【Others Papers】Role of Electron and Hole Trapping in Crystalline and Amorphous Ga₂O₃ in Defect Generation

      Researchers from the University College London have published a dissertation titled "Role of Electron and Hole Trapping in Crystalline and Amorphous Ga₂O₃ in Defect Generation" in ECS Meeting Abstracts.

Abstract

      Crystalline and amorphous Ga₂O₃ has wide applications in power electronics, high-temperature gas sensors, solar-blind UV photodetectors and as a visible-light transparent semiconductor with high conductivity and electron mobility. These properties can be affected by localization of electrons and holes in polaron-like states. We use computational modelling to predict the structure and electronic properties of (self)-trapped holes in the crystalline α and β phases and in the amorphous phase of Ga₂O₃ in comparison with these properties in crystalline and amorphous Al₂O₃. Both materials have qualitatively similar geometric and electronic structures. The calculations are made using density functional theory (DFT) and the nonlocal density functional. Bulk amorphous (a) Ga₂O₃ and Al₂O₃ structures were generated using classical molecular dynamics (MD) and the melt-quench technique and further optimized using DFT. Electrons do not localize in pristine crystalline Ga₂O₃ and Al₂O₃, however, holes form small polarons in both materials. We demonstrate that formation of hole bi-polarons can lead to a significant reduction of barriers for formation of Frenkel pairs following the reaction 2h⁺ ↔ V_O²⁺ + O_i, where V_O²⁺ is the doubly positive O vacancy and O_i is the oxygen interstitial atom.

      In the amorphous phase, electrons can trap on intrinsic low-coordinated Al sites in a-Al₂O₃ caused by disorder as well as at neutral O vacancies, but no electron trapping is found in a-Ga₂O₃. In a-Ga₂O₃, trapped holes are localized around low-coordinated oxygen atoms (two or three coordinated) similar to the behavior seen in crystalline β-Ga₂O₃. Moreover, our calculations predict the formation of stable hole bi-polarons also in the amorphous phase, accompanied by the O-O bond formation. The calculations demonstrate that the hole trapping is spontaneous in both crystalline and amorphous phases with the hole-trapping energies in the amorphous structure on average deeper compared to those found in the crystalline structure. We demonstrate how the two- hole localization leads to Ga-O bond weakening and much lower barriers of 1.5 eV for generation of pairs of V_O²⁺ vacancies and interstitial O_i atoms compared to the crystalline phase. These results are discussed in the context of degradation and dielectric breakdown of Ga₂O₃ films under bias application.

      Thus, despite similarities, Ga₂O₃ and Al₂O₃ demonstrate different behavior. In a-Al₂O₃, both electrons and holes can trap at structural precursor sites forming bi-polarons and both electron and hole bi-polarons significantly reduce barriers for formation of Frenkel pairs of O vacancies and O interstitial ions. However, in Ga₂O₃ only the hole trapping occurs and causes a significant barrier reduction. This difference is caused by significantly shorter Al-O than Ga-O distances, which makes possible the electron trapping at Al-Al bonds formed by 3-coordinated Al ions. These factors should affect the mechanisms and kinetics of degradation and dielectric breakdown in these oxides.

DOI：

https://doi.org/10.1149/MA2025-01351675mtgabs