Overview Direct conversion of thermal to electrical energy using solid state energy conversion devices provides attractive advantages over many conventional energy conversion modules like flexibility of operation as refrigeration or power conversion mode, lack of moving parts, very high reliability and long life [1]. In spite of these advantages, these thermoelectric devices are plagued by very low efficiency. The efficiency of such devices is quantified by a dimensionless figure of merit, ZT = S2σT/κ, where S is the seebeck coefficient, σ and κ are the electrical and thermal conductivities of the material. Commercially available Bismuth Telluride based thermoelectric devices have a ZT of ~1. To improve the efficiency of these devices close to the state of art phase-change based refrigerators, a ZT of 4 is required, which corresponds to an efficiency of 30% of Carnot efficiency [2]. Increasing ZT is extremely difficult due to the coupling between the seebeck coefficient, the thermal and electrical conductivities. There has been several strategies employed to tackle this problem, one of which is to fine tune the other parameters given that atleast one or more of the parameters of ZT is extremely favorable using band gap engineering, nanostructuring etc. Summary of Research Recent reports on large thermoelectric response from Lanthanum doped Strontium Titanate (STO) [3] and multilayered STO based thin films [4], has kindled interest in these oxide based systems. STO offers a very high thermopower due to enhanced effective mass [5] and a rich possibility to fine tune both the electrical and thermal conductivities by various methods like band gap engineering and selective scattering of phonons etc. Doping of STO with La offers control over the electrical conductivity, but the doping should not significantly deteriorate the thermopower, with the increase in carrier concentration. Pulsed Laser Deposition of thin films offers a controlled epitaxial growth of oxide thin films with homogenous concentration profile and excellent crystallinity on appropriate substrates [6]. The project shall broadly address production of epitaxial thin films of La doped STO on epitaxially matching insulating substrates and then controlling various growth and materials parameters to produce high thermoelectric figure of merit. Other interesting reports on superconductivity [7] and high mobility electron gas [8] on oxide interfaces with STO as a component encourages a possibility to explore further science behind this intriguing materials system. Some of the results of the investigations on SrTiO3 led to discovery of introducing oxygen deficiencies in bulk SrTiO3 substrates at temperatures as low as 400C and the necessity to consider the substrate effects while dealing with thin film thermoelectrics [9]. References: 1. A Majumdar, Science, 303, 777 (2004). 2. F J DiSalvo, Science, 285, 703 (1999). 3. T Okuda, K Nakanishi, S Miyasaka and Y Tokura, Phys. Rev. B, 63, 113104 (2001). 4. H Ohta et al., Nature Mater. 6, 129 (2007). 5. Wilfried Wunderlich, Shingo Ohta, Hiromichi Ohta, Kunihito Koumoto, 
Proc. 24th Int. Conf. Thermoelectrics, Clemson USA, 237 (2005). 6. R Ramesh et al., Appl. Phys. Lett, 57, 1505 (1990). 7. N Reyren et al., Science, 317, 1196 (2007). 8. A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). 9. C Yu et. al., Appl. Phys. Lett. 92 (9), 2008, 092118.
Complex Oxides for Thermoelectrics
Members: Jayakanth Ravichandran