(Invited) Review of Recent Progress in Nanoscaled Thermoelectric Thin Films

Monday, 2 October 2017: 14:00
Chesapeake F (Gaylord National Resort and Convention Center)
H. Baumgart (Dept. Electrical & Computer Eng., Old Dominion Univ., Old Dominion University, ECE Department), X. Chen, P. Lin, and K. Zhang (Applied Research Center, Old Dominion University)
In recent years, thermoelectric materials have received increasing attention due to their ability to generate electricity from waste heat recovery. The efficiency of thermoelectric materials systems to produce thermoelectric power is related to the figure of merit ZT and the power factor. The dimensionless thermoelectric figure of merit is expressed as ZT = S2σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity. In the quest to enhance ZT values, a key strategy involves reduction in thermal conductivityk, resulting from phonon scattering by numerous interfaces in low dimensional structures. Promising advances have been achieved with phononic crystal (PnC) nanostructures in thermoelectric materials, because the thermal conductivity of PnC samples is lower compared to non-patterned thermoelectric samples due to phonon-boundary scattering. Metal telluride based compounds such as bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) alloys have reported some of the highest figures of merit and work best for thermoelectric devices used for the temperature range of 200 to 400 K, while lead chalcogenides such as PbTe, PbS, and PbSe are ideal for the temperature range of 350 to 600 K. Here we review advances in the ALD synthesis of composite thermoelectric nanolaminates of alternating Bi2Te3 and Sb2Te3 thin films, and PbSe and PbTe films. During our investigations, we have observed that PbTe/PbSe nanolaminates grown on porous templates have higher Seebeck coefficients than the ones grown on regular planar silicon wafers. The higher Seebeck values result from the lower thermal conductivity k in porous structures, which in turn lead to a higher figure of merit ZT. We demonstrate the feasibility to enhance the figure of merit ZT further by modulating the size and periodicity of the pattern and the thickness of the thermoelectric film in relation to the mean free path (MFP) of the phonons of the thermoelectric material. In our study PbTe and PbSe thin films and nanolaminates were synthesized by Atomic Layer Deposition (ALD) technology on regular planar silicon wafers and on porous templates. Various approaches to enhance the figure of merit ZT and the Seebeck coefficient for ALD PbTe & PbSe films are discussed that result in an effective reduction of the thermal conductivity. In this work, we report on the physical characterization and electrical characterization focusing on Seebeck coefficient measurements in PbTe/ PbSe nanolaminate structures synthesized by ALD on planar Si wafers and on porous Si templates.