Dangling Bond Defects in a-Si,Ge Alloys: A Theoretical Study Using the Tight-Binding Method

Abstract

This paper presents a theoretical study of Si and Ge atom dangling bond defects in a-Si,Ge alloys. We use a tight-binding Hamiltonian, and a structural model based on a cluster Bethe Lattice. The central cluster contains the Si or Ge atom with the dangling bond and all of the possible configurations of nearest neighbor Si and Ge atoms. These clusters are terminated by a Bethe Lattice consisting of virtual atoms having the average properties at the particular alloy composition. We employ self energies and interaction parameters previously used for calculations of the band structures of crystalline Si and Ge and formulated using an sp3s* basis set. We find that for a given set of Si and/or Ge nearest neighbors at each alloy composition, the Ge atom dangling bond state lies deeper in the gap than the corresponding Si atom dangling bond state. However, the spread in defect energies for a Si or Ge atom dangling bond with different nearest neighbors is greater than the separation between the statistically averaged Si and Ge dangling bond state energies. This makes it difficult to compare the results directly with experimental data. Nevertheless, the calculation suggests that the explanation for a difference in dark conductivity activation energies from about 0.8 eV to 0.65 eV in alloys with approximately equal Si and Ge atom concentrations, but grown by different deposition techniques, is related to a shift in Fermi level pinning from Ge to Si atom dangling bond states. The calculated spread in dangling bond energies and overlap between the manifold of Si and Ge states also explains the simultaneous occurrence of Si and Ge atom dangling bonds in a-Si,Ge:H alloys with Ge concentrations ranging to about sixty atomic percent.