Unlocking the Single-Domain Heteroepitaxy of Orthorhombic κ-Ga₂O₃ via Phase Engineering

The emergent ferroelectric property of κ-Ga2O3 is expected to deliver advanced functional memory and ultralow-loss transistors, while the commonly observed rotational domains in κ-Ga2O3 make the origin of ferroelectricity mysterious. In this work, the single-domain heteroepitaxy of orthorhombic κ-Ga2O3 epilayers on sapphire has been demonstrated by the halide vapor-phase epitaxy (HVPE) technique. The optimal temperature of 550 °C is energetically favorable for the stabilization of κ-Ga2O3 on sapphire without impurity phases, and the growth dynamics is dominated by the surface-reaction-limited mechanism. The evolution of microstructures and optical characteristics indicate that the κ–β phase transition occurs at an elevated temperature of over 575 °C together with a remarkable reduction of growth rate. With proper phase engineering, the single-domain κ-Ga2O3 epilayers have been ultimately achieved, exhibiting multisteps resembling a terrace morphology, a relatively low screw dislocation density of 5.2 × 107 cm–2, and reduced band tail subgap states. The single-domain structure of orthorhombic κ-Ga2O3 was identified by the XRD ϕ-scans and transmission electron microscopic analysis. The realization of single-domain epitaxy allows one to uncover the driving force for the intriguing ferroelectric behavior of κ-Ga2O3 and to design power devices with improved performance.