Nanoscale domain wall dynamics in ferroelectric thin films: effects of electro-mechanical field interactions

Abstract

Ferroelectric oxide thin films are currently used in ultra high density non-volatile memories (FeRAM) and Nano/Micro-Electro-Mechanical Systems (NEMS/MEMS). In all these application, the functional property is determined by the ferroelectric and ferroelastic domain wall movement. Moreover, commercially developed systems are based on polycrystalline (or textured) ferroelectric thin films and therefore their performance rely heavily on the microstructural features such as orientation, grain size, grain boundary contribution etc. Furthermore, as the thin films are patterned onto the substrate for any device fabrication, the film-substrate interface defects such as lattice misfit, dislocations etc., highly influence the domain wall behavior.

This dissertation, investigates the nanoscale domain switching behavior in polycrystalline perovskite lead zirconate titanate, Pb(ZrxTi(l-x)03 (PZT) ferroelectric thin films. Systematic Piezoresponse Force Microscopy (PFM) studies provide direct visual evidence of the complex interplay between electrical and mechanical fields in a polycrystalline system, which causes effects such as correlated switching between grains, ferroelastic domain switching, inhomogeneous piezostrain profiles and domain pinning on very minute length scales. Furthermore, the grain to grain long range interaction and ensuing collective dynamics in the domain switching behavior have been investigated using the time resolved PFM and Switching Spectroscopy PFM (SSPFM). Finite element method (FEM) has been employed to quantify the local ferroelectric interaction and assess the several possible switching mechanisms. The experiments find that of the three possible switching mechanisms, namely, direct electromechanical coupling, local built-in electric field and strain, and grain boundary electrostatic charges, the last one is the dominant mechanism.

Having studied in detail the nanoscale domain wall behavior, we are now able to control and engineer the domain behavior in the PZT ferroelectric thin film materials. In the special case of thin film ferroelectrics that have a large ferroelastic self-strain associated with their phase transformation, a key aspect is the interaction of this self-strain with the boundary conditions of the film. In this thesis, by depositing a strongly tetragonal ferroelectric thin film on a soft rhombohedral bottom layer, we show that these elastic interactions result in a ferroelastic domain structure in the tetragonal film that is susceptible to external perturbation. High-resolution piezoresponse force microscopy images demonstrate gross movement (nm scale) of the ferroelastic domains under local bias and shows enhanced electromechanical response through this movement. Band excitation piezoforce spectroscopy investigation presents visual evidence for the local mechanism that underpins the ferroelastic domain wall movement and reveals distinct origins for the reversible and irreversible components of ferroelastic domain motion. We find that while reversible switching is essentially a linear motion of the ferroelastic domains, irreversible switching takes place via domain-wall twists. Critically, real-time images of in-situ domain dynamics under an external bias reveal that the reversible component leads to reduced coercive voltages. Finally, we show that junctions representing three-domain architecture represent facile interfaces for ferroelastic domain switching. The results presented here thus provide (hitherto missing) fundamental insight into the correlations between the physical mechanisms that govern ferroelastic domain behavior and the observed functional response in domain-engineered thin film ferroelectric devices.