Surfaces of Homopolar Amorphous Semiconductors: Definition, Characterization, and Density of Surface States

To rigorously investigate the contribution of surfaces to the density of electronic states of a-Si (Ge), and the effect of the topology on the density of surface states, a surface for amorphous homopolar tetrahedral solids is defined. Continuous random network models are statistically analyzed for homogeneity. Various possible surfaces generated from these models are examined with the result that a spherical surface is found to be most representative of a surface from a homogeneous infinite amorphous solid. The homopolar amorphous surface is characterized by a wealth of steric and dangling bond configurations. Surface atoms can have one, two, or three dangling bonds, and can have from zero to three nearest neighbor surface atoms. The density of dangling bonds is 0.106 bonds/(ANGSTROM)('2). Reconstruction enables a 96% reduction in the density of dangling bonds. The ring structure of the surface atoms is significantly different from the ring structure of the bulk atoms. The topological effects on the density of surface states is exhaustively treated using a s-band Hamiltonian. Rings of different sizes uniquely contribute to the density of states. Other topological properties, such as multiple dangling bonds per surface atom and near neighbor surface atoms are treated. The effects on the density of states by the surface in the valence band and energy gap of a tetrahedral solid is investigated using a two parameter Hamiltonian. The local and configuration averaged density of states are computed for the dangling bond and four back bond hybrids. The ring structure affects the density of surface states in the valence band, but not the more localized energy gap states. The antibonding spectral feature in the energy gap deriving from surface atoms with two or three dangling bonds is independent of all topological effects, while the bonding spectral feature from these same surface atoms is not. The spectral feature due to surface atoms with only one dangling bond is also affected by the topology. Comparison with empirical results verifies the contribution of the dangling bond to ESR signals, and elucidates the origin of the subtle valence band features in the UPS spectra.