低溫製備鈮鎂酸鉛-鈦酸鉛薄膜之微結構、鐵電與介電性質比較

Abstract

由於(1-x)PMN-xPT薄膜屬於高溫退火製程的鐵電材料，且容易出現第二相，因此退火溫度必須高達700 ℃以上才能降低第二相的存在，但也導致(1-x)PMN -xPT薄膜所能相匹配的元件製程較有限。本研究目的為在低溫退火製備(1-x)PMN-xPT薄膜，期望藉由降低退火溫度達到穩定良好的表現性質，適合用於製程中可相匹配的元件方面。且配置不同成分比例的(1-x)PMN-xPT薄膜以比較其差異性。 本研究使用溶膠-凝膠法製備(1-x)PMN-xPT薄膜，並對(1-x)PMN-xPT薄膜進行成相、顯微組織、介電、鐵電性質及熱分析的檢視， 此外本研究選用銦化錫導電玻璃(ITO/glass)基板並鍍上鈦酸鉛鑭(Pb0.86La0.14TiO3，PLT)當作晶種層(seeding layer)形成PLT/ITO/glass基板製備(1-x)PMN-xPT薄膜，主要原因有三：(1)選用PLT/ITO/glass基板製備(1-x)PMN-xPT薄膜較少文獻研究，(2)利用低溫退火製備(1-x)PMN-xPT薄膜可使得元件應用上可達到多元性，(3)基板具有透明特性，期望可應用於光電研究。 本研究製備不同成分比例之(1-x)PMN-xPT，分別為0.9PMN-0.1PT、0.7PMN- 0.3PT及0.55PMN-0.45PT等三種成分。且由於添加高分子PVP的緣故，在燒結過程中高分子揮發掉，使得(1-x)PMN-xPT薄膜的顯微結構形成多孔狀。 藉由顯微結構的觀察、質譜儀分析及持溫時間的試驗得知，含鎂含量較多的0.9PMN-0.1PT薄膜，再經過熱處理退火中，鎂含量也揮發較多。致使0.9PMN- 0.1PT在B-site的晶格缺陷多，使得介面能提高且異質介面多，導致0.9PMN- 0.1PT先以異質成核的方式使介面能下降，再以晶粒成長和粗化的方式使晶粒增加。因此0.9PMN-0.1PT在顯微結構方面以成核占主要部份。反之含鎂含量相對較少的0.7PMN-0.3PT及0.55PMN-0.45PT，鎂揮發也較少，晶格缺陷少使得兩者成分比例的(1-x)PMN-xPT在顯微結構方面會以晶粒成長與緻密化機制占大部分。 本研究另外做了退火燒結的持溫時間增加的試驗，研究結果得知藉由退火持溫時間的增加，可使(1-x)PMN-xPT薄膜晶粒成長、孔隙率降低及緻密化。對於改善鐵電及介電特性，例如：殘留極化值增加、介電常數增加及frequency dispersion現象皆可獲得更佳的性質。

Lead magnesium niobate-lead titanate ((1-x)PMN-xPT) thin films are an important class of ferroelectric materials and high temperature annealing is typically required to produce the perovskite phase in these thin films. Existing studies have suggested that in order to avoid the formation of second phases (e.g., pyrochlore), annealing at more than 700C is commonly needed. The high annealing temperature hinders the integration of ferroelectric/piezoelectric (1-x)PMN-xPT thin films into the production of engineering components. The main purpose of this study is therefore to prepare (1-x)PMN-xPT thin films of various PMN:PT ratios at low annealing temperatures. This is achieved with the aid of a polymer, polyvinylpyrrolidone (PVP). The (1-x)PMN-xPT thin films were prepared by the sol-gel method. PVP was added to the PMN-PT sol before the sol mixture was spin-coated onto the indium tin oxide (ITO)/glass substrates. A lead lanthanum titanate (Pb0.86La0.14TiO3) seeding layer was deposited onto the ITO/glass substrate first in order to promote the crystallization of PMN-PT. Three different thin film compositions were prepared in this study: 0.9PMN-0.1PT, 0.7PMN-0.3PT and 0.55PMN-0.45PT. The crystalline, microstructure, dielectric and ferroelectric properties of the prepared thin films were then characterized. The characterization data indicate that with the aid of PVP, the ferroelectric perovskite phase can be successively produced in the 0.9PMN-0.1PT, 0.7PMN-0.3PT and 0.55PMN-0.45PT thin films with a low annealing temperature of 450 C.This is due to the exothermic reaction/decomposition of PVP during annealing, providing the necessary heat for forming the perovskite phase. However, the addition of PVP renders the thin films to exhibit a porous microstructure. The 0.9PMN-0.1PT thin film in particular exhibits a highly porous microstructure characterized by loose lattices of fused small grains. This is believed to be caused by the evaporation of a large quantity of Mg during annealing, causing the increase in interfacial surface energy. This would in turn promote the fast nucleation of small PMN-PT grains at the expense of densification and grain growth. The density of the 0.9PMN-0.1PT thin film can be improved by lengthening the annealing time or by increasing the amount of Mg (i.e., higher than what is required by the stoichiometric ratio) in the original sol mixture. By lengthening the annealing time, the density and ferroelectric properties of the prepared (1-x)PMN-xPT thin films can be improved. The relative permittivities of the films show a broad peak as a function of temperature; this indicates the relaxor nature of the films. However, the frequency dispersion for the temperature of maximum permittivity (Tmax) is not obvious. This is believed to be caused by the microstructural porosity, weakening the frequency dependence of dielectric behaviors.