Electron and hole transport in compensated amorphous silicon

Abstract

A detailed investigation is presented of electron and hole drift mobilities, μ_e and μ_h , in compensated a-Si over a wide range of the compensated doping ratio, D_c (0.3 vol.p.p.m. to 5000 vol. p.p.m.). μ_e does not show anomalous dispersion and the interpretation of electron results leads to three distinct transport mechanisms. At low D_c , the multi-trapping model applies and μ_e , is determined by the potential fluctuations associated with the randomly distributed ionized dopant sites. Experimental results are in good agreement with predicted fluctuation amplitudes. With increasing D_c , room-temperature electron transport changes to phonon-assisted hopping, at first near the bottom of the tail states and then through the donor states. μ_h shows anomalous dispersion and is controlled by multi-trapping interaction with the tail states at all D_c The dispersion parameter has been investigated as a function of D_c , suggesting that the tail-state distribution is unaffected by compensation. Finally, it is shown experimentally that the distribution of dopant sites is strongly dependent on the deposition parameters and that this can account for the fundamental disagreement in both experimental results and interpretation between the Xerox and Dundee work on compensated a-Si.