應力調節複雜性氧化物之極化性質

Abstract

本論文系研究複雜性氧化物之極化如何受應力影響。論文分為兩部分，第一部分為多鐵材料鐵酸鉍之鐵電極化受應力限制其反轉而改善其鐵電保留之性質；第二部分為混合兩複雜性氧化物，於結構中形成成分梯度變化而產生極化，並利用伴隨成分梯度所產生之應力梯度變化以控制極化方向。首先，多鐵材料之非揮發性電子元件提供了超越目前常見電子元件的可能性。然而，在製作出可用元件前，必須解決鐵電性質的穩定性，如：壓印(imprint)、疲勞(fatigue)與保留(retention)。此實驗中，藉由異質磊晶成長多鐵介晶體(mesocrystal)以達到永久鐵電保留。實驗方法主要透過掃描探針顯微鏡紀錄鐵電極化隨時間之變化並提出可能的解釋，最後再由穿透電子顯微鏡與相場模擬證實鐵電保留之關鍵在於限制自發性極化遲豫(relaxation)時所伴隨的鐵彈性形變。複雜性氧化物具有相當豐富的性質，如：鐵電、超導和巨磁阻等等，吸引了科學家投入此領域研究。為改善或控制其性質，複雜性氧化物被設計並製作出各式各樣結構，從單一材料薄膜到由至少含有兩種材料之異質結構。近年來，樣品成長技術之進步，樣品成長時的精確度可達到單位晶包的尺度。因此，我們提出一種新式的異質結構。憑藉著成長時的控制精確度，創造出於垂直膜面方向上具有組成成分之梯度變化之異質結構。此實驗以鍶鈦氧(SrTiO3)與鑭鋁氧(LaAlO3)作為此新式異質結構之模型。在此垂直膜面方向上之成分梯度將伴隨著應變梯度。而由過去研究中，可預期極化不連續的存在。透過電荷、軌道和晶格間自由度的耦合，將產生一受應變梯度所誘發且控制之極化，稱之為彎電性(Flexoelectricity)。

In this thesis, we study how the polarization in complex oxides are affected by the strain. It is discussed in two parts: in the first part, the reversal of the ferroelectric polarization of multiferroic BiFeO3 (BFO) is constrained to enhance the ferroelectric retention, and in the second part flexoelectricity is created through a compositional gradient. Firstly, Non-volatile electronic devices based on magnetoelectric multiferroics have triggered new possibilities of outperforming conventional devices for applications in information storage, the emerging field of spintronics, and sensors. However, ferroelectric reliability issues, such as imprint, retention, and fatigue, must be solved before the realization of practical devices. In this study, everlasting ferroelectric retention in the heteroepitaxial constrained multiferroic mesocrystal is reported, suggesting a new approach to overcome the failure of ferroelectric retention. Studied by scanning probe microscopy and transmission electron microscopy, and supported via phase-field simulations, the key to the success of ferroelectric retention is to prevent the crystal from ferroelastic deformation during the relaxation of the spontaneous polarization in a ferroelectric nanocrystal. On the other hand, complex oxides present versatile properties, such as ferroelectricity, superconducting, and colossal magnetoresistance (CMR), and have been widely studied. To improve or manipulate the properties, complex oxides are designed and well grown into different structures, from the thin film of a single material to hetero-structures combining at least two materials. With the advance of growth techniques, the accuracy of growth is controlled on the scale of a unit cell via the assistance of reflection high-energy electron diffraction (RHEED). Utilizing the ability to control the growth at the atomic scale, a non-uniformity with compositional gradient along the out-of-plane (OOP) direction is created. The model system is constituted by SrTiO3 (STO) and LaAlO3 (LAO). Under this circumstance of an OOP compositional gradient, a strain gradient ensues. Polarity discontinuities between STO and LAO are also expected compared to the bi-layer system. Through the coupling among the charge, orbital, and lattice, a spontaneous polarization is created. Since the spontaneous polarization is induced and manipulated by the strain gradient, the behavior is named “flexoelectricity”.