: Ferroelectric $PbTiO_3$ thin films with a perovskite crystal structure have much application potential in nonvolatile memory component, infrared sensor, micro piezoelectric actuator and high density data storage. Generally, ferroelectric thin films have been fabricated by high temperature above phase transition temperature ($T_c$) process to form a ferroelectric phase including sol-gel, sputtering, metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD). And this high temperature bring on thermal stress in thin film, inter-diffusion between film and substrate, formation of a-domain, etc.
Hydrothermal synthesis is an attractive method for the deposition of ferroelectric thin film because it can realize a low processing temperature of 200℃ or less. This method uses aqueous chemical reactions to synthesize inorganic materials at relatively low temperature (100~350℃) and high pressure (<15MPa). This technique is simple and low energy consumption experimental set-up that has not been previously reported for the fabrication of perovskite thin film. In ferroelectrics, this has been applied to the production of nano-sized powders. In 1995, epitaxial ferroelectric $PbTiO_3$ thin film was deposited using hydrothermal on $SrTiO_3$. Many researchers have been shown high crystallinity of hydrothermal epitaxy, optimized solution condition, etc. However, much of research on hydrothermal synthesis for the deposition of ferroelectric thin film is in the initial stages. Until now, the question whether hydrothermally synthesized thin films have ferroelectricity has not been directly answered. Domain structure of hydrothermally synthesized thin films also has not been examined yet.
In recent year, many researchers tried to show about relation between ferroelectricity and domain structure. A local ferroelectric characteristic is important because of its device application. The ferroelectricity of the film was investigated using local piezoresponse hysteresis, while $PbTiO_3$ domain structure was investigated using Piezoresponse force microscopy (PFM), Scanning Nonlinear Dielectric Microscopy (SNDM) measurement. A local $PbTiO_3$ film exhibited hysteresis behavior of the piezoresponse, which represents ferroelectricity.
In this study, $PbTiO_3$ thin films were epitaxially grown on Nb doped $SrTiO_3$ single crystal substrates by 160°C hydrothermal synthesis. The crystallinity and the phase are analyzed by TEM, XRD, epitaxial growth mode is observed by SEM and their ferroelectricity was investigated by P-E hysteresis curve, Piezoresponse force microscopy (PFM), Scanning nonlinear dielectric microscopy (SNDM).
Because hydrothermal synthesis is processed in temperature range much below 490℃ which is Curie temperature of $PbTiO_3$, there are only +c,-c domains. This fact is measured by PFM, SNDM domain imaging in this study. Pure $PbTiO_3$ has an Ferroelectric tetragonal structure (Ft) below curie temperature (Tc), which typically has higher polarization. So we anticipate high ferroelectric, piezoelectric property in hydrothermal synthesis $PbTiO_3$ thin film. As-grown $PbTiO_3$ has only +c-domain. And in case of 600℃ heat treated epitaxial $PbTiO_3$ thin film, we find a twinned domain structure in the form of a … c/a/c/a … polydomain by SNDM, PFM image. But microstructural voids raise a leaky problem on film.
Poling switching property image by using PFM, SNDM clearly shows polarization switching in as-grown $PbTiO_3$. 600℃ heat treated epitaxial $PbTiO_3$ thin film also clearly shows polarization switching, but there exist a-domain which did not show polarization switching. SNDM hysteresis curve shows square hysteresis shape and matchs with poling switching image by PFM, SNDM well. From these data, Ec(Cohesive field)is about 70kV/cm
We measured the piezoelectric constant ($d_{33}$) by using the point contact with AFM conductive tip. 600℃ heat treated a-domain of $PbTiO_3$ thin film had a piezoelectric constant of 30~50pm/V, while 600℃ heat treated c-domain epitaxial $PbTiO_3$ thin film had a relatively higher piezoelectric constant of 80~120pm/V. With vacuum anneal the increase of piezoelectric constant represents the improvement of piezoelectricity.
Domain structure of a hydrothermally grown PbTiO3 film is more resembling.
We measure domain width, domain wall width and confirm that relation between domain width and film thickness. And we measured nanoscale domain size in epitaxial $PbTiO_3$ island by hydrothermal synthesis. We measured that 23nm sized ferroelectric domain is minimum size.
