Determination of\nthe β to γ Phase Transformation\nMechanism in Sc- and Al-Alloyed β‑Ga<sub>2</sub>O<sub>3</sub> Crystals

β-Ga<sub>2</sub>O<sub>3</sub> is a promising ultrawide\nbandgap\nsemiconductor for next-generation power electronics, but the unintended\nformation of γ-Ga<sub>2</sub>O<sub>3</sub> in β-Ga<sub>2</sub>O<sub>3</sub> crystals has been observed in a variety of situations.\nSuch defective inclusions, resulting from growth kinetics or ion-induced\ndamage, can degrade the material performance and alter the local electronic\nstructure. Previous studies have only examined the presence of γ-Ga<sub>2</sub>O<sub>3</sub> in β-Ga<sub>2</sub>O<sub>3</sub> thin-film\nstructures. In this work, we observe the ubiquitous formation of a\nthin γ-Ga<sub>2</sub>O<sub>3</sub> layer on the surface of mechanically\nexfoliated melt grown Al- and Sc-alloyed β-Ga<sub>2</sub>O<sub>3</sub> single crystals and characterize the atomic scale structure\nacross the interface using scanning transmission electron microscopy.\nDirect imaging paired with electron diffraction confirms γ-Ga<sub>2</sub>O<sub>3</sub> formation, and orientation relationships are\ndetermined across the interface. Electron energy loss spectroscopy\nidentifies the O K-edge spectral fingerprint of γ-Ga<sub>2</sub>O<sub>3</sub>, while many-body perturbation theory on top of density\nfunctional theory explains the shift of the spectral intensity between\nβ- and γ-Ga<sub>2</sub>O<sub>3</sub> as an interplay of\nexcitonic and electronic effects. Further first-principles studies\nevaluate the role of strain on phase stability and identify that at\nan 8.5% tensile strain, γ-Ga<sub>2</sub>O<sub>3</sub> becomes\nenergetically favored over β-Ga<sub>2</sub>O<sub>3</sub>. Stabilization\nof the β phase of Ga<sub>2</sub>O<sub>3</sub> under compressive\nstress is further confirmed through electron diffraction studies of\nthe regions surrounding Vickers indentations. Phase stability is also\nobserved to be independent of the alloying element. These findings\nconfirm the capability for γ-Ga<sub>2</sub>O<sub>3</sub> to\noccur under extreme environments while also providing evidence that\nstrain is the underlying driving force causing the phase transformation.