【International Papers】 University of Florida 丨 Perspective on comparative radiation hardness of Ga₂O₃ polymorphs

      Researchers from the University of Florida have published a dissertation titled "Perspective on comparative radiation hardness of Ga₂O₃ polymorphs" in Journal of Vacuum Science & Technology A.

Project Support

      The work at UF and PSU was performed as part of Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA), sponsored by the Department of the Defense, Defense Threat Reduction Agency under Award No. HDTRA1-20-2-0002. A. Haque acknowledges the support from the Air Force Office of Scientific Research, Agreement No. FA9550-24-1-0341. The work at NUST MISIS was supported in part by a grant from the Ministry of Science and Higher Education of Russian Federation (Agreement No. 075-15-2022-1113). The work at IMT RAS was supported by the State Task (No. 075-00296-24-01). The authors at NUST MISIS and UF thank Andrej Kuznetsov for discussions on the gamma polymorph. The research at UCF was supported in part by the National Science Foundation (NSF) (Nos. ECCS2310285, ECCS2341747, and ECCS 2427262), US-Israel BSF (Award No. 2022056), and NATO (Award No. G6072; G6194).

Background

      Gallium oxide (Ga₂O₃) is currently attracting much interest for future generations of high-power electronics as well as solar-blind UV photodetectors. The four polymorphs of Ga₂O₃ most investigated are trigonal (α), monoclinic (β), cubic (γ), and orthorhombic (κ). Each of these polymorphic forms exhibit distinct structural and electronic characteristics and they transform into the beta phase at specific temperature ranges. In this paper, we discuss the comparative changes in carrier removal rate and minority carrier diffusion length in the different polymorphs of Ga₂O₃ because of exposure to different types of radiation. 

Abstract

      Gallium oxide (Ga₂O₃) exists in different polymorphic forms, including the trigonal (α), monoclinic (β), cubic (γ), and orthorhombic (κ) phases, each exhibiting distinct structural and electronic properties. Among these, β-Ga₂O₃ is the most thermodynamically stable and widely studied for high-power electronics applications due to its ability to be grown as high-quality bulk crystals. However, metastable phases such as α-, γ-, and κ-Ga₂O₃ offer unique properties, including wider bandgap or strong polarization and ferroelectric characteristics, making them attractive for specialized applications. This paper summarizes the radiation hardness of these polymorphs by analyzing the reported changes in minority carrier diffusion length (L_D) and carrier removal rates under various irradiation conditions, including protons, neutrons, alpha particles, and gamma rays. β-Ga₂O₃ demonstrates high radiation tolerance with L_D reductions correlated to the introduction of electron traps (E2*, E3, and E4) and gallium–oxygen vacancy complexes (V_Ga–V_O). α-Ga₂O₃ exhibits slightly better radiation hardness similar to κ-Ga₂O₃, which also shows minimal L_D changes postirradiation, likely due to suppressed defect migration. γ-Ga₂O₃ is the least thermodynamically stable, but surprisingly is not susceptible to radiation-induced damage, and is stabilized under Ga-deficient conditions. The study highlights the role of polymorph-specific defect dynamics, doping concentrations, and nonuniform electrical properties in determining radiation hardness. We also discuss the effect of radiation exposure on the use of NiO/Ga₂O₃ heterojunction rectifiers that provide superior electrical performance relative to Schottky rectifiers. The presence of NiO does change some aspects of the response to radiation. Alloying with Al₂O₃ further modulates the bandgap of Ga₂O₃ and defect behavior, offering potentially tunable radiation tolerance. These findings provide critical insights into the radiation response of Ga₂O₃ polymorphs, with implications for their use in aerospace and radiation-hardened power electronics. Future research should focus on direct comparisons of polymorphs under identical irradiation conditions, defect identification, and annealing strategies to enhance radiation tolerance.

Conclusion

      This paper summarizes the radiation hardness of different Ga₂O₃ polymorphs (α, β, κ, and γ) by examining the changes in minority carrier diffusion length and defect introduction rates after various types of irradiations (protons, neutrons, alpha particles, and gamma rays). β-Ga₂O₃, the most thermodynamically stable polymorph, has been extensively studied, revealing that L_d values in high-quality epitaxial layers range from 0.15 to 0.6 μm depending on defect concentrations (E2*, E3, and E2). Irradiation with 20 MeV protons decreased L_d, correlating with an increase in electron trap densities (E2*, E3, and E4). The damage constant for 1/L_d² was approximately 3.6 × 10⁻¹³ mm⁻²/cm⁻². Neutron irradiation and alpha particle irradiation also reduced L_d, with varying damage rates. Studies on NiO/β-Ga₂O₃ heterojunctions showed different responses to irradiation compared to Schottky diodes. In comparison, α-Ga₂O₃ exhibits a higher radiation tolerance than the β-phase, with low-energy proton implantation in α-Ga₂O₃ also reducing L_d and inducing a new defect band MCL spectra while κ-Ga₂O₃ demonstrates minimal L_d changes after irradiation, though its properties are complicated by nonuniformity. It exhibited short L_d values (∼70 nm) with minimal changes after 1.1 MeV proton irradiation. γ-Ga₂O₃, being metastable, also exhibits unique defect dynamics. The study highlights the complex interplay among polymorph structure, doping, irradiation type, and the resulting changes in transport properties.

      The literature provides several insights into why different polymorphs of Ga₂O₃ exhibit varying degrees of radiation hardness. The differences in radiation hardness can be attributed to several factors, including structural stability, defect dynamics, doping concentrations, and the nature of the defects introduced by irradiation.

      In summary, different polymorphs exhibit varying degrees of radiation hardness with β-GaO₃ generally being the least radiation-tolerant and γ-Ga₂O₃ the most for proton irradiation. The differences are not large for these conditions and not significant enough to suggest that the other polymorphs are favored for radiation-hard applications. While γ-Ga₂O₃ is very resistant to damage, it has, as yet, not been successfully doped and has limited device applications.