Thin film nanoscale polarization relaxation kinetics

Nanostructured ferroelectric oxides are now a part of high-density memory applications such as console video games, smart cards, probe based hard disk storage as well as materials for Nano/Micro-Electro-Mechanical Systems (NEMS/MEMS). The motion of a ferroelectric domain wall is central to all these applications; it controls retention of data, speed of writing as well as scalability of a written bit. The domain wall can be highly labile and can move distances that can be several times longer than its own width. Thus the fundamental studies, investigating the mechanisms controlling domain wall, particularly the role of dominant microstructural features such as grain boundaries in the structural and temporal evolution as well as the pathways adopted by a written bit in response to external perturbations are of intense scientific interest.
 
Here at UNSW, we perform high-resolution piezoresponse force microscopy (PFM) studies on polycrystalline PbZr0.25Ti0.75O3 (PZT) films deposited on Ir-IrO2 buffered Si wafer by a production-type Metalorganic Chemical Vapour Deposition (MOCVD) tool. Time dependent high-resolution images reveal that the formations of nanometre scale kinks that induce jaggedness are the first events that occur during the relaxation process. This is followed by domain wall flattening; a process where part of the wall moves to minimize the jaggedness. The domain is fairly stable until the next kink is introduced and the process continues. Thus the initial study demonstrates that the domain wall motion in polycrystalline thin films consists of a series of sequential events rather than a smooth and continuous process.
 
Domain Lability and Fluctuation
We use a contact mode based Scanned Probe Microscopy technique; called Piezoresponse Force Microscopy (PFM) to image the domain walls. An AC signal Vac=Vosin(ωt) was applied between the conducting cantilever tip (movable top electrode) and the bottom electrode of the sample to acquire the piezoresponse images with the aid of lock in amplifiers. Then through the same tip, DC bias was applied locally to a grain of interest to switch the polarization state. High resolution (50 nm x 50 nm) PFM images (Fig. 1) were then captured as a function of time to study the nanoscale domain wall motion. Fig. 1 presents visual evidence that the motion of a ferroelectric domain wall proceeds through formation of nanosized kinks and arches which causes domain wall motion to be a jagged process. The experimental results agree with a kinetic model that predicts the wall velocity (Fig. 2) to go through varying maxima as a function of time during relaxation process. Thus this study reveals that 1. the motion of a ferroelectric domain wall in the absence of an electric field in a polycrystalline thin film proceeds through formation of nanoscale kinks and arches 2. progression of a domain wall is not a smooth but a rather jerky process that creates a series of structural evolutions in the domain wall.
 
Figure 1
Figure 1: High-resolution PFM images that clearly reveal the formation and influence of kinks in domain wall motion. (a) is the VPFM image of the virgin state (b) to (l) domain images after the application of dc bias as a function of time. (Cross mark in (a) denotes the place, where the dc bias was applied). Times after which the images are captured are provided at the bottom right.
 
Figure 2
Figure 2: The domain wall velocity calculated as a function of time. Notice the velocity has several maxima thus showing the domain wall motion proceeds through a “stop-and-go” fashion.
 
A. Varatharajan
V. Nagarajan