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Theoretical and experimental advances in Bi2Te3 / Sb2Te3 - based and related superlattice systems M. Winkler1, X. Liu2 , U. Schürmann3, J. D. König1, L. Kienle3, W. Bensch2, H. Böttner1 1

Fraunhofer Institute for Physical Measurement Techniques IPM, Heidenhofstraße 8, D-79110 Freiburg, Germany Institute of Inorganic Chemistry, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 2, 24118 Kiel, Germany 3 Synthesis and Real Structure, Institute for Materials Science, Christian-Albrechts-Universität zu Kiel, Kaiserstr. 2, 24143 Kiel, Germany 2

ABSTRACT Roughly a decade ago an outstanding thermoelectric figure of merit ZT of 2.4 was reported for nanostructured Bi2Te3/Sb2Te3-based thin film superlattice (SL) structures. The published results strongly fueled and renewed the interest in the development of efficient novel nanostructured thermoelectric materials. This review article shall give an overview over the most recent theoretical and experimental advances on Bi2Te3/Sb2Te3 SLs and related superlattice systems. The presented theoretical models are subdivided into electronic and phononic aspects. The experimental results are summarized with regard to the method used. A more detailed elaboration on structural and transport properties is given in the subsequent sections. INTRODUCTION It is well known that roughly 60% of the energy resources are wasted as heat into the environment. A promising technology is the thermoelectric conversion of the waste heat into electricity. To obtain an efficient thermoelectric energy conversion a low thermal conductivity λ, a good electrical conductivity σ, and a large value for the Seebeck coefficient S are required. These material properties can be expressed by the thermoelectric figure of merit ZT which is defined as ZT = σ·S2T/λ. ZT values > 1 are desirable for practical applications. A brief historical timeline for Bi2Te3 –based materials The evolution of Bi2Te3 and related materials as thermoelectrics is displayed in Figure 1. The first welldocumented systematic material screening investigating S and σ of numerous compounds was carried out by Haken in 1910 [1]. Examining the systems Bi-Te and Sb-Te, he identified the phases Bi2Te3 and Sb2Te3 as promising thermoelectrics, starting their success story as room-temperature materials. Around the same time, Altenkirch gave a theoretical analysis of the problem of energy conversion using thermocouples in 1911 [2], showing that the performance of a thermocouple was affected by the involved materials´ Seebeck coefficient, their electrical conductivity and their thermal conductivity, resulting in the equation for ZT. In the meantime, starting in the mid-40s the new material class of (thermoelectric) semiconductors came under research by different groups headed by such names as Schottky, Justi, Lautz, Goldsmid, Telkes and Ioffe [3,4,5,6,7]. In the 1950s, applied thermoelectric research in Europe, the United States and Russia by the pioneers Birkholz, Goldsmid and Ioffe progressed rapidly and first performance data of actual devices was prese