Tuning the Thermal Conductivity of Silicon Phononic Crystals via Defect Motifs: Implications for Thermoelectric Devices and Photovoltaics

High Resolution Image Download MS PowerPoint Slide Phononic crystals are materials with a periodic arrangement of modifications that can be tailored to control their thermal conductivity. Here, we consider thin silicon membranes and structures with holes of different sizes and shapes, forming phononic crystals with different defect motifs. The introduction of intermediate-sized pores in silicon membranes can reduce their lattice thermal conductivity and increase their thermoelectric efficiency as long as the pores are not too small to interfere with electron transport nor too large to cause structural instabilities in the material. We investigate the heat transport properties of pristine and defective membranes, along the [110] and [1̅10] directions, with the homogeneous nonequilibrium molecular dynamics method to determine the thermal conductivity as well as its spectral decomposition. We find that for the hole sizes considered, the thermal conductivity is a logarithmically decreasing function of the defect area, independent of defect shape and transport direction. We also verify that the thermal conductivity of silicon phononic crystals is ultimately limited by the neck size, which is the smallest distance between two adjacent defects, in agreement with the literature. However, our results also show that the dependence of the conductivity with neck size follows a power law along the [110] direction but shows an exponential dependence along the [1̅10] direction. We attribute this difference to the scattering of phonons by surface dimers which originate in the 2 × 1 reconstruction of the silicon [001] surface and are oriented along the [1̅10], acting as resonators that can scatter phonons more efficiently along that direction. Our findings are relevant for the design of thermoelectric devices and thermal barriers in photovoltaic cells.