Long Phonon Mean Free Paths Observed in Cross-plane Thermal-Conductivity Measurements of Exfoliated Hexagonal Boron Nitride

TL;DR

This study measures the cross-plane thermal conductivity of exfoliated hexagonal boron nitride (hBN) flakes, revealing a strong thickness dependence and evidence of long phonon mean free paths, with implications for nanoelectronics thermal management.

Contribution

It provides the first measurements of cross-plane phonon mean free paths in hBN and demonstrates how twist boundaries limit phonon transport in layered materials.

Findings

Thermal conductivity increases forty-fold from 7 nm to 585 nm flakes.

Long phonon mean free paths span hundreds of nanometers in thick flakes.

Twist boundaries significantly reduce cross-plane thermal conductivity.

Abstract

Sub-micron-thick layers of hexagonal boron nitride (hBN) exhibit high in-plane thermal conductivity, useful optical properties, and serve as dielectric encapsulation layers with low electrostatic inhomogeneity for graphene devices. Despite the promising applications of hBN as a heat spreader, the thickness dependence of the cross-plane thermal conductivity is not known, and the cross-plane phonon mean free paths in hBN have not been measured. We measure the cross-plane thermal conductivity of hBN flakes exfoliated from bulk crystals. We find that the thermal conductivity is extremely sensitive to film thickness. We measure a forty-fold increase in the cross-plane thermal conductivity between 7 nm and 585 nm flakes at 295 K. We attribute the large increase in thermal conductivity with increasing thickness to contributions from phonons with long mean free paths (MFPs), spanning many hundreds of nanometers in the thickest flakes. When planar twist interfaces are introduced into the crystal by mechanically stacking multiple thin flakes, the cross-plane thermal conductivity of the stack is found to be a factor of seven below that of individual flakes with similar total thickness, thus providing strong evidence that phonon scattering at twist boundaries limits the maximum phonon MFPs. These results have important implications for hBN integration in nanoelectronics and improve our understanding of thermal transport in two-dimensional materials.