Domain Switching in Polycrystalline Ferroelectric Thin Films

Polycrystalline ferroelectric lead zirconate titanate (PZT) thin films are currently used in high-density memory, microelectromechanical systems and smart cards. However, complex coupling between electrical and mechanical fields exist in polycrystalline materials that lead to nanoscale domain switching behaviors that are difficult to predict. An example is correlated switching, where local switching of one grain brings about drastic domain reorientation within neighboring grains. At UNSW we are using piezoresponse force microscopy (PFM) to generate high-resolution images of the domain structure as well as to induce domain switching in ferroelectric thin films. The piezoresponse is measured both in the out-of-plane (OP) and in-plane (IP) directions.
 
Domain Switching Behaviour
 
1.  Unexpected switching
We observed repeatedly that the OP response, both within the grain of interest and some neighbouring grains switches against the direction of the applied bias. For example in region B (marked in (c)), positioned along a grain boundary of the grain of interest (X), the applied bias switches the OP component from dark to white.
 
2. Ferroelastic switching
90° switching is observed for certain grains, although these are not free from substrate-constraint. For example region C (marked in (c)), where the IP component has changed but the OP component hasn’t.
 
3. Correlated switching
Switching in neighbouring grains, outside the grain to which the bias was applied was often observed. An example can be seen in grain 1 (marked in (a)), where a small region within the grain has switched from dark to bright in the OP image, compare (a) with (c).
 
4. Inhomogeneous switching
When the IP component of polarization is not uniform within a grain and changes to a different non-uniform state after writing. This can also be seen in grain 1, in the same region as the correlated switching, compare (b) with (d)
 
 
Figure 1
 
Figure 1 Shows the piezoresponse images from a typical switching experiment. A bias (-4 V) was applied for 30 seconds at the point marked with an ‘X’ in (a). (a) and (b) are the out-of-plane (OP) and in-plane (IP) images, showing the ferroelectric domain structure, before the bias was applied and (c) and (d) are the images after the bias was applied. The grain structure is as marked out by the blue dotted lines in all images.
 
These findings will be investigated further by using switching spectroscopy PFM and vector PFM capabilities at the Oak Ridge National Laboratory, USA in collaboration with Dr Sergei Kalinin. From these future experiments we hope to map the ferroelectric properties over an area of the film. This information will allow us to determine the underlying causes of the domain switching behaviour described above.
 
S.Wicks
A. Varatharajan
V. Nagarajan
 
Acknowledgements: The work at UNSW is supported by the UNSW Faculty Research Grant and ARC Discovery Grant DP0771432