Competing phases in epitaxial SnO2 thin films deposited on sapphire(0001) substrates using radio-frequency powder sputtering

Abstract

Herein, the competitive crystalline phase between orthorhombic columbite (C–SnO2) and tetragonal rutile (R–SnO2) phases in SnO2 thin films sputter-deposited on sapphire(0001) substrates was investigated. Off-specular X-ray diffraction analysis revealed that only the C–SnO2 was formed initially; however, the growth of the R–SnO2 dominated with increasing thickness. Both phases were epitaxial to the sapphire, and the directional relationship can be summarized as R[100]//C[100]//sapphire[0001] along the surface normal direction and R[010]//C[001]//sapphire [112‾0] along the in-plane direction. In particular, a supercell structure was proposed to accommodate the lattice and symmetry mismatch between C–SnO2 and c-plane sapphire. The electrical conductance of C–SnO2 behaves like a metal as a function of annealing temperature. As the film thickness increased, the optical bandgap gradually decreased from ∼4.68 to ∼3.56 eV owing to the increased amount of the R–SnO2 in the SnO2 thin films. Our results support the crossover of the dominant crystalline phase from the initial C–SnO2 to the later R–SnO2 during the sputter deposition of epitaxial SnO2/sapphire(0001) thin films, which significantly altered the electrical and optical properties.