Nanoepitaxy of SnO2 on α-Al2O3(0 1 2)

Thin tin dioxide films have been grown on r-cut sapphire substrates by atomic-layer chemical vapor deposition (ALCVD), using SnI4 and O2 as precursors. The films prepared at 600 °C were characterized in some detail by X-ray diffraction (XRD), reflection high-energy electron diffraction (RHEED), scanning force microscopy (SFM), and conductivity measurements. The XRD data demonstrated that the films grew epitaxially, had a cassiterite structure, and were (1 0 1)cassiterite∥(0 1 2)sapphire oriented. The in-plane orientation relationships for two mutually perpendicular directions have been determined to be [0 1 0]cassiterite∥[1 0 0]sapphire and [1 0 1̄]cassiterite∥[1̄ 2̄ 1]sapphire. Evidence was found for different epitaxial subdomains. The smoothness of the films has been verified by the RHEED and SFM measurements. The latter showed that ultrathin films with a thickness of ∼10 nm were as smooth as the substrates. The atomic alignment and surface morphology worsened when the ALCVD growth proceeded. It was revealed that the conductivity of the ultrathin films considerably increased under the action of reducing gases in air. This makes the films suitable for application in conductometric gas sensors.