Impeded thermal transport in Si multiscale hierarchical architectures with phononic crystal nanostructures

In-plane thermal conduction and phonon transport in both single-crystalline and polycrystalline Si two-dimensional phononic crystal (PnC) nanostructures were investigated at room temperature. The impact of phononic patterning on thermal conductivity was larger in polycrystalline Si PnCs than in single-crystalline Si PnCs. The difference in the impact is attributed to the difference in the thermal phonon mean free path (MFP) distribution induced by grain boundary scattering in the two materials. Grain size analysis and numerical simulation using the Monte Carlo technique indicate that grain boundaries and phononic patterning are efficient phonon scattering mechanisms for different MFP length scales. This multiscale phonon scattering structure covers a large part of the broad distribution of thermal phonon MFPs and thus efficiently reduces thermal conduction.

Figure 1

SEM images of (a) the whole suspended Si PnC nanostructure and (b) a 2D PnC nanostructure with circular holes aligned as a square lattice ($a=300\mathrm{nm}$ and $r=115\mathrm{nm}$).

Figure 2

(a) Schematic of the μ-TDTR technique and cross section of the suspended structure. (b) Simulated temperature distribution at 15 μs after pulse heating a PnC structure with $a=300\mathrm{nm}$ and $r=115\mathrm{nm}$.

Figure 3

TDTR signals for the unpatterned membrane and the single-crystalline Si PnC nanostructures ($r=73,115,\mathrm{and}126\mathrm{nm}$). The dots and solid lines represent experimental and simulated TDTR signals, respectively.

Figure 4

Measured thermal conductivities for a variety of radii in single- (red circles) and polycrystalline (blue circles) Si PnC nanostructures.

Figure 5

TEM images of (a) top and (b) cross-sectional views for the poly-Si wafer. (c) Histogram of the grain size measured on the surface. The grain size at the bottom part is much smaller due to the conical shape of the grains.

Figure 6

(a) Radius dependence of the measured (red circles) and simulated (orange squares) thermal conductivities. (b) Calculated cumulative thermal conductivities for bulk Si and for single-crystalline Si PnC nanostructures with various radii. The dashed vertical lines are the neck sizes for the PnC nanostructures plotted by the same type of colors.