Energy-Level Spectra of Electrons at the (111), (110), and (100) Surfaces of Silicon and Germanium by Ion-Neutralization Spectroscopy

Abstract

Energy-level spectra obtained for the (111), (110), and (100) faces of Si and Ge by ion-neutralization spectroscopy (INS) bear only a superficial resemblance to the bulk density of states for these solids. In particular, the energies of small peaks in the INS-unfold function vary from one surface to another, and also differ from the energies of bulk critical points determined either by theory or by spectroscopies which more nearly yield bulk properties. A similar statement can be made for the width of the degenerate $p$ band, which is narrower at the surface as revealed by INS than it is in the bulk. Thus, the principal conclusion of this paper is that the local density of states (density of states weighted by local-wave-function magnitude) is significantly modulated as one proceeds from the bulk of a solid to its surface. The surface state observed for Si(111) by photoemission as overlapping the top of the valence band is seen in the INS spectra for both the Si(111)1 and Si(111)7 surfaces. The INS spectra for other Si surfaces and for all of the Ge surfaces studied do not show the kind of definitive evidence for a surface state seen in the Si(111) data although we cannot say that such surface states are not present with reduced intensity.