Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry

Abstract

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.

Reconstructing phonon mean-free-path contributions to thermal conductivity using nanoscale membranes

“…Figure 2(a) shows that the measured diffusivities of the membranes remain constant as a function of the grating spacing, which is indicative of diffusive thermal transport. Figure 2(b) shows the associated thermal conductivities as a function of thickness, compared to other experimental works [27,[34][35][36][37][38][39]. The plotted thermal conductivity for each membrane thickness is the average value calculated from all grating spacings.…”

“…The same calculation using the MFP distribution predicted by Holland [40] in place of that by Esfarjani et al overpredicts thermal conductivity for thicker membranes. Data from other thermal conductivity measurements on supported and unsupported Si thin films are shown for comparison in open symbols [27,[34][35][36][37][38][39]. The thermal diffusivity and conductivity values of bulk silicon are shown for reference in (a) and (b), respectively [31,41].…”

“…Both the signal decay time and the grating period can be measured with high accuracy, with no other parameters involved in the measurement; in particular, precise knowledge of the absorbed laser power and the magnitude of the temperature variation is not required, thus eliminating a major challenge to measuring thermal conductivity in nanoscale objects [26,27]. As no adsorbed metal heater layer is used and the samples are suspended, the measurements and analysis are free from uncertainties due to thermal interface resistance and heat loss to a substrate.…”

Characteristic length of phonon transport within periodic nanoporous thin films and two-dimensional materials

“…In real thin films, it should be noted that certain film-boundary roughness can introduce partially diffusive phonon scattering and thus lower k L at reduced film thickness. [40][41][42][43] When the film thickness is comparable or even smaller than the size of the porous structure, more accurate modeling may further consider the film-boundary scattering of phonons. 13 Assuming smooth film boundaries and rough pore edges, an effective K Pore is extracted by directly comparing the k L predicted by phonon MC simulations and the kinetic relationship.…”

Atomistic origin of the reduced lattice thermal conductivity of silicon nanotubes

“…[8][9][10] Thermal conductivity of Si nanostructures can be significantly influenced by surface roughness, crystalline orientation and diameter, which can modulate the thermal transport properties. [11][12][13][14][15][16] Therefore, the ultralow thermal conductivity of Si nanostructures as thermoelectric materials is highly desirable.…”