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1、湘潭大学 硕士学位论文 （Bi，Nd）（Ti，V）O铁电薄膜的制备及薄膜印 记失效分析 姓名：叶志 申请学位级别：硕士 专业：微电子学与固体电子学 指导教师：唐明华 20070510 湘潭大学硕士学位论文 I 摘 要 近年来，由于含铅量较大的铁电材料在制备和使用的过程中，都会给环境和人类带 来损害。为了保护地球和人类的生存空间，防止环境污染，实现可持续发展的目标，世 界各国的科技工作者正在抓紧研究少铅或无铅的压电、铁电材料。其中铋层状钙钛矿型 （BLSF）铁电薄膜由于其自发极化较大、抗疲劳性高、保持性好以及无铅化学成分等 优点，可望替代铅基材料成为新的铁电随机存储器的专用材料。本文通过实验制备
2、和理 论计算的方法来探索BLSF铁电薄膜的相关特性。 在实验制备方面， 采用化学溶液沉积法(CSD)在 Pt/Ti/SiO2/Si 基片上制备出 Nd3+/V5+ 共掺杂钛酸铋铁电薄膜(Bi4-xNdx)(Ti3-yVy)O12，简记为 BNTV，分别研究了退火温度，钒 掺杂含量对 BNTV 薄膜铁电性及其电学性能的影响。用 X 射线衍射、扫描电子显微镜 确定薄膜样品的成分和微观结构，同时对 BNTV 薄膜的电滞回线(P-E)、电流电压特 性(I-V)、电容电压特性(C-V)、抗疲劳性和电滞回线频率依赖关系进行了测量。结果 表明 BNTV 薄膜的最佳退火温度可达 800C，相对于 BNT 薄膜
3、有所提高，可见钒掺杂 对铋元素的挥发有一定的抑制作用；实验中发现(Bi3.15Nd0.85)(Ti2.91V0.09)O12薄膜具有比 BNT 薄膜更低的漏电流（5.9910-9 A 3V）和更好的抗疲劳特性。因为 BNTV 薄膜中 钒含量的变化会引起薄膜氧空位、空间电荷变化或发生晶格畸变，所以控制适量的钒掺 杂可制备电学性能优良的 BNTV 薄膜。 在理论计算方面，通过对薄膜中偶极子的统计分布函数进行积分的方法，改进经典 的 Preisach 模型，并修正由积分近似值引起的电滞回线的缺陷，用较少的参数仿真出的 电滞回线拥有更饱满、平滑和对称的形状，与 BLSF 薄膜的实验结果相符。由于改进的
4、 模型具有历史电场效应，可以方便、准确地仿真薄膜的非饱和电滞回线，适用于铁电电 容的小信号模拟。引入 Furukawa 提出的方法对我们改进的电滞回线模型数值求解电容 率，得出电容率与电场的关系曲线，即蝴蝶回线。所得的蝴蝶回线在零电场处的奇点比 传统定义的 ( = dP/dE) 要高，更接近实验真实曲线。因此改进模型可以精确、快速地 仿真电滞回线和蝴蝶回线， 对铁电存储器及铁电可调性器件的电路模拟和理论研究具有 一定的应用价值。 另外，基于上述改进的 Preisach 模型，通过扩展单界面层的模型，建立起双界面层 模型来研究电滞回线的印记效应。 将上下界面层的非对称电导率做为联系印记内在原因
5、和外在现象的中间参量，能很好地解释电滞回线中矫顽场的增减，厚度尺寸与电滞回线 偏移效应的依赖关系，电滞回线底部膨胀或紧缩的变形等各种印记失效行为。双界面层 模型仿真出的电滞回线具有电场偏移、回线倾斜、矫顽场变化、形状改变等印记属性， 可以很好地与实验观察结果相吻合。最后，通过模型的理论分析提出减小印记失效的可 行性方法：形成互补的上下界面层使其具有反相相等的电导率。此双界面层印记模型的 提出对洞察铁电薄膜的印记机制和减少印记失效具有重要的指导意义。 关键词：BNTV 铁电薄膜；Preisach 模型；电滞回线；蝴蝶回线；印记 湘潭大学硕士学位论文 II ABSTRACT Recently, o
6、wing to ferroelectric materials and its products containing much lead, it can cause environmental pollution and damage peoples health during manufacture and use. In order to protect our environment from lead pollution, the scientists all over the world are engaged in lead-free or low lead content pi
7、ezoelectric and ferroelectric materials. Among all these materials, bismuth layer-structured ferroelectric (BLSF) thin film have the characteristics of large remanent polarization, high fatigue resistance, good retention, lead-free chemical composition. It has been studied intensively for potential
8、applications in ferroelectric random access memories (FeRAM). In present work, we study the properties of BLSF thin film by experimental and theoretical calculation approach. In the experimental field, thin films of Nd3+/V5+-cosubstituted bismuth titanate, (Bi4-xNdx)(Ti3-yVy)O12 (BNTV), were fabrica
9、ted by chemical solution deposition (CSD) technique. The effect of annealing temperature and V content on the ferroelectric and electrical properties were studied, respectively. The crystallized phase and microstructure were investigated using X-ray diffraction (XRD) and scanning electron microscope
10、 (SEM). At mean time, polarization hysteresis loops (P-E), leakage current-voltage (I-V), and capacitance-voltage loops(C-V), fatigue resistance and the dependence of hysteresis loops upon frequency of the BNTV thin film were measured. As a result, the V5+ substitution of Ti-site had improved the op
11、timal annealing temperature of BNT thin film to 800C. It seems that the effect of V-doped to restrain Bi loss is obvious. Compared with the BNT thin film, it also found that (Bi3.15Nd0.85)(Ti2.91V0.09)O12 thin film have the lower leakage current (5.9910-9 A 3V), better fatigue resistance. The change
12、 of V contents in BNTV thin film may result in oxygen vacancies, space trapped charge and lattice defect. This suggests that the desired ferroelectric thin film with good electrical properties would be obtained if the substituted V5+ contents are well controlled. At the theoretical field, The Preisa
13、ch model was improved through an approximate integral of the statistic distribution function of the dipoles in thin film. Then, the defect of the hysteresis loops, which caused by approximation of integral, was corrected by superposition method. The simulated hysteresis loops show full, smooth and s
14、ymmetric shape. The improved model is able to describe the unsaturated loops due to its history-dependent electric field effect. It is available in the small signal ferroelectric capacitor model. A methodology proposed by Furukawa has been employed to describe the butterfly loops. The simulated loop
15、s singularity at zero bias is much higher than the curve by the definition of permittivity = dP/dE. This case is in good agreement with the experimental result. Thus, the present model is fast, accurate and few undesirable parameters, and therefore suitable for use in circuit 湘潭大学硕士学位论文 III simulato
16、rs and theoretical research in ferroelectric memory and tunable device. Additionally, based on above improved model, we extend single interface layer model to investigate imprint behaviors by establishing two interface layers model. The electrical conductivity of top/bottom layer is used as medi-parameter to correlate the cause and phenomenon of imprint failure. It can explain various phenomenon including the change of coe
