SrTiO3 is a promising n-type oxide semiconductor for thermoelectric energy conversion. Epitaxial thin films of SrTiO3 doped with both La and oxygen vacancies have been synthesized by pulsed laser deposition (PLD). The thermoelectric and galvanomagnetic properties of these films have been characterized at temperatures ranging from 300 K to 900 K and are typical of a doped semiconductor. Thermopower values of double-doped films are comparable to previous studies of La doped single crystals at similar carrier concentrations. The highest thermoelectric figure of merit (ZT ) was measured to be 0.28 at 873 K at a carrier concentration of 2.5 × 10 21 cm −3 .
Epitaxial thin films of strontium titanate doped with different concentrations of lanthanum and oxygen vacancies were grown on LSAT substrates by pulsed laser deposition technique. Films grown with 5-15% La doping and a critical growth pressure of 1-10 mTorr showed high transparency (>70-95%) in the UV-visible range with a sheet resistance of 300-1000 Ω/0. With the aid of UV-visible spectroscopy and photoluminescence, we establish the presence of oxygen vacancies and the possible band structure, which is crucial for the transparent conducting nature of these films. This demonstration will enable development of various epitaxial oxide heterostructures for both realizing opto-electronic devices and understanding their intrinsic optical properties.
Stability issues in thermoelectric Na x CoO 2 thin films have been solved by the addition of an in situ amorphous AlO x capping layer, which prevents previously reported degradation when exposed to air. These chemically stable thin films enable detailed analysis of the intrinsic thermoelectric properties and form a significant progress towards applications. Single phase Na x CoO 2 thin films with a low surface roughness are grown by pulsed laser deposition with either an epitaxial or textured crystal structure on Al 2 O 3 (001) or LaAlO 3 (001) single crystal substrates, respectively. For textured thin films a resistivity and thermopower of 0.99 mV cm and 69 mV K 21 are observed at room temperature, respectively. Based on an estimated thermal conductivity for thin films, the dimensionless figure of merit is very comparable to Na x CoO 2 single crystals, demonstrating the effectiveness of the developed capping layer.
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.
An automated apparatus capable of measuring the electrical conductivity and thermopower of thin films over a temperature range of 300-750 K is reported. A standard dc resistance measurement in van der Pauw geometry was used to evaluate the electrical conductivity, and the thermopower was measured using the differential method. The design of the instrument, the methods used for calibration, and the measurement procedure are described in detail. Given the lack of a standard National Institute of Standards and Technology (Gaithersburg, Md.) sample for high temperature thermopower calibration, the disclosed calibration procedure shall be useful for calibration of new instruments.
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