【作者】 朱哲；

【导师】 郑学军；

【作者基本信息】 湘潭大学 ， 材料科学与工程， 2011， 博士

【摘要】 现代微电子和光电子技术的快速发展使得微型化和集成化成为电子器件发展方向的主流。而一类具有钙钛矿结构的铁电薄膜（如BaTiO₃、PbTiO₃、Pb（Zr, Ti）O₃、（Pb, La）（Zr, Ti）O₃等）,因其自身存在优异的铁电、压电和热释电等性能,从而被广泛用于铁电随机存储器、光波导器件、变频器、传感器等众多领域,已经成为当前新型功能材料研究的热点之一。与此同时,研究铁电薄膜的生长机制并获得优异的制备工艺参数,同时探究制备出的铁电薄膜在不同的环境下物理性质的变化过程,成为目前国际上高度关注的研究课题。在薄膜的生长制备方法中,脉冲激光沉积（PLD）法作为一种传统的薄膜生长技术,因其具备使最终成膜的化学计量比与靶材一致的特点,而被广泛用于多元氧化物薄膜的制备。迄今为止,关于PLD法制备钙钛矿铁电薄膜的实验工作已被不少课题组研究报道,但是对于其原子级别的亚单层生长的研究却不多见。同时,一定参数条件下钙钛矿铁电薄膜在生长过程中的三维生长机制尚不明了,不同制备参数对薄膜二维和三维生长的影响也需要进一步去探究。在研究薄膜生长动力学的理论方法中,动力学蒙特卡罗（KMC）方法是一种常被采用的方法,它的优点是能够从原子尺度上来研究薄膜的生长。以往的文献报道,多数是用KMC方法模拟金属薄膜和半导体薄膜的生长。但是对于具有复杂晶体结构和生长机制的钙钛矿铁电薄膜而言,却很少有人用KMC方法来模拟其生长过程。与此同时,制备过程中铁电薄膜本身的不均匀性和来自基底的机械约束,使得铁电体呈现出一种电畴结构。其次铁电体的物理性质和应用方向又依赖于电畴的类型、结构及其运动规律。因此研究铁电体内部电畴的各种性质并分析其形成机制,是全面了解铁电薄膜各方面性质的基础。本文首先基于传统的依赖于动能的KMC模型,以BaTiO₃薄膜的PLD生长为例,考虑钙钛矿结构的特点建立一个二维KMC模型和一个三维KMC模型,分别模拟了BaTiO₃薄膜的亚单层生长和三维生长的过程,研究了钙钛矿铁电薄膜在各种不同工艺参数下的生长特点。另一方面,利用压电力显微镜（PFM）实时观测了不同外场作用下铁电薄膜的电畴演化。最后通过理论建模和模拟计算,分析了不同外场作用下铁电相变的规律以及压电、介电性质的变化。主要内容如下:（1）利用KMC方法模拟了BaTiO₃薄膜的PLD亚单层生长（覆盖率θ小于0.1）。区别于传统的solid-on-solid （SOS）模型,原子的成键过程被考虑进KMC模型中来模拟具体的钙钛矿结构。在二维KMC建模中,主要考虑原子的沉积、原子的表面扩散和吸附原子的成键三个基本事件。模拟研究了不同脉冲重复频率和脉冲持续时间对岛密度和岛尺寸的影响。结果表明:随着激光脉冲重复频率的增加,岛密度增加而岛尺寸减小。然而当脉冲重复频率位于千赫兹以上,在覆盖率θ<0.1时,岛密度没有变化;在覆盖率θ>0.1时,随着脉冲持续时间的增加,岛密度增加而岛尺寸减小。PLD制备薄膜的过程中,当脉冲重复频率位于千赫兹以上时,就需要重新考虑脉冲持续时间对薄膜生长带来的的影响。（2）建立了一个三维的KMC模型来模拟PLD制备过程中BaTiO₃薄膜的多层生长。在多层生长中,针对BaTiO₃薄膜的多层生长特点,重新考虑三维生长过程中的四个独立事件:原子的沉积、吸附原子的三维扩散、原子的三维成键和原子岛的迁移。区别于传统的SOS模型,特别考虑了原子三维成键和悬挂原子的存在。由于三维生长过程原子向下跃迁的特点,在利用BMH势函数计算扩散激活能时,多考虑了Ehrilich Schwoebel （ES）势垒项,分别模拟了不同的PLD参数对BaTiO₃薄膜多层生长的影响。进一步将反射式高能电子衍射（RHEED）强度的计算考虑到了模拟中,模拟了不同的PLD参数下,BaTiO₃薄膜RHEED曲线的变化趋势。结果表明:（a）薄膜的单层覆盖率约为0.75 ML;并且随着入射原子动能的增加,BaTiO₃薄膜逐渐向着层状生长模式发展且表面粗糙度逐渐降低。（b）在重复频率f = 1Hz、f = 10Hz和f = 50Hz下,薄膜的生长模式分别为层状-岛状、层状-岛状和岛状的生长模式;且随着重复频率的增加,表面粗糙度也逐渐增加。（c）当平均沉积速率F_aver= 0.1ML/s时为层状-岛状生长模式,F a ver= 0.5ML/s时为岛状生长模式, F_aver= 1.0ML/s时为层状生长模式;且表面粗糙度在F_aver= 0.5ML/s时最大, F_aver= 1.0ML/s值为最小;在所选择的三个典型值域下, F_aver= 1.0ML/s是最优的参数值。（d）对于较高的入射原子动能,RHEED强度曲线的振荡逐渐变得有规律且峰值变得越来越强;生长模式对应的是二维生长,且薄膜变得较为平整。（e）随着脉冲重复频率的增加,RHEED强度曲线的振荡逐渐变得不规则且峰值降低,薄膜的生长模式向岛状生长模式转变。（f）在不同平均沉积速率下,随着平均沉积速率的增加,RHEED强度曲线的振荡逐渐变不规则且峰值降低,薄膜的生长模式向岛状生长模式转变。F_aver= 1.0ML/s是最优的参数值,理论模拟的结果和RHEED的实验结果相符合。（3）采用金属有机物分解（MOD）法制备了Bi层状铁电薄膜Bi_3.15Eu_0.85Ti₃O₁₂（BET）和弛豫型铁电薄膜（1-x）Na_0.5Bi_0.5TiO_3-xK_0.5Bi_0.5TiO₃（NBT-KBT100x）。利用PFM实时观测了BET和NBT-KBT100x薄膜的电畴,研究了在不同外场条件下纳米级铁电畴翻转,以及保持性能和印记失效。结果表明:（a）利用PFM在BET薄膜表面加载机械力,观测到薄膜在不同力载荷下的电畴形貌演化。探针加力扫描的过程中,通过对晶粒形变量的分析,发现部分晶粒形变量大于5%。这说明外力对部分晶粒带来了不同方向上压应力和拉应力,从而导致了90°畴变。（b）运用PFM观测了不同组分NBT-KBT薄膜的电畴结构。NBT-KBT17薄膜呈现单畴态的晶粒个数明显增多。选取NBT-KBT17薄膜分别测试了其面内极化分量和面外极化分量,该薄膜的面内压电信号较强,这说明薄膜d₃₁模式下的压电响应明显,可以用来作为相应模式下的俘能器件。（c）选择NBT-KBT17薄膜中一个较大尺寸的单晶,实现了对弛豫性铁电体的电畴写入。将其在大气环境下放置不同时长,发现其保持性能较好,说明该薄膜具有应用于铁电薄膜存储器件的潜在价值。最后分别设定不同扫描速率对NBT-KBT13薄膜大区域面积写畴,发现扫描速率较小的0.1Hz时,写入时间越长,极化翻转的程度越明显。（d）通过PFM探针分别测试了加载作用前后NBT-KBT18薄膜电容器的相位和振幅滞后回线图。结果表明:相位回线向右发生了一定的移动,且振幅的蝶形曲线均在不同方向上发生了移动,同时形状也发生了改变。这说明外加作用力导致了薄膜印记的产生。（4）采用PLD方法在立方Si基底上沉积制备了组分比为Pb(Zr_0.52Ti_0.48)O₃（PZT）的薄膜样品,并对其进行了结构和形貌表征。利用X射线衍射（XRD）技术估算了PZT薄膜的面内应变值。利用PFM对PLD方法制备的PZT薄膜扫描区域中的单个晶粒进行了单畴化处理。并对选择的单晶单畴加载了不同的直流电,观察其在不同电压下的畴变演化,得到了发生180°畴变的阈值。针对实验观察的单晶畴态变化建立非线性热力学理论,模拟了失配应变-外加电场的相图,并和实验现象进行了对比。研究表明:（a）PZT薄膜结晶度较好,具有典型的钙钛矿相且没有明显的焦绿石相,薄膜的厚度为500nm。（b）利用PFM对PLD的单晶区域分别加载了不同的直流电观察其在不同电压下的畴变,最终得到使其发生180°畴变的阈值范围在120-160kV/cm之间。（c）根据布拉格衍射定律,由PZT薄膜样品的XRD谱得到了各个晶面的离面应变,并作统计平均得到平均离面应变。分别利用忽略压电效应和考虑压电效应的面内应变计算公式,估算得到了PZT薄膜的面内应变值为-0.001和-0.002。