Measuring the thermal properties of anisotropic materials using beam-offset frequency domain thermoreflectance

Abstract

Thermoreflectance techniques have become popular to measure the thermal properties of thin films such as thermal conductivity and thermal boundary conductance (TBC). Varying the focused spot sizes of the beams increases the sensitivity to in-plane heat transport, enabling the characterization of thermally anisotropic materials. However, this requires realignment of the optics after each spot size adjustment. Offsetting the probe beam with respect to the pump beam and modulating over a wide range of frequencies (5 kHz to 50 MHz) yield better sensitivity to the thermophysical properties of anisotropic materials without varying the spot sizes. We demonstrate how beam-offset frequency domain thermoreflectance can be used to measure the in- and out-of-plane thermal conductivity as well as the TBC simultaneously from a single data set by working at reduced spot sizes. Lowering the laser spot size allows us to detect signals over a wide range of frequencies and use larger beam offsets, thanks to the increase in the thermoreflectance signal. We measure the anisotropic thermal properties of a range of materials, including single layer Graphene on SiO2, which is of interest for novel electronic devices.