Localized states in amorphous Si and Si,Ge alloys

Abstract

This paper discusses applications of the tight‐binding method to: i) localized anti‐bonding states of Si‐H groups in a‐Si:H, and ii) dangling bond defect states in a‐Si,Ge:H alloys. The a‐Si:H calculations demonstrate that anti‐bonding states of Si‐H groups with bond‐angle distortions of ∼16°–20° or more can be localized below the conduction band edge, and display the trapping properties of floating‐bond defect states. The calculations for the a‐Si,Ge:H alloys show that the average‐alloy bond‐angle disorder, ∼6–8°, can induce a spread in the energies of Si and Ge dangling bond states of ∼0.3 eV, that is larger than that due to chemically‐induced splittings, 0.11–0.15 eV, which include the effects of different distributions of nearest‐neighbor Si and/or Ge‐atoms. The bond‐angle disorder induced dispersion leads to a spectral overlap of the Ge and Si dangling bonds, so that both types of defects can be observed in alloy samples.