Analysis of the superhyperfine structure and thegtensor of defects in amorphous silicon

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Abstract
We point out that the integrated intensity of the EPR superhyperfine structure of the D center in a-Si should provide a definitive test of whether these defects are threefold or fivefold coordinated. We have performed a very precise fit of the D-center EPR spectrum to a powder pattern, incorporating both isotropic and anisotropic broadening but no hyperfine interactions. The difference between the fit and the data thus gives an indication of the hyperfine and superhyperfine contributions to the spectrum and allows us to place a lower limit on the number of atoms involved in the D center. Our limit corresponds to one secondary atom (in addition to the dominant one) and so is not able to distinguish the two cases. On the other hand, the powder pattern fit allows us to obtain, for the first time, the g tensor of the D center. We argue that the symmetry of the g tensor is similar to that of the vacancy in silicon and hence is more like a floating bond than a dangling bond.