Solid H2 in a-Si:H at low temperatures.

Molecular hydrogen in a-Si:H has been detected by the heat released during orthopara conversion. Calorimetric measurements in the temperature range 0.05-5K indicate as much as 0.5 at. % H2 trapped in microvoids at least 10angstroms in diameter. The specific heat is dominated by a broad peak near 3.5 K due to short-range orientational order of the ortho-H2 within each cluster. No sharp order-disorder transition occurs, which would signal the onset of long-range order as in bulk solid H2. The absence of long-range orientational order is attributed to the small size of the clusters and to positional disorder within each cluster. The concentration of H2 can be maximized by a short anneal at 500C, during which Si-H bonds are broken and H diffuses to the microvoids.