The effect of annealing treatment on thermoelectric properties of nanostructured Bi_0.5Sb_1.5Te₃ thin films fabricated by ball milling and thermal evaporation

Bismuth antimony telluride compounds are among the best thermoelectric materials with high efficiency. Many parameters including the chemical composition, microstructure, synthesis procedure, deposition process, thickness, and annealing treatment can influence on thermoelectric properties of thin films. In this work, the effect of the annealing process on morphology and thermoelectric properties of Bi_0.5Sb_1.5Te₃ thin films was investigated. The films were fabricated by ball milling and thermal evaporation process. Subsequently, annealing treatment was performed on some of the thin films at 373 K for 1 h. X-ray diffraction patterns confirmed that the procedure considered in the present research is successful in the synthesis of nanostructured Bi_0.5Sb_1.5Te₃ thin film with a single phase of a ternary alloy. Furthermore, the EDS result demonstrated a slight increase of Te element versus the stoichiometric ratio. Analysis of SEM images showed that the grain size of the thin film materials decreases after the annealing process. Thermoelectric results illustrated that the annealed thin films show a higher carrier concentration (3.41 × 10¹⁴ cm⁻²) at 305 K in comparison to the unannealed ones (3.28 × 10¹⁴ cm⁻²). Furthermore, the Seebeck coefficient decreased from 204 to 168 µV/K at 323 K, as the samples were annealed. However, the power factor of the annealed thin films was as high as 2.8 µW/k².cm at 473 K in comparison with the power factor of the unannealed thin films (1.3 µW/k².cm). In conclusion, the annealing process could improve some properties like electrical conductivity and power factor affecting thermoelectric properties of thin films.