Aspects of Thin-Film Superlattice Thermoelectric Materials, Devices, and Applications
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Aspects of Thin-
Film Superlattice Thermoelectric Materials, Devices, and Applications
Harald Böttner, Gang Chen, and Rama Venkatasubramanian Abstract Superlattices consist of alternating thin layers of different materials stacked periodically. The lattice mismatch and electronic potential differences at the interfaces and resulting phonon and electron interface scattering and band structure modifications can be exploited to reduce phonon heat conduction while maintaining or enhancing the electron transport. This article focuses on a range of materials used in superlattice form to improve the thermoelectric figure of merit. Keywords: thermal conductivity, thermoelectricity.
Introduction Ideas in using superlattices to improve the thermoelectric figure of merit (ZT) through the enhancement of electronic conductivity and reduction of phonon thermal conductivity were first discussed in a workshop by M.S. Dresselhaus, T. Harman, and R. Venkatasubramanian.1 Subsequent publications from Dresselhaus’s group on the quantum size effects on electrons drew wide attention and inspired intense research, both theoretical and experimental, on the thermoelectric properties of quantum wells and superlattices.2 Several groups reported in recent years enhanced ZT in various superlattices such as Bi2Te3/Sb2Te3 and Bi2Te3/ Bi2Se3,3 and PbSeTe/PbTe quantum dot superlattices4 (Figure 1). The large improvements observed in these materials systems compared with their parent materials are of great importance for both fun-
MRS BULLETIN • VOLUME 31 • MARCH 2006
damental understanding and practical applications. Superlattices are anisotropic. Different mechanisms to improve ZT along directions both parallel (in-plane) and perpendicular (cross-plane) to the film plane have been explored. Along the in-plane direction, potential mechanisms to increase ZT include quantum size effects that improve the electron performance by taking advantage of sharp features in the electron density of states,2 and reduction of phonon thermal conductivity through interface scattering.5 Along the cross-plane direction, one key idea is to use interfaces for reflecting phonons while transmitting electrons (phonon-blocking/electrontransmitting),6 together with other mechanisms, such as electron energy filtering7 and thermionic emission,8 to improve electron performance. These mechanisms
have been explored through a few superlattice systems whose constituent materials have reasonably good thermoelectric properties to start with, V–VI materials such as Bi2Te3/Sb3Te3,3,9 IV–VI materials such as PbTe/PbSe,4,9 and V–V materials such as Si/Ge10–12 and Bi/Sb,13 with the most impressive results obtained in Bi2Te3 superlattices3 and PbTe-based quantum dot superlattices.4 The large ZT improvements observed in these superlattices shattered the ZT 1 ceiling that persisted until the 1990s, opening new potential applications in cooling and power generation using solidstate devices. Much research is needed in materials, understanding, and devices to further advance superlatti
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