Potential-barrier measurements at clustered metal-semiconductor interfaces

Photoemission spectroscopy is commonly used to study band bending at clustered metal-semiconductor interfaces. However, these band-bending data have been difficult to interpret because of the nonuniform distribution of pinning sites. In this paper the interpretation of these measurements is investigated in detail. Using three-dimensional Poisson integration programs, we examine the extent to which the band bending can be ascribed to states at the cluster-semiconductor interfaces. First, the potential variations in the semiconductor near an individual cluster are calculated. The depletion of the semiconductor along the surface is found to be significantly less than the one-dimensional depletion length. Next, the surface-potential variations are computed for systems of known cluster morphology, assuming that the pinning states are restricted to the cluster-semiconductor interfaces. From these potentials, the expected band-bending measurements are extracted and compared to experiment. At submonolayer coverages, we find that the experimental band bending is greater than can be attributed to pinning states under the clusters. Thus, this low-coverage band bending is interpreted in terms of surface states between the clusters. Defects, chemisorbed adatoms, and surface unbuckling are discussed as possible sources of these states. Above one monolayer coverage, the clusters cover enough of the surface to uniformly pin the Fermi level. Experimentally, however, a separation between the n- and p-type surface Fermi-level positions persists to the highest coverages. This separation is also interpreted in terms of intercluster surface states.