Thermoelectric and Transport Properties of Vacuum Evaporated Bi2Te3xSe3(1−x) Thin Films

Bi2Te3xSe3(1−x) thin films having nearly equal thicknesses (≈ 2500 Å) were deposited onto unheated well cleaned glass substrates by thermal evaporation in vacuum. The selected samples were annealed at 423 K for 60 min in vacuum and used for the characterization. The deposition conditions like pressure, distance of the substrate and rate of deposition were maintained same, throughout the experiment. X-ray diffraction pattern exhibits polycrystalline rhombohedral structure which is typically represented in the hexagonal lattice, with strong preferential orientation of the crystallites along (1010) direction for all the compositions. The grain size observed from the FESEM images is about 25-50 nm. The films with composition x = 0.2, 0.3, 0.7, 0.8 and 0.9 are just about stoichiometric. It is observed that the resistivity of the films decreases with increase in temperature, consequently all the film samples of annealed Bi2Te3xSe3(1-x) thin films have negative temperature coefficients of resistivity, which suggested their semiconducting nature. The developed thermal emf was measured with respect to the temperature gradient. It is found that, for composition x = 0.1 and 0.2 showed n-type conduction of the samples. Whereas the samples for composition x = 0.3 to 0.9 showed p-type conduction of the Bi2Te3xSe3(1-x) ternary thin films.