NANOSCALE ELECTRIC PHENOMENA AT OXIDE SURFACES AND INTERFACES BY SCANNING PROBE MICROSCOPY Sergei V. Kalinin A DISSERTATION in the Materials Science and Engineering Presented to the Faculties of the University of Pennsylvania in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy 2002 COPYRIGHT SERGEI VASILYEVICH KALININ 2002 ACKNOWLEDGEMENTS First and foremost, I would like to thank Dawn Bonnell for being an outstanding supervisor. She introduced me to the field of nanoscience and has guided my research from the first day at the AFM controls to the finer points of science. To her I owe my devotion to SPMs and all the things nano. Many parts of this dissertation would be impossible if it were not for the collaboration with many people at Penn and elsewhere. I am deeply grateful to Prof. Michael Cohen and Prof. Vasek Vitek (Upenn), Prof. Mark Kachanov (Tufts) and Dr. A.E. Giannakopoulos and Prof. Subra Suresh (MIT) for enlightening discussions on electrostatic theory and continuum mechanics. Collaborations with Albina Borisevich, Matt Suchomel, and Prof. Peter Davies at U.Penn and G. Popov and Prof. Martha Greenblatt at Rutgers have allowed me to satisfy my curiosity and them to learn more about their favorite materials. The discussions with Dr. A. Gruverman (NCSU) were most useful to understand what's going on under the AFM tip. Samples from Steve Dunn (Cranfied) have helped us to jumpstart our ferroelectric lithography program. Marcus Freitag and Prof. Charlie Johnson (UPenn) have introduced me, albeit briefly, to the wondrous nanotubeland. This thesis would be incomplete without the strontium titanate samples from and discussions with Prof. Gerd Duscher (ORNL and NCSU) and Dr. Steve Pennycook (ORNL). The assistance from Dr. Marco Radosavljevic and Juraj Vavro (Upenn) in low temperature transport measurements is greatly appreciated. Last but not least, I would like to acknowledge D. Gorbachev for software support and advice during my four years at Penn and before. Finally, I am extremely grateful to the members of my dissertation committee – Prof. Takeshi Egami, Prof. I-Wei Chan and Prof. Charlie Johnson, who have undertaken the heavy burden of reading this manuscript. During my time at Penn, I constantly felt the support and encouragement from the members of Bonnell group. There are several people in Bonnell group whom I want to thank in particular. Brian Huey and Nick Angert for introducing me to AFM and sharing its small tricks, which would otherwise take years to discover. Tony Alvarez, iii for providing constant support in most endeavors, both in the lab and outside, and sharing the experience on how to live in the USA. Rui Shao, for invaluable contribution in putting our nanoscience on the solid computerized basis and taking on the projects I held in the high esteem. I would also like to thank all other people at Bonnell group, – Jack Smith, Ed Peng, Prof. Jeff Sellar, Carolyn Johnson, Fabio Weibel, Cyrill Ruegg, Dr. Zonghai Hu and Dr. Xiaojun Lei – thank you all. The research at Penn would be impossible without the assistance from the staff members. I am especially grateful to Vladimir Dominko, whose experience and personality rule over the microfabrications laboratory, for assistance in micropatterning and transport measurements; Dr. Jim Ferris and Dr. Doug Yates, for help in electron microscopy and numerous useful discussions; Bill Romanow, for always lending helping hand; Pat Overend and Irene Clements for making life so much easier. Finally, I greatly acknowledge the financial support from NSF Grant DMR 00- 79909 and NSF Grant DMR 00-80863 and DoE grant DE-FG02-00ER45813-A000. iv ABSTRACT NANOSCALE ELECTRIC PHENOMENA AT OXIDE SURFACES AND INTERFACES BY SCANNING PROBE MICROSCOPY Sergei V. Kalinin Dissertation Supervisor: Prof. Dawn A. Bonnell Strong coupling between mechanical, electrical and magnetic properties in oxide materials, heterostructures and devices enable their widespread applications. Achieving the full potential of oxide electronics necessitates quantitative knowledge of material and device properties on the nanoscale level. In this thesis, Scanning Probe Microscopy is used to study and quantify the nanoscale electric phenomena in the two classes of oxide systems, namely transport at electroactive grain boundaries and surface behavior of ferroelectric materials. The groundwork for the application of SPM for the determination of interface I-V characteristics avoiding contact and bulk resistivity effects is established. Scanning Impedance Microscopy (SIM) is developed to access ac transport properties. SIM allowed the interface capacitance and local C-V characteristic of the interface to be determined thus combining the spatial resolution of traditional SPMs with the precision