The long-accepted belief that odd-membered rings in amorphous silicon cause the major differences between its valence-band density of states and that of crystalline silicon is overturned by new x-ray photoemission spectra of the two materials. Through application of a new theoretical approach for calculating the density of electronic states in amorphous semiconductors, we show that the changes actually observed are expected consequences of dihedral-angle disorder.
Our understanding of amorphous silicon (a-Si) rests on knowledge of the structural differences between crystalline (c-) and a-Si, and of the relationship between these differences and electronic properties. 1 The important structural differences between c-and a-Si entail correlations involving more than three atoms, the simplest of which are the dihedral-angle distribution 2 and the relative populations of short Nbond "rings ''3 where N differs from the crystalline value of 6. It is widely agreed that there must be a broad, if not yet precisely quantified, distribution of dihedral angles, but the a priori importance of five-and seven-member rings is much less certain because they are not essential in generating a continuous random network. The effects of ring statistics in the RDF are very subtle and it is generally held that the only direct evidence for odd-membered rings comes from differences between the x-ray photoemission (xPs) valence-band spectra of c-and a-Si. 4 In this paper, we report new xPs data which show that those differences previously attributed to odd-membered rings do not actually occur. Significant differences do, however, exist. We apply a new theoretical technique 5 which enables isolation of the effects of specific structural elements on the density of valence-band states, and show these differences to be just those expected as consequences of dihedral-angle disorder in the amorphous sample. If odd-membered rings are present in a-Si, they do not appear to have a significant effect on the xes valence-band spectrum.
The valence-band xPs spectra of c-Si(lll) and a-Si:H are shown in Fig. 1. These spectra were