Thermoelectric power of amorphous germanium films

Abstract

The temperature dependence of the thermoelectric power (TEP) and the corresponding resistivity of evaporated amorphous (a-)Ge films is studied in the temperature range 140 to 500 K as a function of such deposition parameters as technique of deposition, nature and temperature of the substrate, rate and angle of deposition, and the presence of ambient gases. Also, the effect of ageing and annealing, oxidation, and impurities (Sb, AI, Fe, and Au) is studied. The characteristic behaviour of the TEP is a small (≈0.1 mV/K) negative value at low temperatures (below 300 K) followed by an inversion of the sign and a rapid rise with temperature above the inversion temperature. Whereas the low temperature behaviour of the TEP is not significantly affected, the inversion temperature and the high temperature behaviour are strongly modified by changes in the deposition parameters, post-deposition treatments, and by addition of impurities. In general, the sign, magnitude, and its temperature variation depend both on the impuritiy and its concentration. There is, however, no correlation of these effects with the electronic nature of the impurity. In contrast to the behaviour of the TEP, the dependence of the resistivity on deposition parameters and impurities is significantly smaller. The TEP can be understood in terms of hopping at the Fermi level and bipolar conduction in the extended states of an asymmetrical energy-band diagram whose tailing, extent of localization, and the shape of density-of-states curve (hence the position of Fermi level) depend sensitively on deposition parameters, post-deposition treatments, and impurities.