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

β-Ga 2 O 3 is a promising ultrawide bandgap semiconductor for next-generation power electronics, but the unintended formation of γ-Ga 2 O 3 in β-Ga 2 O 3 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-Ga 2 O 3 in β-Ga 2 O 3 thin-film structures. In this work, we observe the ubiquitous formation of a thin γ-Ga 2 O 3 layer on the surface of mechanically exfoliated melt grown Al- and Sc-alloyed β-Ga 2 O 3 single crystals and characterize the atomic scale structure across the interface using scanning transmission electron microscopy. Direct imaging paired with electron diffraction confirms γ-Ga 2 O 3 formation, and orientation relationships are determined across the interface. Electron energy loss spectroscopy identifies the O K-edge spectral fingerprint of γ-Ga 2 O 3, while many-body perturbation theory on top of density functional theory explains the shift of the spectral intensity between β- and γ-Ga 2 O 3 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-Ga 2 O 3 becomes energetically favored over β-Ga 2 O 3 . Stabilization of the β phase of Ga 2 O 3 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-Ga 2 O 3 to occur under extreme environments while also providing evidence that strain is the underlying driving force causing the phase transformation.