Room-temperature MBE deposition, thermoelectric properties, and advanced structural characterization of binary Bi2Te3 and Sb2Te3 thin films

Highlights: ► MBE deposition at room temperature and subsequent annealing for phase formation. ► This growth procedure easily yielded stoichiometric Sb2Te3 and Bi2Te3 thin films. ► Sb2Te3 films revealed favorable charge carrier density and mobility. ► Advanced characterization of texture by high-energy X-ray diffraction. ► Precise chemical analysis by calibrated EDX spectroscopy and energy-filtered TEM. - Abstract: Sb2Te3 and Bi2Te3 thin films were grown at room temperature on SiO2 and BaF2 substrates using molecular beam epitaxy. A layer-by-layer growth was achieved such that metallic layers of the elements with 0.2 nm thickness were deposited. The layer structure in the as-deposited films was confirmed by X-ray diffraction and was seen more clearly in Sb2Te3 thin films. Subsequent annealing was done at 250 °C for 2 h and produced the Sb2Te3 and Bi2Te3 crystal structure as confirmed by high-energy X-ray diffraction. This preparation process is referred to as nano-alloying and it was demonstrated to yield single-phase thin films of these compounds. In the thin films a significant texture could be identified with the crystal c axis being almost parallel to the growth direction for Sb2Te3 and tilted by about 30° for Bi2Te3 thin films. In-plane transport properties were measured for the annealed films at room temperature. Both films yielded a charge carrier density of about 2.6 × 1019 cm−3. The Sb2Te3 films were p-type, had a thermopower of +130 μV K−1, and surprisingly high mobilities of 402 cm2 V−1 s−1. The Bi2Te3 films were n-type, showed a thermopower of −153 μV K−1, and yielded significantly smaller mobilities of 80 cm2 V−1 s−1. The chemical composition and microstructure of the films were investigated by transmission electron microscopy (TEM) on cross sections of the thin films. The grain sizes were about 500 nm for the Sb2Te3 and 250 nm for the Bi2Te3 films. In the Bi2Te3 thin film, energy-filtered TEM allowed to image a Bi-rich grain boundary phase, several nanometers thick. This secondary phase explains the poor mobilities of the Bi2Te3 thin film. With these results the high potential of the nano-alloying deposition technique for growing films with a more complex layer architecture is demonstrated.