Simultaneous dynamic characterization of charge and structural motion during ferroelectric switching

Abstract

Monitoring structural changes in ferroelectric thin films during electric field-induced polarization switching is important for a full microscopic understanding of the coupled motion of charges, atoms and domain walls. We combine standard ferroelectric test-cycles with time-resolved x-ray diffraction to investigate the response of a nanoscale ferroelectric oxide capacitor upon charging, discharging and switching. Piezoelectric strain develops during the electronic RC time constant and additionally during structural domain-wall creep. The complex atomic motion during ferroelectric polarization reversal starts with a negative piezoelectric response to the charge flow triggered by voltage pulses. Incomplete screening limits the compressive strain. The piezoelectric modulation of the unit cell tweaks the energy barrier between the two polarization states. Domain wall motion is evidenced by a broadening of the in-plane components of Bragg reflections. Such simultaneous measurements on a working device elucidate and visualize the complex interplay of charge flow and structural motion and challenges theoretical modelling.The applications of ferroelectric (FE) materials in technology have many facets based on the fundamental aspects of charge transport, structural changes of the crystal lattice and the motion of domain walls. Sensors and actuators mostly rely on the large piezoelectric coefficient d 33 , which can be particularly high e.g. in relaxor ferroelectrics [1] or at morphotropic phase boundaries [2]. In electronic circuits the large dielectric constant yields a high capacitance C = 0 A/d in capacitors with area A and thickness d. Recently, the concept of negative capacitance has re-attracted attention [3,4]. Considerable research efforts were devoted to finding the minimum thickness for sustaining ferroelectricity [5], not only because d is in the denominator of the capacitance, but mainly because of the quest for FE-RAMs with high storage density and low power consumption [6][7][8]. The switchable FE polarization P S allows storing an information bit as

Time-resolved X-ray diffraction study of the structural dynamics in an epitaxial ferroelectric thin Pb(Zr0.2Ti0.8)O3 film induced by sub-coercive fields

“…[30] The high resolution X-ray diffraction measurement showed that the film is c-axis oriented [28]. The reciprocal space map without applied external field was reported in a recent publication for a different electrode of the same sample [28]. The excellent FE properties of the device with an RC time constant of about 1 µs and a coercivity of about 4.5 V have been discussed in ref.…”

“…The typical full-width-at-half-maximum (FWHM) of the rocking curves was 0.3 • .The X-ray signal is sent to a timecorrelated single photon counting system (PicoHarp 300), which allows reconstructing the rocking curves at different delays by synchronizing the detector trigger with the rising edge of the electrical pulse. [30] The high resolution X-ray diffraction measurement showed that the film is c-axis oriented [28]. The reciprocal space map without applied external field was reported in a recent publication for a different electrode of the same sample [28].…”

“…The interpretation of very large intensity changes up to 40% for the two polarization states solely ascribed to the anomalous scattering must be reconsidered since contributions from disorder cannot be neglected. We have recently reported the structural dynamics accompanying the reproducible repeated electrical switching of FE thin films, relating the observations to the simultaneously measured electrical charging with saturation fields of opposite polarity [28].In the present TR-XRD study of a 250 nm thin Pb(Zr 0.2 Ti 0.8 )O 3 (PZT) FE capacitor, we compare the time-dependent structural dynamics during sub-coercive field pulses to the full switching dynamics under saturation fields. We present the field-and time-dependent lattice response, infer the domain dynamics, and demonstrate the presence of remnant disordered states.…”

“…The interpretation of very large intensity changes up to 40% for the two polarization states solely ascribed to the anomalous scattering must be reconsidered since contributions from disorder cannot be neglected. We have recently reported the structural dynamics accompanying the reproducible repeated electrical switching of FE thin films, relating the observations to the simultaneously measured electrical charging with saturation fields of opposite polarity [28].…”

“…An external field was applied between one Pt electrode and the SRO layer along the [001] direction, which is the FE polarization axis of PZT. A pulse generator (Keithley 3390) was used to apply voltage pulses up to ±8 V with the help of a tungsten needle [28]. X-rays of 9 keV photon energy provided at the XPP-KMC3 endstation at the storage ring BESSY II at the Hemholtz Zentrum in Berlin were employed for the TR-XRD.…”