Reactively sputtered TiO2 layers on SnO2:F substrates: A Raman and surface photovoltage study

DC reactively sputtered TiO2 layers on SnO2:F substrates were investigated by Raman and surface photovoltage spectroscopy. The stoichiometry and layer thicknesses were investigated by elastic recoil detection analysis. The deposition temperature, the O2/(O2 + Ar) ratio and the deposition time were varied systematically. With increasing temperature, the layers become crystalline with the rutile modification dominating. Rutile phase preferentially forms on vertical facets of SnO2 crystallites. Anatase phase starts to form during prolonged deposition and at lower O2/(O2 + Ar) ratios. The energy of the exponential absorption tails below the band gap, a measure of the defect density of the films, is determined by the deposition temperature and not by other parameters if the deposition temperature is relatively high, irrespective of the content of crystalline phases or the value of the band gap. Charge separation takes place at length scales significantly shorter than the layer thicknesses (diffusion length less than 6 nm). TiO2 films sputtered at 380 °C show rectifying behaviour with a carbon contact.