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Two-Dimensional Electron Layers in Perovskite Oxides Cover Two-dimensional electron layers in perovskite oxides Cover Stairstep twinning in lanthanum aluminate (LaAlO3), one of the main constituents of the perovskite heterostructures discussed in this thesis. Image was taken by Michael W. Davidson of the Florida State University and is used with permission. Ph.D. committee Chairman and secretary Prof. dr. G. van der Steenhoven (University of Twente) Supervisor Prof. dr. ing. D.H.A. Blank (University of Twente) Assistant supervisor Dr. ing. A.J.H.M. Rijnders (University of Twente) Members Prof. dr. Ir. H. Hilgenkamp (University of Twente) Prof. dr. P.J. Kelly (University of Twente) Prof. H.Y. Hwang (University of Tokyo, Japan) Prof. J.-M. Triscone (University of Geneva, Switzerland) Prof. dr. J. Aarts (Leiden University) Prof. dr. M.S. Golden (University of Amsterdam) The research described in this thesis was performed with the Inorganic Materials Science group, the MESA+ Research Institute at the University of Twente, the Netherlands and Hwang lab at the University of Tokyo, Japan as a part of the Nanoelectronic Materials flagship (TMM.6996) of the Dutch NanoNed national nanotechnology R&D initiative. This research was supported by NanoNed, a na- tional nanotechnology program coordinated by the Dutch Ministry of Economic Affairs. G.W.J. Hassink Two-dimensional electron layers in perovskite oxides Ph.D. thesis University of Twente, Enschede, the Netherlands. ISBN: 978-90-365-2918-1 DOI: 10.3990/1.9789036529181 Printed by W¨ohrmannPrint Service, Zutphen, the Netherlands Copyright c G.W.J. Hassink, 2009 TWO-DIMENSIONAL ELECTRON LAYERS IN PEROVSKITE OXIDES PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, Prof. dr. H. Brinksma, volgens besluit van het College voor Promoties in het openbaar te verdedigen op vrijdag 20 november 2009 om 15:00 door Gerrit Willem Johannes Hassink geboren op 2 maart 1980 te Ede Dit proefschrift is goedgekeurd door de promotor Prof. dr. ing. D.H.A. Blank en de assistent-promotor Dr. ing. A.J.H.M. Rijnders Contents 1 Attractive repulsion 1 1.1 Introduction . 1 1.2 Outline . 3 1.3 References . 5 2 Two-dimensional electron layers in correlated-electron oxides 7 2.1 Introduction . 7 2.2 Two-dimensional electron gases . 8 2.2.1 Potential wells in semiconductors . 9 2.2.2 Confinement of electrons . 11 2.2.3 Properties and applications of confined electron gases . 13 2.3 Correlated-electron oxides . 14 2.3.1 Independent vs. correlated electrons . 15 2.3.2 Transition metal oxides . 16 2.3.3 Electron correlation effects . 18 2.4 Mott insulator/band insulator heterostructures . 20 2.4.1 LaTiO3/SrTiO3 heterostructures . 20 2.4.2 LaVO3/LaAlO3 heterostructures . 22 2.5 Polar/non-polar interfaces . 23 2.5.1 LaAlO3//SrTiO3 interfaces . 24 2.5.2 Oxygen vacancy dependence . 29 2.6 Concluding remarks . 33 2.7 References . 35 3 Fabrication & characterization of perovskite thin films 45 3.1 Introduction . 45 3.2 Thin film fabrication . 46 3.2.1 Substrates & targets . 46 3.2.2 Principles & advantages of pulsed laser deposition . 47 3.2.3 High-pressure reflective high-energy electron diffraction . 49 3.2.4 Experimental set-up . 51 3.3 Structural & compositional characterization . 51 3.3.1 Atomic force microscopy . 53 3.3.2 X-ray diffraction . 53 3.3.3 (Scanning) Transmission electron microscopy . 53 3.3.4 Electron energy loss spectroscopy . 54 3.4 Electronic & optical characterization . 54 3.4.1 X-ray photoelectron spectroscopy . 54 3.4.2 Electronic transport analysis . 55 3.4.3 UV-vis photoreflectance spectroscopy . 57 3.4.4 UV-vis absorption & ellipsometry . 58 3.5 Concluding remarks . 58 3.6 References . 59 4 Electron localization in LaTiO3/LaAlO3 heterostructures 61 4.1 Introduction . 61 4.2 Sample fabrication . 63 4.3 Two-dimensional Mott insulator . 67 4.3.1 Structural characterization . 67 4.3.2 Comparing LaTiO3/SrTiO3 and LaTiO3/LaAlO3 . 69 4.3.3 Origin & stability of the 2D Mott insulating state . 72 4.4 Single layers of LaTiO3 in LaAlO3 . 76 4.4.1 Structural characterization . 77 4.4.2 Electronic configuration as a function of surface separation 77 4.4.3 Band bending as a function of surface separation . 84 4.5 Conclusions . 90 4.6 References . 92 5 Fabrication & analysis of single LaAlO3//SrTiO3 interfaces 95 5.1 Introduction . 95 5.2 Pressure dependence from literature data . 97 5.3 Alternative model for LaAlO3//SrTiO3 interfaces . 101 5.4 Sample fabrication . 105 5.5 Structural characterization . 107 5.6 Subtle details of the substrate . 108 5.7 Influence of the PLD process . 109 5.7.1 Film thickness . 110 5.7.2 Gas pressure & composition . 112 5.8 LaO//TiO2 interfaces within LaTiO3 . 116 5.9 Conclusions . 120 5.10 References . 123 6 Interacting interfaces in LaAlO3//SrTiO3 heterostructures 129 6.1 Introduction . 129 6.2 Sample fabrication . 131 6.3 Structural characterization . 132 6.4 Transport in double-interface heterostructures . 