The temperature-dependent defect density of a-Si:H calculated from thermally activated conductivity

The thermally activated conductivity of undoped a-Si:H films, prepared by glow discharge and magnetron sputtering, was measured by cooling samples down from 453 K to 306 K at a rate of 0.3 K min-1. This resulted in systematic and reproducible deviations from straight lines in the ln sigma versus 1/T plot, and from these data the Fermi level shift was determined. By means of a density of states model adapted to each sample, combining electrical, optical and ESR measurements, the temperature-dependent defect density was calculated from the charge conservation law. The results are explained by a non-monotonic temperature-dependent defect density below the equilibrium temperature in undoped a-Si:H. The thermally induced changes increase with increasing hydrogen content of the samples. Films of identical hydrogen content show similar behaviour, independent of the preparation technique.