Hole carrier drift-mobility measurements in a-Si: H, and the shape of the valence-band tail

Electron Transport in a-Si:H at Low Temperatures

Time-of-flight and charge collection experiments performed on a-Si:H p-i-n structures in the temperature range 40–300K reveal electron drift mobilities which continuously decrease with decreasing temperature and electric field. At 40K, fields ≥ 2×105 V/cm are necessary to collect almost all carriers. A discussion of the data on the basis of a nearest neighbour hopping model shows the significant influence of the applied field on the hopping down process of electrons. Field-induced re-emission of deep trapped carriers back into shallow tail states establishes a narrow band of energy levels where electrons accumulate and propagate. The band width and energy position of the accumulation levels in the tail depends strongly on the applied field and the distribution of localized states. With increasing electric field, it shifts up in energy closer towards the conduction band edge and changes the hopping mobility for orders of magnitude. The calculated and experimental data are in reasonable agreement with the model of a hybrid distribution of localized states.