Effect of impurities on the thermoelectric power of amorphous germanium

Large and anomalous changes in the thermoelectric power of amorphous (a?) Ge films have been observed on doping with impurities of Al, Fe and Sb. Depending on the concentration, Al and Sb impurities contribute a negative thermoelectric power below 300°K. and a positive thermoelectric power above 300°K. The effect of Fe is very small below 1 at. %. The thermoelectric power attains high temperature-independent positive values for large (> 1 at. %) concentrations of Al. The observed effects of the impurities cannot be understood in terms of the conventional crystalline semiconductor concepts. A band structure model for a-Ge has been proposed to qualitatively understand the changes in the sign and magnitude of the thermoelectric power with temperature.