The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and/or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility (∼ 100 cm 2 /Vs). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering.

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.

No abstract

We report shear modulus (G) and internal friction (Q(-1)) measurements of large-area monolayer graphene films grown by chemical vapor deposition on copper foil and transferred onto high-Q silicon mechanical oscillators. The shear modulus, extracted from a resonance frequency shift at 0.4 K where the apparatus is most sensitive, averages 280 GPa. This is five times larger than those of the multilayered graphene-based films measured previously. The internal friction is unmeasurable within the sensitivity of our experiment and thus bounded above by Q(-1) ≤ 3 × 10(-5), which is orders-of-magnitude smaller than that of multilayered graphene-based films. Neither annealing nor interface modification has a measurable effect on G or Q(-1). Our results on G are consistent with recent theoretical evaluations and simulations carried out in this work, showing that the shear restoring force transitions from interlayer to intralayer interactions as the film thickness approaches one monolayer.

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.

We show that the dominant energy loss mechanism in plate modes of a 1.5 μm thick silicon micromechanical resonator is thermoelastic damping. In situ ultra-high vacuum annealing lowers the dissipation of two neighboring resonance modes (460 and 510 kHz) at 120 K to Q−1≤5×10−7. From 120 to 400 K, the Q−1 of these modes increase at different rates, in quantitative agreement with a modification (that accounts for mode shape) of Zener’s theory of thermoelastic damping.

We report 3ω thermal conductivity measurements of amorphous and nanocrystalline silicon thin films from 85 to 300 K prepared by hot-wire chemical-vapor deposition, where the crystallinity of the films is controlled by the hydrogen dilution during growth. The thermal conductivity of the amorphous silicon film is in agreement with several previous reports of amorphous silicon prepared by a variety of deposition techniques. The thermal conductivity of the as-grown nanocrystalline silicon film is 70% higher and increases 35% more after an anneal at 600 • C. They all have similarly weak temperature dependence. Structural analysis shows that the as-grown nanocrystalline silicon is approximately 60% crystalline, nanograins and grain boundaries included. The nanograins, averaging 9.14 nm in diameter in the as-grown film, are embedded in an amorphous matrix. The grain size increases to 9.65 nm upon annealing, accompanied by the disappearance of the amorphous phase. We extend the models of grain boundary scattering of phonons with two different non-Debye dispersion relations to explain our result of nanocrystalline silicon, confirming the strong grain size dependence of heat transport for nanocrystalline materials. However, the similarity in thermal conductivity between amorphous and nanocrystalline silicon suggests the heat transport mechanisms in both structures may not be as dissimilar as we currently understand.