PbSe and PbTe epitaxial films alloyed with tin: potential thin film materials with high ZT around room temperature?

For room temperature applications, normally V-VI-compounds with ZT values around 1 are used in the field of bulk thermoelectrics. In the case of thin film materials, several challenges arise from the complex crystal structure of these compounds due to strongly anisotropic thermoelectric transport parameters. This limits, or at least complicates the use of V-VI based thin films. Therefore, materials with a comparably simple crystal structure and isotropic transport properties are desirable. These needs are fulfilled in the case of IV-VI-compounds. Unfortunately, those materials suffer from their relatively low ZT of ∼0.2 around room temperature. Here, we report on structural and in particular thermoelectric properties of molecular beam epitaxy grown PbSe and PbTe thin films alloyed with tin. It was found that increasing the tin concentration without changing the other growth parameters results in increased charge carrier concentrations and thermopower values. Special care was taken to evaluate the in-plane thermal conductivity of insulated free standing Pb_1-xSn_xTe and Pb_1-xSn_xSe thin films. As expected, also the lattice thermal conductivity decreases due to alloy scattering. Increasing the tin concentration is known to result in decreasing bandgaps. By this, the optimum operating temperature is shifted towards ambient temperature. All those effects strongly enhance the thermoelectric properties of Pb_1-xSn_xTe and Pb_1-xSn_xSe in the room temperature region. For tin contents of x∼0.08, ZT>0.6 were determined in both cases. Even higher ZT values should be achievable using e.g. adequate doping levels and/or quaternary Pb_1-xSn_xTe_1-ySe_y compounds.