Piezoresponse Force Microscopy Studies Of (Pb_0.76Ca_0.24)TiO₃ Thin Films

Current interest in ferroelectric thin film results from the numerous potential applications in nonvolatile ferroelectric random access memory (FeRAMS), Micro Electro-Mechanical System (MEMS) and micro-electronics that utilize the unique ferroelectric, piezoelectric, dielectric, pyroelectric, photorefractive, electro-optic and non-linear optic properties of the material. As the dimension of ferroelectric devices reduced, physical properties of ferroelectric materials in nanometer scale which including the formation of domain structure, polarization and depolarization of ferroelectric domain attract attention of many researchers in this area.There are many methods to investigate domains such as polarizing microscopy, chemical etching, transmission electron microscopy, scanning electron microscopy and so on. However, these methods suffer from some serious disadvantages involving complex sample preparation, damage to sample, low resolution. PFM developed from AFM has provided a powerful tool for the research of ferroelectric domains.(Pb_0.76Ca_0.24)TiO₃ (PCT) film is a ferroelectric & piezoelectric material with high anisotropy. When prepared in thin-film form it can have a large d33, comparable to bulk material. Using the Sol-gel method, pure teragonal PCT films have been successfully prepared on platinized Si substrates. The films have a dense structure with a relatively smooth surface.Piezoresponse scanning force microscopy was used to observe the nanoscale ferroelectric domain structure, switching and back-switching processes. The relation between domain structure and the size of the grains were obtained.In this paper, we present direct observations of local domain switching. Domain switching is performed on the selected area by applying a bias voltage through the probing tip, and thereafter the piezoresponse image is obtained. With the increase of the polarization voltage, the domain switching expands the whole grain. The ferroelectric films were polarized in nanometer scale under positive or negative voltages applied by the probing tip. The reversing of different domains of the PCT films requires different polarization voltages. Some domains can reverse at low polarization voltage, but others reverse at high polarization. This thesis also reveals that when electric field produced by edge effect in non-scan area was larger than coercive field of this area, domains would reverse. This thesis studies back-switching process through observe piezoresponse images measured in different times after a bias voltage polarization, and the effect of domain structure, domain wall, grain boundaries to back-switching processes.