Electronic transport in doped a-Ge and a-Si prepared by dc cathodic sputtering

Doped a-Ge and a-Si films are prepared by the cathodic sputtering technique with Sb and In impurities for a-Ge, P and B impurities for a-Si. The electrical conductivity σ and thermopower S of different samples are measured as a function of absolute temperature between 150 and 500 K. The effect of doping is to change not only the activation energies Esubσ and Esub(S) but also other parameters in the expression of σ and S. The considerable reduction in the pre-exponential term of σ is found to be much larger for a-Si than for a-Ge. The temperature shift of the Fermi level is taken into account to explain the variations of these parameters. The activation energy of conductivity is higher than that of thermopower by about 0.10 eV for a-Ge and 0.15 eV for a-Si. Experimental data analysis suggests that in both, a-Ge and a-Si, for high doping concentrations the hopping conduction through an impurity band makes a dominant contribution to the current flow below room temperature. (author)