Local polarization dynamics in ferroelectric materials

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IOP PUBLISHING Rep. Prog. Phys. 73 (2010) 056502 (67pp) Local polarization dynamics in ferroelectric materials REPORTS ON PROGRESS IN PHYSICS doi:10.1088/0034-4885/73/5/056502 Sergei V Kalinin 1,5 , Anna N Morozovska 2 , Long Qing Chen 3 and Brian J Rodriguez 4,5 1 The Center for Nanophase Materials Sciences and Materials Sciences and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2 V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 41, pr. Nauki, 03028 Kiev, Ukraine 3 Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA, 16802, USA 4 Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland E-mail: sergei2@ornl.gov and brian.rodriguez@ucd.ie Received 13 July 2009, in final form 7 January 2010 Published 7 April 2010 Online at stacks.iop.org/RoPP/73/056502 Abstract Ferroelectrics and multiferroics have recently emerged as perspective materials for information technology and data storage applications. The combination of extremely narrow domain wall width and the capability to manipulate polarization by electric field opens the pathway toward ultrahigh (>10 TBit inch −2 ) storage densities and small (sub-10 nm) feature sizes. The coupling between polarization and chemical and transport properties enables applications in ferroelectric lithography and electroresistive devices. The progress in these applications, as well as fundamental studies of polarization dynamics and the role of defects and disorder on domain nucleation and wall motion, requires the capability to probe these effects on the nanometer scale. In this review, we summarize the recent progress in applications of piezoresponse force microscopy (PFM) for imaging, manipulation and spectroscopy of ferroelectric switching processes. We briefly introduce the principles and relevant instrumental aspects of PFM, with special emphasis on resolution and information limits. The local imaging studies of domain dynamics, including local switching and relaxation accessed through imaging experiments and spectroscopic studies of polarization switching, are discussed in detail. Finally, we review the recent progress on understanding and exploiting photochemical processes on ferroelectric surfaces, the role of surface adsorbates, and imaging and switching in liquids. Beyond classical applications, probing local bias-induced transition dynamics by PFM opens the pathway to studies of the influence of a single defect on electrochemical and solid state processes, thus providing model systems for batteries, fuel cells and supercapacitor applications. (Some figures in this article are in colour only in the electronic version) 5 Authors to whom any correspondence should be addressed. Contents 1. Physics and applications of ferroelectric materials 2 1.1. Ferroelectric materials and applications 2 1.2. Ferroelectrics at the nanoscale 3 1.3. Local probing of ferroelectric materials 4 2. Principles and instrumental aspects of PFM 5 2.1. Basic principles of PFM 5 2.2. Contact mechanics of PFM 6 2.3. Resolution theory in PFM 8 2.4. Calibration of PFM 20 2.5. Implications for PFM data analysis 22 3. Local polarization switching in ferroelectric materials by PFM 23 3.1. Polarization dynamics at the nanoscale 23 0034-4885/10/056502+67$90.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA
Rep. Prog. Phys. 73 (2010) 056502 S V Kalinin et al 3.2. Experimental aspects of tip-induced polarization switching 23 3.3. Theory of tip-induced polarization switching 30 4. Time and voltage spectroscopies in PFM 37 4.1. Experimental apparatus for PFS and SS-PFM 38 4.2. Phenomenological theory of domain loop formation 40 4.3. Hysteresis loop formation 41 4.4. SS-PFM of polarization dynamics in low-dimensional ferroelectrics 49 4.5. Time resolved spectroscopies of ferroelectrics 51 4.6. Switching in ferroelectric capacitors 54 5. Phase-field simulations of local ferroelectric switching mechanism 54 5.1. Phase-field method 56 5.2. Modeling the electric potential distribution from PFM 57 5.3. Nucleation bias 57 5.4. Mesoscopic switching mechanism in a single domain 58 5.5. Local ferroelectric switching across a ferroelastic twin wall 58 5.6. Nucleation potential distribution in a domain structure 59 6. Advanced topics in PFM of ferroelectrics 60 6.1. Polarization mediated surface chemistry 60 6.2. PFM in a liquid environment 60 6.3. PFM and transport measurements 60 7. Summary 61 Acknowledgments 61 References 62 1. Physics and applications of ferroelectric materials 1.1. Ferroelectric materials and applications Ferroelectric materials have become the prototypical example of functional oxides since the discovery of ferroelectricity in BaTiO 3 (BTO) in the mid-1940s [1, 2]. For several decades, ferroelectric ceramics and single crystals were explored as materials for ultrasonic transducers in SONAR systems and, later, medical ultrasound imaging [3–7]. Correspondingly, much of the early research in the field was driven by the applications of ferroelectrics as electromechanically active materials [8]. The advances in thin film synthesis and microfabrication technologies in the last two decades have resulted in rapid development of electromechanical applications of ferroelectrics on the micrometer scale in microelectromechanical systems and actuators [9–12]. The synergy between the advances in single crystal growth and basic studies of ferroelectrics in the 1950s and 1960s has enabled a broad spectrum of applications in electrooptical systems and photothermal imaging [13–16]. As with electromechanical applications, the current trend in miniaturization of device component size resulted in multiple applications of ferroelectric materials for tunable nanoscale optics, nanophotonics and plasmonics [17, 18]. From the early days of ferroelectrics, much attention has been paid to the applications of ferroelectrics in information technologies. The presence of two or more stable polarization states (figure 1(a)), the ease of polarization switching by electric field and the small domain wall width suggested extraordinarily high storage densities, while coupling between polarization and optical and transport properties held the promise of efficient read-out mechanisms. Since the early 1950s, a number of patents on ferroelectric memory diodes, ferroelectric field-effect transistors and domain wall based storage have been filed [19–21]. However, the large switching biases (≫10 V) required for polarization manipulation in single crystals rendered these applications impractical at that time. The advances in sol–gel film synthesis of thin films with coercive biases well below ∼10 V in the early 1990s rendered the information technology applications of ferroelectrics feasible [22, 23]. The first examples were the ferroelectric random access memories (FeRAM), in which the dielectric in a standard dynamic random access memory (DRAM) capacitor is substituted with a ferroelectric, adding the advantage of non-volatility. Similarly, the combination of a semiconductor channel and a ferroelectric gate enabled ferroelectric gate transistors. The seminal work by Ahn et al has demonstrated the potential of ferroelectric field effect manipulation in superconducting, semiconductor and organic materials [24–26]. Future progress in this area requires ferroelectric–semiconductor integration technologies that preserve interface quality [27–29]. Advances in understanding order parameter coupling in strongly correlated oxides and atomic-level control of film growth in molecular beam epitaxy and pulsed laser deposition (PLD) [30–32] have stimulated interest in magnetoelectric and multiferroic applications [33–37]. Extensive analysis of the potential applications and relevant fundamental scientific issues are now available [38–41]. The recently studied relationship between ferroelectricity and electronic transport suggests tremendous potential for resistive memory applications [42–44]. Extending beyond the realm of electronic devices, the photovoltaic effect on ferroelectric surfaces and polarization-controlled reactivity has also been demonstrated [45–48], and ferroelectric lithography has been used to fabricate nanostructures [49–51]. The fundamental property of ferroelectric materials that enables their application in functional materials and heterostructures is the presence of switchable polarization and associated domain structures. Polarization switching directly underpins the functionality of data storage [13, 52], FeRAMs [30, 53] and electroresistive [42, 54] memories. The motion of domain walls and interface boundaries enables high electromechanical coupling coefficients in polycrystalline ferroelectrics and relaxors. The continuous tendency for miniaturization of electronic and optical components necessitates the understanding of ferroelectric phase stability 2
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