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

Abstract

A detailed study has been performed concerning the temperature and electric-field dependence of hole carrier time-of-flight pulses in amorphous silicon. Various aspects of the experimental behaviour are employed in the estimation of the energy distribution and other characteristics of localized states in the films. Over the depth range 0·2 to 0·45eV, the trap concentration is found to vary more rapidly than the exponential form assumed in earlier studies, and a Gaussian tail is in better agreement with the data. Localized-state capture cross-sections are calculated as 1–3 × 10⁻¹⁶ cm², and are essentially independent of depth over the range studied.

Taking the present data in conjunction with the energy distribution of states as determined by other techniques allows an estimation of the mobility of free holes. The value of about 10cm²V⁻¹s⁻¹ thus obtained is an order of magnitude larger than the previous estimate from time-of-flight measurements, but is comparable to the figures suggested by other transport data.