Title

Direct observations of polarization reversal process in ferroelectric thin films using high speed piezoresponse force microscopy

Date of Completion

January 2008

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Ferroelectric thin films have been widely implicated for use in future ultra-high-density memory devices, using atomic force microscopy (AFM) related techniques for the read/write operations of ferroelectric memory media where formation of a single domain structure with a defined polarization direction acts as a distinct memory bit. Therefore, it is important to understand the mechanisms and kinetics involved in the polarization switching process, which includes the nucleation and growth of ferroelectric domains at the nanoscale. In recent years, the piezoresponse force microscopy (PFM) technique has been widely been used to image, characterize and modify the domain structures in ferroelectric films with this spatial resolution. However, operating speeds for PFM (and AFM in general) remain a continuing limitation for imaging dynamic processes such as domain switching and read/write operations. ^ A simple method to increase PFM characterization speeds by several orders of magnitude is presented here based on standard commercial equipment and AFM probes. Essentially, an AC voltage at a high resonance frequency is applied to a conducting AFM tip, which is in contact with a ferroelectric surface. The tip is rapidly rastered across a surface without force feedback, while the amplitude and phase of the high frequency resonances are detected with a lock-in amplifier. Although the topography is not reliably recorded, stable contrast related to ferroelectric properties is accurately mapped due to the relative insensitivity of many dynamic AFM contrast mechanisms to variations in repulsive contact forces. Images with nanoscale contrast of ferroelectric domains are presented, acquired at complete frame rates as low as 6 seconds per 256x256 pixel image. The mechanism and kinetics involved in the dynamic domain switching process of PZT thin films are therefore uniquely presented with nanoscale and nanosecond resolution. It was found that domain dynamics processes are governed by the spatial distribution of inhomogeneities present in the film. This allowed a spatially resolved analysis of nucleation and growth kinetics based on tracking thousands of individual domain locations, areas, nucleation times, and growth rates. Finally, an exponential behavior is observed for individual domain growth, and inversely for nucleation time, as a function of electric field. ^