Transport properties of highly doped polycrystalline and amorphousSnO2films

The Hall effect in the temperature range 80–500 K has been measured in sprayed ${\mathrm{SnO}}_2$ films deposited at low substrate temperatures, 220≤$T_s$≤440 °C. The data indicate the classical behavior of degenerate semiconductors for polycrystalline films prepared at $T_s$≥350 °C. With the lowering of $T_s$, the conduction ceases to be metallic either due to the incorporation of excess Cl donor impurities or the appearance of the amorphous state when $T_s$<300 °C. We interpret the Hall-effect data by a generalized two-band model where a fraction of the electronic states remains strongly localized. Quantitative estimates are given for two types of disorder: a distribution of Cl impurity atoms in a polycrystalline matrix, and an amorphous host-lattice structure. The mobility of amorphous ${\mathrm{SnO}}_2$ was measured down to 80 K and the values between 0.4 (80 K) and 1.5 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{\mathrm{-}1}$ ${\mathrm{s}}^{\mathrm{-}1}$ (500 K) are compatible with electron transport via extended states in both the conduction-band and impurity-band regimes.