Computational Study of the Electronic Performance of Cross-Plane Superlattice Peltier Devices

We use a state-of-the-art non-equilibrium quantum transport simulation code, NEMO1D, to address the device physics and performance benchmarking of cross-plane superlattice Peltier coolers. Our findings show quantitatively how barriers in cross-plane superlattices degrade the electrical performance, i.e. power factor. The performance of an In0.53Ga0.47As/In0.52Al0.48As cross-plane SL Peltier cooler is lower than that of either a bulk In0.53Ga0.47As or bulk In0.52Al0.48As device, mainly due to quantum mechanical effects. We find that a cross-plane SL device has a Seebeck coefficient vs. conductance tradeoff that is no better than that of a bulk device. The effects of tunneling and phase coherence between multi barriers are examined. It is shown that tunneling, SL contacts, and coherency only produce oscillatory behavior of Seebeck coefficient vs. conductance without a significant gain in PF. The overall TE device performance is, therefore, a compromise between the enhanced Seebeck coefficient and degraded conductance. INTRODUCTION The dimensionless figure of merit, 2 ZT S GT K = , is the primary material parameter governing the maximum thermoelectric (TE) efficiency. Here T is the temperature, S is the Seebeck coefficient, G is the electrical conductance, and K is the thermal conductance, which is the sum of the electronic contribution, Ke , and the lattice thermal conductance, Kph . Most recent improvements in ZT have been achieved by phonon engineering to reduce the lattice thermal conductivity [1-3]. One way is to use thin film superlattices (SLs), which has led to significant reduction in the lattice thermal conductivity and, therefore, enhanced TE performance [4]. The possibility of enhancing the electronic component (SG, power factor: PF) of TE performance by using SL devices has been studied. First quantitative calculations for in-plane direction in SLs were done by Hicks and Dresselhaus in 1993 [5,6] and showed promising results. For crossplane transport in SL, it has been predicted that energy filtering will lead to significant increases in ZT under a certain condition [7]. A single barrier and multi-layer thermionic refrigeration were proposed [8,9]. Experimentally, researchers have shown the increase in S by filtering out low energy electrons, but a limited increase in power factors due to the decrease in electrical conductivity [10-12]. Although there have been a number of studies, it is still not clear how a SL affects the electronic performance i.e. PF. This work explores the physics of transport in single barrier and multi-barrier (i.e. SL) TE devices using a sophisticated quantum transport model, NEMO-1D[1315]. A clear understanding of how barriers affect the PF is essential for developing single barrier or multi-barrier TE devices with enhanced PF and is the objective of this paper. 509