Excess specific heat in evaporated amorphous silicon

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_Si, and Raman spectra. Increasing T_S results in a more ordered amorphous network with increases in n_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) T3 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 n₀ and the excess T3 specific heat c_ex suggesting that they have a common origin.