Densities of valence states of amorphous and crystalline III-V and II-VI semiconductors

The densities of valence states (DOVS) of the amorphous and crystalline forms of GaP, GaAs, GaSb, InP, InAs, InSb, AlSb, ZnTe, and CdTe have been determined from the energy-distribution spectra of photoelectrons emitted by high-energy photons (16. 9, 21. 2, 40. 8, and 1486. 6 eV). In general the DOVS of the amorphous forms can be represented by a broadened version of those of the corresponding crystalline forms. Fine structure which appears in the upper valence bands of the crystalline materials, due to critical points at L, X, and W, is completely washed out in the amorphous phase. The core-level spectra have nearly the same positions and widths in the amorphous as in the crystalline modifications. This fact indicates that the fluctuations in the Coulombic environment about each type of atom are small, suggesting that the structure is homogeneous and contains an insignificant number of odd-membered rings. The plasma frequencies, determined from the plasma-loss spectra associated with core levels, are the same in the amorphous as in the crystalline phases to within 3%. This fact enables us to conclude that the densities of both modification differ by less than 6%. We present a simple bond-charge model which can simulate realistically the density of valence states of germanium and zinc-blende-type semiconductors. The valence bands at any point of the Brillouin zone are obtained in this model as the solution of a 44 secular equation. Within this model, the structure of the top p-like valence bands depends primarily on overlap between second-neighbor bonds. Thus fluctuations in the position of second neighbors can be invoked to explain the smearing of the fine structure of these bands in the amorphous modifications. A simple model which relates the chemical shifts of the compounds to their ionicity is also discussed.