THE IMAGE POTENTIAL IN SCANNING TUNNELING MICROSCOPY OF SEMICONDUCTOR SURFACES

A semiclassical screening theory for semiconductors is used to treat the image potential throughout a one-dimensional metal-vacuum-semiconductor junction in the absence of surface states. The image interaction due to induced surface charge at the semiconductor-vacuum interface produces an effective band bending in the semiconductor interior, whose influence on electron tunneling through the junction is investigated. In particular, we examine the effect of this extra potential on STM measurements of the apparent tunneling barrier at unpinned semiconductor surfaces, and consider n-type, H-terminated Si(111) as a specific example. In the instance where tip-induced band bending may be neglected, we find the role of the image potential in metal-vacuum semiconductor tunneling to be qualitatively similar to its role in metal-vacuum-metal tunneling: The experimentally determined barrier height-obtained from the logarithmic derivative of the tunneling current with respect to tip-sample separation-deviates little from the electron affinity of the semiconductor, while the theoretical barrier height exhibits a stronger dependence on the vacuum gap width. The origin of this behavior appears to lie in the fact that while the image interaction in the semiconductor may be long range, the image induced tunneling barrier is not.