Solid hydrogen in hydrogenated amorphous silicon.

Several orientationally ordered phases have been observed in bulk samples of solid H(2) at low temperatures. The effects of finite sample size and degree of crystallinity on this very interesting type of ordering have not been addressed so far. In the present report we draw attention to an unsuspected candidate for such studies--hydrogenated amorphous silicon. Calorimetric measurements at low temperatures (0.1-5K) indicate the existence of ~1 at.% molecular hydrogen trapped in the microvoids of a-Si:H.