Manipulation of physical properties in oxide thin films by a local inversion symmetry breaking induced by flexoelectricity

Abstract

대칭의 개념은 물리학에서 놀라운 역할을 가지고 있다. 물질이 가지고 있는 대칭의 종류에 따라서 물질이 가질 수 있는 물리적 질서나 특성이 결정된다. 결정 대칭성에 따라서 고체는 잘 알려진 결정질 점군으로 분류될 수 있는데, 1830년에 32가지 유형의 형태학적 결정군으로 분류될 수 있다는 것이 증명되었다. 어떤 결정군에 속해있는지를 통해 복굴절과 같은 광학성 특성이나 압전성과 같은 전기기계적 결합이 나타날 수 있는지 여부를 알 수 있다. 전기적 특성과 기계적 특성 사이의 상관성은 매우 흥미로운 물리적 현상이며, 미시적인 크기의 전자기계부터 압전 엑츄에이터, 모터, 센서, 에너지 발전기, 전동기 단백질 및 세포막과 같은 생물 시스템에 이르기까지 다양한 분야에서 사용되고 있다. 전기 기계적 상관성의 가장 보편적인 예는 균일한 변형이 가해질 때 전기장이 물질 내부에 생성되는 압전성이다. 반면에, 불균일한 변형이 물질에 가해지는 경우에도 물질 내부에 전기장이 형성되게 되는데 이를 변전효과라고 한다. 변전효과는 전기적 분극과 불균일한 변형 사이의 상관관계로 정의될 수 있다. 이 현상의 중요성으로서 변전효과가 압전효과보다 더 보편적으로 나타난다. 즉, 더 많은 결정에서 나타날 수 있는 효과이다. 압전성은 32개중에 20개의 결정군에서 나타날 수 있는 반면, 변전성은 32개 모두의 결정군에서 나타날 수 있는 현상이다. 또한 변전효과는 물질의 크기가 작아지면 작아질수록 효과의 크기가 커진다는 재미있는 특성을 가지고 있다. 이러한 특성 때문에 압전효과가 사용되는 장치를 대체할 수 있는 효과로서 매우 큰 물리적 산업적 중요성을 가지고 있다. 본 박사논문에서는 변전효과에 의해 유도되는 반전 대칭 깨트림을 이용하여 산화물 박막의 물리적 특성을 제어하는 것이 가능하다는 것을 보여줄 것이다. 특히, 국소적인 영역에 날카로운 원자힘 현미경의 탐침을 이용하여 압력을 가하는 기술을 사용함으로써 원하는 영역에만 국소적으로 물리적 특성의 변화를 야기해낸 일들을 소개할 것이다. 나의 연구들은 산화물 박막에서 순수한 힘만을 가지고 흥미로운 물리적 현상 또는 특성을 제어하고 관찰하는 것이 가능하다는 것을 시사한다.

Over the last decade, flexoelectric effect at the nanoscale in solid has shown their wide potential both scientifically and technologically. There are already some patents for commercial electronic devices to replace piezoelectric devices such as sensor, and actuator. Also, they have shown rich and intriguing physical phenomena, including the recent hot topic of domain wall and photovoltaic. Despite these extensive studies on flexoelectric effect, I believe that many interesting issues still remain untouched. In this thesis, two novel findings has been addressed: trailing flexoelectric field for manipulation of multiaxial ferroelectric, control of the electrical state in a dielectric with flexoelectric origin. Firstly, I have demonstrated that in multiaxial ferroelectric epitaxial films, the trailing flexoelectric field generated by mobile AFM tip pressing can be used as an effective tool for engineering domain structures. We have suggested several advantageous features over the electrical way. Also, we have overcome the serious drawback of mechanical switching of ferroelectric polarization with AFM tip that is switching is unidirectional, i.e. only switching of polarization up to down is possible. I believe that the finding of this mechanism of trailing flexoelectric will open a great possibility to study exotic ferroelectric domain by creating it without applying electrical bias. Also, we demonstrated flexoelectric control of the electrical state in ultrathin dielectric films. We explained this huge resistivity change in terms of tunable depolarizaton field by controlling the amount of flexoelectric polarization generated inside of the ultrathin dielectric films. Achieving breakdown using very small electric bias, we could prevent extrinsic effect such as Joule heating and permanent damage on the sample even though we applied a huge electrostatic field by means of flexoelectricity. This work overcomes a long-standing dilemma: the electrical-state switching in dielectrics requires strong fields, but when applied by strong static fields, dielectrics inevitably suffer from irreversible damage. Utilizing universal flexoelectricity, we could develop a general approach to apply non-destructive, strong electrostatic fields in various insulating systems, such as the Mott insulator. Also, our results can be extended to realize future novel devices such as flexoelectric switch and transistor. Lastly, there can be many interesting topics we can further study with this AFM tip pressing technique. Since force applied by AFM tip will generate local strain gradients, thus breaking of local inversion symmetry, any change of physical properties or intriguing phenomena induced by the inversion symmetry breaking such as band gap opening in graphene or photovoltaic in centrosymmetric materials can be studied. As those studies would be original in the aspect that mechanism is novel flexoelectricity, it would provide good chances for high-impact publications.

The concept of symmetry plays an incredible role in physics. What kinds of symmetries the material possess determine the physical orders or properties the material can have. Depending on crystal symmetries, solid can be classified into well-known crystallopic point group. In solids, there exist 32 types of morphological crystalline symmetries derived in 1830 from a consideration of observed crystal forms. The point group of a crystal determines the directional variation of physical properties that arise from its structure, including optical properties such as birefringency, or electromechanical coupling such as piezoelectricity. Couplings between electrical and mechanical properties are quite intriguing physical phenomena and have been used in many applications ranging from microelectromechanical systems to biological systems such as a piezoelectric actuator, motor, sensor, energy generator, electromotor proteins, and cellular membranes. The most popular example of electromechanical couplings is piezoelectricity in polar systems (where the space inversion symmetry is broken), in which the homogeneous strain can induce electric fields and vice versa. On the other hand, there can occur a novel electromechanical coupling known as flexoelectricity in the presence of a strain gradient (i.e., inhomogeneous strain). The flexoelectricity can be defined as a coupling between polarization and strain gradient. One important aspect of this phenomena is that the flexoelectricity is more universal phenomena than the piezoelectricity. While piezoelectricity can arise in 20 point groups, flexoelectricity can arise in all 32 point groups as the strain gradients itself spontaneously break the inversion symmetry. Other than that, flexoelectricity has another important aspect. Flexoelectricity becomes larger and larger as the scale being reduced since the strain gradient is inversely proportional to the size of the sample given that the applied strain is fixed. Because of those two advantages mentioned above, flexoelectricity can play an important role in both nanoscale physics and application as it might be possible to replace the devices in which the piezoelectricity is in use. In this thesis, I will show that it is possible to tune the physical properties of oxide thin films by breaking the space inversion symmetry induced by flexoelectricity. Specifically, I exploited the technique so-called atomic force microscope (AFM) tip pressing that is applying pressure using a sharp AFM tip to induce local strain gradients, thus, break the local inversion symmetry. With this technique, local control of physical properties of oxide thin films was possible and three works related to AFM tip pressing will be addressed. Our studies on oxide thin films suggest that many interesting control of local physical properties can be possible by means of pure mechanical force.