134 6.4.1 Inclusion of a LaTiO3 intra-block interface . 135 6.4.2 Overview of the transport properties . 136 6.4.3 Modelling the interface interaction . 139 6.4.4 Multi-band transport properties at low temperature . 148 6.4.5 Robustness of double-interface heterostructures . 155 6.5 Optical investigations of double-interface heterostructures . 156 6.5.1 Photocarrier dynamics from photoconductivity relaxation . 157 6.5.2 Photoreflectance investigation of band transitions . 159 6.6 Conclusions . 162 6.7 References . 164 7 Epilogue 169 Summary 171 Samenvatting 175 Dankwoord 181 Chapter 1 Attractive repulsion 1.1 Introduction From the earliest days of the natural sciences, humanity has tried to understand and classify the world around us. The 'classical' Elements in Greek, Chinese and Japanese philosophy are examples of this. Even electric charge had a place in this picture; in fact, the very word 'electron' is derived from the Greek word for amber, 'elektron', as they knew that when you rub a piece of amber with a fur skin, a static electric charge will develop[1]. Later on, the alchemists of the Middle Ages would take this philosophical basis and, combined with continuing technological development, slowly start to turn these natural philosophies into the natural sciences we know today. Driven by the same wish to understand the world around them as the ancient Greeks and modern scientists, they strove to understand matter and energy. Indeed, their very goal was the ability to control matter to such a point that one could turn common materials into gold. Since the first half of the last century we actually have the knowledge how to achieve that goal (though it is tremendously inefficient, costing much more than the possible gains)[2]. This was the result of a huge increase in the understanding of matter following the development of quantum mechanics at roughly the same time. Slowly, metals, semiconductors and simple insulators became understood as the field of solid state science was developed. This understanding of these simple materials, though, is based on a simple as- sumption: that the outer electrons of the atoms making up these materials are independent from each other[3, 4]. This simplification to only include electron-ion and ion-ion interaction greatly simplifies the calculations and allows for a surpris- ingly comprehensive theory of metals, most semiconductors and simple insulators. Indeed, its success greatly contributed to the technological advances that occurred during the last century. The fast development of IC technology described by 1 Moore's law[5] was possible only because of the almost equally fast increase in materials knowledge. However, there are many materials and properties that cannot be described using this independent electron approximation. For these materials the electron-electron interaction has to be explicitly included. These correlated electron materials have always been of interest (for example, lodestone, Fe2O3, was already known and used as a compass in the 4th century in China[6]), but more recently these materi- als and their applications have become of more and more technological importance because of their diverse and often desirable properties. It is often thought that the only way to continue Moore's law is to switch to new, correlated, materials as the end of the semiconductor parameter space is coming closer and closer. Most of the correlated electron materials research focuses on the transition metal oxides[7], of which the previously mentioned lodestone is one. Superconductors[8, 9], transparent conductors[10], high dielectric constant insulators[11], colossal mag- netoresistance sensors[12]; all these, and more, can be found within this group of materials. And all these attractive properties are due to the Coulomb interactions within the compounds. The ultimate example of this correlated nature is the Mott insulator. From independent-electron band theory we know that a partially filled band should conduct. In transition metal oxides, the situation can be different. Where in a normal metal the electrons are 'spread out' over the entire space occupied by the metal, in a transition metal oxides the narrow orbitals force the electrons to move by hopping from ion to ion. If, however, there is already an electron present on the target site, there will be a Coulomb repulsion between the stationary and moving electron. If this energy becomes too large to overcome, hopping will not occur and thus the material will become insulating. These Mott insulators are the 'parent' compounds from which most oxide super- conductors are derived through doping, either with electrons or with holes. At the same time, they provide very interesting systems to study electron correlation itself as well. The Coulomb force balance can easily be disturbed by pressure[13] or doping[14].
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