Calculation of the densities of states of a-Si 1 − yC y(:H) alloys and comparison with measured X-ray spectra

Abstract

A simplified model based on the Tight-Binding Coherent-Potential Approximation method that does not account for topological disorder, is employed to calculate the s and p-symmetry densities of states of a-Si 1 − y C y (:H) alloys and crystalline SiC. Valence and conduction partial Si and C densities of states are obtained which allow us to observe that sp 3 hybridization is existing in these compounds. The calculations also show that in a-Si 1 − y C y alloys, the total band gap increases by about 0.7 eV when going from y=0.1 to y=0.287 due to a progressive shift of both valence and conduction edges below and above the top of the valence band, respectively. For crystalline SiC, the gap increases by about 1.5 eV with respect for example to a-Si 0.9C 0.1. In hydrogenated alloys, the calculations display a steepening of the valence and conduction band edges as compared to non-hydrogenated alloys confirming thus the `healing' effect with H incorporation. The soft X-ray Si Kβ emission spectra are simulated by convolution of the occupied Si 3p distribution with appropriate Lorentzian and Gaussian functions which account for the lifetime of the inner level involved in the X-ray transition and the instrumental function, respectively. The calculated spectra are in good agreement with experimental results on samples of similar nominal composition despite the fact that the theoretical model does not account for topological disorder.