Thermopower behaviour of amorphous versus crystalline Ge and GeTe films

Abstract

In continuation of our studies of the structural, optical and electrical properties of amorphous (a-) versus crystalline (c-) Ge and GeTe films, the present paper reports the temperature dependence of the thermoelectric power (α) of these films relative to evaporated gold films. In c-films, α increases nearly linearly with temperature below about 300°K (metallic or degenerate semiconducting behaviour) and then rises rapidly to a saturation value. The linear temperature dependence is characteristic of metals or degenerate semiconductors. The rapid rise is associated with rapid changes of mobility in the degenerate c-films. The thermoelectric data of GeTe films support our earlier conclusion that only a single valence band contributes to the transport process, and further, that the maximum of the valence band occurs at k ≠ 0. The thermoelectric data of a-films bear no correlation with those of c-films. For a-GeTe films, a constant and positive value of α ~ 0.88 mV/°K is observed in the measurable temperature range 175°-375°K. In a-Ge films, α is negative below 300°K with a constant value of about 0.08 mV/°K below ~250°K. It reverses sign above 300°K and rises rapidly to a value of about 0.96 mV/°K at ~420°K and then decreases above 420°K, most probably due to alloying at the Au-Ge interface. The behaviour of thermopower in amorphous films cannot be understood by conventional transport theories. A qualitative interpretation is possible on the basis of heat transport by a hopping process and a speculative energy band diagram.