Bi 2 Te 3 : Structural Modulations in Epitaxially Grown Superlattices and Bulk Materials
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Bi2Te3: Structural Modulations in Epitaxially Grown Superlattices and Bulk Materials Nicola Peranio1, Oliver Eibl1, and Joachim Nurnus2 1 Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, D-72076 Tübingen, Germany 2 Fraunhofer Institut Physikalische Messtechnik, Heidenhofstr. 8, D-79110 Freiburg, Germany ABSTRACT Multiquantum well structures of Bi2Te3 are predicted to show an enhancement of the thermoelectric figure of merit ZT. Electron-conducting Bi2Te3 thin films and superlattices (SL) with a period of 12 nm were epitaxially grown on BaF2 substrates by molecular beam epitaxy. The microstructure was investigated by transmission electron microscopy. The SL could be imaged with strong contrast yielding a period of 12.0±0.5 nm. The SL is slightly bent with an amplitude of 30 nm and a wave length of 400 nm. Threading dislocations were found with a density of 2·109 cm-2. The SL interfaces are strongly bent close to threading dislocations, undisturbed regions have a maximum lateral size of 500 nm. A structural modulation (nns) with a wave length of 10 nm was found in Bi2Te3 thin films, superlattices and bulk materials. The structural modulation is found to be of general character for Bi2Te3 materials and is superimposed to the average structure. It was analysed in detail by stereomicroscopy in bulk material yielding a pure structural modulation with a displacement vector parallel to [5,-5,1] and a wave vector parallel to (1,0,10). The investigations showed the presence of none, one or two (nns). The number of (nns) and thereby the thermoelectric properties might be controlled by the growth parameters. Phonons should be scattered on the sinusoidal strain field of the (nns) yielding (i) a significantly decreased thermal conductivity, (ii) a reduced dimensionality and (iii) anisotropic transport coefficients in the basal plane. INTRODUCTION The thermoelectric figure of merit ZT was predicted to increase in low-dimensional structures based on Bi2Te3 superlattices (SL) compared to bulk materials (ZT=1 at 300 K) [1]. Hole-conducting Bi2Te3/Sb2Te3 SL epitaxially grown on GaAs substrates by metallorganic chemical vapor deposition MOCVD showed a spectacular ZT of 2.4 [2,3]. The subject of this paper are electron-conducting Bi2Te3 thin films and Bi2(Te1-xSex)3/Bi2(Te1-ySey)3 SL epitaxially grown on BaF2-substrates [4-6]. Measurements of transport coefficients [4-7] (table I) yielded a reduced lattice thermal conductivity but also an unfavourable smaller power factor for the superlattices. Therefore, the thermoelectric figures of merit could not be increased and remained close to those in bulk materials. Microstructural analysis is required to explain the poor enhancement of ZT in n-type Bi2Te3 thin films and SL. TEM is particularly useful for studying these materials since it combines structural and chemical analysis and allows to image directly the SL and dislocations by their strain contrast. Structural analysis of n-type and p-type Bi2Te3 bulk materials yielded a dislocation density
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