Intensity dependence of diffusion length in amorphous silicon by surface photovoltage measurements

Abstract

The dependence of minority carrier diffusion length Lp on light intensity in undoped amorphous silicon can be obtained by surface photovoltage (SPV) measurements alone. The sample is first illuminated with a light bias but unbiased electrically. Then it is voltage-biased electrically, but unbiased optically. By combining SPV measurements made under both conditions, and interpreting with the aid of a regional approximation theory, the effect of the surface space-charge region can be allowed for, and the intensity dependence of Lp revealed. In the equation for light dependence Lp∝I S, S was found experimentally to vary from −0.038 to −0.015, depending on the sample. This relatively weak dependence means that the main cause of the observed change in the apparent diffusion length as the bias light intensity increases can be ascribed to the collapse of the surface space-charge region. We also obtain estimates of the characteristic energy slopes of the density of states near the center of the gap. Values of 290–737 mV were found for a series of samples. This corresponds to an almost uniform distribution in energy within a few tenths of a volt on the valence-band side of the equilibrium Fermi level.