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.
The ubiquitous low-energy excitations, known as two-level tunneling systems (TLSs), are one of the universal phenomena of amorphous solids. Low temperature elastic measurements show that e-beam amorphous silicon (a-Si) contains a variable density of TLSs which diminishes as the growth temperature reaches 400 °C. Structural analyses show that these a-Si films become denser and more structurally ordered. We conclude that the enhanced surface energetics at a high growth temperature improved the amorphous structural network of e-beam a-Si and removed TLSs. This work obviates the role hydrogen was previously thought to play in removing TLSs in the hydrogenated form of a-Si and suggests it is possible to prepare "perfect" amorphous solids with "crystal-like" properties for applications.
A silicon nitride membrane-based nanocalorimeter is described for measuring the heat capacity of 30 nm films from 300 mK to 800 K and in high magnetic fields with absolute accuracy approximately 2%. The addenda heat capacity of the nanocalorimeter is less than 2 x 10(-7) J/K at room temperature and 2 x 10(-10) J/K at 2.3 K. This is more than ten times smaller than any existing calorimeter suitable for measuring thin films over this wide temperature range. The heat capacities of thin Cu and Au films are reported and agree with bulk values. The thermal conductivity of the thin low stress silicon nitride is substantially smaller than thicker membranes while the specific heat is enhanced below 20 K. Design of the nanocalorimeter will be discussed along with fabrication details and calibration results.
In e-beam evaporated amorphous silicon (a-Si), the densities of two-level systems (TLS), n 0 and P, determined from specific heat C and internal friction Q −1 measurements, respectively, have been shown to vary by over three orders of magnitude. Here we show that n 0 and P are proportional to each other with a constant of proportionality that is consistent with the measurement time dependence proposed by Black and Halperin and does not require the introduction of additional anomalous TLS. However, n 0 and P depend strongly on the atomic density of the film (n Si ) which depends on both film thickness and growth temperature suggesting that the a-Si structure is heterogeneous with nanovoids or other lower density regions forming in a dense amorphous network. A review of literature data shows that this atomic density dependence is not unique to a-Si. These findings suggest that TLS are not intrinsic to an amorphous network but require a heterogeneous structure to form.
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