Universal heat flow behavior in highly-confined semiconductor nanosystems

Nanostructured semiconductors can exhibit thermal properties unachievable in bulk systems, and will play a crucial role in next-generation nanoelectronics and energy efficient devices. However, first principles models are too computationally challenging for 3D nanostructured geometries, while mechanistic approaches make overly-simplistic assumptions about the nature of phonon-boundary interactions. Here, we study heat flow in a 3D nanostructured silicon metalattice, using an infrared pump laser to excite the sample and an extreme ultraviolet probe to monitor its relaxation. Analyzing surface acoustic waves launched by laser-excited metallic gratings, we first nondestructively extract porosity and elastic properties, and validate using electron tomography images [1]. We then model the heat flow dynamics, which obey a Fourier-like relation with an ultra-low apparent thermal conductivity. Through an analogy to rarefied gas flow in porous media, we explain this and similar measurements of phonon transport in silicon nanomeshes, porous nanowires and nanowire networks [2]. We isolate a geometry-dependent permeability contribution to thermal conduction from an intrinsic viscous component, which scales universally with porosity across all systems where feature sizes are much smaller than the dominant phonon mean free paths. This leads to an analytic description of thermal conduction in highly-confined silicon nanosystems, enabling their representation as effective media for engineering design applications.

 

[1] ACS AMI 14, 41316 (2022). [2] arXiv:2209.11743