Electrical Properties of SnO2 Polycrystalline Thin Films and Single Crystals Exposed to O2- and H2-Gases

Abstract

The electrical conductivity σ and carrier density n have been measured in the range 77–670 K for the SnO₂ single crystal and thin films prepared by the vapour deposition technique. It is found that the SnO₂ thin film possesses a carrier density of 10¹⁹ cm^-3, three orders of magnitude larger than that in the as-grown SnO₂ single crystal, and exhibits electrical properties characteristic of a degenerate semiconductor. Both single crystals and thin films undergo irreversible changes in electrical properties upon heat treatment in vacuum or exposure to O₂- or H₂-gases. This occurs as a result of reduction and oxidation of SnO₂. However, the oxidation and reduction proceed in thin films at much lower temperatures than in single crystals. The unique gas-sensing character of thin films was interpreted by assuming that the oxidation and reduction preferentially proceed along the grain boundaries in the thin film.