Use of hydrogenation in the study of the transport properties of amorphous germanium

Amorphous germanium films were prepared by rf sputtering in the presence of hydrogen or deuterium gas, resulting in samples containing up to 8 at.% of these elements. Their dc conductivity and thermoelectric power were measured over a large temperature range (25-525 K). At low temperatures, increasing hydrogenation decreases the conductivity by five orders of magnitude while leaving its temperature dependence virtually unaltered. The thermoelectric power is negative (of order - 100 μV/K), has little temperature dependence, and only a weak dependence on hydrogen content. These results are interpreted with a model in which conduction takes place between nearest-neighbor sites of a narrow band. The model allows an estimate of 4 Å to be deduced for the localization length, an estimate which does not require assumed values for free parameters. The films were remeasured after annealing to 250°C. In the temperature range of about 280 to 500 K, the thermoelectric power is characterized by only one activation energy, typically 0.17 eV, whereas the conductivity displays two distinct activation energies, one for $T < 420$ K of about 0.35 eV, and one for $T > 420$ K of about 0.27 eV. It is suggested that this conduction occurs by small polaron hopping.