Strained and Unstrained Bi(1-x)Sb(x) Superlattice Thermoelectrics
Final progress rept. 1 May 1997-31 Dec 2000
NORTHWESTERN UNIV EVANSTON IL DEPT OF PHYSICS AND ASTRONOMY
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The electronic band structure of artificially ordered superlattice alloys SLA of Bi and Sb, prepared by alternately depositing thin layers of Bi and Sb as a superlattice, could be modified from a semimetal to a semiconductor by changing the period of the superlattice. This new SLA opens a possibility to engineer an alloy material to get more desirable thermoelectric properties, because a thermal conductivity reduction, due to an increase in phonon scattering from the superlattice interfaces, is expected certain electronic properties are altered at the same time. To check the performance of an in-plane BiBiSb cooling device, anisotropy in the films was investigated. Bi showed a strongly anisotropic and field-direction-dependent magneto-Seebeck effect. The magneto-transport properties of Bi1-xSbsub x films and BiCdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis and multicarrier fitting to the magnetic-field-dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. Using the local density relativistic full-potential linear muffin-tin orbital method, we found that the internal displacement changes the Bi electronic structure from a metal to a semimetal and that an increase of the trigonal shear angle can lead to a semimetal-semiconductor transition in Bi. Post annealing of Bi films near its melting point improved the magnetoresistance ratios drastically, which can be ascribed to the electron mobility enhancement. The calculation of the electronic structure of Bi2Te3 suggested that the quasi-two dimensional crystal structure of Bi2Te3 gives a large, but finite, anisotropy in the effective mass which enhances the thermoelectric figure of merit.
- Electricity and Magnetism