A new approach to the interpretation of transport results in a-Si

Abstract

The paper deals with two related problems which in the past have led to major difficulties in the interpretation of transport results for a-Si. The first concerns the temperature shifts of reference energies with respect to the state distribution, the second the comparison of transport results from different specimens. In the analysis we take into account the temperature shift of both the Fermi energy and the mobility edge, ε_c(T), with respect to a common reference point at ε_A, the onset of the tail states. A simple model calculation indicates that the electron overlap energy increases with T; thus to satisfy the localization criterion, ε_C(T), will have to move towards ε_A with increasing T. The temperature coefficient for ε_c(T) is shown to be approximately proportional to ε_c(0) — ε_a, the extension of the tail states at T = 0. This introduces an important new parameter into the model, which is likely to be sensitively dependent on preparation conditions. The above analysis has been applied in this paper to the extensive experimental results available for undoped glow-discharge a-Si. It is shown that the large observed range of the conductivity pre-exponential factor σ₀ reflects the distribution in ε _c (0)— ε _A values; the new approach also removes the problems in the energy normalization of field effect data. A critical test is the prediction that the conductivity activation energy at high T should show a discontinuity at which it decreases by ε _c (0)— ε _A This is fully confirmed by experimental results in this and other laboratories. Preliminary measurements on the. photoconductive threshold as a function of T are consistent with other data for the shift of ε_c(T). Finally the implications for the interpretation of thermoelectric power and drift mobility results are briefly discussed.