Enhancing thermoelectric power generation efficiency with molecular beam epitaxial TbAs/III-V semiconductor nanocomposites

Date
2011
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University of Delaware
Abstract
TbAs:GaAs and TbAs:In0.53Ga0.47As nanocomposites were fabricated and characterized for the purpose of increasing efficiency for thermoelectric power generation. An efficient thermoelectric generator will exhibit a high thermoelectric figure of merit, ZT=S2sT/?, where S is the Seebeck coefficient, s is electrical conductivity, T is temperature, and ? is thermal conductivity. In previous studies, ErAs and ErSb nanoparticles which precipitated in epitaxial InGa(Al)As and InGa(Al)Sb served to successfully increase ZT by reducing thermal conductivity (through phonon scattering), increasing Seebeck coefficient (through energy- dependent electron scattering), and increasing electrical conductivity (through nanoparticle donation of electrons). Theoretical predictions suggest that replacing Er with Tb at the optimum concentration will serve to further increase ZT, particularly by way of increasing the thermoelectric power factor (S2s). These materials were synthesized by co-depositing Tb during III-V semiconductor growth in an ultra-high vacuum molecular beam epitaxial deposition system. At rare earth concentrations exceeding the solid solubility limit of the matrix, randomly-dispersed nanoparticles of approximately uniform size form throughout the film, as seen by scanning transmission electron microscopy. In addition, the structural, compositional, electrical, and thermal properties of the materials in both growth series (TbAs:GaAs and TbAs:InGaAs) were measured with an array of techniques. TbAs nanoparticles in GaAs served to reduce thermal conductivity by a factor of about five with 1.8 at.% TbAs, and electrical measurements were consistent with theoretical energy band predictions and behavior of past similar materials. The success of these TbAs:GaAs materials led the way for growth of TbAs:InGaAs which has now been shown to exhibit highly-desirable thermoelectric properties. The room temperature thermal and electrical characterization of TbAs:InGaAs revealed a maximized room temperature power factor and ZT at a TbAs concentration of 0.3 at.%. The results were similar to those found in comparable ErAs:InGaAs materials with the exception that ErAs nanoparticles precipitated at smaller concentrations than TbAs due to an apparent increase in the solid-solubility of Tb in InGaAs. Measurements of select TbAs:InGaAs samples up to 600K revealed significant increase of power factor over similar well-performing ErAs materials and ZT reached 1.1 at 600K for 0.8% TbAs:InGaAs. This material’s temperature- dependent ZT was comparable to that of the highest-ZT ErAs:InGaAs sample (0.2% ErAs); however, a major difference is that increased efficiency due to Tb is caused primarily by power factor enhancement while Er mainly served to reduce thermal conductivity. As a result, future work will involve co-depositing both Er and Tb in InGaAs in an attempt to exploit the electrical and thermal benefits each rare earth element provides, perhaps resulting in an overall very efficient thermoelectric material.
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