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

The 2 ω 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 2 ω 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 3 ω method. In some cases, however, it is difficult to apply the conventional 2 ω method to the κ measurement of high thermal resistance films such as general thermoelectric films due to its non-linear TR signal response to (2 ω ) −0.5 . Here, we present a 2 ω 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 BaF 2 (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 BaF 2 (111) substrates. These results demonstrate that our method can be a powerful tool to measure the thermal conductivity of thermoelectric films.