Thermal and Electrical Transport in Oxide Heterostructures By Jayakanth Ravichandran A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Applied Science and Technology in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Ramamoorthy Ramesh, Co-Chair Dr. Arunava Majumdar, Co-Chair Professor Junqiao Wu Professor Oscar Dubon Professor Joel Moore Fall 2011 Thermal and Electrical Transport in Oxide Heterostructures Copyright 2011 by Jayakanth Ravichandran 1 Abstract Thermal and Electrical Transport in Oxide Heterostructures by Jayakanth Ravichandran Doctor of Philosophy in Applied Science and Technology University of California, Berkeley Professor Ramamoorthy Ramesh, Co-Chair Dr. Arun Majumdar, Co-Chair This dissertation presents a study of thermal and electrical transport phenomena in heterostructures of transition metal oxides, with specific interest in understanding and tailoring thermoelectricity in these systems. Thermoelectric energy conversion is a promising method for waste heat recovery and the efficiency of such an engine is directly related to a material dependent figure of merit, Z, given as S2σ/κ, where S is thermopower and σ and κ are electrical and thermal conductivity respectively. Achieving large figure of merit has been hampered by the coupling between these three thermoelectric coefficients, and the primary aim of this study is to understand the nature of thermoelectricity in complex oxides and identify mechanisms which can allow tuning of one or more thermoelectric coefficients in a favorable manner. Un- like the heavily studied conventional thermoelectric semiconductors, transition metals based complex oxides show conduction band characteristics dominated by d-bands, with much larger effective masses and varying degrees of electron correlations. These systems provide for exotic thermoelectric effects which are typically not explained by conventional theories and hence provide an ideal platform for exploring the limits of thermoelectricity. Meanwhile, oxides are composed of earth abundant elements and have excellent high temperature stability, thus providing compelling technologi- cal possibilities for thermoelectrics based power generation. In this dissertation, we address specific aspects of thermoelectricity in model complex oxide systems such as perovskite titanates and layered cobaltates to understand thermal and thermoelectric behavior and explore the tunability of thermoelectricity in these systems. The demonstration of band engineering as a viable method to tune physical prop- erties of materials is explored. The model system used for this case is strontium titanate, where two dopants such as La on the Sr-site and oxygen vacancies are employed to achieve band engineering. This method was used to obtain tunable transparent conducting properties and thermoelectric properties for heavily doped strontium titanate. The second aspect investigated is the use of strongly correlated 2 materials for thermoelectricity. The cobaltates, specifically layered cobaltates, show large thermopower even at very large carrier densities. The coupling of thermopower and electrical conductivity is shown to be weaker for a strongly correlated material such as cobaltate, which opens up possibilities of complete decoupling of all three ther- moelectric coefficients. Finally, the thermal properties of complex oxides, specifically in perovskite titanates, is addressed in detail. Thermal conductivity is demonstrated to be a sensitive probe for defects in a system, where processing conditions play a sig- nificant role in modulating the crystallinity of the material. The perovskite titanate superlattice system of strontium titanate and calcium titanate is used beat alloy limit. It also shows interesting period thickness dependent thermal properties. The possible origin of this effect is briefly discussed and future directions for this research is also elaborated in detail. i In memory of my grandfather ii Table of Contents List of Figures v List of Tables xi 1 Introduction 1 1.1 Background and Motivation . 1 1.1.1 Introduction . 1 1.1.2 Thermoelectric effects . 3 1.1.3 Thermoelectric energy conversion and figure of merit . 4 1.1.4 Theoretical aspects of thermoelectric transport . 6 1.2 Towards a high figure of merit . 11 1.2.1 Figure of merit for bulk materials . 11 1.2.2 History of thermoelectricity . 14 1.2.3 New physical principles towards high figure of merit . 16 1.2.4 Thermoelectric phenomena in oxides . 17 1.3 Summary . 19 2 Materials synthesis and characterization 20 2.1 Thin film growth { Pulsed laser deposition . 20 2.2 Characterization techniques . 22 2.2.1 Reflection high energy electron diffraction . 22 2.2.2 X-ray diffraction and reflectivity . 24 2.2.3 Atomic force microscopy . 27 2.2.4 Transmission electron microscopy . 28 2.2.5 Rutherford backscattering . 28 2.3 Transport and spectroscopic measurements . 29 2.3.1 Electrical transport . 