Conductivity and thermoelectric power of amorphous germanium and amorphous silicon

A series of amorphous germanium ($a-\mathrm{Ge}$) and amorphous silicon ($a-\mathrm{Si}$) films was prepared by rf sputtering. For the $a-\mathrm{Ge}$ samples the temperature dependence of the conductivity measured down to 25 K is in general agreement with the earlier results of Knotek and Hauser. The thermoelectric power measured down to 40 K is negative (∼ - 80 μV/K) and virtually temperature independent for $70\mathrm{K}<T<\mathrm{}300\mathrm{} \mathrm{K}$. For $T<\mathrm{}70\mathrm{}$ K, it becomes rapidly more negative with decreasing temperature. The temperature dependence of the conductivity in the $a-\mathrm{Si}$ samples is similar to that found in $a-\mathrm{Ge}$ but the thermopower, which is negative and changes by little from 200 to 300 K, becomes more positive as the temperature is decreased. An attempt is made to interpret these data using the variable-range hopping theory and three simple density-of-states models. The attempt is unsuccessful. However, the Ansatz that conduction occurs in a narrow band (width ∼ 0.01 eV) does explain the features of the thermopower data and suggests that for these samples conduction at $T>\mathrm{}70\mathrm{}$ K is by hopping between nearest-neighbor sites.