Recently, there has been considerable interest in ferroelectric films as a medium for non-volatile memory devices. In addition, the domain wall thickness of a typical ferroelectric material is only a few lattices (≤1 nm). In particular, much attention has been paid to investigating ultrahigh-density (over 1 $Tbit/inch^2$) data storage using scanning probe techniques, such as Scanning Probe Microscope (SPM)-based ferroelectric data storage. The general requirements of non-volatile data storage media are fast operating times, small bit size, and reliability related to long-term data retention and fatigue. However, most of the studies on domain switching behavior have focused on domain size and stability in relation to writing time and writing voltage. There have been few studies related to retention loss, which is defined as a decrease in a reversed domain size with time by the lateral movement of the $textit{c/c}$ domain wall in the absence of an external field. Thus far, studies on such retention loss have been confined to polycrystalline films or 90˚ twinned domain structured films. However, polycrystalline or twinned domain structured ferroelectric films are not useful for a data storage media application because both the grain and twin boundaries act as defects that inhibit polarization switching.
The switchable electric polarization (spontaneous polarization, $P_s$) of ferroelectric thin film is ideal for use in devices that require nonvolatile memory. Conventionally, this polarization was obtained via the paraelectric (PE) to ferroelectric (FE) phase transition. Most of the ferroelectric films have been fabricated in a polycrystalline structure because the deposition was often carried out using nonepitaxial substrates, like silicon, for their applications in the monolithic integrated devices. On the other hands, Pb-based ferroelectric thin films have been grown heteroepitaxially on the cubic single crystals, such as MgO and $SrTiO_3$ (STO), using radio-frequency (rf) magnetron sputter deposition, metal-organic chemical vapor deposition (MOCVD), and pulsed laser ablation. All of these techniques require a high processing temperature (more than 500 ℃) above the Curie temperature ($T_c$) for crystallization. During fabrication, a $textit{c/a/c}$-polydomain structure forms during the PE to FE phase transition to relieve the misfit strain energy between the cubic PE and the tetragonal epitaxial FE phase. In this case, the $textit{a}$-domains form through four coherent {101} twinnings, and the twin planes become the $textit{c/a}$-domain boundary. Investigations have shown that $textit{a}$-domain switching is difficult even at very large external field strengths; that is, as mentioned above, manipulating the domain structure in ferroelectric thin films is difficult. Moreover, lead and oxygen ion vacancies occur in the thin film. These drawbacks reportedly affect the reliability of ferroelectric film devices due to weakness such as a fatigue behavior. Therefore, a fabrication method using a processing temperature lower than $T_c$ is needed to restrict the $textit{a}$-domain formation for high-performance engineering applications. However, it is a formidable challenge to fabricate heteroepitaxial ferroelectric thin films at a processing temperature lower than $T_c$ while avoiding phase transitions.
Hydrothermal epitaxy is a technique that utilizes aqueous chemical reactions to synthesize inorganic materials in the form of epitaxial films on structurally similar single-crystal substrates under elevated pressure (below 15 MPa) and at low temperature. In this study, heteroepitaxial $PbTiO_3$ (PTO) and $PbZr_{1-x}Ti_xO_3$ (PZT) thin films were fabricated on a cubic Nb-doped (100) $SrTiO_3$ (NSTO) single crystal substrate, which was each used as both a substrate and an $textit{n}$-type semiconductor bottom electrode, by hydrothermal epitaxy under a hydrostatic pressure at a temperature (160℃ and 210℃) below $T_c$ without including phase transition. The PTO and PZT capacitors with a Pt top electrode and an $textit{n}$-type Nb-doped $SrTiO_3$ bottom electrode showed a sharp P-E hysteresis curve, and the remanent polarization, $2P_r$, of the PTO and PZT capacitors were about 144 μC/㎠ and 63 μC/㎠, respectively. On the basis of piezoresponse force microscopy (PFM) and transmission electron microscopy observations, it was revealed that heteroepitaxial PTO and PZT thin films had a switchable $textit{c}$ mono-domain at the as-fabricated state. In addition, our PZT capacitor with an ordinary Pt top electrode and an Nb-doped $SrTiO_3$ semiconductor bottom electrode revealed fatigue-free behavior in up to $10^{11}$ switching cycles.
In this study, dot-domains with an average diameter of 36 nm and square-domains with sizes of 1 μ㎡ and 25 μ㎡ were switched using an atomic force microscopy (AFM) conductive tip on a heteroepitaxial PTO film with a +$textit{c}$ mono-domain, which was fabricated via hydrothermal epitaxy below Curie temperature; then, the retention loss behaviors were examined.
