Effect of rapid thermal annealing on thermoelectric properties of bismuth telluride films grown by co-sputtering

Thermoelectric bismuth telluride (Bi2Te3) films were prepared by co-sputtering of bismuth and tellurium targets and changes in their characteristic properties after rapid thermal annealing were investigated. Films with four different compositions were elaborated by adjusting the powers for each target. Rapid thermal treatments in range of 300-400 deg. C were carried out during 10 min under reductive atmosphere (flowing Ar gas with 10% H2). Electrical transport properties of the films were determined by Hall effect measurements. As increasing the annealing temperature, carrier concentrations of the films were decreased while their mobility increased. These changes were clearly observed for the films close to the stoichiometric composition. Seebeck coefficients of all samples have negative values, indicating the prepared films have n-type conduction, and their magnitude was increased after the thermal treatment. Rapid thermal annealing was found to be very effective in improving the thermoelectric properties of Bi2Te3 thin films. Recrystallization of Bi2Te3 phase has caused the enhancement of thermoelectric properties, along with the decrease of the carrier concentration. Maximum of Seebeck coefficient and corresponding power factor were obtained for the stoichiometric film annealed at 400 deg. C (about -128 μV/K and 9 x 10-4 W/K2 m, respectively). At further higher temperatures, mound defects were observed on surface due to the evaporation of Te elements and consequent disruption of film's structure