentration. Pure beta and gamma-phase (AlGa)(2)O-3 films were grown on (010) and (100) beta-Ga2O3 substrates, respectively. Nitrogen doping of (010) beta-(AlxGa1-x)(2)O-3 was achieved using N2O, and higher Al composition and lower substrate temperature led to higher N incorporation. The results demonstrate the potential for growing high Al content beta-(AlGa)(2)O-3 films using N2O as an oxygen source.
JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A
(2023)
Article
Physics, Applied
Youngbin Yoon, Min Ju Kim, Byung Jin Cho, Myunghun Shin, Wan Sik Hwang
Summary: Monoclinic gallium oxide (β-Ga2O3) is of interest in the scientific community for its application in power electronics, particularly in devices requiring handling of high voltage. Research has shown that 8-nm-thick Sn-doped polycrystalline β-Ga2O3 thin films have a large optical bandgap and high voltage handling capacity, making them suitable for various fields beyond power electronics.
APPLIED PHYSICS LETTERS
(2021)
Article
Nanoscience & Nanotechnology
Timothy A. Morgan, Justin Rudie, Mohammad Zamani-Alavijeh, Andrian V. Kuchuk, Nazar Orishchin, Fikadu Alema, Andrei Osinsky, Robert Sleezer, Gregory Salamo, Morgan E. Ware
Summary: The band offsets for the beta-(Al0.21Ga0.79)(2)O-3/beta-Ga2O3 (010) heterojunction were experimentally measured, and the resulting band alignment was determined to be of type II. These findings provide valuable data for the design and prediction of beta-(AlxGa1-x)(2)O-3 heterojunction-based devices.
ACS APPLIED MATERIALS & INTERFACES
(2022)
Article
Nanoscience & Nanotechnology
Arkka Bhattacharyya, Carl Peterson, Takeki Itoh, Saurav Roy, Jacqueline Cooke, Steve Rebollo, Praneeth Ranga, Berardi Sensale-Rodriguez, Sriram Krishnamoorthy
Summary: We demonstrate a new substrate cleaning and buffer growth scheme in beta-Ga2O3 epitaxial thin films using MOVPE. By growing a low-temperature un-doped Ga2O3 buffer followed by a transition layer to high-temperature Si-doped Ga2O3 channel layers, continuous growth of the channel structure was achieved. The parasitic Si channel at the epilayer-substrate interface was effectively compensated through solvent cleaning and hydrofluoric acid treatment. This substrate cleaning combined with the LT buffer scheme shows the potential for designing Si-doped beta-Ga2O3 channels with exceptional transport properties.
Article
Materials Science, Coatings & Films
Michael A. Mastro, Charles R. Eddy, Marko J. Tadjer, Jennifer K. Hite, Jihyun Kim, Stephen J. Pearton
Summary: Recent advancements in bulk crystal growth of beta-Ga2O3 have led to commercialization of large-area substrates and high-quality films on (010) substrates, revealing subnanometer-scale facets and larger ridges on the surface. Recommendations for standardizing substrates to control ridge formation have been proposed based on density function theory calculations.
JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A
(2021)
Article
Physics, Applied
Nidhin Kurian Kalarickal, Ashok Dheenan, Joe. F. F. McGlone, Sushovan Dhara, Mark Brenner, Steven. A. A. Ringel, Siddharth Rajan
Summary: We presented the design and fabrication of beta-Ga2O3 self-aligned lateral MOSFETs with a heavily doped beta-Ga2O3 cap layer. The fabricated device achieved a record high DC drain current density of 560 mA/mm at 5V drain bias, although the current density was limited by excessive self-heating. However, pulsed I-V measurements showed a record high current density of 895 mA/mm and a high transconductance of 43 mS/mm, indicating reduced self-heating in the device. These findings are promising for the development of high power density devices based on beta-Ga2O3.
APPLIED PHYSICS LETTERS
(2023)
Article
Materials Science, Multidisciplinary
Gokhan Atmaca, Ho-Young Cha
Summary: This study evaluates the effects of adding an ultrathin spacer layer and a back-barrier layer to double-channel heterostructure modulation-doped field-effect transistors (MODFETs) based on delta-doped beta-AlxGa1-x2O3/beta-Ga2O3 heterostructures. The results show that the proposed double-channel heterostructure MODFET has higher drain current and g m peak compared to the single-channel case.
PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE
(2022)
Article
Engineering, Electrical & Electronic
Dawei Wang, Dinusha Herath Mudiyanselage, Houqiang Fu
Summary: This work systematically explores the design space for delta-doped beta-(AlxGa1-x)(2)O-3/Ga2O3 high electronmobility transistors (HEMTs) using TCAD simulation, investigating the effects of different delta-doping concentrations, positions, and widths on device performance. High delta-doping concentrations can improve transconductance and channel mobility but may degrade other performance indicators such as breakdown voltages (BVs).
IEEE TRANSACTIONS ON ELECTRON DEVICES
(2022)
Article
Physics, Applied
A. F. M. Anhar Uddin Bhuiyan, Zixuan Feng, Lingyu Meng, Andreas Fiedler, Hsien-Lien Huang, Adam T. Neal, Erich Steinbrunner, Shin Mou, Jinwoo Hwang, Siddharth Rajan, Hongping Zhao
Summary: This work investigates the structural and electrical properties of metalorganic chemical vapor deposited Si-doped beta-(AlxGa1-x)(2)O-3 thin films grown on (010) beta-Ga2O3 substrates as a function of Al composition. The results show that increasing Al composition leads to a decrease in Si incorporation efficiency, an increase in C and H impurity levels, and lower net carrier concentrations in the films. Higher Al compositions also result in cracking in the films due to lattice mismatch with the substrate. The study highlights the importance of Al composition in determining the properties of Si-doped beta-(AlxGa1-x)(2)O-3 films.
JOURNAL OF APPLIED PHYSICS
(2022)
Article
Materials Science, Multidisciplinary
A. F. M. Anhar Uddin Bhuiyan, Zixuan Feng, Lingyu Meng, Hongping Zhao
Summary: The study systematically investigates the effects of growth parameters on the structural and surface morphological properties of beta-(AlxGa1-x)(2)O-3 thin films grown via MOCVD on (010) beta-Ga2O3 substrates. It is found that a higher [TMAl]/[TEGa + TMAl] molar flow ratio results in higher Al incorporation in the films, while higher growth temperature leads to lower growth rates with substantial surface roughening. Increasing chamber pressure lowers growth rates and Al incorporation in the AlGaO films, indicating a narrower MOCVD growth window as the Al composition increases.
JOURNAL OF MATERIALS RESEARCH
(2021)
Article
Engineering, Electrical & Electronic
Youngbin Yoon, Sunjae Kim, In Gyu Lee, Byung Jin Cho, Wan Sik Hwang
Summary: Polycrystalline n-type beta-Ga2O3 thin films with a thickness of 100 nm were successfully fabricated using sputtering and spin-on-glass Sn-doping techniques. These thin films were utilized as an active layer for power electronics and ultraviolet optoelectronics, showing high breakdown voltages and suitability for solar-blind photodetectors.
MATERIALS SCIENCE IN SEMICONDUCTOR PROCESSING
(2021)
Article
Physics, Applied
Siyuan Xu, Lining Liu, Guangming Qu, Xingfei Zhang, Chunyang Jia, Songhao Wu, Yuanxiao Ma, Young Jin Lee, Guodong Wang, Ji-Hyeon Park, Yiyun Zhang, Xiaoyan Yi, Yeliang Wang, Jinmin Li
Summary: In this study, a fin field-effect transistor (FinFET) based on a β-Ga2O3 nanowire with a diameter of approximately 60nm on a Si substrate was demonstrated. The device showed excellent performance with high on/off ratio, low leakage current, and low subthreshold swing. Simulation results were consistent with experimental measurements, and trap-related 1/f noise and 1/f^2 noise were identified through low frequency noise analysis.
APPLIED PHYSICS LETTERS
(2022)
Article
Physics, Condensed Matter
Ravi Ranjan, Nitesh Kashyap, Ashish Raman
Summary: In this work, a novel Gate-all-around beta-(AlGa)(2)O-3/Ga2O3 HEMT device with Al2O3 as a passivation layer and using gate connected field plate technique has been proposed. The proposed device shows improved breakdown voltage and out power density. In-depth investigation reveals that at a field plate length of 2.5 μm, the device exhibits the peak breakdown voltage and out power density. The effects of gate electrode are comprehensively studied by varying gate length and work function.
MICRO AND NANOSTRUCTURES
(2022)
Article
Optics
Gaofeng Deng, Katsuhiko Saito, Tooru Tanaka, Qixin Guo
Summary: The study of temperature-dependent photoluminescence behaviors and temperature sensing properties of Er-doped (AlGa)(2)O-3 films with different Al contents reveals an enhancement in green emissions and thermal sensitivity with increasing host bandgap and Al content. The research provides insights for developing optical temperature sensors with high luminescence efficiency and sensitivity.
