Thermal-equilibrium processes and electronic transport in undoped hydrogenated amorphous silicon

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Abstract
The temperature and time dependence of the d.c. conductivity of undoped hydrogenated amorphous silicon is presented. Measurements of the electronic transport are reported, with particular emphasis on the effects of annealing and cooling the samples. Two regimes of behaviour are observed. When samples are rapidly cooled from 200°C below a temperature T E∼145°C a non-equilibrium dark conductivity, higher than that corresponding to slow cooling, is observed. The electronic and atomic structure then slowly relax and the time dependence of the excess conductivity is well described by a stretched exponential function. The second regime above T E corresponds to a relaxation time short compared to experimental times and the conductivity is independent of which order the annealing temperature is chosen. Thus the thermal equilibrium processes observed in undoped samples are qualitatively very similar to those observed in doped samples as recently reported in the literature.