For electron irradiation, vertical rectifiers fabricated on epi Ga2O3 (Si-doped, 10 μm) on bulk (001) β- Ga2O3 substrate (Sn-doped, 650 μm) with Ti/Au as the ohmic contact for the full back area and Ni/Au for Schottky contacts in the front. The rectifiers were subject to 1.5 MeV electron irradiation at fluences from 1.79 × 1015 to 1.43 × 1016 cm-2 at a fixed beam current of 10-3 A. The electron irradiation caused a reduction in carrier concentration in the epi Ga2O3, with a carrier removal rate of 4.9 cm-1. The electron-induced damage would cause the 2kT region of the forward current-voltage characteristics increase. For the highest electron fluence, the diode ideality factor increased around 8% along with two orders of magnitude increase in on-state resistance. There was a significant reduction in reverse bias current that scaled with electron fluence. The diode on/off ratio was measured when switching from +1 V to -40 V. At -10 V reverse bias voltage, the on/off ratio was severely degraded by electron irradiation, decreasing from ~107 in the reference diodes to ~2 × 103 for the 1.43 × 1016 cm-2 fluence. The reverse recovery characteristics showed little changes even at the highest fluence, with the reverse recovery time in the range 21-25 nsec for all rectifiers. This phenomenon can be explained by the short minority carrier lifetime. which controls the carrier storage time in Ga2O3 intrinsic layer.[3] Also, based on the diode on/off ratio, the primary cause of the degradation was introduced by the reduction of majority carrier concentration.
Regarding proton irradiation, β- Ga2O3 vertical rectifiers were subject to 10 MeV proton irradiation at a fixed fluence of 1014 cm-2. The electrical performance of vertical rectifiers was measured before and after proton irradiation, as well as subsequent annealing up to 450°C. The carrier removal rate was determined to be 235.7 cm-1, which is comparable to the carrier removal rate for GaN-based heterostructures under the same proton energy. Point defects introduced by the proton damage create trap states that reduce the carrier concentration in the Ga2O3. Even annealing at 300°C produces a recovery of approximately half of the carriers in the Ga2O3, while annealing at 450°C almost restores the reverse breakdown voltage. The diode on/off ratio of the rectifiers switched from +1 V to -40 V. The diode on/off ratio was severely degraded by proton irradiation and this was only partially recovered by 450°C annealing. The minority carrier diffusion length decreased from ~340 nm for the reference diode to ~315 nm after the proton irradiation.
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- Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. Masui and S. Yamakosh, Semicond. Sci. Technol., 31, 034001 (2016).
- Higashiwaki, K. Sasaki, A. Kuramata, T. Masui and S. Yamakoshi, Appl. Phys. Lett., 100, 013504 (2012).
- I. Stepanov, V.I. Nikolaev, V. E. Bougrov and A.E. Romanov, Rev. Adv. Mater. Sci., 44, 63 (2016).