Local polarization dynamics in ferroelectric materials

More documents

Recommendations

Info

Rep. Prog. Phys. 73 (2010) 056502 S V Kalinin et al Figure 29. (a) Environmental EM observations of the water meniscus at the tip–surface contact junction as a function of humidity. (b) Hysteresis of meniscus size for increasing and decreasing humidity. (c) The kinetics of the alkanethiol drop spreading in the dip-pen lithography experiment. The kinetic law limited by surface diffusion is strongly reminiscent of ferroelectric domain switching kinetics observed in ambient environment. Inset shows the alkanethiol dot pattern. Panels (a), (b) reproduced from [299], copyright 2005, American Chemical Society. Panel (c) reproduced from [300], copyright 2002, American Physical Society. surfaces [281, 282, 286–288]. These mechanisms result in the formation of a high-density hydroxylated layer and interstitial protons and oxygen vacancies. The effects of these charges have been observed by variable-temperature surface potential measurements by KPFM of ferroelectrics [288, 289], resulting in surface domain-related electrostatic potential contrast retention above the Curie temperature [281] and temperature-induced domain potential inversion [282]. Polarization-controlled physisorption has recently been studied by Altman and Bonnell groups [47, 290, 291] using electron spectroscopies and KPFM. The effect of surface charges on domain structures and stability has been studied in detail by the Argonne group [292, 293]. Finally, the effect of screening charges on PFM contrast [283, 294] and polarization switching [295] has recently been studied using ultra-high vacuum PFM. Beyond chemisorption, for high partial pressure of water vapor (e.g. corresponding to atmospheric conditions) the hydrophilic surfaces develop thin (∼1–10 nm) wetting layers stabilized by disjoining pressure. These layers can support surface conductance through charged ion motion and have been directly observed in SPM experiments [276, 296]. The proximity of the tip results in the condensation of liquid at the tip–surface junction due to capillary and electrocapillary effects [297, 298]. Recent observations by environmental EM have provided spectacular depiction of the water droplet at the tip–surface junction and its slow formation and relaxation kinetics as a function of water partial pressure [299]. The full effect of surface contamination on domain dynamics is unclear, primarily because the charges are not observable directly in the PFM experiment and can be detected only by complementary measurements. The full set of observed data suggests that in poorly conductive materials, the surface charging and local charge injection may affect switching dynamics. Humidity-dependent liquid necks form at the tip–sample contact which changes the tip–sample contact area and can affect the lateral expansion of growing domains [301]. Interestingly, the observed nearly logarithmic kinetics of surface diffusion is highly reminiscent of the switching kinetics in the films, suggesting a relationship between the two. Furthermore, charge injection and the diffusion of charged species can lead to backswitching [302] and the formation of bubble domains [303, 304]. The mechanism is illustrated in figure 30. Here, the application of electric field to the probe results in both polarization switching and the formation of a charged surface layer on the surface, with effective overscreening of polarization charge. On continuous application of the bias, the charged area propagates on the surface. Note that thermodynamical analysis of polarization switching suggests that switching is possible only when the polarization charge is screened on the surface; hence the kinetics of screening charge spreading can be the controlling factor in domain growth kinetics observed in ambient. Finally, on switching the bias with the tip still in contact with the surface, the screening charges directly below and in close proximity to the tip are immediately compensated. However, the charges far from the surface remain, resulting in an electric field of opposite orientation to the original. This field can result in backswitching, as shown in figure 30 [301–304]. Finally, the aspect of ambient experiments that should be mentioned specifically is the electrochemical reactions at the tip–surface junction. Tip-induced electrochemistry is by now a well-developed paradigm, with multiple examples on metals, solid ionic electrolytes and semiconductors 29
