Thermal properties of a-Si:H at low temperatures.

The electrical and optical properties of hydrogenated amorphous silicon have been studied extensively because of the passivating effect of H on the intrinsic defects in a-Si. The thermal properties, on the other hand, have received very little attention, particularly at temperatures below a few Kelvins. We report a quasi-linear specific heat and a T(2) thermal conductivity below 1K which indicate a density of two-level tunneling systems (TLS) very similar to that in fused silica. We have also observed a release of heat from the sample at low temperatures which depends on the thermal history on cooling from 300K. It is attributed to ortho-para conversion of molecular hydrogen within the sample.