Nanoalloyed Bi 2 Te 3 , Sb 2 Te 3 and Bi 2 Te 3 /Sb 2 Te 3 Multilayers

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Nanoalloyed Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 Multilayers M. Winkler 1, Jan D. Koenig 1, S. Buller 2, U. Schuermann 3, L. Kienle 3, W. Bensch 2, H. Boettner 1 1

Fraunhofer-Institute for Physical Measurement Techniques IPM, Thermoelectric Systems, Heidenhofstraße 8, 79110 Freiburg, Germany 2 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 ABSTRACT In this work, thin films of Bi2Te3 and Sb2Te3 were synthesized by the nanoalloying approach: Nanoscale layers of the elements Element nanoscale layers of Bi, Sb and Te are stoichiometrically deposited on a cold substrate using a MBE setup and subjected to an annealing process in which a solid state reaction yielding Bi2Te3 and Sb2Te3 takes place. Besides the two binary compounds, nanoscale multilayer (ML) stacks of 9 nm Bi2Te3/9 nm Sb2Te3 were created. The electrical transport properties of the binary compounds were determined in dependence of composition. Compound formation was directly observed in temperature-dependent in-situ XRD scans and was found to start at ~100 °C. The stability of the Bi2Te3/Sb2Te3 ML nanostructure against temperature-driven interdiffusion during annealing was examined by SIMS and TEM for an annealing temperature of 150 and 250 °C, respectively. A comparative TEM study of the as grown and annealed state is presented. INTRODUCTION For decades, the figure of merit ZT of thermoelectric materials did not exceed values of ~1. Research in the field of low-dimensional thermoelectric compounds showed that the thermoelectric performance can be enhanced significantly by nanostructuring [1]. In 2001, a breakthrough value for ZT of ~2.4 was obtained for p-conducting V2VI3-superlattices (SLs) [2]. This was achieved by a significant reduction of lattice thermal conductivity while maintaining a high electrical conductivity in the cross-plane direction of the SL. However, there has been no verification or reproduction of the mentioned results since 2001, inciting an interest in detailed systematic investigations necessary for a better understanding of the high performance of those SLs. The SLs consisted of stacks of Bi2Te3 and Sb2Te3. Thus, an investigation of the properties of these two binary compounds which constitute the building blocks of the SL structure is a good starting point for the synthesis and understanding of the extraordinary properties of those SLs. EXPERIMENTAL DETAILS The thin films are fabricated by a synthesis method called “nanoalloying” which has been proven to create thin films with good thermoelectric properties [3]. Layers of the different elements with a thickness in the (sub-) nm range are stoichiometrically deposited on a cold substrate and are subjected to a low-temperature annealing process to induce compound formation via a solid state

reaction. This approach originates from the method of modulated elemental reactants introduced by