Updated on 2022/11/29


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 2

  1. 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 )   2022.1

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    Language:Japanese   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


  2. 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