Electron and Proton Irradiation Damage in β-Ga2O3 Vertical Rectifiers
Authors: Yang Jiancheng , Chen Zhiting , Ren Fan , Pearton Stephen J. , Yang Gwangseok , Kim JiHyun , Lee Jonathan , Flitsiyan Elena , Chernyak Leonid , Kuramata Akito 
Published: February 2020

Wide bandgap semiconductors materials have always been the interest of their ability that can withstand high radiation fluences for the military, nuclear-industry monitoring, satellites and other space-borne applications. Monoclinic β phase Ga2O3 is attracting great attention for power electronics and solar-blind photodetectors because of its large bandgap (~4.9 eV) and a critical electric field strength of 8 MV/cm.[1,2,3] Also, the large diameter bulk β- Ga2O3 is commercially available with a controllable n-type doping for its epitaxial layers. Given its high bond strength and expected high vacancy formation energy, β- Ga2O3 is likely to be very radiation hard based on the reduced density of atomic displacements expected for a given energy of non-ionizing radiation exposure.[4] The cosmic rays, trapped radiation in the Earth’s radiation belts, and solar particle events are three primary sources of space irradiation. The cosmic rays consist predominantly of protons; the radiation belts contain energetic protons and electrons while solar particle events are mostly lower energy protons and electrons, in which the average integral proton fluxes at 10 MeV are of order 1014 cm-2.[1] Thus, it is important to understand the effect of energetic proton irradiation on Ga2O3.
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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