Intense small wave-vector scattering from voids in amorphous silicon: A theoretical simulation

Voids of various sizes have been introduced into amorphous-silicon models that were generated with molecular-dynamics simulations. The presence of the voids leads to a rapidly increasing structure factor for wave vectors below 1 A${̊}^{\mathrm{-}1}$ that is similar to the intense small-angle scattering observed in experiments. The voids cause only small changes in the vibrational densities of states. The presence of voids decreases the local strain in the a-Si networks, leading to a substantial reduction in the number of five-coordinated defect sites and a somewhat lower bond-angle strain from the a-Si model without voids. By comparing the properties of voids in crystalline and amorphous structures, we find the effect of decreasing the bond-angle disorder is to make the TO peak of the densities of states narrower and stronger, in agreement with Raman measurements.