Heteroepitaxial Growth of α-Ga₂O₃ by MOCVD on a-, m-, r-, and c-Planes of Sapphire

Ga2O3 thin films were deposited simultaneously on (112̅0) a-plane, (101̅0) m-plane, (0001) c-plane, and (011̅2) r-plane sapphire substrates using metal–organic chemical vapor deposition (MOCVD) and characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). The different surface energy and strain conditions imposed by each sapphire plane make the choice of substrate orientation critical to the stabilization of the α-phase. β-Ga2O3 nucleation was found to be preferential over α-Ga2O3 on sapphire orientations with <11̅00> α-Al2O3 present (c- and a-planes) when grown under the same conditions. In contrast, α-Ga2O3 is preferred during the initial stages of growth on the r- and m-plane, although suppression of island growth is required to prevent the formation of inclined facets on which β-Ga2O3 might nucleate. Transmission electron microscopy (TEM) provided a direct confirmation of this growth for r-plane substrates. Classical nucleation theory was applied to rationalize these observations and guide the search for the growth window of α-Ga2O3. As a result, decreasing the VI/III ratio and increasing the TEGa flow rate were found to be effective in realizing phase-pure α-Ga2O3 on a-plane sapphire by MOCVD with good structural quality (62 arcsec full width half-maxima of X-ray rocking curve), though the equivalent growth on c-plane substrates yielded mixed-phase β- and κ-Ga2O3─another metastable phase of Ga2O3, instead. Growth on the m-plane resulted in the smoothest surface morphology and thickest phase-pure α-Ga2O3 film, indicating that it is the most promising substrate orientation for future device manufacturing.