Understanding Extrinsic and Intrinsic Anisotropic Heat Transport in Crystalline Materials

Although thermal transport in cubic crystals is isotropic, thermal transport could be highly anisotropic, depending on crystallographic orientations. The anisotropy in thermal transport could originate from either intrinsic (e.g., crystal structures) and extrinsic (e.g., defects) factors. In this talk, I will present our recent works that advance the fundamental understanding of intrinsic and extrinsic anisotropy in thermal transport in crystalline materials. I will first discuss the anisotropy thermal transport in single-crystal InN films induced by dislocations. I will report strong thermal transport anisotropy governed by highly oriented threading dislocation arrays along the cross-plane direction in micron-thick, single-crystal indium nitride (InN) films. We find that the cross-plane thermal conductivity is more than tenfold higher than the in-plane thermal conductivity at 80 K when the dislocation density is on the order of ~3×10¹⁰ cm^-2. This large anisotropy is not predicted by the conventional models. With enhanced understanding of dislocation-phonon interactions, our results open new regimes for tailoring anisotropic thermal transport with line defects, and will facilitate novel methods for directed heat dissipation in thermal management of diverse device applications. Then, I will discuss the intrinsic thermal transport anisotropy in layered, two-dimensional (2D) crystals such as black phosphorus (BP) and MoTe₂. We find that the anisotropy in the cross-plane direction behaves differently from the anisotropy in the in-plane directions. We find that, contradictory to common beliefs, anisotropy in the heat capacity and relaxation times also contribute significantly to the overall anisotropy in thermal transport. We will discuss the origin of the anisotropy in more details in the talk.