29 2.3.2 Thermal transport . 32 2.3.3 High temperature thermoelectric measurements . 34 2.3.4 Optical spectroscopy . 40 2.4 Summary . 41 iii 3 Band engineering of strontium titanate 42 3.1 Introduction . 42 3.2 Electronic structure of SrTiO3 ...................... 43 3.3 Doping of SrTiO3 ............................. 45 3.3.1 A-site doping . 45 3.3.2 B-site doping . 46 3.3.3 Oxygen vacancy doping . 46 3.4 The concept of double doping . 47 3.5 Controlling electrical conductivity and optical transparency using dou- ble doping . 47 3.5.1 Background and motivation . 47 3.5.2 Experimental methods . 48 3.5.3 Results and discussions . 49 3.5.4 Summary . 52 3.6 Tuning the electronic effective mass using double doping in SrTiO3 . 53 3.6.1 Background and motivation . 53 3.6.2 Experimental methods . 54 3.6.3 Results and discussions . 55 3.6.4 Summary . 59 3.7 High temperature thermoelectric response of double doped SrTiO3 . 60 3.7.1 Background and motivation . 60 3.7.2 Experimental methods . 61 3.7.3 Results and discussions . 62 3.7.4 Summary . 67 3.8 Summary and future outlook . 68 4 Thermal transport in perovskite titanate films and superlattices 70 4.1 Introduction . 70 4.2 Fundamentals of thermal conductivity . 70 4.2.1 Principles . 70 4.2.2 Phonon scattering mechanisms . 74 4.2.3 Thermal limits of solids . 76 4.3 Thermal conductivity as a metric for crystalline quality of SrTiO3 . 77 4.3.1 Background and motivation . 78 4.3.2 Experimental methods . 78 4.3.3 Results and discussions . 80 4.3.4 Summary . 81 4.4 Thermal transport in superlattices . 82 4.4.1 Introduction . 82 4.4.2 Interface scattering { fundamentals and theoretical models . 82 4.4.3 Survey of previous work . 83 4.5 Thermal transport across SrTiO3/CaTiO3 superlattices . 84 iv 4.5.1 Background . 84 4.5.2 Experimental methods . 85 4.5.3 Results and discussions . 85 4.5.4 Summary . 90 4.6 Summary and future outlook . 91 5 Thermoelectricity in strongly correlated oxides 92 5.1 Introduction . 92 5.2 Thermoelectricity of cobalt oxides . 92 5.2.1 Introduction . 92 5.2.2 Hubbard model . 93 5.2.3 Survey of previous work . 94 5.3 Size effects on thermoelectricity in Bi2Sr2Co2Oy . 94 5.3.1 Introduction . 95 5.3.2 Experimental methods . 95 5.3.3 Results and discussions . 96 5.3.4 Summary . 101 5.4 Summary and future outlook . 101 6 Conclusion and future directions 102 Bibliography 106 v List of Figures 1.1 Sankey diagram of the 2009 US annual energy flow showing the pri- mary energy sources, their end uses and losses. Courtesy: Lawrence Livermore National Laboratory . 2 1.2 Diagram of the circuit on which Seebeck discovered the Seebeck effect. A and B are two different metals. Courtesy: Wikipedia . 3 1.3 Schematic of a thermoelectric generator made of n- and p-type semi- conductors connected to an external load R. 5 1.4 Schematic figure showing the density of states as a function of energy for a three dimensional isotropic electron distribution. The calculated differential electrical conductivity and differential thermopower as a function of the position of chemical potential are shown. When the chemical potential is deep in the band, we get metallic behavior and hence high electrical conductivity and low thermopower, but in the case of chemical potential at the edge of the band or in the gap, we get semiconducting behavior and hence, low electrical conductivity and high thermopower. The total electrical conductivity and thermopower can be obtained by evaluating the area under the curve for the differ- ential counterparts. 11 1.5 Schematic figure showing the variation of electrical and thermal con- ductivity and thermopower as a function of carrier concentration (den- sity). 14 1.6 Schematic showing the evolution of ZT over the last 60 years for a temperature range of 150{1200 K. 15 1.7 Schematic demonstration of electron filtering using a metal semicon- ductor metal heterostructure. 17 1.8 A plot of the product of powerfactor (S2σ) and temperature against the filling for NaxCoO2. The filling can tune the correlation and span phases from band insulator to a Mott insulator. Figure from Ref. [33] 18 2.1 Schematic of the pulsed laser deposition with in-situ characterization { reflection high energy electron diffraction (RHEED). Image courtesy: Jeroen Huijben. 21 vi 2.2 The intensity variation of the specular spot as 4 monolayers of CaTiO3 and 4 monolayers of SrTiO3 are grown on a SrTiO3 substrate. The RHEED pattern for the SrTiO3 substrate and the film after growth of (4 monolayers of CaTiO3 + 4 monolayers of SrTiO3) x 65 superlattice. 23 2.3 The XRD pattern of the film aligned to the out-of-plane direction of the substrate. The substrate used here is DyScO3 (110) and the film grown is 15% La doped SrTiO3 at base pressure. 25 2.4 The reciprocal space map for a superlattice with a period consisting of 1 unit cell of SrTiO3 and 1 formula unit cell of CaTiO3 with a total thickness of ∼ 200 nm on a SrTiO3 substrate.