  The structural relaxation of pure amorphous silicon (a-Si) and hydrogenated
amorphous silicon (a-Si:H) materials, that occurs during thermal annealing
experiments, has been analysed by Raman spectroscopy and differential scanning
calorimetry. Unlike a-Si, the heat evolved from a-Si:H cannot be explained by
relaxation of the Si-Si network strain, but it reveals a derelaxation of the
bond angle strain. Since the state of relaxation after annealing is very
similar for pure and hydrogenated materials, our results give strong
experimental support to the predicted configurational gap between a-Si and
crystalline silicon.
