Updated on 2023/09/29


OKA Toru
Institute of Materials and Systems for Sustainability TOYODA-GOSEI GaN Leading Innovative R&D Industry-Academia Collaborative Chair Designated professor
Designated professor

Degree 1

  1. 博士(工学) ( 2003.3   京都大学 ) 


Papers 4

  1. Effects of proton irradiation-induced point defects on Shockley-Read-Hall recombination lifetimes in homoepitaxial GaN p-n junctions

    Narita Tetsuo, Kanechika Masakazu, Tomita Kazuyoshi, Nagasato Yoshitaka, Kondo Takeshi, Uesugi Tsutomu, Ikeda Satoshi, Kosaki Masayoshi, Oka Tohru, Suda Jun

    APPLIED PHYSICS LETTERS   Vol. 122 ( 11 )   2023.3

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    Publisher:Applied Physics Letters  

    This work examined the intentional generation of recombination centers in GaN p-n junctions on freestanding GaN substrates. Irradiation with a 4.2 MeV proton beam was used to create a uniform distribution of vacancies and interstitials across GaN p+/n- and p-/n+ junctions through anode electrodes. With increasing proton dose, the effective doping concentrations were found to be reduced. Because the reduction in the doping concentration was much higher than the hydrogen atom concentration, this decrease could not be attributed solely to carrier compensation resulting from interstitial hydrogen atoms. In fact, more than half of the electron and hole compensation was caused by the presence of point defects. These defects evidently served as Shockley-Read-Hall (SRH) recombination centers such that the SRH lifetimes were reduced to several picoseconds from several hundred picoseconds prior to irradiation. The compensation for holes in the p-/n+ junctions was almost double that for electrons in the p+/n- junctions. Furthermore, the SRH lifetimes associated with p-/n+ junctions were shorter than those for p+/n- junctions for a given proton dose. These differences can be explained by variations in the charge state and/or the formation energy of intrinsic point defects in the p-type and n-type GaN layers. The results of the present work indicate the asymmetry of defect formation in GaN based on the fact that intrinsic point defects in p-type GaN readily compensate for holes.

    DOI: 10.1063/5.0141781

    Web of Science


  2. Reliability issues of gate oxides and p-n junctions for vertical GaN metal-oxide-semiconductor field-effect transistors (Invited)

    Narita Tetsuo, Kikuta Daigo, Ito Kenji, Shoji Tomoyuki, Mori Tomohiko, Yamaguchi Satoshi, Kimoto Yasuji, Tomita Kazuyoshi, Kanechika Masakazu, Kondo Takeshi, Uesugi Tsutomu, Kojima Jun, Suda Jun, Nagasato Yoshitaka, Ikeda Satoshi, Watanabe Hiroki, Kosaki Masayoshi, Oka Tohru


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    Publisher:IEEE International Reliability Physics Symposium Proceedings  

    We focus on reliability issues of gate oxides and p-n junctions to realize vertical GaN metal-oxide-semiconductor field-effect transistors (MOSFETs). An annealed AlSiO gate oxide on GaN displayed a lifetime of over 20 years at 150 °C and suppressed positive bias instability in MOSFETs. The key to high channel mobility and stability under positive gate bias is the interface structure designed to minimize oxide border traps. We also evaluated the reliability of GaN p-n diodes (PNDs) on freestanding GaN substrates with different threading dislocation densities. The reverse leakage for PNDs involving threading dislocations was explained by variable-range hopping, while the reverse leakage for dislocation-free PNDs was dominated by band-to-band tunneling. The fabricated PNDs demonstrated excellent robustness under high-temperature reverse bias. However, after continuous forward current stress, reverse leakage pathways were formed at threading screw dislocations, which should be minimized in future GaN substrates.

