MOCVD Growth of κ‑Ga₂O₃ on Al-Rich Al _x Ga_1-x N Templates: Phase Diagram and Microstructural Evolution

Orthorhombic κ-phase Ga₂O₃ is a metastable, ferroelectric polymorph of Ga₂O₃ with a large spontaneous polarization, offering a pathway to polarization-induced high-mobility 2D electron gases via κ-Ga₂O₃/Al _x Ga_1-x N heterostructures. Here, we map the metal-organic chemical vapor deposition (MOCVD) growth window of κ-Ga₂O₃ on Al-rich Al _x Ga_1-x N-on-sapphire template layers (x = 0.5 and 0.75) by systematically varying the gallium and oxygen precursor flow rates and benchmark these results against cogrown films on c-plane sapphire. On Al_0.5Ga_0.5N, κ-Ga₂O₃ can be grown over a wide range of conditions extending to low growth rates and high VI/III regions, whereas on sapphire substrates the κ-phase favorable window is restricted to the high growth rate and low VI/III regime only. Microstructural and phase evolution analyses by X-ray diffraction (XRD) and transmission electron microscopy (TEM) of the κ-Ga₂O₃ films grown on Al_0.5Ga_0.5N confirms that growth initiates as a phase-pure monoclinic β-Ga₂O₃ layer; κ-Ga₂O₃ nucleation starts between 20 and 45 nm of layer thickness and becomes the only phase growing around ∼100-250 nm thick, resulting in a phase-pure κ-Ga₂O₃ top surface. A similar progression was observed for growth on x = 0.75% Al _x Ga_1-x N template layers.