Determination of the β to γ Phase Transformation Mechanism in Sc- and Al-Alloyed β-Ga2O3 Crystals

Abstract

β-Ga₂O₃ is a promising ultrawide bandgap semiconductor for next-generation power electronics, but the unintended formation of γ-Ga₂O₃ in β-Ga₂O₃ crystals has been observed in a variety of situations. Such defective inclusions, resulting from growth kinetics or ion-induced damage, can degrade the material performance and alter the local electronic structure. Previous studies have only examined the presence of γ-Ga₂O₃ in β-Ga₂O₃ thin-film structures. In this work, we observe the ubiquitous formation of a thin γ-Ga₂O₃ layer on the surface of mechanically exfoliated melt grown Al- and Sc-alloyed β-Ga₂O₃ single crystals and characterize the atomic scale structure across the interface using scanning transmission electron microscopy. Direct imaging paired with electron diffraction confirms γ-Ga₂O₃ formation, and orientation relationships are determined across the interface. Electron energy loss spectroscopy identifies the O K-edge spectral fingerprint of γ-Ga₂O₃, while many-body perturbation theory on top of density functional theory explains the shift of the spectral intensity between β- and γ-Ga₂O₃ as an interplay of excitonic and electronic effects. Further first-principles studies evaluate the role of strain on phase stability and identify that at an 8.5% tensile strain, γ-Ga₂O₃ becomes energetically favored over β-Ga₂O₃. Stabilization of the β phase of Ga₂O₃ under compressive stress is further confirmed through electron diffraction studies of the regions surrounding Vickers indentations. Phase stability is also observed to be independent of the alloying element. These findings confirm the capability for γ-Ga₂O₃ to occur under extreme environments while also providing evidence that strain is the underlying driving force causing the phase transformation.