【作者】 吴云翼；

【导师】 张端明；

【作者基本信息】 华中科技大学 ， 凝聚态物理， 2008， 博士

【摘要】 随着铁电存储器材料研究的深入,含铋层钙钛矿结构铁电薄膜（BLSF）近年来引起了人们的极大兴趣。其中,Bi₄Ti₃O₁₂（BTO）及其改性铁电薄膜是应用于铁电存储器的最热点材料之一。本文首先概述了铁电体的基本特性以及铁电薄膜在电存储器中的应用。BTO被认为是最具有应用前景的铁电材料之一。然而,剩余极化较小以及较差的抗疲劳特性限制了其在工业中的广泛应用。针对BTO薄膜存在的主要问题,本文研究A位和B位掺杂以及A/B位共掺杂对BTO薄膜微结构和铁电性能的影响;在此基础上,考虑到在薄膜高温制备中Bi₂O₃易挥发问题,通过增加Bi₂O₃过量的方法来研究Bi_3.25La_0.75Ti₃O₁₂（BLT）薄膜结构和铁电性能的变化规律;另外我们还研究缓冲层对BLT薄膜晶粒取向和微结构的影响。研究结果获得了克服以上问题的优化工艺,包括最佳Bi₂O₃过量值以及最佳的掺杂方式和掺杂量,并且发现缓冲层能够改善BLT薄膜的晶粒取向和漏电流特性。主要研究结果和结论如下:将不同Bi₂O₃过量条件下制备的BLT薄膜在600℃-800℃下进行后期退火。研究结果显示:BLT薄膜的极化行为同时受到退火温度和Bi₂O₃过量值的影响。由于适当的Bi₂O₃过量能够补偿热处理过程的铋损失,从而可以提高BLT薄膜的极化和介电特性;但是,加入过多的Bi₂O₃会形成杂相,反而会削弱BLT薄膜的极化和介电特性。在所有BLT薄膜样品中,7.5 mol%Bi₂O₃过量所制得的BLT薄膜表现出最大的剩余极化值（2Pr）25.26μC/cm²和相对较小的矫顽电压（Vc）3.62 V,以及最大的介电常数值252.6和较小的介电损耗0.066。同时,7.5 mol%Bi₂O₃过量的BLT薄膜表现出最好的抗疲劳特性,1×10⁹读/写转换周期后,样品只有9.2%的初始极化的退化。在不同温度下退火制备A位Pr掺杂Bi_2.9Pr_0.9Ti₃O₁₂（BPT-0.9）薄膜。在此基础上,在750℃下讨论退火时间不同对BPT-0.9薄膜铁电特性的影响,从而确定最佳的工艺参数。研究发现,当退火时间比较短的情况下（10分钟）,薄膜XRD图中衍射峰强度相对较弱,峰也相对较宽,表明薄膜的结晶程度较低;随着退火时间的延长,衍射峰强度逐渐高,峰也变尖锐,表明BPT-0.9薄膜结晶程度的提高。然而,当退火时间超过25分钟,XRD图中出现了杂相烧绿石相,这表明高温下过长时间的退火不利于薄膜单相结晶。溅射时基片温度的不同对薄膜的结晶取向也会产生影响。在不同基片温度（500℃-750℃）下沉积Bi_3.63Pr_0.3Ti₃O₁₂（BPT-0.3）薄膜。研究发现当基片温度低于650℃情况下,薄膜表现出（117）方向取向;随着基片温度的升高,（00l）系列的衍射峰逐渐变强,并且当基片温度升高到750℃,BPT-0.3薄膜表现出c轴优先取向。制备B位Zr掺杂的Bi_3.25La_0.75Ti_3-xZr_xO₁₂（BLTZ-x,x=0、0.2、0.5、0.8、1.2和1.6）薄膜。随着Zr掺量的增加,BLTZ薄膜的剩余极化值逐渐增加,当x=0.2时达到最大;然而随后随着Zr⁴⁺掺量的继续增加,剩余极化值开始减小;不同Zr掺量的BLTZ薄膜矫顽电压V_c没有明显的变化。疲劳特性测试表明,与没有掺杂的BLT薄膜相比,适当Zr⁴⁺替代Ti⁴⁺的BLTZ-0.2薄膜表现出更好的抗疲劳特性。利用固相反应法制备了B位Mn掺杂Bi_3.25La_0.75Ti_3-xMn_xO₁₂（BLTM-x,x=0.02、0.04、0.06和0.08）陶瓷。研究结果发现,随着Mn掺杂量的增大,BLTM的居里温度（T_c）逐渐降低;另一方面,BLTM的剩余极化却逐渐增大,这可能是由于晶粒取向变化和尺度的增大引起的。研究发现,用Pt等金属作为衬底存在一些缺陷:一是容易在BLT薄膜和Pt界面上由于晶格失配产生大的应力;二是高温处理中在薄膜和Pt界面间容易产生互扩散。我们通过在BLT薄膜和Pt界面间加入一层缓冲层（Bi₂O₃或TiO₂）,可以改变BLT薄膜的结晶取向,并且可以改善界面互扩散现象,从而提高BLT薄膜的铁电特性。更多还原

