TUNNELING SPECTROSCOPY ON SEMICONDUCTORS WITH A LOW SURFACE-STATE DENSITY

A detailed study of tunneling spectroscopy concerning semiconductors w ith a low surface state density is presented. For this purpose, I-V cu rves under dark conditions and under illumination were measured on the (0001) van der Waals surface of a p-type WS2 single crystal, which is known to be free of intrinsic surface states. The measurements are in terpreted by an analytical one-dimensional metal-insulator-semiconduct or model, which shows that the presence of the finite tunneling curren t has to be considered in the calculation of the tip-induced bandbendi ng. Rectification of the dark I-V curves is explained by the absence o f an inversion layer at the semiconductor surface. In contrast, the I- V curves measured for different light intensities and tip-sample separ ations indicate the existence of an optically induced inversion layer. Since no surface recombination needs to be considered to model these spectra, we conclude that bulk recombination, diffusion and direct tun neling of photogenerated minority charge carriers are the dominant pro cesses for semiconductors with a low density of surface states. In con trast to the standard interpretation of tunneling spectroscopy, which can be applied to semiconductors with a high surface state density, ou r results clearly show that in this case the normalized differential c onductivity (dl/dU)/(I/U) cannot be used to determine the energetic di stribution of the local surface state density. (C) 1997 American Vacuu m Society.