Thickness-Dependent\nCross-Plane Thermal Conductivity\nMeasurements of Exfoliated Hexagonal Boron Nitride

Abstract

Submicrometer-thick\nlayers of hexagonal boron nitride (hBN) exhibit\nhigh in-plane thermal conductivity and useful optical properties,\nand serve as dielectric encapsulation layers with low electrostatic\ninhomogeneity for graphene devices. Despite the promising applications\nof hBN as a heat spreader, the thickness dependence of its cross-plane\nthermal conductivity is not known, and the cross-plane phonon mean\nfree paths (MFPs) have not been measured. We measure the cross-plane\nthermal conductivity of hBN flakes exfoliated from bulk crystals.\nWe find that submicrometer thick flakes exhibit thermal conductivities\nup to 8.1 ± 0.5 W m^–1 K^–1 at 295 K, which exceeds previously reported bulk values by more\nthan 60%. Surprisingly, the average phonon mean free path is found\nto be several hundred nanometers at room temperature, a factor of\n5 larger than previous predictions. When planar twist interfaces are\nintroduced into the crystal by mechanically stacking multiple thin\nflakes, the cross-plane thermal conductivity of the stack is found\nto be a factor of 7 below that of individual flakes with similar total\nthickness, thus providing strong evidence that phonon scattering at\ntwist boundaries limits the maximum phonon MFPs. These results have\nimportant implications for hBN integration in nanoelectronics and\nimprove our understanding of thermal transport in two-dimensional\nmaterials.