  Hexagonal boron nitride (h-BN) has received great interest in recent years as
a wide bandgap analog of graphene-derived systems. However, the thermal
transport properties of h-BN, which can be critical for device reliability and
functionality, are little studied both experimentally and theoretically. The
primary challenge in the experimental measurements of the anisotropic thermal
conductivity of h-BN is that typically sample size of h-BN single crystals is
too small for conventional measurement techniques, as state-of-the-art
technologies synthesize h-BN single crystals with lateral sizes only up to 2.5
mm and thickness up to 200 {\mu}m. Recently developed time-domain
thermoreflectance (TDTR) techniques are suitable to measure the anisotropic
thermal conductivity of such small samples, as it only requires a small area of
50x50 {\mu}m2 for the measurements. Accurate atomistic modeling of thermal
transport in bulk h-BN is also challenging due to the highly anisotropic
layered structure. Here we conduct an integrated experimental and theoretical
study on the anisotropic thermal conductivity of bulk h-BN single crystals over
the temperature range of 100 K to 500 K, using TDTR measurements with multiple
modulation frequencies and a full-scale numerical calculation of the phonon
Boltzmann transport equation starting from the first principles. Our
experimental and numerical results compare favorably for both the in-plane and
through-plane thermal conductivities. We observe unusual temperature-dependence
and phonon-isotope scattering in the through-plane thermal conductivity of h-BN
and elucidate their origins. This work not only provides an important benchmark
of the anisotropic thermal conductivity of h-BN but also develops fundamental
insights into the nature of phonon transport in this highly anisotropic layered
material.

Mesoscale and Nanoscale Physics