Design of Heterostructures for High Efficiency Thermionic Emission
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Design of Heterostructures for High Efficiency Thermionic Emission Zhixi Bian and Ali Shakouri Electrical Engineering Department, University of California Santa Cruz, CA 95064, U.S.A. ABSTRAT We use two heterostructure designs to improve the energy conversion efficiency of solidstate 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. INTRODUCTION The performance of a thermoelectric device is mostly determined by the dimensionless figure of merit ZT. ZT is defined as ZT = (S 2σ / k )T , where T is the absolute temperature, S is the Seebeck coefficient, and σ and k are the electrical and thermal conductivities, respectively [1]. Solid-state thermionic energy converters are expected to offer a larger thermoelectric power factor ( S 2σ ) than uniform bulk materials due to the selective emission of hot electrons while maintaining a similar electrical conductivity as highly degenerate emitter materials [2, 3]. However, it has been shown that such electronic-transport enhancing technique can only improve the thermoelectric property up to a point [4]. Instead, the advantage of conventional planar superlattices or multilayers is in the reduction of phonon transport and the parasitic heat loss [5, 6]. The main shortcoming of planar barriers is that they only transmit electrons whose kinetic energy in the direction perpendicular to the barrier is large enough. To use the total kinetic energy to overcome the potential barrier, we proposed to use controlled roughness at the heterostructure interfaces to break the in-plane translational invariance and lateral momentum conservation [7]. The nonepitaxial interface scattering and lateral momentum nonconservation have been verified by ballistic electron emission microscopy for some metal/semiconductor interfaces [8, 9]. However, the enhancement of the total thermionic emission current needs to be further quantified for these experiment
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