Thermoelectric Transport in a ZrN/ScN Superlattice

MONA ZEBARJADI 0 1 ZHIXI BIAN 0 1 RAJEEV SINGH 0 1 ALI SHAKOURI 0 1 ROBERT WORTMAN 0 1 VIJAY RAWAT 0 1 TIM SANDS 0 1 0 1.Department of Electrical Engineering, University of California , Santa Cruz, CA 95064 , USA . 2.Birck Nanotechnology Center , Purdue University , West Lafayette, IN, USA. 3. 1 Zebarjadi, Bian, Singh, Shakouri, Wortman, Rawat, and Sands Metal/semiconductor superlattices have the potential for a high thermoelectric figure of merit. The thermopower of these structures can be enhanced by controlling the barrier height using high-energy electron filtering. In addition, phonon scattering at interfaces can reduce the lattice contribution to the thermal conductivity. In this paper, we present theoretical and experimental studies of the thermoelectric transport in ZrN/ScN metal/semiconductor superlattices. Preliminary measurement results show an exponential increase in the cross-plane electrical conductivity with increasing temperature, which indicates the presence of the barrier. Fit of the Boltzmann transport-based model with the data indicates a barrier height of 280 meV. The cross-plane Seebeck coefficient of the sample is also measured by combining Seebeck voltage transient measurements with the thermal imaging technique. A Seebeck coefficient of 820 lV/K at room temperature is extracted, which is in good agreement with the simulation result of 800 lV/K. Theoretical calculations predict that the ZrN/ScN structure can exhibit a ZT of 1.5 at 1300 K assuming lateral momentum is conserved and that a ZT of 3 is achievable if the lateral momentum is not conserved. - The figure of merit, ZT, is a dimensionless parameter that determines the energy conversion efficiency of thermoelectric devices. ZT is defined as rS2T=j; where r is the electrical conductivity, S is the Seebeck coefficient, j is the thermal conductivity, and T is the absolute temperature. In recent years, new approaches have been under investigation to enhance ZT by using low-dimensional thermoelectric materials. At low dimensions, the Seebeck coefficient can be enhanced due to the abrupt change of the density of states1 or as a result of hot electron filtering by potential barriers perpendicular to electron transport.2 The thermal conductivity can be decreased by using interfaces to scatter phonons more effectively than electrons, for example, in the case of phonon (Received July 8, 2008; accepted December 17, 2008; published online January 19, 2009) blocking superlattices3 or embedded nanoparticles.4,5 Many of the new thermoelectric materials having large ZT values are limited to operating temperatures below 500 C due to their instabilities and device contact degradation at higher temperatures. Therefore, there is a need for high-ZT materials for thermoelectric energy conversion that can operate under large temperature gradients from room temperature to about 1000 C.6 Rocksalt nitride metal/ semiconductor materials are physically and chemically stable at very high temperatures. Here, we have investigated rocksalt nitride metal/semiconductor superlattices as novel thermoelectric metamaterials that utilize the thermoelectric enhancement of thermionic emission at heterointerfaces. The electron energy filtering effect of potential barriers is expected to increase the Seebeck coefficient while maintaining an adequate electrical conductivity due to the large density of states in the metal layers of the material. The abundant heterointerfaces along the transport direction also suppress the lattice thermal conductivity. Previous modeling of similar metal/semiconductor superlattices has shown that these superlattice structures can enhance the thermoelectric properties to achieve ZT values in excess of 2.6 One of the major challenges in realizing metal/ semiconductor superlattices is identifying suitable material combinations that can be grown in the form of superlattices and maintain morphological stability at high operating temperatures. The considerations of melting point (Tm > 2600 C), crystallographic compatibility, thermal expansion coefficients, and electronic properties have led to the selection of (Zr,W)N/ScN as the desirable metal/ semiconductor combination. In this paper, we report the growth and characterization of ZrN/ScN samples. We apply a modified thermionic transport model to fit the experimental electrical conductivity. We then use the model to predict the performance of the structure at different temperatures and with different barrier heights. Coherent epitaxial multilayers of 4-nm-thick ZrN and 6-nm-thick ScN were grown on rocksalt MgO substrates by DC magnetron sputtering in an argon/ nitrogen ambient. Samples were etched to form pillars 1 lm in height. Au/Cr layers 100 nm thick were subsequently deposited on the pillars to form the electrical contact layer. The bulk properties of ZrN and ScN were characterized inside a thermostat under vacuum in the temperature range of 300 K to 800 K. Figure 1 shows the experimental data for the bulk ZrN and ScN. The electrical conductivity data along with the Hall measurement data was used to set the relaxation times for each layer in the modeling of ZrN/ScN superlattices. The electrical resistance of the ZrN/ScN superlattices was measured in the cross-plane direction using the four-wire method in the range of 300 K to 475 K. The electrical conductivity was extracted considering the geometry of the sample. Fig. 1. Conductivity and Seebeck coefficient of bulk ScN and ZrN. Fig. 2. Thermal imaging of the sample in the heating mode at room temperature and under an applied current of 150 mA. Cross-plane Seebeck coefficient was measured by combining the transient voltage measurement and the thermal imaging technique. A current pulse of 150 mA was applied to the sample in the cooling and heating modes. The transient voltage response was measured with an oscilloscope. The voltage signal consists of the resistive voltage and the Seebeck voltage due to Peltier cooling/heating and Joule heating. The resistive component of the total voltage drops instantaneously while the Seebeck voltage exhibits a slower decay time due to the thermal response of the thermoelectric device. The Seebeck voltage due to the Peltier effect was extracted by subtracting the Seebeck voltage amplitude in the cooling mode from the amplitude in the heating mode. Thermal imaging was utilized to measure the surface temperature of the top contact for an identical current amplitude. The thermal imaging results are shown in Fig. 2. A Seebeck coefficient of 820 lV/K is extracted from these data at room temperature. THEORETICAL MODEL A theoretical model was developed to calculate the thermoelectric properties of a superlattice structure. This model was based on a modified Boltzmann transport equation.7 A transmission coefficient due to the quantum-mechanical reflection coefficient was added to include the effect of quantum wells on the transport. The details of this formalism a (...truncated)