Excess low temperature heat capacity in a-Si:H thin films prepared by hot-wire CVD

Thin films of $a$-Si:H prepared by hot-wire chemical vapor deposition (HWCVD) have previously been found to have orders magnitude lower density of two-level systems (TLS) as measured by internal friction. These TLS are thought to be responsible for the low temperature universality seen in thermodynamic measurements in all amorphous materials. We present heat capacity measurements using our MEMS nanocalorimeter on a range of $a$-Si and $a-$Si:H films prepared by e-beam evaporation and HWCVD. The tunneling model predicts that a reduction in the TLS should result in Debye-like heat capacity. However, we find that the low temperature heat capacity of $a$-Si:H prepared by HWCVD is two orders of magnitude larger than crystalline silicon at 2K.