계면 응력 상태에 따른 비스무스 철 산화물 에피택시 박막의 자기적 특성 강화 현상 연구

Abstract

Strain engineering has obtained a great attention and achievement in recent science and technology since strain turned out to be a very important parameter that may have a significant influence upon material properties. The achievements were accompanied by the development of thin film growth techniques. In Si-based technologies, for example, strained-silicon has improved the performance of transistors pretty much. For optical devices, strain can modulate the band gap structure and this may be led to the creation of new optical properties or the reduction of the loss of energy conversion efficiency. In piezoelectric materials, mechanical stress induces a polarity in the materials and an external electric field can cause mechanical strain. When a ferromagnetic material undergoes strain in the form of thin films, the magnetic transition temperature can be changed. For ferroelectric thin films, strain can lower the ferroelectric-to-paraelectric transition temperature, change the spontaneous polarization direction, and enhance spontaneous polarization itself of a ferroelectric material. Further more, strain can affect the cross coupling between order parameters in multiferroic materials that have ferroelectric, (anti)ferromagnetic, and ferroelastic order parameters simultaneously. Despite of its importance, however, strain has been in a significant controversy that if the strain can enhance material properties or not. For some ferroelectric or ferromagnetic materials, recent research has proposed that strain cannot enhance ferroelectric polarization and the enhancement of magnetic moments is due to the compositional disorder such as oxygen non-stoichiometry by taking advantage of some theoretical calculations. Epitaxially constrained BiFeO3, as a well-known room temperature multiferroic, has also been in the center of a world-wide conflict since it has been reported that compressive strain on a BiFeO3 thin film can give rise to a huge enhancement of polarization and magnetization. Although thermodynamic calculations have proved the feasibility of the enhancement, this controversial issue is still gathering scientific and technological interest. In this dissertation, therefore, strain-dependent multiferroic properties of epitaxial BiFeO3 thin films were studied. Especially, the enhancement of magnetization in compressively strained BiFeO3 thin films were considered in terms of crystal structures. The variation of orbital hybridization and magnetic spin structures were examined and proposed as the origin of the enhancement with some theoretical analysis. In addition, the reliability of ferroelectric properties of a compressively strained BiFeO3 thin film as a new ferroelectric device was investigated.First, the origin of enhanced magnetization in epitaxially constrained BiFeO3 thin films with rhombohedral symmetry is investigated. By utilizing pulsed-laser-deposition method, we could fabricate epitaxial BiFeO3 thin films with rhombohedral symmetry on SrTiO3 (111) substrate. From the obtained lattice parameters, interface misfit strain was calculated and the enhanced magnetic properties were explained as a function of the misfit strain. In addition, x-ray absorption spectra revealed the hybridization of Fe 3d and O 2p orbitals and we could correlate the hybridization with the pronounced enhancement of magnetic properties by introducing spin canting. Second, the enhancement of magnetic properties by epitaxial constraints, one of the controversial issues for epitaxial BiFeO3 thin films with (001)c-orientation, was approached in view of the variation of antiferromagnetic spin axis. For this purpose, two different kinds of in-plane strain were induced by adjusting the surface miscut which is nearly zero (low) and 4 (high) along [110] direction. BiFeO3 thin films were grown epitaxially with compressive strain and showed x-ray linear dichroism (XMLD) for both cases. Being grown on the low miscut surface, XMLD is reduced as the film thickness increases, which indicates that antiferromagnetic spin axis evolves from out-of-plane direction to in-plane direction. However, there is no dependence upon azimuthal angle, which is deduced to be caused by the complicated mosaic domain structure. On the other hand, the epitaxial BiFeO3 thin films on high miscut surface exhibit in-plane spin anisotropy. As the film thickness increases, the antiferromagnetic spin axis rotates from [001] direction to [-110] direction. This homogeneous spin alignment induces the pronounced enhancement of magnetization compared to the films on low miscut surface.Finally, the engineering aspects of compressively strained BiFeO3 epitaxial thin films on a metal electrode layer are dealt with in terms of device reliability. For this purpose, epitaxial BiFeO3 thin films were prepared on epitaxially grown Pt layers and electric fatigue resistance and charge retention were examined. These films show an outstanding fatigue free characteristic up to 10^12 bi-polar switching cycles with good charge retaining ability up to 10^4 seconds, while polycrystalline BiFeO3 thin films are known to have severe electric fatigue after 10^6 cycles.

계면 응력 상태에 따른 BiFeO3 다강체 에피 박막의 자기적 특성의 변화 양상을 계면의 misfit strain에 대하여 분석하였고 그 기원을 미시적인 관점에서 찾아보고자 하였다. 또한 BiFeO3 에피 박막의 강유전 소자 특성을 저장된 전하 정보의 신뢰성 측면에서 평가하고 그 원인을 분석하고자 하였다.먼저, 벌크 상태에서 가지는 R3c 구조를 그대로 가지면서 에피 성장한 BiFeO3 박막에 대해서 자기적 특성을 측정한 결과, 박막의 두께가 얇아질수록 즉, misfit strain이 커질수록 포화 자화 값이 커지는 양상이 발견되었다. 엑스선 흡수 스펙트럼을 분석한 결과 이러한 포화 자화 값 증가가 균일하지 않은 Fe의 산화 상태에 따른 것이 아니라 misfit strain에 따른 구조 변화에 기인하는 것으로 밝혀졌다. 열역학적 관점에서 살펴본 결과 misfit strain이 작은 영역에서는 스핀 싸이클로이드가 균일한 스핀 배열 상태로 전이하면서 숨겨져 있던 자기적 특성이 나타난다는 결론을 도출할 수 있었고, misfit strain이 큰 영역에서는 Fe 3d &#8211

O 2p 오비탈 혼성에 의해서 in-plane 내의 spin canting 각도가 작아져서 포화 자화 값이 더 크게 나타나는 결론을 이끌어 낼 수 있었다. [001] 방향으로 성장된 BiFeO3 에피 박막의 경우, 마찬가지로 misfit strain이 커질 때 포화 자화 값이 크게 증가하는 결과를 얻을 수 있었다. 특히 이 경우 표면에서 [110] 방향으로 4도 정도 기울어진 기판을 사용하여 단일 분역 구조를 형성시켰을 경우 약 다섯 배 정도 포화 자화 값이 커지는 것을 알 수 있었다. 나아가서 엑스선 흡수 스펙트럼 분석 결과, 반강자성 스핀 방향이 misfit strain이 작아지면서 in-plane 쪽으로 회전하고 in-plane 내에서도 이방성이 나타나는 점으로부터 포화 자화 값 증가 현상의 원인을 유추할 수 있었다. 끝으로 Pt/MgO 하부 전극 기판 구조 위에 에피택시 성장한 BiFeO3 박막의 경우, 비교적 낮은 온도인 500 도에서 에피 성장이 가능하였으며, 다결정 박막과 달리 10^12 사이클까지 스위칭을 반복하여도 전기적인 피로 현상이 나타나지 않았고, 저장된 전하 정보도 10^4 초까지 유실되지 않는 우수한 메모리 특성을 보였다. 이러한 점들로부터 BiFeO3 물질이 강유전 메모리 소자로서의 응용 가능성이 매우 높음을 알 수 있다.