Local polarization dynamics in ferroelectric materials

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