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 SiO 2 , which is of interest for novel electronic devices.

Simultaneous measurement of anisotropic thermal conductivity and thermal boundary conductance of 2-dimensional materials

“…We model the graphene layer as having a thickness of 0.335 nm and negligible out-of-plane thermal resistance and treat this together with the TBC of Ni/graphene and the TBC of graphene/SiO2. We performed measurements at several pump-probe beam offsets, and the fitted value for ∥ of graphene is 636 ± 140 W/mK and the TBC across Ni/graphene/SiO2 is 17 ± 0.2 MW/m 2 K. The measured in-plane thermal conductivity is in good agreement with our previous measurement of supported graphene through beam offset FDTR 19 and with other literature values 25,42,43 , though previous measurements were done using Al or Ti transducers. This further strengthens Yang's observation that the in-plane thermal conductivity of graphene is independent of the metal contact 25 .…”

“…As an example, the resulting uncertainty histograms of the measured in-plane thermal conductivity and effective conductance of Ni/graphene/SiO2 for 20layer h-BN sample is shown in Figure 4. The confidence of the results could be increased by performing several independent measurements 19 .…”

“…However, in TDTR the instrumentation is comparatively expensive, and the measurements are typically modulated at frequencies below 20 MHz, setting a lower limit to the thermal penetration depth and in turn limiting the ability to measure thin samples. Frequency domain thermoreflectance (FDTR) is cost effective as it does not require an ultrafast laser or electro-optic modulators, is easier to set-up as it does not use a long mechanical delay stage, and it allows modulating the measurement over a wide range of frequencies 19,23,24 .…”

Anisotropic Thermoreflectance Thermometry: A contactless frequency-domain approach to study anisotropic thermal transport

“…The development of novel experimental methodologies to study anisotropic thermal transport has recently become a relevant research objective. A considerable number of experimental techniques and methodologies 9,[14][15][16][17][18][19][20][21][22][23][24][25][26][27] based on variations of the 3-omega method, 28 time-domain thermoreflectance, 29 and frequency-domain thermoreflectance 30 have been developed for this purpose, demonstrating their capability to obtain the components of κ i j . The main differences between these approaches are the dimensionality of the heat source (line or spot), and their contact or contactless fashion (electrical resistor or focused optical spot).…”

“…On the other hand, contactless techniques such as those reported in Refs. [9,[17][18][19][20][21][22][23][24][25][26][27] are based on small Gaussian or ellipse-shaped focused spots, i.e., sensitive to all crystallographic directions simultaneously. Although this is not an intrinsic impediment to obtain κ i j , it substantially complicates the analysis of the measured data with respect to the case of an elongated line-shaped heat source (sensitive to only two cyrstallographic directions simulataneously), as it is evident from Refs.…”