鐵系金屬氧化物材料其成長機制與熱、電激發響應之研究

Abstract

本論文係研究鐵系金屬氧化物（metal ferrite）其成長行為，結構和物理性質間的基礎探討。本論文將分成三個部分，茲簡述如下：第一部分探討具樹突狀結構之四氧化三鐵於溶液中的成長行為，包括成長動力學與機制；另外也探討成長過程的相變化行為。第二部分則研究受形變鐵酸鉍薄膜之相變化過程與其受溫度激發響應之焦電性質。第三部分則是觀察受形變鐵酸鉍薄膜受電場激發下，相變化的過程及其相關鐵電電阻轉換行為。 首先，樹突狀礦物的成長一直被視為非平衡態的過程且廣泛地存在於自然界中，但對其動力學與成長機制的基礎了解卻因為直接觀察技術的缺乏而受到侷限。故本實驗將利用液態臨場穿透式電子顯微鏡，探討液態中樹突狀四氧化三鐵奈米結構的成核成長。我們觀察從奈米顆粒轉變成樹突狀結構的成長動力學是反應控制的動力學型態和古典的理論不同。此外，總體的尖端分岔動力學是由相鄰的分支對於外圍前驅物的競爭所決定的，此亦為一種圖靈不穩定(Turing Instability)機制。再更深入的結構分析，我們發現成長四氧化三鐵的過程，直接和間接兩種結晶過程會共存，此現象可以藉由熱力學的角度進行解釋。本研究對於樹突狀四氧化三鐵的分岔和結晶機制提供新的闡議，相信此結果可運用至自然界中其他礦物。 第二部分，近期材料的激發響應已受到廣大的矚目，其為能源轉換的必要條件。而隨著奈米科技的進步，激發響應所伴隨的原子移動而引發的相變化需要於奈米尺度下更細部的探討。近期，鐵酸鉍的應力工程因為它的能源轉換潛力已成為核心的研究目標，如焦電和形狀記憶元件等。本研究中，藉由熱的刺激，鐵酸鉍混合相會展現可逆的相轉變並伴隨優異的焦電響應。我們利用臨場高解析穿透式電子顯微鏡觀察T相和R相之間的相變化過程需要一過度相。此外，相變化的起源是來自於熱力學穩定和基板引發形變之間的交互作用；同時我們也藉由相場模擬去證實整個相轉變的過程。此研究成果對於受形變鐵酸鉍薄膜激發響應的原子行為提供新的基礎瞭解。此外也展現此特殊材料具有應用於新穎能源收集的潛力。 最後，近年來非揮發性記體受到極大的矚目，特別是最為成熟的鐵電隨機存取記憶體因其高讀寫速度和低能耗而最具代表性。儘管其為最為成熟的非揮發記憶體，但仍有待解決的議題和挑戰。近期，鐵酸鉍的應力工程因為其獨特的性質而成為廣泛的研究對象。在本實驗中，我們探討受形變鐵酸鉍薄膜的可轉換二極體特性，具有應用於高密度記憶體的潛力。藉由臨場通電實驗，我們證實鐵電極化反轉和鐵電電阻轉變之間的關係。進一步的結構分析也觀察到受外加電場下的相轉變過程是從混合相變成T相。此結果引發了多種電阻組態。綜觀此特性可用蕭特基能障的改變去解釋。藉由調控內部鐵電極化和外加電場的方向能改變不同的電阻值。藉由材料內部空間的開發，此概念將對高密度記體的研究提供新的方式。

In this thesis, we focus on the investigation of fundamentals in complex oxide (metal ferrite) regarding the growth behaviors and correlation between structure and physical properties. There are three parts as discussed below. One is the investigation of growth behavior of spinel magnetite with dendritic architecture in solution and corresponding phase transition during the formation procedure. The second part is the study of phase transition and corresponding pyroeleteic response with thermal stimuli in strained BiFeO3 thin film; the third part is the study of phase transition with electrical stimuli and the correlated potential multilevel ferroelctric resistive switching in BiFeO3 with MPBs features. In first part, dendritic mineral growth, recognized as a paradigm of non-equilibrium process, is commonly found in nature, but fundamental understanding of branching kinetics and formation mechanism is still limited due to the lack of real-time imaging capability during growth. Here, by using in situ transmission electron microscopy (TEM), we study the nucleation and growth of dendritic magnetite nanostructures from solution. Growth kinetics of magnetite, from particle to dendrite, show the reaction like limited kinetics model along with distinct fractal dimensions, in contrast with classical model. Overall tip splitting kinetics are determined by the competition between the adjacent branches to the precursors with Turing-like instability mechanism. We also demonstrate the coexistence of direct and indirect crystallization pathways, as explained through the thermodynamic consideration. Our findings provide new insights into the understanding of dendritic magnetite branching and crystallization mechanism, which we believe is applicable to general mineral dendrite systems in nature. In second part, great attention is paid to that stimulus response of materials, which is a prerequisite for energy harvesting applications. Driven by advances in nanotechnology, the atomic motions, involved in phase transitions and associated with stimulus responses, require detailed investigation on the nanoscale. Recently, strain engineering of BiFeO3 has become the subject of broad research interest due to its promising potential in energy conversion applications, such as piezoelectric, pyroelectric and shape memory effect (SME) devices. In this study, an excellent pyroelectric response is associated with reversible phase transitions in mixed-phase BiFeO3 films using thermal stimuli. Using an in situ high-resolution (HRTEM), we observed that phase transition between rhombohedral-like (R-like) and tetragonal-like (T-like) BiFeO3 involved the migration of the phase boundary, which is a prerequisite for the growth of the T-like phase and requires an intermediate phase. Moreover, the origin of the phase transition is attributed to competition between thermodynamic stability and substrate-induced strain, as suggested by phase-field simulations. The results provide a fundamental understanding of the atomic processes that underlie the stimulus response of the strained BiFeO3 films and demonstrate the potential of this extraordinary material for novel energy harvesting applications. In third part, recent quest for non-volatile memories has attracted tremendous attention, especially in mature ferroelectric random access memory (FeRAM) with properties of high read/write speed and low power consumption. Despite its promise, some scientific issues are still encountered. Strain engineering of BiFeO3 has recently become the subject of broad research interest because of its intriguing properties. In this study, we demonstrate the switchable diode characteristics in highly strained BFO thin films. Using a unique in situ TEM, we verify the correlation between ferroelectric resistive switching with possible multilevel states and polarization reversal. Structural investigation confirms the phase transition from mixed phase to pure T-like phase by external bias, which is the origin of the multilevel states. The switchable diode with resistive switching can be explained in terms of the variation of the barrier height, controlled by ferroelectric polarization and polarity of the external bias. This research model, i.e., engineering of the room inside, may offer an approach toward high-density memories.