Influence of hopping conduction on the thermopower of indium oxide thick films

We have studied the temperature (T) dependent behaviors of thermopower (S) and resistivity (ρ) of a series of indium oxide (In2O3) thick films prepared at different oxygen partial pressures. For the films deposited at lower oxygen partial pressures, the ρ − T curves display metallic characteristics, and the relation between thermopower and temperature is dominated by the electron diffusion process. For the films deposited at higher oxygen partial pressures, the Mott type and Efros-Shklovskii type variable-range-hopping (VRH) processes are observed in the resistivity data. The thermopower obeys S ∝ T1∕2 from ∼50 to ∼200 K, which can be explained by Mott VRH process. At lower temperatures, the existence of phonon-drag causes S to change from negative to positive with decreasing temperature and then present a peak value with further decreasing temperature. Our results demonstrate the validity of theoretical predictions concerning three dimensional Mott VRH thermopower.