Anisotropic thermoreflectance thermometry: A contactless frequency-domain thermoreflectance approach to study anisotropic thermal transport

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Pérez, L. A., Xu, K., Wagner, M. R., Dörling, B., Perevedentsev, A., Goñi, A. R., Campoy-Quiles, M., Alonso, M. I., & Reparaz, J. S. (2022). Anisotropic thermoreflectance thermometry: A contactless frequency-domain thermoreflectance approach to study anisotropic thermal transport. In Review of Scientific Instruments (Vol. 93, Issue 3, p. 034902). AIP Publishing and may be found at https://doi.org/10.1063/5.0066166.We developed a novel contactless frequency-domain thermoreflectance approach to study thermal transport, which is particularly convenient when thermally anisotropic materials are considered. The method is based on a line-shaped heater geometry, produced with a holographic diffractive optical element, instead of using a spot heater as in conventional thermoreflectance. The heater geometry is similar to the one used in the 3-omega method, however, keeping all the technical advantages offered by non-contact methodologies. The present method is especially suitable to determine all the elements of the thermal conductivity tensor, which is experimentally achieved by simply rotating the sample with respect to the line-shaped optical heater. We provide the mathematical solution of the heat equation for the cases of anisotropic substrates, thin films, and multilayer systems. This methodology allows an accurate determination of the thermal conductivity and does not require complex modeling or intensive computational efforts to process the experimental data, i.e., the thermal conductivity is obtained through a simple linear fit (“slope method”), in a similar fashion to the 3-omega method. We demonstrate the potential of this approach by studying isotropic and anisotropic materials in a wide range of thermal conductivities. In particular, we have studied the following inorganic and organic systems: (i) glass, Si, and Ge substrates (isotropic), (ii) β-Ga2O3 and a Kapton substrate (anisotropic), and (iii) a 285 nm thick SiO2 thin film deposited on a Si substrate. The accuracy in the determination of the thermal conductivity is estimated as ≈5%, whereas the temperature uncertainty is ΔT ≈ 3 mK.EC/H2020/648901/EU/Finding a needle in a haystack: efficient identification of high performing organic energy materials/FOREMA