Ferroelectric thin films with perovskite structure have great advantages on the fabrication of nonvolatile memory components, pyroelectric sensor, and microactuator. For engineering applications, ferroelectric thin films require micro- or even nano-sized patterning. There are two methods for manufacturing micro- and nanocell; top-down and bottom-up approaches. Within the top-down approach, focused ion beam patterning, electron-beam direct writing, and nanoimprint lithography are implied, whereas several physical and chemical self-patterning routes are implied in bottom-up approach. Top-down approaches are very well suited to fabricate structures with extremely good spatial resolution and positioning precision, but these approaches have necessarily high deposition temperature or need high temperature treatment above phase transition temperature $(T_c)$ for the crystallization. This high temperature above $T_c$ results in the thermal stress, lead or oxygen volatilization, inter-diffusion between film and cell and substrate, and formation of a-domain, etc. Also, top-down approaches have potential possibility in side wall damage of the cell. On the other hand, bottom-up approaches are not based on carving thin films, but rather on building structures from the bottom using atoms and molecules. The primary disadvantage of the bottom-up approaches is the random positioning of the obtained structures that will make a precise interconnection of them virtually impossible.
Hydrothermal epitaxy is suitable for micro- and nanocell fabrication since it is basically low temperature process below phase transition temperature $(T_c)$ and advantageous for bottom-up approaches. 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. But recently, for the first time, P-V hysteresis curve was obtained with a hydrothermally synthesized $PbTiO_3$ thin film on a Nb-doped $SrTiO_3$ substrate[21].
In this study, $PbTiO_3$ film and micro-cell was epitaxially grown by hydrothermal synthesis below phase transition temperature on Nb-doped $SrTiO_3$ single crystal substrates used as both a substrate and a n-type semiconductor bottom electrode.
PTO film with a thickness of approximately 1.2 μm was synthesized by reacting 0.2g anatase $TiO_2$ powder with 1g $Pb(NO_3)_2$ in 20ml of 8M KOH solution at 160℃ for 16 hours. The XRD θ-2θ profile and the (101) pole figure of the PTO thin films show that our PTO thin films were heteroepitaxially grown by means of hydrothermal epitaxy on a NSTO substrate. On the basis of TEM and PFM analyses, it was confirmed that no a-domain formed in the heteroepitaxial PTO film. That is, the heteroepitaxial PTO thin film must be grown with a stable +c-domain structure formed in the PTO film. The polarization-voltage (P-V) hysteresis curve was obtained from the PTO capacitor. Compared to the spontaneous polarization $(P_s: 57 μC/ ㎠)$ of PTO crystal fabricated by the conventional high temperature method, the PTO thin film has large remanent polarization $(2Pr: 144 μC/ ㎠)$ resulting from the single +c-domain structure.
PTO micro-cells with sizes of 3×3, 5×5, 10×10 μm2 were fabricated by reacting 0.2g anatase $TiO_2$ powder with 1g $Pb(NO_3)_2$ in 20ml of 6M KOH solution at 170℃ for 6 hours. SEM plane view and cross-section view show that $PbTiO_3$ micro-cells were 2-dimensionally arrayed by bottom-up approach on NSTO substrate. From the AFM line scans of PTO micro-cell, we could find that as the cell size decreases, scaling ratio (distance/height) also decreases. Scaling ratio is related to the degree of in-plane strain relaxation. As the scaling ratio decreases, in-plane strain in patterned film is relaxed due to the reduced substrate clamping effect. So, piezoresponse hysteresis curve show that as scaling ratio decreases, coercive voltage is decreasing and piezoresponse signal is increasing. From the piezoresponse hysteresis curve and polarization reversal experiment, we could evaluate the ferroelectricity of 2-D arrayed PTO micro-cells.
