No abstract

We report TOF measurements on high quality intrinsic a-Ge:H and a-SiGe:H films of E04=1.4eV in temperature ranges of 200 to 280 and 230 to 300K, respectively. Complete charge collection is achieved in all measurements. For a-Ge:H films, the (gt)e product obtained from the Hecht plot is (5±3)x10-8 cm 2 /V above 240K and decreases at lower temperatures. The electron transit signal is dispersive at all temperatures. The c obtained from tt,,V-'l/A is 0.23 at 200K and approaches 1.0 at 260K. The electron drift mobility l9d shows activated behavior, with an energy of 0.37±0.05eV, and has an extrapolated room temperature value of 0.03 cm 2 Ns. Compared to a-Ge:H, l9d of a-SiGe:H alloy samples is lower by one order of magnitude but has a similar activation energy. These results are consistent with a band tail hopping transport model.

The activated conductivity of a-Ge:H between room temperature and 460K was investigated using heating and cooling rates in the range between .001 and 0.1 K/s. A splitting of the cooling curves obtained at different rates, which defines the so called equilibrium temperature TE, is observed mainly between 420 and 430K. Taking into consideration that TE depends on the maximum cooling rate, the present results are in good agreement with those reported by Eberhardt et al. The higher cooling rate always leads to the lower conductivity at any temperature below TE. These effects can be rationalized in terms of a reversible shift of the Fermi level towards midgap at higher temperature. Though reversible changes of the mobility cannot be excluded, they cannot account for our set of experimental data. Rather, changes in the density of electronic states within the mobility gap can explain the effects observed.