Thermal Conductivity Measurement of Thermoelectric Thin Films by a Versatility-Enhanced 2ω Method

The 2ω method is a technique to measure the cross-plane thermal conductivity, κ, of a film sample based on sinusoidal Joule-heating of a metal film deposited on the sample surface and its thermoreflectance (TR) measurement of surface temperature cooling due to the heat dissipation. The 2ω method is being paid attention because it is more cost-effective and easier to use than the conventional time domain thermoreflectance (TDTR) method or the 3ω method. In some cases, however, it is difficult to apply the conventional 2ω method to the κ measurement of high thermal resistance films such as general thermoelectric films due to its non-linear TR signal response to (2ω)^−0.5. Here, we present a 2ω method based on a versatile TR signal analysis which enables the κ measurement of high thermal resistance film more explicitly than the conventional analysis based on a linear TR signal response. This method determines explicitly the thermal conductivities of PbTe films and PbTe/GeS superlattices grown on BaF₂(111) substrates by hot wall epitaxy: κ = 2.1 ± 0.13 Wm⁻¹ K⁻¹ and κ = 0.71 ± 0.05 Wm⁻¹ K⁻¹, respectively. Furthermore, a significant impact of PbTe film crystallinity on thermal conductivity is demonstrated by comparative measurements between polycrystalline PbTe film and epitaxial PbTe film grown on the BaF₂(111) substrates. These results demonstrate that our method can be a powerful tool to measure the thermal conductivity of thermoelectric films.