多鐵材料鐵酸鉍磊晶薄膜應變誘發形態相邊界之研究

Abstract

現代的功能型智慧材料，如鐵磁或者壓電材料，許多都是利用化學合成金屬的典型方式，使多相的材料演化同時具有兩相存在的邊界，這樣的一個邊界通常具有很大的外延反應。舉凡參雜 Mn之磁性材料中浮現的龐磁阻效應、參雜的銅氧化物材料中的高溫超導體，或者具有壓電效應的弛緩性鐵電材料。舉例來說，如Pb(Zrx,Ti1-x)O3 (PZT)、Pb(Mg0.33,Nb0.67)O3-PbTiO3 (PMN-PT)、Pb(Zn0.33,Nb0.67)O3-PbTiO3 (PZN-PT)在所組成相結構的邊界中，可以測量到非常大的壓電係數，這些包含Tetragonal相與Rhombohedral相的相邊界中，由於巨大的壓電效應，使得PZT、PMN-PT、PZN-PT這些材料帶來廣泛的應用範圍，包含從微尺寸定位器到太陽能感應制動器。 本研究是報告屬於鈣鈦礦的鐵酸鉍- BiFeO3 (BFO) ，經由磊晶薄膜成長於LaAlO3 (LAO)基板上，由於薄膜上應變的誘發，薄膜可同時擁有 Tetragonal相以及Rhombohedral相，兩相的相邊界存在著’Morphotropic Phase Boundary’ (MPB) 特性。相較於眾多文獻報告中的PZT存在之MPB性質，BFO在自身單成份的薄膜裡便能擁有MPB特性，值得一提的是，BFO不似PZT一般含高污染的鉛 (Pb) 元素，在本研究的量測中，BFO鐵電、壓電特性的量測中，更勝PZT一籌，因此，BFO壓電薄膜無疑提供了無鉛世界中一種非常好的選擇。 在本研究中，我們試著以一些儀器來分析在BFO薄膜表面上這樣的界面，研究這種形成的動態過程。首先我們使用Laser MBE磊晶成長BFO薄膜，並搭配高壓RHEED的監控系統，因此，我們得到RHEED 繞射圖與振盪曲線，藉以釐清究竟是什麼溫度下，形成這麼一個過渡相。再者，我們利用溫度變化的RSM倒晶格分佈圖，來理解相與相邊界結構上的演化。最後掃描式探針顯微鏡 (SPM) 將會為我們掃取AFM表面形態圖與PFM鐵電區域分析圖，搭配溫度變化的參數來檢視相邊界靜態與彈性上的變化，所有在這界面上與動力學有關的範圍，如結構、表面相形態、鐵電區域特性的形成，將一一發表在這篇研究報告上。

Modern functional materials, for example ferromagnets and piezoelectrics, are typically chemically complex and exhibit the co-existence of multiple phases that evolve as a consequence of chemical alloying. In such materials, huge responses to external stimuli are often found at phase boundaries . Examples of the discovery of such behavior include the emergence of colossal magnetoresistance in doped manganites, high temperature superconductivity in doped cuprates, and large piezoelectric esponses in relaxor ferroelectrics . he large piezoelectric coefficients in Pb(Zrx,Ti1-x)O3 (PZT), Pb(Mg0.33,Nb0.67)O3-PbTiO3 (PMN-PT), and b(Zn0.33,Nb0.67)O3-PbTiO3 (PZN-PT) systems, for example, occur in compositions that lie at the boundary between two crystal structures, a rhombohedral-to-tetragonal phase boundary . These giant piezoelectric responses have made PZT, PMN-PT, and PZN-PT the materials of choice for a variety of applications ranging from micro-positioners to acoustic sensing in sonar. In this report, we demonstrate that epitaxial strain can be used to drive the formation of a morphotropic phase boundary (MPB) and exhibit tetragonal phase as well as rhombohedral phase in pervoskite-BiFeO3 (BFO) thin film grown on LaAlO3 (LAO) substrate. Comparing to MPBs which are often observed in mixed perovskites such in PZT family, the observation of BFO boundaries can exist in a single-component. BFO deserves to be mentioned with huge piezoelectrics and its lead-free component. the observation of such boundaries in a BFO thin film is of great interest for potential applications in next generation . In this study, we have tried to figure out the kinetics on formation of such a new interface by using various techniques. First of all, we grow BFO films on LaAlO3 substrates controlled by pulsed laser deposition with high pressure RHEED. We have also used RHEED patterns as well as oscillations to identify the temperature of phase transitions. Reciprocal space mapping as a function of temperature is used to understand the structural evolution of this phase boundary. In final part, topography and ferroelectric domain patterns as a function of temperature have also been probed by scanning probe microscopy (SPM) to study the electrostatic and elastic boundary conditions of this new interface. The correlation between structure, surface topography, and ferroelectric domain pattern will be addressed to understand the kinetics on formation of this new interface.