JOURNAL OF LUMINESCENCE
(2021)
Article
Physics, Applied
Yuichi Oshima, Katsuaki Kawara, Takayoshi Oshima, Mitsuru Okigawa, Takashi Shinohe
JAPANESE JOURNAL OF APPLIED PHYSICS
(2020)
Article
Engineering, Electrical & Electronic
Yuichi Oshima, Katsuaki Kawara, Takayoshi Oshima, Mitsuru Okigawa, Takashi Shinohe
SEMICONDUCTOR SCIENCE AND TECHNOLOGY
(2020)
Article
Physics, Applied
Aboulaye Traore, Maria Gouveia, Hironori Okumura, Cedric Mannequin, Andrea Fassion, Takeaki Sakurai
Summary: Photo-induced conductivity transients are reported for unintentionally doped beta-Ga2O3 and n-type beta-(Al0.16Ga0.84)(2)O-3, increasing their conductivity with sub-bandgap light. The conductivity increase remains after light is turned off and slowly decreases. Optical cross-sections and absorption peaks of photo-ionized defects have been measured and calculated, indicating broad optical absorption peaks inducing the measured photoconductivity in both materials.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2021)
Article
Physics, Applied
Hironori Okumura
Summary: The critical layer thickness for (AlGa) (2) O-3 heteroepitaxial growth on beta-Ga2O3 (010) substrates via plasma-assisted molecular beam epitaxy was reported, along with the electrical properties of heavily tin-doped (AlGa) (2) O-3 layers. The aluminum composition in the (AlGa) (2) O-3 layers was reproducibly controlled within 19% by changing aluminum fluxes. Pseudomorphic growth of (Al0.12Ga0.88) (2) O-3 and (Al0.15Ga0.85) (2) O-3 layers of certain thicknesses on beta-Ga2O3 (010) substrates was achieved.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2021)
Article
Physics, Applied
Hironori Okumura, Yasuhiro Watanabe, Tomohiko Shibata, Kohei Yoshizawa, Akira Uedono, Hiroki Tokunaga, Shuuichi Koseki, Tadanobu Arimura, Sami Suihkonen, Tomas Palacios
Summary: We report on the diffusion of impurities in ion implanted AlN layers after thermal annealing. By annealing at 1600 degrees C, silicon and magnesium atoms were found to diffuse into the AlN layer, while less change was observed for germanium atoms. Silicon implantation resulted in the introduction of vacancy-related defects. Higher annealing temperatures (>1300 degrees C) reduced the vacancy-related defects and led to the diffusion of oxygen atoms from the substrate due to sapphire decomposition. By controlling the annealing temperature and the distribution of silicon and oxygen concentrations, we were able to achieve reproducible electrical conductance in silicon-implanted AlN layers.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2022)
Article
Chemistry, Multidisciplinary
Laura E. Ratcliff, Takayoshi Oshima, Felix Nippert, Benjamin M. Janzen, Elias Kluth, Rudiger Goldhahn, Martin Feneberg, Piero Mazzolini, Oliver Bierwagen, Charlotte Wouters, Musbah Nofal, Martin Albrecht, Jack E. N. Swallow, Leanne A. H. Jones, Pardeep K. Thakur, Tien-Lin Lee, Curran Kalha, Christoph Schlueter, Tim D. Veal, Joel B. Varley, Markus R. Wagner, Anna Regoutz
Summary: Ga2O3 and its polymorphs have great potential for electronic structure engineering. In this study, a robust atomistic model of gamma-Ga2O3 is developed using density functional theory and machine-learning approach, which is validated by experimental results. This work is of significant importance for understanding the electronic structure of complex, disordered oxides.
ADVANCED MATERIALS
(2022)
Article
Physics, Applied
Takayoshi Oshima, Yuichi Oshima
Summary: Selective area growth of beta-Ga2O3 was demonstrated by HCl-based halide vapor phase epitaxy on SiO2-masked (001) and (010) beta-Ga2O3 substrates. High-aspect-ratio structures with (100) sidewall facets were observed for stripe windows along [010] and [001] directions on the respective substrates. These structures can be applied to trenches and fins for beta-Ga2O3-based power devices.
