Using a combination of positron annihilation and x-ray-diffraction techniques, we have shown that low hydrogen concentration hot wire chemical vapor deposition grown a− S i: H forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. On annealing up to 400 C, the amorphous network is seen to relax and the first stages of recrystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low activation energy, around 0.1 eV, and is probably caused by a reconfiguration of hydrogen-terminated dangling, bond defects. The formation of microvoids and the recrystallization can both be interpreted by the migration of unterminated dangling-bond defects.