Measurement of thickness dependent anisotropic thermal conductivities in highly anisotropic semiconductors

Semiconductors with highly anisotropic thermal conductivity are a substrate in device designs with highly efficient heat dissipation. In this study, we measured the anisotropic thermal conductivities as a function of thickness in various highly anisotropic semiconductors using time domain thermoreflectance technique (graphite, h-BN and MoS₂). Our results show that in graphite, as the thickness of the flake reduces both in-plane and out-of-plane thermal conductivities deviates significantly from the bulk value due to the presence of phonons with large mean free path; On the other hand, due to the presence of phonons with shorter mean free path, MoS₂ flakes with thicknesses similar to graphite showed thermal conductivities comparable to its bulk value. When the thickness of the flake is reduced and is comparable with the mean free path of the phonons, heat transport is not diffusive anymore and becomes quasi-ballistic. Understanding the thermal anisotropy in two-dimensional materials as a function of their thickness has relevant applications in various fields such as microelectronics, thermoelectric applications, etc.