Rep. Prog. Phys. 73 (2010) 056502 Figure 30. (a) Antipoling effect observed on epitaxial PZT. (b) Schematic representation of the effect: (left) poling at high voltage and (right) collection of surface charges and formation of opposite field, leading to polarization switching. The domain wall between the upper and lower regions reduces the electrostatic energy of the head-to-head configuration. In the center schematic the contributing potentials from the spherical model are presented. (c) Illustrates the surface charge distribution at the beginning and at the end of a biasing experiment, and after the bias is turned off. (a), (b) Reproduced with permission from [304], copyright 2005, American Physical Society. known by now [305–307]. While the electrochemistry of classical ferroelectrics such as PZT and BTO in ultrathin films and in the presence of water vapor is not studied, experimental observations suggest that tip-induced electrochemical processes are extremely common. Typically, their onset is associated with the formation of ‘switched’ patterns with low contrast and highly disordered and broad domains walls. Often the process is associated with the formation of topographic features following the bias pattern. For high biases and humidities, the complete breakdown of the film and the formation of particulates are observed. Recent measurements suggest that the onset of the electrochemical processes is related to the leakage current flow. Note that these processes are closely related to the transport properties of ferroelectrics, actively studied in the context of resistive switching [42, 267, 268, 308], and hence must be considered in this context. 3.3. Theory of tip-induced polarization switching The analysis of the tip-induced switching behavior by PFM necessitates the comparison between macroscopic and microscopic switching as a first step of theoretical analysis. The seminal work by Landauer has demonstrated that the experimentally observed switching fields correspond to S V Kalinin et al impossibly large (∼10 3 –10 5 kT) values for the nucleation activation energy [79]. This discrepancy is resolved by postulating the presence of discrete switching centers initiating low-field nucleation. The spatial and energy distribution of the nucleation center has become the central component of all statistical theories for polarization switching in macroscopic ferroelectrics [66, 95]. Notably, this behavior is universal to all first order phase transitions including solidification, martensitic transformations and phenomena such as plastic deformation and electrochemical reactions, in which the formation of a new phase is always initiated at a specific defect site or is controlled by defect dynamics. Given that typical spacings between extended defects are on the order of 10–100 nm, even high-quality epitaxial films will have multiple defects within a typical device structure such as a micrometer-sized capacitor. In comparison, in an SPM experiment, the field is concentrated to a small (10–30 nm) volume within a material that may possess small number of defects or be (extended) defect free. This opens the pathway for probing defectmediated switching in ferroelectrics on a single defect level. The original work by Landauer [79] utilized the rigid ferroelectric approximation (polarization is P or −P , domain walls are infinitely thin) to describe thermodynamics of domain formation in a uniform field. This approach was extended in a series of groundbreaking works by Molotskii et al [309–313] to the analysis of the domain formation induced by the inhomogeneous electric field of a biased SPM probe. This thermodynamic analysis was further developed by Morozovska et al to account for the finite electric field below the probe, surface and bulk screening and presence of defects [211, 314]. Below, we develop a general description of switching thermodynamics in the rigid approximation. 3.3.1. General formalism of the problem. The free energy of a nucleating ferroelectric domain is [314] (r,U) = d (r) + Dom (r,U) , (3.1a) where the domain geometry is described by the N-dimensional parameter vector, r (which can be, e.g., coordinates of all points on a domain wall, or the radius and length in the semiellipsoidal approximation) and U is the electric bias applied to the local probe in the proximity of the surface. The first term in equation (3.1a) is the contribution from the frozen disorder (i.e. random electric fields in the material), d (r), within the volume of the domain. The second term in equation (3.1a) is the thermodynamic work of switching Dom (r,U) = S (r) + p (r,U) + D (r) . (3.1b) It comprises the contributions of positive domain wall energy, S (r) = ψ S S, where S is the domain surface and ψ S is the domain wall energy density, p (r,U) is the energy density due to the probe field and D (r) is the depolarization energy. The analysis of the switching process can be simplified for a rigid piezoelectric, for which effective material constants are independent of the electric field. In this case, the interaction energy with the probe tip electric field, E p 3 (x), 30
Page 1 and 2: Home Search Collections Journals Ab
Page 3 and 4: Rep. Prog. Phys. 73 (2010) 056502 S
Page 15 and 16: Rep. Prog. Phys. 73 (2010) 056502 i
Page 29: Rep. Prog. Phys. 73 (2010) 056502 S
Page 33 and 34: Rep. Prog. Phys. 73 (2010) 056502
Page 37 and 38: Rep. Prog. Phys. 73 (2010) 056502 a
Page 49 and 50: Rep. Prog. Phys. 73 (2010) 056502 D
Page 51 and 52: Rep. Prog. Phys. 73 (2010) 056502 (
Page 57 and 58: Rep. Prog. Phys. 73 (2010) 056502 f
Page 61 and 62: Rep. Prog. Phys. 73 (2010) 056502 d
Page 63 and 64: Rep. Prog. Phys. 73 (2010) 056502 W
Page 65 and 66: Rep. Prog. Phys. 73 (2010) 056502 [
Page 67 and 68: Rep. Prog. Phys. 73 (2010) 056502 [

Local polarization dynamics in ferroelectric materials

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?