of conventional electrical measurements. SPM of SrTiO3 grain boundaries in conjunction with variable temperature impedance spectroscopy and I-V measurements allowed to find and theoretically justify the effect of field suppression of dielectric constant in the vicinity of the electroactive interfaces in strontium titanate. Similar approaches were used to study ferroelectric properties and ac and dc transport behavior in a number of polycrystalline oxides. Polarization-related chemical properties of ferroelectric materials were investigated and quantified, leading to the discovery of the effects of potential retention above Curie temperature and temperature induced potential inversion. The origins of these phenomena were traced to the interplay between fast polarization and slow v screening charge dynamics. Piezoresponse Force Microscopy (PFM) was used to study the polarization dynamics. An extensive description of contrast mechanisms in PFM conveniently represented in the form of "Contrast Mechanism Maps" was developed to relate experimental conditions such as tip radius and indentation force with the dominant tip-surface interactions. This topic was further developed to study the photochemical activity on ferroelectric surfaces as a function of domain orientation and use PFM to create predefined domain structures paving the way for photochemical assembly of metallic nanostructures on ferroelectrics. vi 1. INTRODUCTION One of the most fascinating aspects of chemistry and physics of oxide materials is a wide variety of the properties they exhibit. While traditional semiconductor materials typically exhibit a single functionality and the coupling between mechanical, electrical and magnetic properties is relatively weak, this is not the case for the oxide materials. Century-old examples include strong electromechanical coupling in ferroelectric and piezoelectric materials that enable multiple applications as sensors, actuator and transducers.1 More recent examples include perovskite manganites, in which the interplay between magnetic ordering and transport properties gives rise to the effect of colossal magnetoresistance and enables their potential applications for magnetic field sensors and magnetic heads.2 Oxide systems allow novel paradigms for the electronic devices. The high degree of spin polarization in manganites provides one of the possible material bases for spintronics devices.3 High temperature superconductors (HTSC) lend themselves for superconductive electronics; alternatively, mesoscopic quantum effects in Josephson junctions enable quantum-computing applications.4 Switchable polarization in ferroelectric materials might enable non-volatile memory devices.5 This multitude of properties comes with the price. In traditional semiconductors, only few parameters must be controlled to achieve reliable device performance. Extensive knowledge has been accumulated on suitable microfabrication routes that do not degrade material properties and allow assembly of systems with complex device functionality. In comparison, the tendency to form oxygen vacancies and to develop concentration gradients due to dopant segregation in oxides significantly hinders the preparation of device-ready materials. Significant progress has been achieved in the last decade using Molecular Beam Epitaxy (MBE) and Pulsed Laser Deposition (PLD) techniques, which for the first time allowed preparation of epitaxial oxide films with extremely low defect density and enabled subatomic control of the composition, sparking a new interests for these applications. It can be expected that oxide heterostructures and devices will significantly further existing technology. Indeed, in addition to the wide spectrum of physical properties of oxides per se, careful control and engineering of oxide interfaces presents 1 multiple new opportunities. To mention a few, it was reported recently that multiplayer structures based on SrTiO3/BaTiO3 multilayer allow materials with advanced dielectric 6 properties. Other examples include interfacial magnetism in the CaMnO3/CaRuO3 system,7 ferroelectric-semiconductor heterostructures,8 and so on. An extremely important class of oxide interfaces is constituted by grain boundaries.9 While the electronic applications described above are a nascent, albeit rapidly developing field, grain boundaries in bulk ceramic materials have been extensively studied and used for a better half of the century. In polycrystalline semiconductive oxides, the formation of electroactive interfaces due to dopant or vacancy segregation or the presence of interface states is a ubiquitous phenomena. Structure and topology of electroactive interfaces are known to influence greatly, and, in some cases, govern the properties of material.