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Demonstrated high ZT1.7 at 800K for the first time in nanostructured III-V material (0.12-0.5% ErAs:InGaAs). Successful electron transport modeling can explain temperature dependent Seebeck coefficient and mobility without fitting parameters. For the case of 0.8% TbAs:InGaAs, a ZT of 1.04 at 600K was demonstrated. Interestingly power factor enhancement for TbAs is much bigger than that for ErAs nanoparticles. However ErAs nanoparticles give higher thermal conductivity due to their smaller size (1nm vs. 2-5nm). Detailed experimental optimization of p-ErSb:InxGa1-xSb lead to ZT=0.57 at 530K. In the case of metal/semiconductor multilayers, observed giant cross-plane Seebeck coefficients of 2,560 microV/K at room temperature and 16,640 microV/K at 360 K, and extremely low room temperature thermal conductivity of 0.92 W/mK for La0.67Sr0.33MgO3/LaMnO3 perovskite oxide metal/semiconductor superlattices. Excellent agreement with the first principle calculations. First measurements of Seebeck for nitride superlattice (130 microV/K at 700K). Matches theoretical Seebeck at high-temperatures. This corresponds to ZT2.5 with measured thermal conductivity and theoretical electrical conductivity. Finally, demonstrated fast Seebeck mapping with a novel multi-probe scanning Seebeck system. This is now used by BSST (Gentherm) to study the uniformity of variety of TE materials. Finally performed a detailed cost/efficiency trade off analysis for TE power generation and first analytical calculation of stress in multi-leg TE modules. The new model shows that the shearing stress under temperature gradient can be significantly reduced with low fractional area coverage of TE elements.

We present a detailed theory on electron energy filtering by the nonplanar potential introduced by dispersed nanoparticles or impurities in bulk materials for enhancement of the thermoelectric power factor. When electrons with energies below a certain cut-off energy are prevented from participating in conduction through the material, the Seebeck coefficient and thus the thermoelectric power factor can be drastically enhanced. Instead of using planar heterostructures which require elaborate epitaxial techniques, we study embedded nanoparticles or impurities so that the conservation of lateral momentum limiting electron transport at heterointerfaces is no longer a limiting factor. Based on the Boltzmann transport equations under the relaxation time approximation, the optimal cut-off energy level that maximizes the power factor is calculated to be a few k(B)T above the Fermi level, and is a function of the scattering parameter, Fermi level, and temperature. The maximized power factor enhancement is quantified as a function of those parameters. The electronic thermal conductivity and Lorenz number are also shown to be suppressed by the electron filtering to further enhance the thermoelectric figure of merit. We find that the power factor of PbTe at 300 K could be enhanced by more than 120% when the cut-off energy level is 0.2 eV or higher and the carrier density higher than 5 x 10(19) cm(-3). Finally we propose the use of distributed resonant scatterings to partially realize the nonplanar electron filtering in bulk materials.

The maximum cooling temperature of a thermoelectric refrigerator made of uniform bulk material is limited by its dimensionless figure-of-merit ZT. Cascaded stages are typically needed in order to obtain a higher cooling temperature. Multiple stage configurations have disadvantages of device complexity, and reduced efficiency due to the non-ideal heat spreading between different stages. In this paper, we prove that the maximum cooling temperature can be increased by using a single stage made of inhomogeneous material. This optimization is different from conventional graded materials where there is a large temperature gradient and local material properties are optimized in order to achieve the highest ZT at the local temperature under operation. The new optimization is attributed to the redistribution of the Joule heating and Peltier cooling profiles along the current and heat flow directions. The cooling efficiency can also be increased by a moderate amount. Numerical simulations are used to optimize the doping profile for a thermoelectric cooler based on single crystal silicon., The original document contains color images.

The transient Harman technique is used to characterize the cross-plane ZT of InGaAs/InGaAlAs superlattice structures with embedded ErAs nanoparticles in the well layers. ErAs nanoparticles have proven to substantially reduce the thermal conductivity while slightly increasing the electrical conductivity of bulk InGaAs. The InGaAs/InGaAlAs superlattice structure was designed to have a barrier height of approximately 200meV. Although ErAs nanoparticles provide free carriers inside the semiconductor matrix, additional doping with Si increased the Fermi energy to just below the barrier height. The bipolar transient Harman technique was used to measure device ZT of samples with different superlattice thicknesses in order to extract the intrinsic cross-plane ZT of the superlattice by eliminating the effects of device Joule heating and parasitics. High-speed packaging is used to reduce signal ringing due to electrical impedance mismatch and achieve a short time resolution of roughly 100ns in transient Seebeck voltage measurement. The measured intrinsic cross-plane ZT of the superlattice structure is 0.13 at room temperature. This value agrees with calculations based on the Boltzmann transport equation and direct measurements of specific film properties. Theoretical calculations predict cross-plane ZT of the superlattice to be greater than 1 at temperatures greater than 700K., The original document contains color images. Presented at the MRS Fall Meeting, held in Boston, MA in November 2005. Published in the Proceedings of the MRS Fall Meeting, 2005.

We use two heterostructure designs to improve the energy conversion efficiency of solid-state thermionic devices. The first method is to use a non-planar heterostructure with roughness in order of electron mean free path. This has some combined benefits of increased effective interface area, and reduced total internal reflection for the electron trajectories arriving at the interface. Monte Carlo simulations of various geometries show that the electrical conductivity and thermoelectric figure of merit can be improved for non-planar barrier compared to the planar counterpart. The second method is to use planar high barrier heterostructures with different effective masses for charge carriers in emitter and barrier regions. When an electron passes from a lower effective mass emitter and arrives at a barrier with higher effective mass, since both the lateral momentum and total energy are conserved, part of the lateral energy is coupled to the vertical direction and the electron gains momentum in the direction perpendicular to the interface to enter the barrier region. For high potential barriers, the improvement of thermionic current is about the same as the ratio of the effective masses of the two materials, which can be a factor of 5-10 for typical heterostructure material systems., Presented at the Fall Meeting of the Materials Research Society (MRS), held in Boston, MA, Nov 2005. Published in the Proceedings of the MRS Fall Meeting, Nov 2005. The original document contains color images.