Study of the interface electronic structure of a model metal-semiconductor interface

Abstract

We present a study of the interface electronic structure of a model simple-cubic metal and a compound semiconductor of the cesium-chloride structure. Both the metal and the semiconductor are described in terms of nearest-neighbor tight-binding models which allow closed-form analytical results for the surface (i.e., solid-vacuum interface) Green's functions of the individual semi-infinite solids. We specifically consider the interface created by coupling the (100) surfaces of the metal and the semiconductor, for a fourfold atomic coordination, via an appropriate metal-semiconductor coupling matrix. The interface Green's function matrix is obtained in terms of the metal and semiconductor surface Green's functions and the metal-semiconductor coupling matrix by solving the appropriate Dyson equation. Results for the dispersion curves of localized interface states, their density, and the charge distribution are presented for (i) different metal-semiconductor coupling strengths keeping the position of the Fermi energy in the semiconductor band gap fixed and (ii) fixed metal-semiconductor coupling strength, but different self-consistent positioning of the Fermi energy in the semiconductor band gap. On-site Coulomb repulsion on the surface metal and semiconductor atoms is taken into account and the position of the Fermi energy in the semiconductor band gap self-consistently determined by ensuring overall charge neutrality via the Friedel sum rule. The results show consistent trends and correlations between the character of the interface states and their density and the metal-semiconductor coupling strength. Similarly, correlations with the position of the Fermi energy are also found. In particular, it is found that the interface states account for approximately two thirds of the bonding charge (i.e., the charge shared by the metal and semiconductor atoms at the interface). The other one third of the charge is found to be associated with bulklike continuum states. This suggests that care must be exercised in drawing inferences from cluster or finite-layer slab calculations of metal-semiconductor interfaces, since the size of the cluster or number of layers required to reproduce the bulk-like continuum states is not known a priori.