Sn4+ charge density variation in In2O3

Thin tin-doped indium oxide films have a low resistivity (10-3-10-4 W·cm) and a high luminous transmittance (>80%) [1]. Therefore they find an application as electrodes in flat panel displays, solar cells, etc.
The lowest resistivities (in the order of 1·10-4 W·cm) are reported for ITO films prepared by sputtering of ceramic targets which are expensive and mechanically fragile. The stable process with metallic targets can be much more favorable for industrial application, because of low cost and high deposition rates. The production of ITO films with low resistivity by reactive sputtering usually requires postdeposition annealing step or deposition on heated substrates [1].
A number of publications were devoted to investigate ITO powders with different Sn content [2] and ITO films subjected to oxidizing and reducing annealing [3]. These films were prepared from ceramic targets. Mössbauer spectroscopy in combination with X-ray diffraction and Hall measurements have been applied in these studies on ITO to investigate the chemical state of Sn atoms.
Similar investigations on the ITO films prepared by reactive magnetron sputtering at elevated substrate temperatures are missing up to now. Therefore indium-tin oxides (ITO) films were deposited by reactive middle frequency dual magnetron sputtering at heated substrates.
The doping of Sn in In2O3 was investigated using the surface sensitive 119mSn Conversion-Electron Mössbauer Spectroscopy (CEMS) to investigate the chemical state of tin.