Proton and X-Ray Irradiation Effects on Al₂O₃ / β-Ga₂O₃ MIS Capacitors

β-Ga₂O₃ electronic devices are of great interest for space applications where high energy radiation can impact device performance. While β-Ga₂O₃ has been predicted to have higher resistance to displacement damage (DD) compared with conventional semiconductors, β-Ga₂O₃ FET devices include MIS gate structures, which can be affected by total ionizing dose (TID) effect like any FET technology. Here we report a systematic study that compares proton and X-ray irradiation to interrogate TID and DD effects from both sources. The impact of X-rays and proton irradiation are individually investigated to separate the displacement damage and TID effects. Two sets of metal-insulator-semiconductor (MIS) capacitors were prepared with in-situ MOCVD-grown Al₂O₃ (15nm) / β-Ga₂O₃ thin films in two growth temperatures (650°C and 900°C). Ni gate contacts were deposited on the insulator, and Ti/Al/Ni/Au contacts on the substrate formed the Ohmic contacts. The devices were irradiated with 1.8 MeV proton radiation with fluence steps from 5 ×10¹² cm^-2 to 3 ×10¹⁴ cm^-2. During proton irradiation, a gate bias of -3 V, 0 V, and 2.5 V was applied to separate devices to vary the electric field strength and polarity across the insulator. Similar biasing conditions were applied separately while irradiating with 10 keV X-ray up to 1 Mrad(SiO₂).
Firstly, it is shown that both TID and DD effects are caused by protons. TID is observed and characterized through flatband voltage shifts (ΔV_FB). As total fluence increases, ΔV_FB up to 1.1 V is observed after proton irradiation. DD due to proton damage is confirmed by measuring the carrier removal (CRR ~200 cm^-1) and a concomitant increase in deep-level defect concentrations determined by deep-level defect spectroscopy methods (DLTS and DLOS) that probe the Ga₂O₃ region beneath the insulator. In contrast, X-ray irradiation only shows a TID effect with minimal carrier removal, as expected. The amount of ΔV_FB contributed from the change in C-V characteristics by displacement damage is an order of magnitude lower than the TID effect. By comparing ΔV_FB from devices with each biasing condition, we observed that the TID effect increases with field strength across the Al₂O₃ insulator. This is consistent with the columnar model of recombination whereby a higher field prevents the recombination of ionized electron-hole pairs (EHP), leaving net positive charge trapped in the Al₂O₃ layer. We find that the TID effect caused by 1.8 MeV proton and 10keV X-ray irradiation both track similar trends with increasing fluence and dosage, respectively. The data reveals that the ΔV_FB caused by X-ray from 20k to 500k rad(SiO₂) is equivalent to that of 1.8 MeV proton from 1 x10¹² to 2 x10¹⁴ cm^-2 fluence. This relation can be used to draw equivalence between the two types of radiation in terms of TID-induced damage to Al₂O₃ / β-Ga₂O₃ MIS devices. This work acknowledges the collaborative effort of the Ion Beam Laboratories of the Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This work is supported by the Consortium for Enabling Technologies & Innovation, the Air Force Center of Excellence and the Air Force Office of Scientific Research.