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) 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(ex) suggesting that they have a common origin. DOI: 10.1103/PhysRevLett.110.135901