Ferroelectric $PbTiO_3$ 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 $PbTiO_3$ 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 this study, the domain structures in heteroepitaxial $PbTiO_3$ films fabricated by hydrothermal method at various conditions below Curie temperature (=490℃) were observed. The film synthesized at 160℃ had c-domains and were hardly observed with a-domain structures, despite the film synthesized at 200℃ were observed with c/a-domain structures. This continuous film had a small volume fraction of a-domains with in the +c-mono domain matrix. The mechanisms of domain structure formation in these materials can be well explained with the help of Landau-Ginzburg-Devonshire (LGD) phenomenological theory. Theoretical study of this phenomenon using LGD theory, with consideration of the experimental results, was presented. We described the hydrostatic pressure ― film thickness dependence of equilibrium domain structures in $PbTiO_3$ films fabricated by hydrothermal epitaxy below $T_c$ on the grounds of the calculated elastic Gibbs free energy.
Heteroepitaxial $PbTiO_3$ film on an Nb-doped (001) $SrTiO_3$ single crystal substrate was fabricated by hydrothermal epitaxy at 200℃. X-ray ω-scan and piezoresponse force microscopy confirmed that the film had a c/a/c/a poly-domain structure without phase transition. After heat treatment of this film at 600℃, we observed the lattice parameter and domain structures by TEM and <111> pole figure. We found that a {211},{111} twin structure existed along the (1 $\overline{1}$ 1) plane within the a-domain and predicted that these {211} twin structures would be due to the interaction of two different variants of a-domains in presence of a c-domain in the heteroepitaxial $PbTiO_3$ film. It is suggested that observed the {111} twin structure within the a-domain, which was formed through the phase transition, were created due to a large in-plane shear stress enough to create [111] twin structure.
And we also investigated the chemical etching behavior of heteroepitaxial $PbTiO_3$ films on Nb-doped $SrTiO_3$ substrate with mono +c-domain fabricated by using hydrothermal epitaxy. The as-fabricated films were patterned lithographically into submicron-sized patterns with various lateral sizes using KOH solution, and the etching mechanism is suggested. Our study reveals the feasibility of fabricating patterned nanodomains in $PbTiO_3$ films by Scanning Probe Microscopy (SPM) domain engineering and a chemical etching process. Further, our study shows that an analysis on the distribution and morphology of polarized ferroelectric nanodomains in $PbTiO_3$ is possible through a precise control over the applied bias voltage, pulse width and time.
비교적 큰 잔류 분극 값 ($2P_r$ of 144 $μC/cm^2$ and 63 $μC/cm^2$) 을 가지고, 국부적인 영역에서의 강유전, 압전특성 또한 가지는 수열합성을 통해 제조되는 $PbTiO_3$ 박막은 합성 초기 아일랜드에서 rectangular, pyramidal facet이 생성함과 동시에 +c-domain으로 분역구조가 형성됨을 관찰하였고, 합성 시의 응력요인과 용액 전하요인이 +c-domain형성에 영향을 미침을 고찰했다.
수열합성 조건에서 a-domain이 생성되는 것은 합성온도와 압력조건 그리고 두께에 따라 영향을 받음을 관찰할 수가 있었다. Landau Ginzburg Devonshire 현상학적 열역학식을 이용하여 수열합성으로 제조되어지는 막에서 수열압과 수열압에 의해 기판에 붙잡혀 (constrained) 분극값의 재계산되는(renormalized) 에너지를 고려하여 계산했다. 계산 결과 200℃ 수열 합성조건 약 200nm두께에서 a-domain이 생성됨을 확인할 수 있었다. 수열합성으로 a-domain이 존재하는 상태에 600℃ 열처리를 한 경우 기존에 관찰되지 않은 특이한 분역 구조가 관찰하였고 [110] surface band 분역이라 명명하였다. 이 [110] surface band 분역은 211 쌍정 분역(twin domain)으로 정방정에서 찌그러진 (tetragonally distorted) 구조로 TEM과 극점도를 통해 예상할 수 있었다. 이 {211} 쌍정 분역은 primary a-domain과 secondary a-domain 사이의 shear deformation에 의해 생성된다고 생각할 수 있다. 분극 이온 dipole이 negatively charged surface를 형성할 경우 에칭 용액의 positively charged proton의 흡착이 빨리 일어나 positively charged surface 보다 etch rate이 훨씬 커서 분극 분역에 따른 단차가 생성된다. 에칭 용액으로는 KOH 수용액을 사용하여 OH-이온이 positively charged surface 를 에칭하는 원리를 이용하여, 즉 이러한 선택적 에칭을 이용하여 $PbTiO_3$ film의 +c/a/-c 분역구조의 관찰할 수 있었다. 선택적 에칭을 이용해 cell을 제조할 수 있음을 확인하였다.
