Sputtering growth and optical properties of [100]-oriented tetragonal SnO2 and its Mn alloy films

SnO 2 and its Mn alloy thin films have been grown on Al2O3(0001) substrates by reactive radio-frequency magnetron sputtering performed in the presence of O2 gas. The prepared films showed preferred orientation in the [100] direction of the rutile structure of SnO2. The O2/Ar gas-flow ratio maintained during the sputtering was found to significantly affect the crystalline quality and stoichiometry of the films. The optical constants of the SnO2 and Sn1−xMnxO2 (x⩽0.27) films were measured by spectroscopic ellipsometry in the 2–5 eV photon energy region. The band-gap energy of SnO2 is determined to be 3.86 eV and that of Sn1−xMnxO2 increases for x⩽0.11 and then decreases for the higher x, exhibiting a negative bowing. The initial increase of the band-gap energy is attributable to the hybridization between localized Mn d states located near the band gap and O p-like valence bands. The decrease of the band-gap energy is interpreted as due to the SnO2–MnO2 alloying effects.