    DOI: 10.1109/IRPS48203.2023.10118047

    Web of Science


  3. Identification of type of threading dislocation causing reverse leakage in GaN p-n junctions after continuous forward current stress

    Narita Tetsuo, Kanechika Masakazu, Kojima Jun, Watanabe Hiroki, Kondo Takeshi, Uesugi Tsutomu, Yamaguchi Satoshi, Kimoto Yasuji, Tomita Kazuyoshi, Nagasato Yoshitaka, Ikeda Satoshi, Kosaki Masayoshi, Oka Tohru, Suda Jun

    SCIENTIFIC REPORTS   Vol. 12 ( 1 ) page: 1458   2022.1

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    Language:English   Publisher:Scientific Reports  

    Power devices are operated under harsh conditions, such as high currents and voltages, and so degradation of these devices is an important issue. Our group previously found significant increases in reverse leakage current after applying continuous forward current stress to GaN p–n junctions. In the present study, we identified the type of threading dislocations that provide pathways for this reverse leakage current. GaN p–n diodes were grown by metalorganic vapor phase epitaxy on freestanding GaN(0001) substrates with threading dislocation densities of approximately 3 × 105 cm−2. These diodes exhibited a breakdown voltage on the order of 200 V and avalanche capability. The leakage current in some diodes in response to a reverse bias was found to rapidly increase with continuous forward current injection, and leakage sites were identified by optical emission microscopy. Closed-core threading screw dislocations (TSDs) were found at five emission spots based on cross-sectional transmission electron microscopy analyses using two-beam diffraction conditions. The Burgers vectors of these dislocations were identified as [0001] using large-angle convergent-beam electron diffraction. Thus, TSDs for which b = 1c are believed to provide current leakage paths in response to forward current stress.

    DOI: 10.1038/s41598-022-05416-3

    Web of Science



  4. Increase of reverse leakage current at homoepitaxial GaN p-n junctions induced by continuous forward current stress

    Narita Tetsuo, Nagasato Yoshitaka, Kanechika Masakazu, Kondo Takeshi, Uesugi Tsutomu, Tomita Kazuyoshi, Ikeda Satoshi, Yamaguchi Satoshi, Kimoto Yasuji, Kosaki Masayoshi, Oka Tohru, Kojima Jun, Suda Jun

    APPLIED PHYSICS LETTERS   Vol. 118 ( 25 )   2021.6

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    Language:Japanese   Publisher:Applied Physics Letters  

    Reliability tests involving the application of high electrical stresses were employed to assess GaN-based vertical p-n junctions fabricated on freestanding GaN substrates with threading dislocation densities less than 104 cm−2. Electric field crowding at the device edges was eliminated by employing a shallow bevel mesa structure, thus allowing an evaluation of the reliability of the internal p-n junctions. The p-n diodes exhibited reproducible avalanche breakdown characteristics over the temperature range of 25-175 °C. No degradation was observed even during tests in which the devices were held under a reverse bias near the breakdown voltage. Despite this high degree of reliability in response to reverse bias stress, a small number of diodes were degraded during continuous forward current tests, although the majority of diodes remained unchanged. The reverse leakage current exhibited by degraded diodes was increased with an increase in the forward current density within the range of 50-500 A/cm2, while the breakdown voltages were unchanged in response to current stress. The leakage level increased exponentially with an increase in the total amount of injected carriers but eventually plateaued. In the degraded p-n diode, a luminous point in an emission microscope corresponded to one of the threading dislocations observed in the synchrotron x-ray topography, indicating that a specific dislocation played as a leakage path after injecting carriers.

    DOI: 10.1063/5.0053139

    Web of Science


Presentations 2

  1. Progress and Challenges of Vertical GaN Power Devices on GaN Substrates Invited International conference

    Tohru Oka

    The 2022 International Meeting for Future of Electron Devices, Kansai  2022.11.28  IEEE EDS Kansai Chapter

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    Event date: 2022.11

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

    Venue:Online & Avanti Kyoto Hall,Kyoto,Japan   Country:Japan  

  2. 縦型GaNパワーデバイス開発の現状と今後の展望 Invited

    岡 徹

    Keithley Days 2022  2022.9.1  テクトロニクス&ケースレー

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    Event date: 2022.9

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

    Venue:online   Country:Japan