The steady-state and transient photoconductivity of sputtered hydrogenated amorphous Ge ($T_s=300\mathrm{}$ K) was studied as a function of wavelength, light intensity, and temperature. The reduction of low-temperature dark conductivity and the shift to higher energies of the photoconductivity edge with hydrogenation are consistent with the notion that hydrogen eliminates states from the pseudogap. The magnitude and the temperature dependence of the drift mobility between 77 and 300 K was inferred from the measurements. Above about 200 K the mobility had an activation energy of 0.1 eV, while below 200 K it was only weakly $T$ dependent. This variation with $T$ is theoretically predicted from small-polaron theory, viz., hopping with an activation energy of half the polaron binding energy above $T_0=\frac{\hslash {\omega }_0}{2k}$, where $\hslash {\omega }_0$ is the optical phonon energy, and hopping at low $T$ with an activation energy equal to the disorder energy. From these data the disorder energy is estimated to be of the order of 0.01 eV. To account for a weak dependence of the lifetime on temperature and on hydrogenation, we propose a model in which the photocarriers from small polarons which degrade in energy while they are moving in the applied field.

• Received 24 March 1977

DOI:https://doi.org/10.1103/PhysRevB.16.1564