A Route to Oxide Electronic Materials*

Perovskite oxides offer diverse compositions, structures, and properties that can be carefully controlled through epitaxial synthesis methods, presenting vast opportunities in electronic or optoelectronic applications. SrTiO₃ exhibits a fascinating range of electronic phenomena, including quantum electronic transport and superconductivity. Doping SrTiO₃ via Nb substitution holds promise in thermoelectrics and as electrodes for ferroelectric or multiferroic oxides. A key challenge has been in achieving epitaxial synthesis of SrTiO₃ over large areas. Synthesis methods have been constrained by the substrate dimensions traditionally employed for perovskite epitaxy. We have employed solid phase epitaxy to synthesize low-defect-density γ-Al₂O₃ on commercial α-Al₂O₃ substrates, available in large dimensions, underpinning the feasibility of large-area epitaxial synthesis. The (111) surface of cubic γ-Al₂O₃ has a lattice constant twice that of typical pseudocubic perovskites and thus provides an appropriate template for growth of perovskites. Epitaxial growth of Nb-doped SrTiO₃ on γ-Al₂O₃ yields thin films with improved structural and electronic properties compared to those grown on α-Al₂O₃, including electronic conductivity that is two to three orders of magnitude higher. The epitaxial formation of SrTiO₃ (111) on γ-Al₂O₃ (111) heralds a noteworthy instance of nonisomorph epitaxial growth between a perovskite and a spinel, bridging the two predominant oxide material classes.