（d）针对实验中的单晶单畴区域,将探针施加的外场近似为均一的,考虑进两个新的相:ac^-和r^–相,其中面外极化分量为负从而和电畴中不同明暗颜色衬度的极化分量相匹配。利用非线性热力学理论建立模型,模拟得到了失配应变-外加电场相图和极化分量-外加电场曲线。在失配应变-外加电场相图中,找到对应于面内应变-0.002发生ac^-相到c⁺相转变的电场值为139kV/cm。对比PFM的实验结果,发生畴变的阈值范围在120-160kV/cm之间,而139kV/cm正好落在这个范围之内,说明理论阈值和实验阈值符合的较好。（5）建立唯象热力学模型,讨论了生长在正交基底的外延PZT薄膜在非平衡双轴失配应变下外加应力和外加温度对PZT薄膜相变和物理性质的影响。结果表明:（a）在失配应变-失配应变相图中,随着Ti组分的逐渐增加,r和α1c相所在区域渐渐缩小,相应的其他c、α1和α1α2相所在的区域则慢慢扩大。在失配应变-介电系数关系曲线中,薄膜的介电系数随e₂的变化是非单调的。在r和c相中,介电系数ε₃₃随着失配应变的减小而增加。在失配应变-压电系数关系曲线中,薄膜沿e₁正方向（ e₁ = 0.005）被拉伸的情况下,处于单斜r相的薄膜沿非平衡双轴失配应变e₂的方向;由压缩逐渐向拉伸的转变过程中,压电系数d₃₃也随之逐渐增大,并且随着组分x的增大而增大。（b）固定失配应变e₁ = 0.005,模拟得到了外延生长在正交基底上PZT薄膜失配应变-外加应力的相图。随着Ti组分的不同,出现了新的α1c相和四方相α1,且新相区域也会随之变化。在外加拉应力下c相是薄膜较易出现的相,而在外加压应力下薄膜易处于α1α2相。（c）研究了不同温度场对非平衡双轴失配应变下PZT薄膜的相图。当组分x≤0.7时,相图中多出了正交α1c相。由于非平衡双轴失配应变的存在,使得从顺电相向r相发生相变的过程中,多出了正交α1α2相。同时另一新的四方α1相也出现在沿失配应变e₂正方向被拉伸的区域。随着Ti组分的增加,单斜r相的面积缩小且位置下移。之前存在的四相点也变为了三相点,而正交α1α2相则向相图的中心位置移动。更多还原

【Abstract】 With the rapid development of modern microelectronics and optoelectronic technology, electronic devices move toward the miniaturization and integration. Perovskite ferroelectric thin films, such as BaTiO₃, PbTiO₃, Pb（Zr,Ti）O₃, （Pb, La）（Zr,Ti）O₃, have attracted much attention and become the research hotspot within new functional materials due to the excellent ferroelectric, dielectric, pyroelectric, electro-optic, acousto-optic, and piezoelectric properties. Meanwhile, studying the ferroelectric thin films growth mechanism, getting excellent preparation parameters, and exploring the physical properties of the ferroelectric thin films under the different environment have been the research topic in academic interests.Pulsed laser deposition （PLD） with remaining the target stoichiometry is a conventional technique, which is widely used for the growth of complex oxygen thin films. Although, a few studies about PLD growth of perovskite ferroelectric thin film have been reported, the growth mechanism of initial growth stage has rarely been investigated. Moreover, the effects of various experimental parameters on two and three dimensional growth of perovskite ferroelectric thin film still need to be further understood so that the three-dimensional growth mechanism can be figured out. Among the therotical ways to study the growth process, Kinetic Monte Carlo （KMC）, which can provide atomic-scale point of views on the growth of thin films, was usually used to simulate the growth of semiconducting and metal thin films. However, the simulation works of perovskite ferroelectric thin films growth were not easy to be done because of the complexities of lattice structure and growth mechanism. Meanwhile, for ferroelectric thin films, the domain state is produced by its own uniformity and mechanical constraint. The structure, type, number and evolution of domain decide physical properties and the applied direction of ferroelectric thin films. Therefore, further research on the real time observation of domain structures is the foundation of understanding the properties of ferroelectric thin films. In this thesis, first of all two and three dimensional KMC models were constructed to simulate the submonolayer and multilayer growths of perovskite ferroelectric BaTiO₃ thin film via PLD respectively, for various experimental parameters. Secondly, the domain evalution of ferroelectric thin films is observed by using Piezoelectric Force Microscopy （PFM） under the different external fields. Finally, the phase transition, dielectric properties and piezoelectric properties of ferroelectric thin films are investigated theorically. The main contents are given as follows.