Transport properties of highly doped polycrystalline and amorphous SnO2 films.

Abstract

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\ensuremath{\le}${T}_{s}$\ensuremath{\le}440 \ifmmode^\circ\else\textdegree\fi{}C. The data indicate the classical behavior of degenerate semiconductors for polycrystalline films prepared at ${T}_{s}$\ensuremath{\ge}350 \ifmmode^\circ\else\textdegree\fi{}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 \ifmmode^\circ\else\textdegree\fi{}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 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ (500 K) are compatible with electron transport via extended states in both the conduction-band and impurity-band regimes.