【作者】 杨斌；

【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】 近年来,含铋层钙钛矿结构的铁电薄膜(BLSF)引起了人们的极大兴趣。其中,稀土元素Nd掺杂的钛酸铋Bi3.15Nd0.85Ti3O12(BNT)薄膜是应用于铁电存储器的最热点材料之一。本文采用化学溶液沉积方法,分别在Pt(111)/Ti/SiO2/Si(001)和TiO2(101)/ Pt(111)/Ti/SiO2/Si(001)衬底上制备了BNT薄膜,研究了前驱体溶液浓度、退火条件和衬底材料对薄膜晶粒形貌和取向,以及铁电性能的影响,并对这些影响,从物理机制上进行了讨论。在大量实验的基础上,本论文确认了一种制备较大剩余极化强度的BNT薄膜的优化工艺为:使用TiO2种子层,在750℃条件下快速退火。本论文首次开展了对BNT薄膜的晶粒形貌和取向的深入探讨。其中,晶粒形貌主要受成核机制的影响。对于生长在Pt电极上的BNT薄膜,当结晶温度较低时,体成核、界面成核同时发生,薄膜由细小晶粒组成;当结晶温度较高时,成核势垒低的界面成核成为主要的成核方式,薄膜晶粒呈柱状。而对于生长在TiO2种子层上的BNT薄膜,由于其界面处的成核势垒较高,不利于界面成核成为主要的成核方式,因而不利于柱状晶粒的形成。BNT薄膜的晶粒取向同时受成核机制和晶粒生长的影响。对于直接生长在Pt电极上的BNT薄膜,它受三方竞争的影响:界面成核——利于c轴取向的晶粒、晶粒生长——利于a轴取向的晶粒,以及体内成核——利于随机取向的晶粒。但对于生长在TiO2种子层上的BNT薄膜,其晶粒取向只受两方面竞争的影响:界面成核和晶粒生长——利于a轴取向的晶粒,以及体内成核——利于随机取向的晶粒。同时,本论文的研究还表明,BNT薄膜的铁电性能,尤其是剩余极化强度,主要随其a轴取向度的增大而增强,但同时也受其他因素,如晶粒大小的影响。晶粒尺寸较小的BNT薄膜,其剩余极化强度较小,矫顽场较大。总之,要获得剩余极化强度较大的BNT薄膜,应一方面提高薄膜、衬底界面处的a轴取向晶粒的成核,另一方面促进薄膜晶粒的柱状生长。最后,本论文还提出了反铁电耦合的铁电双层膜和相应超晶格系统的模型,其特点是考虑了组成薄膜体内的极化梯度和表面效应,并且组成材料是一级相变材料。基于该模型,我们在Landau-Ginzburg热力学理论的框架下,研究了铁电双层膜和相应超晶格系统的电滞回线和尺寸驱动相变行为。结果表明,铁电双层膜和相应的超晶格系统的组分膜的性质和厚度比,以及组分膜之间的反铁电耦合强度,都会影响其电滞回线的形状,同时,组分膜之间的反铁电耦合的强度,还会对系统的尺寸驱动相变行为产生重大的影响。当耦合较弱时,系统具有尺寸驱动相变行为,而当耦合较强时,该行为消失。总而言之,本论文研究的创新之处在于:1、首次系统研究了BNT薄膜的晶粒形貌,尤其是发现了TiO2种子层的使用不利于柱状晶粒的形成;2、首次系统研究了BNT薄膜的晶粒取向,发现晶粒取向是成核机制和晶粒生长机制竞争的结果;3、发现BNT薄膜的铁电性能同时受薄膜取向和晶粒尺寸的影响;4、提出了反铁电耦合的铁电双层膜和相应超晶格系统的模型,其特点是组成材料为一级相变材料,考虑了组分膜内的极化梯度和表面效应。随后,在Landau- Giznburg热力学理论的框架下,研究了系统的电滞回线和尺寸驱动相变行为。这一研究,提出了一种新型的材料设计,具有一定的应用前景。更多还原

【Abstract】 Bismuth layered structure ferroelectrics (BLSFs) thin films have been widely investigated recently. Among them, Bi3.15Nd0.85Ti3O12 (BNT),as a typical kind of layer-structured ferroelectrics, has attracted much attention due to its potential applications in nonvolatile ferroelectric random access memory (NvFRAM) devices.In this thesis, we fabricated BNT thin films by chemical solution deposition method on both Pt(111)/Ti/SiO2/Si(001) and TiO2(101)/ Pt(111)/Ti/SiO2/Si(001) substrates. The effects of the precursor solution concentration, annealing conditions and the propertiy of substrates on the morphology and orientation of grains in those films were addressed, based on the classical nucleation theory. And using Landau-Ginzburg thermadynamics theory, the hysteresis loops of first-order ferroelectric superlattices are simulated, by taking into acount the graded polarization and the surface effect. Moreover, the effects of the antiferroelectric coupling on the size effect of the system were investegated. The main results are as follows:The grain morphology of BNT thin films is mainly determined by the characteristics of the nucleation event; when crystallization occurs at a low temperature, thus the barrier heights for both the interface nucleation and the bulk nucleation can be surmounted, and the film displays microstructure with fine grains. In the opposite case, lower energy interface nucleation events dominate the thin film microstructure and the film is mainly columnar in nature.The grain orientation of BNT thin films are decided by both the nucleation and the grain growth. On the one hand, the c-axis-oriented-grain nucleation is favored due to its lower surface energy and the better mismatch with Pt. But the grains are randomly orientated when the nucleation is occurred in the bulk of films. On the other hand, the a-axis-oriented grains are favored owing to their fasted grain growth speed along the film normal. Anyway, a competition exists between the nucleation and grain growth in determining the film orientation. But for films grown on the seeding layer of TiO2, the low degree of c-aixs-orientation is obtained because the a-axis-oriented grains are favored in both the nucleation and the growth. While, a high degree of (117)-orientation is still obtained even when annealed under 700℃using the rapidly thermal annealing. This results from the fact that the TiO2 seeding layer helps the occurance of bulk nucleation, which favors the randomly oriented grains. Until the annealing with high temperature of 750℃and rapidly thermal annealing is adopted, BNT thin films with a high degree of a-axis-orientation can be obtained.The ferroelectric properties of BNT thin films, especially their ramnent polarization, has close relation with their orientation, i.e., the higher degree of a-axis-orientation, the stronger the remnant polarization. But at the same time, the property is also affected by the grain size, because it is more difficult to reorientate the polarization in the smaller grains by the applied electric field, and so the coercive field increases. For first-order ferroelectric superlattices, their loop patterns vary between typically antiferroelectric and typically ferroelectric depending on the thickness ratio, coupling constant, thickness and extrapolation length. The antiferroelectric coupling has a great effect on the phase transition of a bilayer. It is shown that the size-driven phase transition can’t be observed in a ferroelectric bilayer in the case of strong antiferroelectric coupling.The innovations of the thesis are as follows:1. Study systematically the effects of fabrication conditions on the morphology of grains in BNT thin films;2. Study systematically the effects of fabrication conditions on the orientation of grains in BNT thin films and address the mechanism ;3. Put forward the opitimized fabrication conditions to get BNT thin films with excellent properties;4. Simulate the hysetesis loops of first-order ferroelectric bilayers or superlattices and their size effect, by taking into account the surface effect and polarization variation in component films.更多还原