  We present a computational study of small-angle X-ray scattering (SAXS) in
amorphous silicon ($a$-Si) with particular emphasis on the morphology and
microstructure of voids. The relationship between the scattering intensity in
SAXS and the three-dimensional structure of nanoscale inhomogeneities or voids
is addressed by generating ultra-large high-quality $a$-Si networks with
0.1-0.3% volume concentration of voids, as observed in experiments using SAXS
and positron annihilation spectroscopy. A systematic study of the variation of
the scattering intensity in the small-angle scattering region with the size,
shape, number density, and the spatial distribution of the voids in the
networks is presented. Our results suggest that the scattering intensity in the
small-angle region is particularly sensitive to the size and the total
volume-fraction of the voids, but the effect of the geometry or shape of the
voids is less pronounced in the intensity profiles. A comparison of the average
size of the voids obtained from the simulated values of the intensity, using
the Guinier approximation and Kratky plots, with those from the spatial
distribution of the atoms in the vicinity of void surfaces is presented.
