Identification of polymorphs in epitaxial SnO2 thin films deposited on sapphire (0001) substrates

In this study, synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM) methods were combined to identify polymorphs in SnO₂ thin films deposited on sapphire (0001) substrates using radio-frequency powder sputtering. A wide range of off-specular Bragg peaks—including higher-order reflections—were examined via high-resolution, in-plane XRD analyses for precise phase identification. Evidently, the orthorhombic columbite (C-SnO₂) and tetragonal rutile (R-SnO₂) phases coexisted in the as-deposited films. Because both the phases were aligned with their (200) planes along the surface normal, their out-of-plane Q_z components appeared nearly identical. However, the in-plane Q_x and Q_y components were distinguishable. The lattice constants were estimated from the in-plane Bragg peak positions, and the corresponding strain states in ultrathin films (<10 nm) were determined. In the early stage of growth, the C-SnO₂ and R-SnO₂ domains exhibited opposing strains—compressive and tensile strains, respectively—because of extended domain matching epitaxy, which accommodated lattice mismatch and governed the stabilization of each polymorph. The coexistence of the two phases at the atomic scale was further supported by cross-sectional high-resolution TEM analysis. These findings provide new insights into the strain-driven stabilization of polymorphs and the structural evolution of epitaxial SnO₂ thin films on symmetry-mismatched substrates.