Epitaxial Growth and Thermoelectric Properties of Bi 2 Te 3 Based Low Dimensional Structures
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Epitaxial Growth and Thermoelectric Properties of Bi2Te3 Based Low Dimensional Structures Joachim Nurnus, Harald Beyer, Armin Lambrecht, Harald Böttner Fraunhofer Institut Physikalische Messtechnik, Heidenhofstr. 8 D-79110 Freiburg i. Br., Germany ABSTRACT Bi2Te3 based low dimensional structures are interesting material systems to increase the thermoelectric figure of merit ZT by either the expected reduction of the thermal conductivity or by a possible power factor enhancement due to quantum confinement. Due to low lattice mismatch Bi2(Te1-xSex)3, PbSe1-xTex, as well as Pb1-xSrxTe, and BaF2 are suitable for Bi2Te3 based low dimensional structures. Especially due to their significantly enhanced band gap lead chalcogenide compounds like Pb1-xSrxTe (Pb0.87Sr0.13Te: 0.6 eV) are well-suited barrier materials in MQW structures. Alternatively the insulator BaF2 can be used for that purpose. Here we report mainly on results of different superlattice structures mentioned above grown by molecular beam epitaxy (MBE) on BaF2(111). The structural properties of these layers were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and secondary ion mass spectroscopy (SIMS). Structural performance and thermoelectric properties of different Bi2Te3 based superlattices were reported and compared with regard to their superlattice parameters. INTRODUCTION Calculations predict an enhancement of the thermoelectric figure of merit in low dimensional-structures -multi-quantum wells (MQW) and super-lattices (SL)- in comparison to bulk materials [1, 2]. Further calculations deal with those structures based on Bi2Te3 and PbTe [3]. Because of their high bulk figures of merit both materials are well-suited for thermoelectric applications. The predicted huge increase in ZT for 2D structures results on one hand from the large density of electron states due to quantum confinement. On the other hand the thermal conductivity is reduced by interface scattering in the boundary regions between the low dimensional structure forming layers [4]. A prerequisite for proving both approaches experimentally is the availability of epitaxial systems with compatible properties. These can be derived in a first approach from only a few physical material properties of the compounds. Figure 1– data taken from [5, 6] - shows the basic epitaxial map of the interesting compounds. In analogy to the experimental results concerning epitaxial deposition in the lead chalkogenide based pseudomorphic systems [6] one can expect similar behaviour for Bi2Te3-based epitaxy for the continuous series of Bi2Te3/Bi2Se3 solid solutions. In spite of extended miscibility gaps, which are partially known for the Sr-free systems of Bi2Te3/PbTe and Bi2Te3/PbSe [5] and which should be expected for BaF2/Bi2Te3 system, in all likelihood epitaxial growth will be possible as lattice parameters fit well in the hexagonal growth planes accordingly, see figure 1. Our concept is to structure these multifaceted possibilities into systems either suited for MQW’s or SL’s as out
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