The specific heat of pure and hydrogenated amorphous silicon
- Author(s): Queen, Daniel Robert
- Advisor(s): Queen, Daniel R
- et al.
At low temperature, amorphous materials have low energy excitations that result in a heat capacity that is in excess of the Debye heat capacity calculated from the sound velocity. These excitations are ubiquitous to the glassy state and occur with roughly the same density for all glasses. The specific heat has a linear temperature dependence below 1K that has been described by the phenomenological two-level systems (TLS) model in addition to a T3 temperature dependence which is in excess of the T3 Debye specific heat. It is still unknown what exact mechanism gives rise to the TLS but it is assumed that groups of atoms have configurations that are close in energy and, at low temperature, these atoms can change configurations by tunneling through the energy barrier separating them. It has been an open question as to whether tetrahedrally bonded materials, like amorphous silicon, can support TLS due to the over-constrained nature of their bonding.
It is shown in this work that amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H) have specific heat CP in excess of the Debye specific heat which depends on the details of the growth process. There is a linear term that is due to TLS in addition to an excess T3 contribution. We find that the TLS density depends on number density of atoms in the a-Si film and that the presence of hydrogen in a-Si:H increases CP further. We suggest that regions of low density are sufficiently under-constrained to support tunneling between structural configurations at low temperature as described by the TLS model. The presence of H further lowers the energy barriers for the tunneling process resulting in an increase in TLS density in a-Si:H. The presence of H in a-Si:H network is found to be metastable. Annealing causes H to diffuse away from clustered regions which reduces the density of TLS. A low temperature anomaly is found in the a-Si:H films in their as prepared state that is of unknown origin but appears to take the form of a broadened Schottky anomaly. This feature is removed upon annealing. We find that there is a clear link between density of TLS and the excess T3 heat capacity. This correlation is not currently addressed by models of the glassy state.
Additionally, a reversible increase in the heat capacity is found upon light soaking in both a-Si and a-Si:H. This increase occurs over the entire measured temperature range 2-300K and can be removed by annealing at 200°C. The light soaked heat capacity at low temperatures has the form of the low energy excitations that are found in glasses. We suggest that the dangling bond defects responsible for the Staebler-Wronski effect are a consequence of the photo-induced structural states.