（1） We proposed an energy-dependent kinetic Monte Carlo （KMC） approach to simulate BaTiO₃ thin film growth via PLD within the submonolayer regime, in which the coverageθis less than 1. Distinguishing with the traditional solid-on-solid （SOS） model, the adatom bonding is specially considered to describe the atom combine according to the perovskite structure, and the PLD growth of the perovskite thin film on the surface of square lattice substrate of homoepitaxial system which is considered as three stochastic incidents such as the deposition, diffusion, and bonding of adatoms. Varying the values of the laser repetition rate and pulse duration, the relative curves of the island density and island size vs coverage were obtained. The simulation results show that the island density increases while the island size decreases with the pulse frequency. When the pulse repetition rate is less than 1 KHz, there is no obvious variation for the curves of the island density and island size vs coverage. However, when the pulse repetition rate is larger than 1 KHz, the island density is no change forθ< 0.1, and with the pulse duration the island density increases while the island size decreases forθ< 0.1. In conclusion, it indicates that if pulse repetition rate is elevated to kilohertz or higher, pulse duration should be considered in growth model at the cases.（2） An energy-dependent KMC approach is proposed to simulate the multilayer growth of BaTiO₃ thin films via pulsed laser deposition, in which the four steps, such as the deposition of atoms, the diffusion of adatoms, the bonding of adatoms, and the surface migration of adatoms, are considered. Distinguishing with the traditional SOS model, the adatom bonding and the overhanging of atoms, according to the perovskite structure, are specially adopted to describe the ferroelectric thin film growth. The activation energy is considered from the interactions between the ions, which are calculated by BMH potential and Ehrilich Schwoebel （ES） is first considerd in calculations because of the adatoms hopping to subjacent atomic layers. From the simulation, the relationship between growth modes and the different PLD parameters is acquired. Moreover, the Reflection High-Energy Electron Diffraction （RHEED） is calculated in the simulation based on the three dimensional KMC model and the effects of different PLD parameters on the RHEED intensity are studied. The main conclusions are as follows. （a） The saturated coverage of each atomic layer is about 0.75 ML. With the increasing of incident kinetic energy, the growth mode of BaTiO₃ thin film can be transformed from the 3D growth to the layer-by-layer growth, and the surface roughness decreases. （b） With the increasing of laser repetition rates, the growth modes transformed from the layer-by-layer growth to the 3D growth, and the surface roughness increases. （c） For the mean deposition rates 0.1, 0.5 and 1.0 ML/s, the growth modes are respectively 2D-3D growth, 3D growth and 2D growth. The maximum value of surface roughness is for 0.5 ML/s, while the minimum value is for 1.0 ML/s. （d） For a higher incident kinetic energy, the RHEED intensity becomes stronger and the surface morphology becomes smoother. （e） With the increasing of laser repetition rates, the growth modes transformed from the layer-by-layer growth mode to the island growth mode, and the surface roughness increases. （f） With thehigher mean deposition rates, we conclude that the growth modes have the trendency to the layer-by-layer growth.