Metal-semiconductor contacts: electronic properties

Abstract

Rectification in metal-semiconductor contacts was first described by Braun in 1874. We owe the explanation of this observation to Schottky. He demonstrated that depletion layers exist on the semiconductor side of such interfaces. The current transport across such contacts is determined by their barrier heights, i.e., the respective energy difference between the Fermi level and the edge of the majority-carrier band. Since Schottky had published his pioneering work in 1938 the mechanisms, which determine the barrier heights of metal-semiconductor contacts, have remained under discussion. In 1947, Bardeen attributed the failure of the early Schottky-Mott rule to the neglect of electronic interface states. The foundations for a microscopic description of interface states in ideal Schottky contacts was laid by Heine in 1965. He demonstrated that a continuum of metal-induced gap states (MIGS), as they were called later, derives from the virtual gap states of the complex semiconductor band-structure. Neither this MIGS model nor any of the many other monocausal approaches, the most prominent is Spicer's Unified Defect Model, can explain the experimental data. In 1987, Mönch concluded that the continuum of MIG states represents the primary mechanism, which determines the barrier heights in ideal, i.e., intimate, abrupt, and homogeneous metal-semiconductor contacts. He attributed deviations from what is predicted by the MIGS model to other and then secondary mechanisms. In this respect, interface defects, structure-related interface dipoles. interface strain, interface compound formation, and interface intermixing, to name a few examples, were considered.