Nonuniform surface potentials and their observation by surface sensitive techniques

Abstract

Various models of Schottky barrier formation have been proposed in the last few years which involve metallurgical interactions at the metal–semiconductor interface. Most of these models involve nonuniform lateral variations in the surface potential. For metallic clusters and/or anion clusters, these variations involve a relatively large size scale (tens to hundreds of angstroms). For interface defect formation, the suggestion of cluster formation energy as the driving force for defect formation could also lead to a nonuniform distribution of pinning sites on a similar size scale. We have studied the effects of various spatial distributions of pinning sites (e.g., surface defects, clusters of anions and/or adsorbed metal atoms) and variations of their energy levels upon surface potentials and their depth distribution via a two-dimensional finite difference program that integrates Poisson’s equation. Our results suggest that surface sensitive spectroscopies provide a less than exact measure of pinning levels in many such cases. For example, for 1018 n-GaAs and pinning site separation of 100 Å, our calculations imply a Kelvin probe band-bending result for the surface potential of ≊0.15 V less than the ‘‘pinning’’ value (for pinning values of 0.8 and 0.6). Furthermore, such nonuniform distributions alter the effect of probe depth. The surface potential determined via e.g., UPS, with a 20 Å mean free path, would differ from the ‘‘true’’ surface averaged value by ≊0.09 eV for a uniform 0.8 V surface; while this difference is ≊0.07 for 100 Å separations, and ≊0.05 for 400 Å separations, the ratio of this λ dependent term to the total measured band bending increases as the distance between pinning sites increases. Note that a pinning level 0.8 eV below the conduction band minimum leads (under the assumptions of 100 Å between sites, probe depth ≊20 Å, and 1018 cm−3 bulk doping) to a ‘‘measured’’ surface potential within 0.6 eV of the conduction band minimum. These results should impact the interpretation of surface dependent studies of Schottky barrier formation, especially for specific models of such formation.