ICT2014 Abstracts

In 2001, outstanding properties were reported  for Bi₂Te₃/Sb₂Te₃ thin film superlattice (SL) structures grown by metal-organic chemical vapor deposition (MOCVD) [[1]]. In order to gain more insight into the path-breaking ZT value of 2.4 and the underlying mechanisms, investigations aiming at reproducing, re-examining and possibly improving this kind of nanolayered structures were carried out.

Two synthesis methods were used: 1.) Nanoalloying, i.e. the deposition of element films and subsequent annealing to induce compound formation by sputtering / MBE setup and 2.) Epitaxial deposition by MBE. We present a comprehensive study on transport properties, structural properties and thermal stability of the SLs by (HR)XRD, SIMS, SEM and (S)TEM. Supplementary theoretical calculations concerning the electrical anisotropy of Bi₂Te₃ / Sb₂Te₃ SLs will also be shown.

The nanoalloyed SLs are easy to fabricate on many substrate materials, display a high structural quality and very pronounced c-axis texture, high PFs of up to 40-50 µW/cmK² and thermal conductivities down to 0.40 W/mK. However, the thermal stability of the nanostructuring is inferior to epitaxial material.

The epitaxially deposited Bi₂Te₃/Sb₂Te₃ SL films display very sharply defined interfaces down to period lengths as small as 6 nm. This is the first reported reproduction of Bi₂Te₃/Sb₂Te₃ SLs with such small period lengths. Low lattice thermal conductivities down to ~ 0.3 W/mK were determined. Remarkably, due to charge carrier compensation effects only low Seebeck coefficients and carrier mobilities were observed, preventing high ZT values. In-situ XRD and TEM heating experiments showed a decay of the nanostructure above temperatures of ~ 350 °C. Annealing the films led to a huge improvement of the power factor to values of 46 µW/cmK².