ELECTRONIC-STRUCTURE OF CARBON-RICH A-SI1-XCXNY ALLOYS - THE ROLE OF HYDROGEN AND GRAPHITIC CARBON

We calculated the electronic structure of carbon-rich a-Si1-xCxHy, an alloy for which there is conflicting experimental evidence for the deg ree of disorder and the fraction of graphitic carbon atoms present. Ou r calculation takes into account valence disorder due to carbon and hy drogen as well as shore-range order effects. We deduce that, if the al loy is heavily hydrogenated, then Si-Si bonds exist in considerable am ount up to x = 0.75 at least. Furthermore the fraction of graphitic ca rbon atoms increases slowly up to the value of x at which a maximum in the band gap occurs and then it increases rapidly as some experiments seem to confirm. Our calculated band gap as a function of x is in rea sonable agreement with experiment and our peaks in the density of stat es (DOS) correlate well with the peaks of the photoemission spectra wh ose orbital character we can deduce. We note that our conclusion on th e presence of Si-Si bonds is independent of our method of calculating the DOS and derives from our matching of atoms to the corresponding bo nds. Finally estimates of the fracton of graphitic atoms at each x are made.