He Diffraction from Semiconductor Surfaces

Abstract

The outermost layers of many single crystal materials undergo reconstruction, i.e., the geometry at the surface differs from that expected for an ideal termination of the bulk. Most of these reconstructions result in a change in the size of the repeating surface unit cell and therefore can immediately be identified by the simple observation of the diffraction spot pattern in a low-energy electron diffraction (LEEP) apparatus. In particular the common semiconductors, which are primarily covalent materials with strong directional bonding, all undergo surface reconstruction with some faces exhibiting one or more metastable surface structures. These may be considered to be driven by the high potential energy of the half-filled orbitals generated when the bulk bonds are broken to make surface atoms. The resulting periodicities of these surfaces in many cases have been known from LEED for over twenty years. Despite numerous experimental and theoretical studies, the geometrical configurations of almost all these surfaces are still to be resolved. In Table 1 are listed a selection of the reconstructions observed for some low index faces of common semiconductors. Out of this list, the geometries of GaAs(110) and Si(111)2×1 are the only ones generally accepted as well determined.