Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering

Abstract

A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively.

Thermal transport size effects in silicon membranes featuring nanopillars as local resonators

“…We observe, expectedly, k Memb to drop significantly with decreasing d due to dispersion modification as well as the reduction of phonon mean free paths (MFP) caused by diffuse boundary scattering at the surfaces as a result of their reconstruction at the equilibrium state [23]. These effects been studied experimentally in the literature in the context of a thin silicon layer on a substrate [24], freestanding silicon membranes [12,25], and also silicon nanowires [26]. The k NPM curves are observed to similarly increase with increasing unit-cell size (due to increasing thickness) until gradual saturation.…”

“…A recent molecular dynamics study examined a wider range of geometric dimensions on the same pillared silicon membrane configuration and demonstrated a reduction in the thermal conductivity by roughly a factor of 3 [11]. The resonance-hybridization mechanism described above may be used in conjunction with boundary scattering from rough surfaces to lower k [12,13].…”

“…A recent molecular dynamics study examined a wider range of geometric dimensions on the same pillared silicon membrane configuration and demonstrated a reduction in the thermal conductivity by roughly a factor of 3 [11]. The resonance-hybridization mechanism described above may be used in conjunction with boundary scattering from rough surfaces to lower k [12,13].For the concept of an NPM to be realized in practice, the nanostructured material's characteristic length scales, such as the membrane thickness and the nanopillar height and cross-sectional area, need to be at least on the order of a few tens of nanometers−which is substantially larger than the length scales considered in the previous studies mentioned above. In this Letter, we study the effect of increasing size, in different ways, on the performance of an NPM.…”

Phonon and heat transport control using pillar-based phononic crystals

“…Xiong et al [32] demonstrated that nanopillars not only generate local resonances but also scatter the phonons and reduce their mean free path by one order of magnitude. Similarly, Neogi et al [61] concluded that the reduction in thermal conductivity is mainly caused by the diffuse scattering of phonons on nanopillars whereas local resonances play only a minor role. Recently, Honarvar and Hussein [34] too found that nanopillars shorten both phonon mean free path and lifetimes by the diffuse scattering, but argued that local resonances still play the leading role in the thermal conductivity reduction.…”

“…Thus, the observed reduction in thermal conductivity was attributed to the surface roughness and the amorphous layer under the pillars. Indeed, amorphous layers at the surfaces are known to reduce the thermal conductivity of nanostructures [61,64–66] and can diffusely scatter phonons stronger than just surface roughness [67]. …”