High-Temperature PbTe Thin Films for Use in Cascade Thermoelectric Power Generation
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High-Temperature PbTe Thin Films for Use in Cascade Thermoelectric Power Generation J.B. Posthill, J.C. Caylor, P.D. Crocco, T.S. Colpitts, and R. Venkatasubramanian RTI International, Research Triangle Park, North Carolina 27709-2194 Abstract PbTe-based thin films were deposited by thermal evaporation at temperatures ranging from ambient temperature to 430ºC on different vicinal GaAs (100) substrates and BaF2 (111). This materials system is being evaluated as a potential candidate thermoelectric material for a mid-temperature stage in a cascade power generation module. Pure PbTe, PbSe, and multilayer PbTe/PbSe films were investigated. All films deposited on different vicinal GaAs (100) substrates were found to be polycrystalline when deposited at 250ºC or lower. A subtle effect of substrate orientation and multilayer periodicity appears to contribute to the more randomly oriented polycrystallinity, which also lowers the thermal conductivity. These results are compared with PbTe epitaxial results on BaF2 (111). Introduction PbTe is an established thermoelectric material that has been available for decades and is utilized now in commercial and space-based applications [e.g., 1, 2]. Typically, bulk PbTe-based pellets of several millimeter dimensions are used in module fabrication. However, recent studies on the deposition of PbTe-based thin films have indicated that substantial improvements in the properties that contribute to an increased thermoelectric figure of merit, ZT. In particular, the work of Harman, et al. has shown the efficacy of MBE-grown self-assembled PbTe/PbSeTe-based quantum dot structures for Seebeck enhancement and thermal conductivity reduction [3]. These are ~ 5% lattice-mismatched epitaxial layers grown on BaF2 (111). Additionally, the thin film superlattice results from Lambrecht, et al. have shown a reduction in thermal conductivity for PbTe / PbSexTe1-x as well as Bi2(SexTe1-x)3 / Bi2(SeyTe1-y)3 superlattice structures [4]. This research was initiated because this previous work has indicated that PbTe-based thin films (< 100µm) have the potential to be used in a higher temperature stage of a cascade thermoelectric module that uses the significantly more efficient RTI Bi2Te3/Sb2Te3 superlattice CVD-grown films at lower temperature [5]. For a small weight penalty at the module level, an increase in overall module efficiency can be achieved in addition to now being able to use a higher temperature heat source -- both characteristics can enable significantly lower overall system weight savings combined with higher potential power output. Experimental Approach A cryo-pumped bell-jar vacuum system was adapted for thermal evaporation of PbTe and PbSe and a dopant source. The base pressure in the system is in the mid-10-8 Torr range. Based on after-deposition processing considerations, GaAs was considered to be a
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reasonable candidate for a substrate. Initial results were obtained by deposition on GaAs (100) at ambient temperature with subsequent annealing. Pure films and PbSeTe all
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