（3） MOD method are used to prepare the Bi_3.15Eu_0.85Ti₃O₁₂（BET） and （1-x）Na_0.5Bi_0.5TiO_3-xK_0.5Bi_0.5TiO₃（NBT-KBT100x） ferroelectric thin films. The microstructure and ferroelectric properties of thin film are also characterized. The initial domain structures of ferroelectric thin films are obtained by PFM. Meanwhile, the domain evolution of ferroelectric thin films is observed by PFM under different external electric fields and forces. The main results are as follows. （a） We have observed 90°domain switching due to the external mechanical forces exerted by the SFM tip on the surface of BET thin film. With the increase of the mechanical force along the certain scan direction, some deformed grains are stretched up and the polarization is reversed to out-of-plane direction, corresponding to the phase transition from r phase to c phase. With the opposite scan direction, the grains are compressed and the polarization was reversed to in-plane direction, corresponding to the phase transition from c phase to aa phase. When the strongest force is applied, the weak piezoelectric signal is observed, and it is coincident with the previous experimental result. Meantime, the movement of 90°domain wall driven by the mechanical force is observed. （b） The surface morphology and domains of NBT-KBT100x with different components are investigated by PFM images. The results show that the NBT-KBT17 thin film presents the most single domain grains. Moreover, the LPFM amplitude and phase images of NBT-KBT17 thin film are also obtained, which indicate that the piezoelectric response is obvious to be energy harvesting device with d₃₁ mode. （c） The single grain of NBT-KBT17 thin film is selected to write the domain by opposite DC voltages. The written single grain was then keeping in environment for different duration to detect the retention of the thin film and the retention loss is low. At last, the different scan rates are applied on the NBT-KBT13 thin film. The results show that the polarization area of NBT-KBT13 thin film is much stronger at the lower scan rate 0.1 Hz. （d） The imprint of ferroelectric capacitor is investigated under the external mechanical force. The phase and amplitude-electric voltage hysteresis loops of NBT-KBT18 ferroelectric capacitor were obtained under the mechanical force. The observations indicate that the hysteresis loop and butterfly curve make a mobile, which means the imprint was produced by the external force.（4） Pb(Zr_0.52Ti_0.48)O₃ （PZT） ferroelectric thin film is prepared by PLD and the micro structure is characterized. The in-plane strain is evaluated by XRD.The single domain treatment on the selected single grain was performed by the negative DC bias in order to obtain the single-domain state, and the opposite color contrasts within the selected grain in piezoelectric phase images of PZT ferroelectric thin film were observed by PFM. Based on nonlinear thermodynamic theory, theα1c^- and r^- phases with the negative P 3 component are introduced to describe the electric-generated domain switching, and the external misfit strain-electric field phase diagram and