When an external electric field is applied via the AFM tip, the electric field distribution in the film is inhomogeneous. In addition, the AFM tip with an external electric field scanned the film surface; therefore, it was assumed that the $textit{c/c}$ domain wall was not perpendicular to the film surface but curved and it extended to the bottom electrode. The electrostatic energy caused by the depolarization fields makes the curved $textit{c/c}$ domain wall energy higher than the straight $textit{c/c}$ domain wall energy (i.e. the domain wall instability). The free energy of the - $textit{c}$ domain within the volume, including the curved $textit{c/c}$ domain wall, will increase more than that of the original +$texit{c}$ domain. Also, the square-domain clearly shrank toward the bottom electrode by ∼1 nm after switching. The switched - $textit{c}$ domain is in a compressive strain; the further increase of the free energy is caused by the strain-polarization coupling energy. The driving force of the retention loss, i.e. the lateral movement of the domain wall of the square-domains, is derived from the summation of the depolarization field energy and the strain-polarization coupling energy. However, the dot-domains switched by the pulse voltage of 2 ms nearly maintain their original size after $5.3\times10^6$ s. Therefore, it is expected that the dot-domains are cylindrical domains that have an almost straight $textit{c/c}$ domain wall and are extended to the bottom electrode.
The presented data and theoretical analysis showed that the retention loss phenomena of the domain reversed using an AFM tip were closely related to the shape of the $textit{c/c}$ domain wall and the strain-polarization coupling effect of the reversed domain. Conclusively, it was suggested that the cylindrical domain, which had an almost straight $textit{c/c}$ domain wall and extended to the bottom electrode on the given thin film, would be free from retention loss.
최근에 비휘발성 저장 매체로서 강유전체 박막에 대한 연구가 활발하게 진행되고 있다. 일반적인 강유전체의 분역벽의 두께가 수 격자 정도로 얇다. 따라서 SPM을 활용한 초고밀도 정보 저장 (SPM-based data storage)에 관련된 연구가 많은 주목을 받고 있다. 비휘발성 정보 저장 매체의 조건으로는 빠른 작동 시간, 작은 bit 크기, 그리고 retention과 fatigue와 관련된 높은 신뢰성이 요구된다. 그러나 강유전체 분극 반전에 관련된 대부분의 연구가 writing 시간과 전압에 대해서 분역의 크기에 대해서만 이루어지고 있다. 분역의 안정성, 즉, retenion loss에 관련된 연구는 매우 드문 실정이다. 일부 관련된 연구가 c/a 다분역 구조 또는 다결정 강유전체 박막에서 이루어진 바는 있으나, 이는 활용가능성이 매우 떨어진다. 결국, c 단분역 구조를 갖는 단결정 강유전체 박막에 대한 연구는 전무한 실정이다. 본 연구는 단결정 강유전체 박막을 제조하기 위해서 수열합성법을 활용하였다. 이는 비교적 저온의 범위에서 박막의 source를 포함하는 수용액 내에 단결정 기판을 넣어 증착하는 방법이다. 본 연구에서는 (001) Nb-doped SrTiO3 단결정을 활용하여 PbTiO3 와 PZT 박막을 제조하였다. 제조된 박막의 강유전 특성은 P-E 이력곡선과 PFM을 이용한 분극 반전 실험을 통해서 확인하였다. 확인 결과 매우 뚜렷한 강유전성을 보였고, 박막의 표면을 향하는 분극만을 갖는 c 단분역 구조를 갖는 단결정 박막이 제조되었다. 본 연구에서는 36 nm의 평균 직경을 갖는 dot 분역과 square 형태의 분역을 전도성을 갖는 AFM tip을 이용하여 반전시킨 후 시간에 따른 분역의 크기 변화를 관찰함으로써 retention loss 특성을 평가하였다. 외부 전기장이 AFM tip을 통해서 가해지는 경우, 박막 내부의 전기장은 불균질하게 적용된다. 따라서 반전된 분역과 초기 분역을 구분하는 c/c 분역벽이 박막의 두께 방향에 대해서 수직적이지 못하고 곡선의 형태를 나타내는 것을 가정할 수 있다. 이와 같이 곡선의 형태를 갖는 분역벽을 따라서 head-to-head 분극 구조가 나타나게 되어 반분극 전계가 생성된다. 따라서 곡선 c/c 분역벽을 포함하는 영역에 대해서 자유 에너지가 높아지는 효과를 유발시킨다. 이러한 분역벽의 불안정성에 의해서 반전된 분역의 크기가 작아지는 결과가 나타나게 되었다. 본 연구에서 나타낸 결과와 논의로부터 AFM tip에 의해서 반전된 분역에 retention loss 현상은 c/c 분역벽의 구조와 매우 밀접한 연관이 있는 것을 확인할 수 있었다. 결과적으로 주어진 박막에서 AFM tip을 이용하여 분역 반전을 일으킬 때, 직선적인 c/c 분역벽을 갖는 cylinder 형태의 분역이 되도록 반전 시켜야 retention loss로부터 자유로울 수 있을 것이다.
