Chapter 5 Surface States on Semiconductors

Abstract

This chapter discusses the different aspects of the surface states on semiconductors. A surface of a crystal breaks the three-dimensional bulk periodicity and there is an associated change in the electronic structure. The Bloch states of the interior of the crystal have to be matched to states decaying exponentially from the surface if the energy is below the vacuum level. The matching conditions can give rise to an increase or decrease in the surface local density of states for a certain electron energy. States with significantly increased amplitude at the surface, relative to the periodic bulk values, are called surface resonances. Surface states in the band gap of semiconductors will affect the position of the Fermi-level at the surface. It is found that for surfaces with a very high density of surface states in the gap, the Fermi-level can appear pinned at a certain position in the gap irrespective of the doping. It is observed that concurrent with the Fermi-level pinning there is a band bending in the near surface region that depends strongly on the doping.