Structural and thermoelectric properties of epitaxially grown Bi2Te3 thin films and superlattices

Abstract

Multi-quantum-well structures of Bi2Te3 are predicted to have a high thermoelectric figure of merit ZT. Bi2Te3 thin films and Bi2Te3∕Bi2(Te0.88Se0.12)3 superlattices (SLs) were grown epitaxially by molecular beam epitaxy on BaF2 substrates with periods of 12 and 6nm, 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 35μW∕cmK2. The lattice thermal conductivity varied between 1.60W∕mK for Bi2Te3 thin films and 1.01W∕mK for a 10nm 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 Bi2Te3 thin film and SL the dislocation density was found to be 2×1010cm−2. Bending of the SL with amplitudes of 30nm (12nm SL) and 15nm (6nm SL) and a wavelength of 400nm was determined. Threading dislocations were found with a density greater than 2×109cm−2. The superlattice interfaces are strongly bent in the region of the threading dislocations, undisturbed regions have a maximum lateral sie of 500nm. Thin films and SL showed a structural modulation [natural nanostructure (nns)] with a wavelength of 10nm and a wave vector parallel to (1,0,10). This nns was also observed in Bi2Te3 bulk materials and turned out to be of general character for Bi2Te3. 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.

Impact of the Topological Surface State on the Thermoelectric Transport in Sb2Te3 Thin Films

“…However, in contrast to previous theoretical model calculations, 7,8 decreased values of S were found experimentally [9][10][11] for Bi 2 Te 3 and Sb 2 Te 3 thin films and were recently corroborated by both model 12,13 and ab initio calculations 4 of our groups.…”

“…[9][10][11] By means of ab initio electronic structure calculations based on density functional theory we discussed the thermoelectric properties of the p-type TI Sb 2 Te 3 for various film thicknesses and temperatures. The topologically protected surface-state leads to metallic conduction of the thin films even in the semiconducting regime.…”

Review of 3D topological insulator thin‐film growth by molecular beam epitaxy and potential applications

“…Of course, those numbers also highly depend on the growth temperature and rate, which will be discussed later. Although it is difficult to gain a quantitatively assessment on the growth on different substrates at this time, it should be pointed out that in spite of the Van der Waals nature of growth, the lattice-matched substrates do give a better surface morphology, as in the case of Bi 2 Se 3 grown on CdS (-0.24%) [11,28] and InP (0.24%) [50] and Bi 2 Te 3 grown on BaF 2 (0.05%) [42]. This result seems to indicate that the strain caused by the lattice mismatch still affects the film quality to some extent.…”

“…This is called Van der Waals epitaxy [40,41], which relaxes the lattice-matching condition required for most common epitaxial growth of covalent semiconductors and their heterostructures. Because of this, a variety of substrates have been chosen for the growth of TIs, despite the large lattice mismatch between the films and the substrates, and relatively good epitaxial films have been achieved from various reports [11,[22][23][24][25][26][27][28][42][43][44][45][46][47][48][49]. Their lattice constants and their mismatches to different TIs are shown in Fig.…”

Influence of strain on anisotropic thermoelectric transport in Bi2Te3and Sb2TeHinsche1, Yavorsky2, Mertig3 et al. 2011Phys. Rev. B

“…It suggests that cross-plane transport along the direction perpendicular to the artificial interfaces of the SL reduces phonon heat conduction while maintaining or even enhancing the electron transport 3 . While some effort in experimental research was done [8][9][10][11][12][13] , only a few theoretical works discuss the possible transport across such SL structures 14,15 . While Park et al 14 discussed the effect of volume change on the in-plane thermoelectric transport properties of Bi 2 Te 3 , Sb 2 Te 3 and their related compound, Li et al 15 focussed on the calculation of the electronic structure for a Bi 2 Te 3 /Sb 2 Te 3 -SL, stating changes of the mobility anisotropy estimated from effective masses.…”