Electronic properties of a realistic model of amorphous silicon

Abstract

We present the results of a first-principles calculation of the electronic density of states for a large, realistic model of amorphous silicon (a-Si). The structural basis of the calculation is a 216-atom, fully-fourfold-coordinated model of a-Si with periodic boundary conditions, whose pair-correlation function, average geometric distortions, and vibrational properties are in good agreement with experiment. The calculation is carried out by using the recursion method in conjunction with the tight-binding linear muffin-tin orbitals scheme in its simplest form. To assess the accuracy of our results, we apply the method to a crystalline silicon cluster and obtain a density of states similar to that obtained with the conventional Brillouin-zone summation method. The valence-band density of states calculated for the a-Si model shows good overall agreement with that measured for a-Si by x-ray photoelectron spectroscopy. The Si 3p valence-band peak is found to shift by 0.5 eV to lower binding energy compared with its position in the crystal, and the Si 3s and ‘‘s-p’’ hybrid bands of the crystal merge into one broad band in the amorphous model. We examine the local density of states of several atoms in the center of the cluster and the charge content of the corresponding atomic spheres to determine the effects of geometric distortion on the density of states and the degree of local distortion-induced static charge transfer. The rms static charge deviation is 0.14e, which is in good agreement with the experimental value of 0.11e. We find that neither particular features in the local densities of states nor the charge deviations from the mean of atoms in the center of the a-Si unit cell are correlated with the surrounding local geometric or topological distortions. Sharp peaks in the spectral functions of the a-Si model indicate remnants of the E-k dispersion relations for both s- and p-like states near k≊0. The peaks broaden for larger values of k, in agreement with the dispersion behavior in disordered semiconductors suggested by Ziman.