Ferroelectric $Pb(Zr,Ti)O_{3}$ (PZT) thin films with a perovskite crystal structure have potential application in nonvolatile memory components, infrared sensors, micro- piezoelectric/electrostrictive actuator and electrooptic devices for data storage and displays. Generally, ferroelectric thin films have been fabricated by a number of different chemical and physical deposition methods including sputtering, sol-gel, and pulsed laser deposition(PLD), etc. All the techniques have necessarily high deposition temperature or thermal treatment above phase transition temperature($T_{c}$), for the crystallization. This high temperature above $T_{c}$ results in the thermal stress, which inevitably produces the problems such as lead volatilization, thermal deformation, and film delamination due to thermal expansion mismatch. In order to elucidate the thermal stress, we fabricated PZT thin films by the hydrothermal synthesis having a low deposition temperature below $T_{c}$.
The hydrothermal synthesis is a technique that uses aqueous chemical reactions to synthesize inorganic materials at relatively low temperature (100~350℃) and high pressure(<15MPa). In ferroelectrics, this has been applied to the production of nano-sized powders with high crystallinity. In 1989, ferroelectric thin film ($BaTiO_{3}$) was deposited using the reaction of Ti sourced substrate, for the first time. In 1991, PZT thin film was synthesized by the developed 'two-step' process. Recently, the piezoelectricity/electrostriction of PZT thin film was confirmed. This has been applied to the electro-driven devices such as cylindrical micro-ultrasonic motor and rod-shaped touch sensor. However, much of research on hydrothermal synthesis for the deposition of ferroelectric thin films is in the initial stages. Until now, the question whether hydrothermally synthesized thin films have ferroelectricity has not been directly answered. In particular, few reports have indicated the nature of domain structures though knowledge of the domain structures is necessary for understanding the macroscopic behavior of ferroelectric material.
In present work, we fabricated isolated PZT single crystal and thin films on Ti substrate by hydrothermal synthesis. And their ferroelectricity was investigated by local piezoresponse and P-E curve. Domain structure was observed by High resolution Transmission Microscopy(HRTEM) and Scanning Nonlinear Dielectric Microscopy(SNDM). Also, the vacuum anneal and granular structure were introduced for the increase of thin film density.
Isolated PZT single crystal and thin films having micro-sized hexahedron structure were fabricated on Ti substrate at 160℃ below $T_{c}$. Isolated PZT single crystal and thin films revealed the butterfly curve of the piezoresponse, which represents the ferroelectricity. From the HRTEM observations of a crystallite in PZT thin film, we observed a twinned domain structure in the form of a ...c/a/c/a... polydomain pattern. On the basis of the HRTEM, we proposed a three-dimensional domain structure; the c/a domain walls form along coherent {101} planes in the tetragonal cell. This structure was experimentally verified at isolated PZT single crystal and thin film by SNDM.
In PZT thin films having micro-sized hexahedron structure, we observed the disappearance of the ferroelectricity at large area(electrode diameter=0.6mm) by P-E measurement. We considered that this disappearance was caused by the leaks of porous hexahedron structure. Thus, the granular structure and vacuum anneal were introduced for the increase of film density. Granular structured PZT thin film has a slim hystresis behavior (Remanent polarization(Pr)=0.8μ/㎠) in granular structure combined with vacuum anneal has the increase of Pr (12.5μC/㎠), which represents the improvement of ferroelectricity.
We measured the piezoelectric coefficients ($d_{33}$) with microstructure and vacuum anneal using the point contact of AFM with conductive tip. Hexahedron structured PZT thin film had a piezoelectric coefficient of 77.0 pm/V, while granular structure had a relatively higher piezoelectric coefficient of 89.7 pm/V. In granular structure combined with vacuum anneal has the increase of piezoelectric coefficient (187.3 pm/V), which represents the improvement of piezoelectricity.
