XXXVI Reunión Bienal de la Real Sociedad Española de Física

The Influence of Defects and Epitaxial Stress on the Thermal Conductivity of Oxide-based Thin-Films

Not scheduled

Aula Magna (Santiago de Compostela, Facultade de Química)

Quantum Materials and Technologies (GEFES)

Description

The extremely large electron mobility in lightly doped SrTiO3 (STO) [1], along with its characteristic band degeneracy at the center of the Brillouin zone [2], makes this oxide an interesting material from the thermoelectric point of view. Ohta et al. showed that its thermoelectric efficiency can be largely increased by reducing its dimensionality [3]. This, along with the possibility to support a two-dimensional electron gas at the interface with LaAlO3, launched the interest in the thermoelectric properties of SrTiO3 thin-films. However, an experimental determination of the complete figure of merit of this system, including the effect of epitaxial stress on the electronic and thermal properties of well characterized thin-films, is lacking. Here we report the thermoelectric properties of SrTiO3 (STO) and CaTiO3 (CTO) thin-films prepared by Pulsed Laser Deposition (PLD), under different degrees of epitaxial stress, as well as different deposition conditions. As we have previously reported [4], it is possible to tune the properties of the thin films during the deposition process, by modifying the oxygen pressure inside the PLD chamber. Another important factor that can determine the magnetotransport and thermoelectric properties of these films is the epitaxial stress induced to the thin films by deposition on top of different substrates (such as STO, (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT) LaAlO3 (LAO) and DyScO3 (DSO)). In order to determine the thermoelectric efficiency of these oxides, it is crucial to measure the thermal conductivity of the films and for this reason, we have used the Frequency Domain Thermoreflectance (FDTR) [5] technique. It is a novel, very fast (single spot measurement taking ~10min) and non-contact method, which requires minimum transducer layer depositions and makes it possible to measure a vast range of materials, including thin film and single crystal-bulk conductive materials. Measurements of the thermal conductivity by FDTR show that cationic, as well as anionic defects play a very important role in the thermal conductivity of these films. Moreover, we observed a clear influence of epitaxial stress on the thermal transport, which can be used to further optimize the thermoelectric performance of the films. We show that different growth conditions allow the control of the final concentration of defects in the sample [4], which also determine the electrical conductivity and Seebeck coefficient. These results show that a careful control of the concentration of defects (namely cation and anion vacancies) during growth, combined with the effect of the epitaxial stress, can be used to optimize the thermoelectric figure of merit of these oxides. Acknowledgments This work was supported thethe Ministerio de Eonomía y Competitividad of Spain (Project No. MAT2016-80762-R) the Consellería de Cultura, Educación e Ordenación Universitaria (ED431F 2016/008, and Centro singular de investigación de Galicia accreditation 2016-2019, ED431G/09) and the European Regional Development Fund (ERDF). References [1] D. W. Reagor, V. Y. Butko, Nature Materials 4, 593 (2005) [2] A. F. Santander-Syro et al., Nature 469, 189 (2011). [3] H. Ohta , S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Y. Nakanishi, Y. Ikuhara, M. Hirano, H. Hosono & K. Koumoto, Nature Materials 6, 129 - 134 (2007) [4] A. Sarantopoulos, E. Ferreiro-Vila, V. Pardo, C. Magén, M. H. Aguirre, and F. Rivadulla, Phys. Rev. Lett. 115, 166801 [5] JA Malen, K Baheti, T Tong, Y Zhao, JA Hudgings, A Majumdar, Journal of Heat Transfer 133 (8), 081601