Epitaxial growth of conducting perovskite SrTiO₃ on (0001) c-axis sapphire using a γ-Al₂O₃ intermediate layer

Epitaxial growth of complex oxides on large-area wafers, such as sapphire and silicon, represents a key step toward scalable oxide device production. Solid phase epitaxy allows the synthesis of γ-Al₂O₃ on α-Al₂O₃ and provides a template with a matched lattice constant and appropriate cubic symmetry for subsequent heteroepitaxial growth of perovskite complex oxides. Nb-doped SrTiO₃ thin films were deposited epitaxially on (111)-oriented γ-Al₂O₃ intermediate layers on (0001) c-axis-oriented sapphire α-Al₂O₃ crystals using pulsed laser deposition. The Nb:SrTiO₃ thin films with a thickness of 53 nm, grown at 700 °C on γ-Al₂O₃, reached fully relaxed lattice parameters and were epitaxially oriented with respect to the substrate. Nb:SrTiO₃ layers deposited using identical deposition conditions directly on α-Al₂O₃, without the γ-Al₂O₃ intermediate layer, were polycrystalline. The sheet conductivity of Nb:SrTiO₃ grown on γ-Al₂O₃/α-Al₂O₃ is more than ten times higher than that of Nb:SrTiO₃ grown directly on α-Al₂O₃ without the γ-Al₂O₃ layer. The results point to new directions for the integration of (111)-oriented pseudocubic perovskite complex oxides and the integration of epitaxial complex oxides over larger areas using α-Al₂O₃ single-crystal substrates.