Nonexponential distributions of tail states in hydrogenated amorphous silicon

The analysis of the time-of-flight (TOF) photocurrent transients that leads to the identification of exponential band tails in hydrogenated amorphous silicon (a-Si:H), also predicts an electric-field dependence of the carrier drift mobilities. To account for a-Si:H samples, prepared in different ways and deposition rates, that do not show this field dependence of the drift mobility, general analytic expressions for the trap-limited-band-transport TOF signals were examined with nonexponential tail-state distributions. It is found that the use of a Gaussian tail-state component makes it possible to match the experimental curves when a field-independent mobility is measured. For samples that are prepared at, or close to, the conditions used for the plasma-enhanced chemical vapor deposition of standard "device-quality" a-Si:H, a purely exponential distribution does afford a good description of the experimental data. However, for samples prepared at high deposition rates in an expanding thermal plasma, or for polymorphous silicon, a significant Gaussian component is resolved in the tail-state distribution. These components can be linked to the presence in the amorphous matrix of nanoscopic more ordered silicon regions.