Thermoelectric figure of merit ofBi/Pb1−xEu<

Abstract

An enhanced thermoelectric figure of merit Z 3D T is predicted for Bi/(111)Pb 1Ϫx Eu x Te superlattices. The values of Z 3D T obtained for xϷ1 superlattices are 2.31, 1.55, and 1.61 at 300, 150, and 77 K, respectively, showing that they are promising candidates for thermoelectric elements in the temperature range 77-300 K.Even with x as small as 0.1, where the conduction-band offset ⌬E c is estimated to be 0.25 eV, the predicted Z 3D T values are 1.75, 1.16, and 1.18 at 300, 150, and 77 K, respectively. It is proposed that other families of Bi-based superlattices, such as Bi/͑111͒CdTe superlattices, should also be good candidates for low-temperature thermoelectric elements.The use of superlattice structures to design useful thermoelectric materials with a large thermoelectric figure of merit ZT (ϭS 2 T/ , where S, , , and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively͒ has attracted significant interest in the thermoelectric materials community. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The basic strategies for enhancing ZT using low-dimensional structures are based on ͑1͒ the use of an enhanced density of states for electrons ͑or holes͒ near the band edge to increase the magnitude of the Seebeck coefficient ͉S͉ at a given carrier concentration and ͑2͒ the use of increased boundary scattering of phonons at the quantum well-barrier interfaces in the superlattice to reduce the lattice thermal conductivity ph relative to the bulk values. 11,16 Recently, the original proposal by Hicks et al. 7,11,13,16 has been extended to more realistic systems, such as GaAs/AlAs ͑Refs. 1-3͒ and Si/Ge ͑Refs. 1,4͒ short-period superlattices, and an enhanced threedimensional figure of merit (Z 3D T) for the whole superlattice was predicted relative to the ZT's for the corresponding bulk materials.The key strategy for the successful design of thermoelectric materials using a superlattice structure is to find materials with highly anisotropic constant energy surfaces for the quantum well layers and materials for the barrier layers that are chemically ͑and structurally͒ compatible with the quantum well material and have large values for the energy band gaps to provide sufficient conduction-and valence-band offsets for the superlattices. Bismuth ͑Bi͒ is a semimetal that has various unique properties, such as a highly anisotropic Fermi surface, large electron and hole mobilities, and a small lattice thermal conductivity. [17][18][19][20][21] These features of Bi make it potentially a very desirable material for thermoelectric applications, especially in its semiconducting form. 7,8,17 A semimetal-semiconductor transition in Bi has been predicted and experimentally demonstrated using low-dimensional structures, such as quantum wells and wires. [22][23][24][25] The twodimensional nature of the electron transport in Bi/͑111͒PbTe superlattices has also been reported experimentally. [26][27][28] Here we report a theoretical investigation of the thermoelectric p...

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