The electronic and atomic structure of hydrogenated amorphous Si—C alloys

Abstract

The electronic structure of amorphous Si—C (a-Si_1-xC_x) and hydrogenated amorphous (a-Si_1-x C_x: H) alloys has been calculated as a function of composition, chemical ordering, C coordination and H configuration using the tight-binding method. The maximum in the optical bandgap observed in a-Si_1-x C_x: H at around x = 0·6 is not due to chemical ordering but to a change in band edge character, from Si-Si bond states for x < 0·6 to C sp² π states for x > 0·6. The gap is controlled by the degree of clustering of sp² sites for x > 0·6. Interpretation of the optical gap, photoemission and X-ray emission data suggests that a moderate degree of chemical ordering exists in a-Si_1-xC_x: H and is higher in a-Si_1-xC_x. Hydrogenation widens the gap over the entire composition range, in Si-rich alloys by a recession of the valence band and in C-rich alloys by reducing the cluster sizes. The position of defect states due to Si and C dangling bonds is calculated and combined into a band model for the alloys. Experimental defect concentrations are found to depend on the width of the valence band tail and the bandgap for Si-rich alloys, consistent with the weak-bond-to-dangling-bond conversion model.