  The paper presents an $ab$ $initio$ study of temperature-induced
nanostructural evolution of hydrogen-rich voids in amorphous silicon. By using
large $a$-Si models, obtained from classical molecular-dynamics simulations,
with a realistic void-volume density of 0.2%, the dynamics of Si and H atoms on
the surface of the nanometer-size cavities were studied and their effects on
the shape and size of the voids were examined using first-principles
density-functional simulations. The results from $ab$ $initio$ calculations
were compared with those obtained from using the modified Stillinger-Weber
potential. The temperature-induced nanostructural evolution of the voids was
examined by analyzing the three-dimensional distribution of Si and H atoms
on/near void surfaces using the convex-hull approximation, and computing the
radius of gyration of the corresponding convex hulls. A comparison of the
results with those from the simulated values of the intensity in small-angle
X-ray scattering of $a$-Si/$a$-Si:H in the Guinier approximation is also
provided, along with a discussion on the dynamics of bonded and non-bonded
hydrogen in the vicinity of voids.