APPLIED PHYSICS EXPRESS
(2022)
Article
Physics, Applied
Aboulaye Traore, Hironori Okumura, Takeaki Sakurai
Summary: Photoconductivity transients in an unintentionally doped (UID) n-type beta-Ga2O3 layer were investigated, and the origin and characteristics of persistent photoconductivity (PPC) were explored. The photoconductivity was found to be related to the photoionization of two distinct deep levels, and PPC was observed due to thermal capture barriers preventing the relaxation of photo-generated electrons back to the deep levels.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2022)
Article
Physics, Applied
Takayoshi Oshima
Summary: In this study, a patterned metal on a beta-Ga2O3 substrate surface was used as an embedded photomask to demonstrate the applicability of backside-exposure lithography to beta-Ga2O3 substrates. Self-aligned photoresist patterning, lift-off, and etching processes were demonstrated for an AZ5214E photoresist in both the positive and negative processing modes. The findings suggest that backside-exposure photolithography is a promising fabrication approach for future beta-Ga2O3-based devices.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2023)
Article
Physics, Applied
Hironori Okumura
Summary: Solid-state materials with a large bandgap energy, such as heavily Si-doped alpha-Al2O3, show improved electrical conductivity and reduced resistivity after post-thermal annealing.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2022)
Article
Physics, Applied
Hironori Okumura, Akira Uedono
Summary: Mg ions were implanted in 1 μm thick AlN layers grown on sapphire substrates. The Mg implantation with a total dose of 5 x 10(14) cm(-2) introduced Al-vacancy related defects, which were decreased by annealing at temperatures over 1400 degrees C in an N-2 ambient. We found that annealing temperatures over 1400 degrees C were necessary for an electrically conductive Mg-implanted AlN layer. The Mg-implanted AlN layer annealed at 1500 degrees C showed 1.1 nA at a bias of 100 V at room temperature and 7 nA at a bias of 10 V at 300 degrees C.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2023)
Article
Physics, Applied
Hitoshi Takane, Takayoshi Oshima, Katsuhisa Tanaka, Kentaro Kaneko
Summary: Selective-area growth of r-SnO2 was achieved on a SiO2-masked r-TiO2 (110) substrate. The heteroepitaxy process led to the growth of islands with {100}-, {110}-, and {011}-faceted sidewalls, which matched the equilibrium shape of the rutile structure. Coalescence of the islands formed a flat (110) top surface on the striped window, followed by lateral overgrowth. Cross-sectional transmission-electron-microscopy observation revealed that misfit dislocations propagated perpendicular to the facet planes due to the image force effect, and the dislocation density decreased significantly in the wing regions.
APPLIED PHYSICS EXPRESS
(2023)
Article
Physics, Applied
Takayoshi Oshima, Yuichi Oshima
Summary: We have demonstrated the effectiveness of plasma-free HCl gas etching for fabricating high-aspect-ratio fins/trenches on a SiO2-masked (010) beta-Ga2O3 substrate. The etching process resulted in the formation of holes or trenches with etching-resistant (100)- and ((1) over bar 01)-faceted sidewalls. By etching in the striped windows along [001], we achieved fins and trenches with flat (100)-faceted vertical sidewalls and slightly rough {310}-faceted inclined bottom corners. The vertical-to-horizontal etching ratio was as high as about 11-14. Our findings suggest that HCl etching is a promising method for such fabrication without plasma damage.
APPLIED PHYSICS EXPRESS
(2023)
Article
Physics, Applied
Takayoshi Oshima, Yuichi Oshima
Summary: In this study, SiO2-masked (001) beta-Ga2O3 substrates were dry etched in HCl gas flow at a high temperature without plasma excitation. The etching selectively formed holes or trenches with inner sidewalls of (100) and/or {310} facets, which are planes of lowest surface energy density and oxygen-close-packed slip planes, respectively. The (100) faceted sidewalls were flat and close to the substrate surface normal. This dry etching method shows promise for fabricating plasma-damage-free trenches and fins for beta-Ga2O3-based power devices.
APPLIED PHYSICS LETTERS
(2023)
Article
Physics, Applied
Jiro Nishinaga, Manabu Togawa, Masaya Miyahara, Kosuke Itabashi, Hironori Okumura, Masataka Imura, Yukiko Kamikawa, Shogo Ishizuka
Summary: Radiation tolerance of Cu(In,Ga)Se-2 (CIGS) solar cells was investigated for application in extremely-high-radiation environments. The cells deteriorated after high-energy proton irradiation but could still generate power. Recombination centers in CIGS layers increased after irradiation, but heat-light annealing partially passivated them. Dark annealing also had a beneficial effect on passivating recombination centers, even with thicker CIGS layers.
JAPANESE JOURNAL OF APPLIED PHYSICS
(2023)
