THE ELECTRONIC-STRUCTURE OF A-SIGE ALLOYS - A CLUSTER SIMULATION

The electronic structure of 14 amorphous tetrahedral clusters, with 21 semiconducting atoms and 28 hydrogen saturators, of the type a-Si1-xG ex are studied using the pseudopotential SCF Hartree-Fock approximatio n. The Ge concentrations studied are 0, 5, 19, 38, 43, 57, 62, 81, 95 and 100%. For four concentrations (38, 43, 57 and 62%), two types of c lusters are studied, Si-like and Ge-like, to investigate the influence of the size of the cluster on the gap. The results obtained indicate a diminishing magnitude of the gap as the Ge concentration increases, and for concentrations between 38 and 62%, a symmetric behavior of the gap is found centered at 50%; the variation of the gap in this region is less than 9%. These results are compared to the experimental ones reported in the literature. The effect of hydrogen on the size of the gap has also been studied both for pure Si and Ge and it is found that the gap increases when four hydrogens are substituted for the central atom in an otherwise unaltered cluster, in agreement with the experim ent. The effect of dangling bonds is also reported. (C) 1998 Elsevier Science B.V. All rights reserved.