Bi_2Te_3: Structural Modulations in Epitaxially Grown Superlattices and Bulk Materials

摘要

Multiquantum well structures of Bi_2Te_3 are predicted to show an enhancement of the thermoelectric figure of merit ZT. Electron-conducting Bi_2Te_3 thin films and superlattices (SL) with a period of 12 nm were epitaxially grown on BaF_2 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 2·10~9 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 Bi_2Te_3 thin films, superlattices and bulk materials. The structural modulation is found to be of general character for Bi_2Te_3 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.