【Abstract】 Bismuth layered structure ferroelectrics（BLSFs） thin films have been widely investigated recently.Among them,as a typical kind of layer-structured ferroelectrics, Bi₄Ti₃O₁₂（BTO） ferroelectric thin film has attracted much attention due to their potential applications in nonvolatile ferroelectric random access memory（NvFRAM） devices.In this thesis,we summarized the basic properties of ferroelectrics and application of ferroelectric film in memory.BTO has been considered one of the promising ferroelectirc materials.However,the low 2Pr and poor fatigue property of BTO thin film limit its industrial application.Aiming at those problems,the dopping effects of A-site and B-site substitution and A/B-sites cosubstitution of BTO thin films were discussed.Considering the easy volatilization of Bi₂O₃ in process of heat treatment,we studied the effect of Bi₂O₃ excess content on the microstructure,ferroelectric and dielectric properties of the Bi_3.25La_0.75Ti₃O₁₂（BLT） films.In addition,the influences of buffer layers on structure and electrical properties of BLT thin film were investigated.The research result basicly attained what we expected.The main results are as follows:The deposited BLT were annealed at 600℃—800℃for 20 minutes respectively. Study result reveals that the polarization behavior is affected by both the annealing temperature and the amount of Bi₂O₃ excess content.Excess Bi₂O₃ added to the BLT system can enhance the polarization and dielectric properties due to compensation of Bi loss occurred during calcinations and weaken the polarization and dielectric properties due to formation of a secondary phase such as bismuth oxide.For example,The BLT film sample derived from 7.5 mol%exhibits a remnant polarization（2Pr） 25.26μC/cm²,and coercive voltage（Vc） of 3.62 V,together with a dielectric constant of 252.6 and a dielectric loss of 0.066.Also,the BLT thin film sample derived from 7.5 mol%Bi₂O₃ excess shows the best fatigue resistance characteristics and about 9.2%polarization degradation after 1×10⁹ read/write switching cycles.A-site Praseodymium doped BTO thin films with composition Bi_2.9Pr_0.9Ti₃O₁₂ （BPT-0.9） were prepared after post annealed at different temperature.Based on it,for BPT-0.9 film annealed at 750℃,the effect of different annealing time on ferroelectric properties was discussed.It was found that when annealed for a short time（10 min）, relatively broaden peaks were observed for the BPT-0.9 thin film,implying low crystallinity.With the increase of the annealing time length,the diffraction peaks become stronger,indicating the growth in crystallinity in the BLT films.However,when the annealing time is above 25 min,a pyrochlore phase peak appears in X-ray diffraction patterns.It indicates that too long annealing time is unfavorable to crystallinity of BPT-0.9 films.Bi_3.63Pr_0.3Ti₃O₁₂（BPT-0.3） films were deposited by RF-magnetron sputtering with substrate temperature varied from 500 to 750℃and the effect of substrate temperature on growth orientation was investigation.Films deposited below 650℃showed no preferred c-axis orientation and（117）peak is the strongest peak.As the substrate temperature was increased,however,the intensity of the film（00l）-plane peaks increased leading to c-axis preferred orientation in the film at 750℃.B-site Zr-doping Bi_3.25La_0.75Ti_3-xZr)xO₁₂（BLTZ-x,x=0,0.2,0.5,0.8,1.2 and 1.6） were prepared successful.With the increasing of dopant concentration,the remanent polarization（2Pr） first increases then decrease while there is almost no obvious difference in the Vc values.In comparison with no Zr-doped BLT,the fatigue test indicates that appropriated Zr-doped BLTZ-0.2 exhibite better fatigue resistance characteristics.B-site Mn-doped Bi_3.25La_0.75Ti_3-xMn_xO₁₂（BLTM-x,x=0.02,0.04,0.06和0.08） were prepared by a solid-state reaction technique.It was found that The Curie temperature of Mn-doped BLT was steadily shifted to lower temperature with increasing Mn-doping content.It was the increase in crystal structure symmetry that led to a decrease in the Curie temperature of the samples.In addition,the remnant polarization of BLTM samples increased with the Mn-doping content,which might be due to modification in the orientation of grains and enhance of grains size.Previous research found there are some disadvantages when used Pt as substrate, such as larger stress due to lattice dismatch and interdifusion between Pt and ferroelectric film when heat treatment.Through introduce buffer layer,such Bi₂O₃ or TiO₂,could change crystallization orientation and limit interdifusion.Because of those would improve the ferroelectric properties of BLT films.更多还原