Influences of donor/acceptor impurities in Bismuth Telluride

The role of anion and cation doping in Bi2Te3 on thermoelectric property have been examined for various doping concentration. Here, the samples have been synthesized in the form of single crystals as well as in the form of thin films. Zone melting method has been employed to grow single crystals. The structural characterization of the grown compound have been characterized by EDX and XRD technique and either Se or Fe doping has not cause a major change in the c/a ratio. Prior to the subjected transport properties characterization, the surface morphology has been exposed by optical microscopy and by force microscopy and electron microscopy at the microscopic level and observed growth mechanisms, dislocations, stacking faults, and deformation. The dislocation density and micro hardness have found to be increased with the increasing Se and Fe concentration. All the crystals exhibited semiconducting behavior as confirmed from the temperature dependence of electrical conductivity and seebeck coefficient. Crystals with doping of Se shows n-type of conduction whereas Fe doping implies p-type conduction by Hall coefficient measurement which further confirm by Seebeck coefficient measurements. Transport parameters shown strong dependency on scattering mechanism; carrier concentration as well as defects increased whereas carrier mobility decreased with the doping concentration. In the case of thermo electric measurements, the values of seebeck coefficient have increased with concentration for both Se and Fe doping over the measured temperature range. Although, figure of merit(ZT) found quite low in Fe doped Bi2-yFeyTe3 crystals due to very high thermal conductivity.
In the case of the thin film, the deposition conditions have optimized, specifically in regards of appropriate substrate temperature for good structural thin films, for thermal evaporation and for e-beam deposition of the Se and Fe doped Bi2te3 films for three different sets of film thickness. The structure and morphology of the films have characterized by electron microscopy and found good structural film with the increased grain size at elevated substrate temperature for Bi2Te3-xSex.