Two-Level Systems and Growth-Induced Thermodynamic Metastability in Hot-Wire Deposited Hydrogenated Amorphous Silicon

Hydrogenated amorphous silicon (a-Si:H) prepared by hot-wire chemical vapor deposition (HWCVD) shows a large specific heat C$_{\mathrm{P}}$ at low temperature T, despite low values of tunneling level states as measured by internal friction. C$_{\mathrm{P}}$ is significantly larger than the Debye specific heat calculated from the sound velocity, characteristic of glasses with two-level systems (TLS). The as-prepared films have an additional Schottky-like anomaly at low temperature that is associated with metastable hydrogen in the amorphous network. Annealing at 200 \textdegree C, well below the growth temperature, irreversibly reduces C$_{\mathrm{P}}$ by over an order of magnitude below 12 K, eliminating the Schottky-like anomaly. Based on the linear term in C$_{\mathrm{P}}$, the TLS density in this annealed state is orders of magnitude larger than expected based on internal friction Q$^{\mathrm{-1}}$ measurements, which are unchanged by the anneal. This large TLS density is suggested to result not from a local Si-H excitation, but instead from atomic scale regions best described as Si-H complexes in the a-Si network. Comparison of heat capacity to internal friction suggests that these TLS are decoupled from acoustic excitations.