Unexpected Origin of Deep Level Compensation by Proton Irradiation in Al-Rich AlGaN

Al-rich AlGaN with band gap energy (Eg) = 4.5 - 6.2 eV is of interest for power electronics because the two- and three-dimensional power switching figures-of-merit scale aggressively with Eg. AlGaN-based electronics are also appealing because resilience to radiation also scales with Eg. The ionization energy required to create an electron-hole pair increases with Eg, and the atomic displacement energy increases rapidly with decreasing lattice constant (i.e., increasing Eg). Thus, AlGaN is a candidate for next-generation, compact, rad-hard power electronics. However, AlGaN-based transistors are immature compared to GaN or SiC, and the specific effects of various radiation environments on AlGaN electronic properties have not been well established.

This study reports the impact of 2.5 MeV proton (p⁺) irradiation up to 1e14 cm² fluence on carrier removal and deep level defect introduction in n-type, Si-doped Al_0.7Ga_0.3N grown by metal-organic vapor phase epitaxy on AlN-on-sapphire templates. Al_0.7Ga_0.3N exhibited a carrier removal rate ~ 155 cm^-1 compared to ~ 250 cm^-1 that is typically reported for n-type GaN, confirming a greater resilience to displacement damage effects. Deep level optical spectroscopy (DLOS) was used to monitor Al_0.7Ga_0.3N defect introduction. DLOS interrogated defect states 1.2 eV and deeper in the band gap relative to the conduction band edge (Ec). As-grown Al_0.7Ga_0.3N displayed deep levels at ~ Ec – 2.4 eV, 3.4 eV and 4.4 eV. The Ec – 4.4 eV deep level density (Nt) increased with p⁺ fluence, however, its Nt increased by only 5e15 cm^-3 compared to a reduction in free carrier concentration of 1.4e16 cm^-3. This observation suggests that additional defect states lying shallower than Ec - 1.2 eV must also be acting as compensators.

Deep level transient spectroscopy (DLTS) was used to examine such shallower defect states. DLTS on as-grown Al_0.7Ga_0.3N found a deep level at Ec – 0.79 eV with Nt ~ 4.5e14 cm^-3. Conversely, DLTS of the Al_0.7Ga_0.3N irradiated with 1e14 cm^-2 fluence showed deep levels at Ec – 0.55 eV (Nt = 4.3e14 cm^-3), Ec - 0.8 eV (Nt = 7.1e15 cm^-3) and Ec - 1.2 eV (Nt = 4.0e15 cm^-3). Differences in Arrhenius plots suggest that the Ec – 0.79 eV and Ec – 0.8 eV deep levels are distinct. Importantly, DLTS reveals that the ascendant Ec – 0.8 and 1.2 eV deep levels with p⁺ irradiation had a combined Nt that accounts for the discrepancy between carrier removal and deep level introduction measured by DLOS alone.

It is unexpected that the major compensators in n-Al_0.7Ga_0.3N with Eg = 5.1 eV would form states in the upper part of the band gap because defect formation energy typically favors acceptor-like defect energy levels lying below mid-gap. Consideration of kinetically-driven defect formation by irradiation and theoretical studies of native defects in AlN provides an explanation. Density functional theory investigation [Yan et al., APL 105 111104 (2014)] of nitrogen vacancies (V_N) and oxygen substituting on V_N (O_N) found that either can form acceptor-like defect states near 1 eV below Ec, but their formation energy was too high to expect significant concentration to incorporate during growth. Nonetheless, p+ irradiation can readily introduce V_N and subsequent O_N formation, providing an explanation for carrier removal in Al_0.7Ga_0.3N studied here. It is notable that V_N and O_N are regarded as shallow donors in GaN, while DFT and DLOS/DLTS find that they become deep acceptors in AlN. This finding highlights the need for fresh examination of defect properties when extending from GaN to AlN.

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