Room-temperature MBE deposition, thermoelectric properties, and advanced structural characterization of binary Bi ₂Te ₃ and Sb ₂Te ₃ thin films

Sb ₂Te ₃ and Bi ₂Te ₃ thin films were grown at room temperature on SiO ₂ and BaF ₂ 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 Sb ₂Te ₃ thin films. Subsequent annealing was done at 250 °C for 2 h and produced the Sb ₂Te ₃ and Bi ₂Te ₃ 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 Sb ₂Te ₃ and tilted by about 30° for Bi ₂Te ₃ 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 × 10 ¹⁹ cm ^-3. The Sb ₂Te ₃ films were p-type, had a thermopower of +130 μV K ^-1, and surprisingly high mobilities of 402 cm ² V ^-1 s ^-1. The Bi ₂Te ₃ films were n-type, showed a thermopower of -153 μV K ^-1, and yielded significantly smaller mobilities of 80 cm ² 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 Sb ₂Te ₃ and 250 nm for the Bi ₂Te ₃ films. In the Bi ₂Te ₃ 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 Bi ₂Te ₃ 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.