Understanding subtle changes in medium-range order in amorphous silicon.

Based on a detailed study of the radial distribution function (RDF) of a model for amorphous silicon (a-Si), we address the relation between short-range rearrangements and an increase in medium-range order induced by thermal relaxation. Recent experimental measurements have shown that a small peak appears in the RDF around 4.7 Å upon annealing, along with other subtle changes, and this is attributed to ordering among the dihedral angles. We show that, although this is a possible explanation, an increase in short-range order (up to second neighbors) is not only necessary for these changes to occur, but could also be their sole cause. To clarify the nature of disorder in the amorphous system, correlations among dihedral and tetrahedral angles are examined. The bivariate probability distribution of these two variables reveals small correlations between dihedral and tetrahedral angles, associated with the staggered and eclipsed conformations. In the first case, bond angles around 112.5[Formula: see text] are favored versus 120[Formula: see text] in the second case. Bond angles between 95[Formula: see text] and 100[Formula: see text] are less probable in both conformations.