the electric field-polarization components curve are simulated at the simplification of uniform stress/electric distribution for the single-domain state of a single grain. The main results are as follows. （a） PZT thin film is well crystallized with random orientation and no pyrochlore phase and the thicknesses of PZT thin film is estimated as 500 nm. （b） The piezoelectric phase images of the selected grain were observed by PFM under different DC voltages and the range of threshold electric field of 180°domain switching is 120-160 kV/cm. （c） According to the Bragg diffraction law, the out-plane strain S_⊥^hikili can be obtained by XRD. The mean out of plane strain can be evaluated by statistical average. Then, the in-plane strains measured by XRD are -0.001 for neglecting piezoelectric effect and -0.002 for considering piezoelectric couple. （d） The external stress and electric fields in PFM are reasonably regarded as the uniform distribution. Because of the opposite color contrasts within the selected grain in PFM, theα1c^- and r^- phases with negative P₃ component are introduced to describe the electric-generated domain switching. For the selected grain with the single-domain states, the misfit strain-external electric field phase diagram and the external electric field-polarization components curve are constructed based on nonlinear thermodynamic theory. In the misfit strain-external electric field phase diagram, the threshold of phase transition E₃ fromα1c^- phase to single-domain c⁺ phase is 139 kV/cm at the misfit strain -0.002, which is within the range of 120-160 kV/cm for electric-generated 180°domain switching. The in-plane strain -0.002 evaluated by XRD is reasonable considering the piezoelectric coupling and the simulation results are in agreement with the experimental observations.（5） The tradional nonlinear thermodynamic theory, which was used to simulate the phase transition of the ferroelectric thin film grown on the cubic substrate, is now amended to explore the effect of nonequally biaxal misfit strain, external stress and external temperature on phase states and physical properties of epitaxial Pb(Zr_1-xTi_x)O₃（PZT） thin films with different components grown on anisotropic substrates. The“misfit strain- misfit strain”, the“misfit strain-external stress”and“misfit strain-temperature”phase diagrams are constructed aiming at single-domain PZT thin films grown on anisotropic substrates and the dielectric and piezoelectric responses under nonequally biaxal misfit strains are studied. The main contents are: （a） In misfit strain- misfit strain phase diagram, the nonequally biaxal misfit strains in the film plane may lead to the appearance of new phases:α1c andα1 phases. With the increasing of Ti contents, the areas of r andα1c phases decrease while the areas of c,α1 andα1α2 increase. An appropriate external stress could be influence the dielectric and piezoelectric responses of the film. （b） In misfit strain-external stress phase diagram for the known misfit strain e₁ = 0.005, the compressive stress may lead to the appearance of c phase while the tensile stress may lead to the appearance ofα1α2 phase. （c） In misfit strain-temperature phase diagram for the known misfit strain e₁ = 0.005, theα1c phase occurs at x≤0.7 and disappears gradually at x>0.7. The quadruple point is changed to the triple point under the nonequally biaxal misfit strains.更多还原