Solid Hydrogen in Hydrogenated Amorphous Silicon

Abstract

The thermal properties of hydrogenated amorphous Si at temperatures between 0.1 and 5 K are shown by calorimetry to be dominated by the presence of molecular hydrogen ${\mathrm{H}}_2$ in microvoids. A spontaneous release of heat due to conversion of $o$-${\mathrm{H}}_2$ to $p$-${\mathrm{H}}_2$ is observed from which the concentration (0.5 at.%) and bimolecular conversion rate (0.025 ${\mathrm{h}}^{-1\mathrm{}}$) are obtained. The specific heat is time dependent, varies roughly as $T^2$ between 0.1 and 3 K, and is attributed to a highly broadened orientational ordering transition of ${\mathrm{H}}_2$ in restricted geometry.