Structural and thermoelectric properties of epitaxially grown Bi_2Te_3 thin films and superlattices

摘要

Multi-quantum-well structures of Bi_2Te_3 are predicted to have a high thermoelectric figure of merit ZT. Bi_2Te_3 thin films and Bi_2Te_3/Bi_2(Te_(0.88)Se_(0.12))_3 superlattices (SLs) were grown epitaxially by molecular beam epitaxy on BaF_2 substrates with periods of 12 and 6 nm, respectively. Reflection high-energy electron diffraction confirmed a layer-by-layer growth, x-ray diffraction yielded the lattice parameters and SL periods and proved epitaxial growth. The in-plane transport coefficients were measured and the thin films and SL had power factors between 28 and 35 μW/cm K~2. The lattice thermal conductivity varied between 1.60 W/mK for Bi_2Te_3 thin films and 1.01 W/mK for a 10 nm SL. The best figures of merit ZT were achieved for the SL; however, the values are slightly smaller than those in bulk materials. Thin films and superlattices were investigated in plan view and cross section by transmission electron microscopy. In the Bi_2Te_3 thin film and SL the dislocation density was found to be 2 X 10~(10) cm~(-2). Bending of the SL with amplitudes of 30 nm (12 nm SL) and 15 nm (6 nm SL) and a wavelength of 400 nm was determined. Threading dislocations were found with a density greater than 2 X 10~9 cm~(-2). The superlattice interfaces are strongly bent in the region of the threading dislocations, undisturbed regions have a maximum lateral sie of 500 nm. Thin films and SL showed a structural modulation [natural nanostructure (nns)] with a wavelength of 10 nm and a wave vector parallel to (1,0,10). This nns was also observed in Bi_2Te_3 bulk materials and turned out to be of general character for Bi_2Te_3. The effect of the microstructure on the thermoelectric properties is discussed. The microstructure is governed by the superlattice, the nns, and the dislocations that are present in the films. Our results indicate that the microstructure directly affects the lattice thermal conductivity. Thermopower and electrical conductivity were found to be negatively correlated and no clear dependence of the two quantities on the microstructure could be found.