Ferroelectric thin films have attracted much attention for their potential applications to nonvolatile memory components, infrared sensor, piezoelectric actuator and electro-optic device. Especially, Single crystal ferroelectric thin films are preferred to polycrystalline films for a range of optical and electronic applications in order to take advantage of their anisotropic properties. For example, the pyroelectric properties of $PbTiO_{3}$ are optimal along the (001) polar direction: thus single crystal films are preferred for pyroelectric devices. Further, devices that use optical transmission require films that do not contain grain boundaries that scatter light. A number of techniques have been employed to grow either oriented or epitaxial ferroelectric thin film on single crystal substrate. Deposition methods include sputtering, pulsed laser deposition, and chemical vapor deposition. 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, lead or oxygen volatilization, inter-diffusion between film and substrate, and formation of a-domain, etc. Then, our interest is in using hydrothermal routes to grow epitaxial thin films at relatively low temperatures.
Hydrothermal synthesis is a technique that involves the growth of oxides directly from aqueous solutions, generally at elevated temperatures (100-400℃) and pressures (0.1-15MPa). This technique offers the advantages of simplicity, low cost, high product purity, and the ability to control particles size. Since synthesis occurs at low temperatures, problems such as inter-diffusion and interfacial reaction which are detrimental to device properties can be eliminated. However, it has been reported that hydrothermal synthesis has a disadvantage such as a low uniformity coverage. In 1995, epitaxial $PbTiO_{3}$ thin film was deposited using hydrothermal method on $SrTiO_{3}$ by A. T. Chien and F. F. Lange. Unfortunately, there has been very little electrical and ferroelectric characterization performed on hydrothermally synthesized $PbTiO_{3}$ thin films where the low synthesis temperatures and aqueous environment would likely affect both microstructure and properties.
In this study, epitaxial $PbTiO_{3}$ thin films were fabricated by means of hydrothermal synthesis on a Nb(0.5wt%) doped (001)single crystal $SrTiO_{3}$ substrate at $230^\circ C$. $Nb-SrTiO_{3}$ used as a substrate and an n-type semiconductor bottom electrode. Moreover, $Nb-SrTiO_{3}$ substrate with surface steps of both 0.4nm step height and 100nm step spacing was applied to improve uniformity coverage of $PbTiO_{3}$ film. The films were heat-treated at 300℃ for 3h. To characterize electric and ferroelectric properties, platinum by means of the top electrode was sputtered on the epitaxial $PbTiO_{3}$ thin film using a shadow mask. The area of the top electrode was $3.8×10^{-4}㎠$.
By means of TEM(transmission electron microscopy)and PFM(piezoelectric force microscopy) analyses, it was confirmed that no a-domain formed in the epitaxial $PbTiO_{3}$ thin film. This result clearly indicated that the hydrothermally synthesized epitaxial $PbTiO_{3}$ thin films did not undergo phase transition. Furthermore, it was observed by means of PFM that the epitaxial $PbTiO_{3}$ thin film had a single +c-domain structure at as-synthesized state.
The eptixial $PbTiO_{3}$ thin film had a dielectric constant of 374 at 1kHz. The spontaneous polarization ($P_{s}$) and the remanent polarization ($P_{r}$) and the coercive field ($2E_{c}$) of the epitaxial $PbTiO_{3}$ thin film were 96 $\mu C/cm^{2}$, 80μ C/cm^{2}$, and 215 kV/cm, respectively. The large polarization results from the single c-domain structure. Form the measurement of the P-E hystetesis loop, an imprint phenomenon was observed in the epitaxial $PbTiO_{3}$ thin film. This induced by an asymmetric electrode configuration. In the fatigue test, $P^{*}_{r}$ - $P^{^}_{r} of the epitaxial $PbTiO_{3}$ thin film after $10^{10}$ cycles was a little degradation. However, the remanent polarization of epitaxial $PbTiO_{3}$ thin film was as large as ever. This hydrothermally synthesized epitaxial $PbTiO_{3}$ thin film is anticipated to be very suitable for high-performance engineering applications.
본 연구에서 상전이 온도 이하에서 수열합성법을 이용하여 $PbTiO_{3}$ 박막을 $(001)Nb-SrTiO_{3}$ 단결정 기판 위에 에피텍셜하게 성장 시켰다. 이는 XRD, TEM을 통해서 확인 하였다. $Pt/PbTiO_{3}/Nb-SrTiO_{3}$ 캐패시터 구조에서 P-E 이력곡선을 측정하여 강유전 특성을 평가 하였다. 80 μC/㎠의 큰 잔류 분극 값을 확인하였다. 이는 상전이를 거치지 않아 박막 내에 a-domain 없이 외부 전계에 반전되는 c-domain만 존재 하고, 결함들이 최소화 되어 큰 잔류분극 값을 가지는 것으로 생각된다. 다만 n-type 반도체인 하부전극의 영향으로 (-)방향으로 임프린트 현상이 나타났다. 그리고 증착된 $PbTiO_{3}$박막의 분역 구조는 up-polarization의 single c-domain구조를 가짐을 PFM을 통하여 확인 하였다. 이는 P-E 이력곡선을 통한 결과와는 반대방향을 나타낸다. 즉 수열합성법에 의해 $PbTiO_{3}$박막이 증착 될 때는 어떠한 영향으로 준안정 상태인 up-polarization상태를 이루고 있다가 Pt전극이 증착 되고 전압을 가할 경우 본래의 안정한 상태인 down-polarization으로 되는 것이다. 이는 수열합성 조건에 의해 영향을 받는 것으로 예상되나 아직 명확한 분석을 하지는 못한 실정이다. $Pt/PbTiO_{3}/Nb-SrTiO_{3}$ 캐패시터의 fatigue 실험을 하였다. $10^{8}$후에 $2P_{r}$값이 감소하기 시작한다. 그러나 매우 큰 초기 분극 값에 의해 $10^{10}$승 까지의 bipolar voltage pulse후에도 잔류 분극 값은 54.6 μC/㎠의 큰 값을 유지 하였다. 이는 소자 응용에 있어서 큰 장점이 될 것이다.
