Artificially Atomic-scale Ordered Superlattice Alloys for Thermoelectric Applications
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Artificially Atomic-scale Ordered Superlattice Alloys for Thermoelectric Applications S. Cho*, Y. Kim*, A. DiVenere*, G. K. L. Wong*, A. J. Freeman*, J. B. Ketterson*, L. J. Olafsen**, I. Vurgaftman**, J. R. Meyer**, C. A. Hoffman**, and G. Chen*** * Dept. of Physics & Astronomy, Northwestern University, Evanston, IL 60208 ** Naval Research Laboratory, Washington, D.C. 20375-5338 *** Mechanical & Aerospace Engineering Dept., Univ. of California, Los Angeles, CA 90095. ABSTRACT We report artificially atomic-scale ordered superlattice alloy systems, new scheme to pursue high-ZT materials. We have fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using MBE, confirmed by the presence of XRD superlattice satellites. We have observed that the electronic structure can be modified from semimetal, through zero-gap, to semiconductor by changing the superlattice period and sublayer thicknesses using electrical resistivity, thermopower, and magneto-transport measurements. InSb/Bi superlattice alloys have also been prepared and studied using XRD and thermopower measurements, which shows that their thermoelectric transport properties can be modified in accordance with structural modification. This superlattice alloy scheme gives us one more tool to control and tune the electronic structure and consequently the thermoelectric properties. INTRODUCTION Recently, a major effort has been directed to finding high ZT materials (here 2 ZT=(S σ/κ)Τ, where S is the thermopower, σ is the electrical conductivity, and κ is the thermal conductivity). On one side, new bulk materials have been synthesized and tested, and on the other side, (quantum well) superlattice or nanowire structures have been studied. In a conventional quantum well superlattice geometry, enhanced ZT values due to quantum confinement have been reported. [1,2] However, the effective overall ZT is then reduced due to thermal back-flow through the barrier layers in quantum well structures. [3] An alternative approach for achieving high ZT materials, atomic-scale ordered superlattice alloys, is presented in this article. The advantage of this approach is that it allows engineering the electronic band structure by forming an ordered alloy with the goal of achieving better thermoelectric properties and/or reduced lattice thermal conductivity due to the boundary scattering from the layers. In an effort to explore this approach, Bi/Sb superlattice alloys have been fabricated using MBE. The atomic arrangement of an ordered superlattice alloy of Bi and Sb, in comparison with a random alloy, is shown in Fig. 1. Bi and Sb are semimetals with a rhombohedral structure. Bi has a small energy overlap between the conduction and valence bands, high carrier mobilities, and small effective masses. These properties have encouraged many researchers to use Bi for quantum size effect studies. The Bi1-xSbx random (conventional) alloy is a semiconductor for 0.07 ≤ x ≤ 0.22 (Eg,max ~ 18 meV), but otherwise is a semimetal. [4-15] With properties such as a small ba
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