SolidH2ina-Si:H at low temperatures

Molecular hydrogen in a-Si:H has been detected by the heat released during ortho-para conversion. Calorimetric measurements in the temperature range 0.05–5 K indicate as much as 0.5 mol % ${\mathrm{H}}_2$ trapped in microvoids at least 10 Å in diameter. The specific heat is dominated by a broad peak near 3.5 K due to short-range orientational order of the ortho-${\mathrm{H}}_2$ within each cluster. No sharp order-disorder transition occurs, which would signal the onset of long-range order as in bulk solid ${\mathrm{H}}_2$. 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 ${\mathrm{H}}_2$ can be maximized by a short anneal at 500 °C, during which Si–H bonds are broken and H diffuses to the microvoids.