Thermoelectricity of Double Doped Strontium Titanate

Lanthanum (La) doped Strontium Titanate (SrTiO₃) is amongst the most promising n-type thermoelectric materials for power generation. We report a double doping method for thin films of SrTiO₃ (STO), grown by Pulsed Laser Deposition (PLD), where doping of STO in the Sr-site by Lanthanum is accompanied by doping with oxygen vacancies. In the past theoretical predictions have shown that introducing oxygen vacancies in STO produces a high-effective mass defect band just below the conduction band edge, explaining the high seebeck coefficient observed in oxygen deficient STO. Based on careful transport measurements, we show that it is possible to obtain enhanced thermoelectric power factor by double doping, using La and oxygen vacancies in these thin films. With the aid of optical spectroscopic measurements, we establish the presence of the impurity band created by the vacancies and validate their role in the enhanced thermoelectric performance with structural and transport measurements. The presence of oxygen vacancies also serves to decrease the thermal conductivity due to effective phonon scattering.