Interface Formed via Spontaneous Phase Transition from α- to κ-Ga₂O₃

In this work, we investigated the heterointerface formed between metastable α-Ga₂O₃ (corundum) and κ-Ga₂O₃ (orthorhombic). When κ-Ga₂O₃ was grow on c-plane sapphire at temperatures above 570 °C via mist chemical vapor deposition, α-Ga₂O₃ initially formed, followed by a spontaneous phase transition from α- to κ-Ga₂O₃, resulting in an atomically abrupt interface. Despite this abrupt interface, the process was uncontrolled, leading to an uneven interface on a micrometer scale. To address this, the growth process was optimized, identifying strain and temperature as the key control parameters to achieve a uniform and atomically abrupt interface. The band offset at the κ-/α-Ga₂O₃ interface was analyzed using X-ray photoelectron spectroscopy. The results revealed a type-II band alignment, with the valence and conduction bands of κ-Ga₂O₃ positioned approximately 1.00 and 0.80 eV higher, respectively, than those of α-Ga₂O₃. This was further corroborated by density functional theory calculations. The observed band offset at the pristine κ-/α-Ga₂O₃ interface holds promise for advanced device designs, offering potential applications in future electronic and optoelectronic technologies.