Cleaved Silicon and Germanium {111} Surfaces

Abstract

Cleaved Si(111) and Ge(111) surfaces exhibit 2 × 1 reconstructions. Initially, a buckling of the top atom-layer was thought to explain the respective atomic rearrangement in the surface. However, this simple model had’to be discarded since the shifts experimentally observed with Si(2p) and Ge(3d) core levels were much smaller and the dangling-bond bands much wider than what was to be expected from calculations for buckled surfaces. Later on, the 2 × 1 reconstructions on Si and Ge(111) surfaces were proposed to consist of zigzag chains along a 〈110〉 direction which are joined to the underlying bulk by five-and seven-member rings while six-member rings are characteristic for the bulk of diamond-structure solids. The formation of such chains was proposed to proceed by a generation of stacking faults. Chains of surface atoms intuitively account for wider bands of dangling-bond surface states since the atoms are then more closely spaced. Additionally, the chains have to be tilted. This is again intuitively concluded from the surface core-level shifts experimentally observed and also resulted from evaluations of experimental I/W _P curves of LEED spots by using dynamical theories of LEED. Tilted chains as stable atomic arrangements on Si(111) surfaces were also obtained from static minimizations of the total energy as well as from a molecular-dynamics approach. The dispersion curves of the dangling-bond surface states on Si(111)2 × 1 surfaces and, what is even more important, the width of their band gap are excellently described by a many-body calculation which assumed tilted chains and considered exchange and correlation effects by using the GW approximation.