The specific heat $C$ of $e$-beam evaporated amorphous silicon ($a$-Si) thin films prepared at various growth temperatures $T_S$ and thicknesses $t$ was measured from 2 to 300 K, along with sound velocity $v$, shear modulus $G$, density $n_{\mathrm{Si}}$, and Raman spectra. Increasing $T_S$ results in a more ordered amorphous network with increases in $n_{\mathrm{Si}}$, $v$, $G$, and a decrease in bond angle disorder. Below 20 K, an excess $C$ is seen in films with less than full density where it is typical of an amorphous solid, with both a linear term characteristic of two-level systems (TLS) and an additional (non-Debye) $T^3$ contribution. The excess $C$ is found to be independent of the elastic properties but to depend strongly on density. The density dependence suggests that low energy glassy excitations can form in $a$-Si but only in microvoids or low density regions and are not intrinsic to the amorphous silicon network. A correlation is found between the density of TLS $n_0$ and the excess $T^3$ specific heat $c_{\mathrm{ex}}$ suggesting that they have a common origin.

• Received 3 May 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.135901