Structural sensitivity of transport and optical properties of amorphous GeTe

Abstract

The temperature dependence of the thermoelectric power and electrical conductivity of thermally evaporated a-GeTe films as a function of substrate temperature, presence of oxygen during evaporation, angle of deposition, and post-deposition, annealing has been studied. The conductivity and thermoelectric power are insensitive to these deposition parameters. This behaviour appears to be the consequence of the existence of a small concentration of voids and dangling bonds in a-GeTe as suggested by electronmicroscopy studies of these films. The electrical conductivity varies exponentially with inverse temperature with an activation energy of 0.38 eV in the temperature range 150 to 400 K. The thermoelectric power is positive and various inversely with temperature in the range 200 to 400 K. These results suggest that conduction in a-GeTe is predominantly due to holes excited in the extended states in the valence band of an asymmetric energy-band diagram and the contribution due to hopping conduction is negligible.