BiFeO_3薄膜的溶胶凝胶方法的制备、掺杂及电磁性质的研究
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摘要
多铁材料又被称为磁电材料,是指在一定的温度范围内表现出共存的铁电有序和磁性有序的材料,该材料被认为同时在铁电和磁性器件方面具有重要的应用前景。因为磁性和铁电性的共存又使得多铁材料具有磁电效应,磁电效应在磁电传感器,波导,磁电开关和调制解调器等方面有重要应用;磁性和铁电性的共存又导致了该材料中的磁和电的耦合,这种耦合效应使得在该材料中电场可以诱导磁化,磁场可以诱导极化。这一特性被认为在磁读电写的新型记忆元件方面有诱人的应用前景。因此,近几年来,多铁材料受到的空前广泛的关注。
BiFeO3是一种具有钙钛矿结构的多铁材料,其居里温度为Tc=1103 K,尼尔温度TN=643 K,室温下表现出共存的铁电性和弱的反铁磁性。因为最近通过脉冲激光沉积方法实现了异质外延生长而获得了极大的铁电性和强的磁性而尤其受到关注。本文研究了BiFeO3薄膜的溶胶-凝胶制备方法,对其进行了A位和B位的替代改性,并对室温下的铁电性,磁性,介电性质以及漏导性质进行了研究。
本文研究的主要内容如下:
(1)采用溶胶-凝胶方法成功的在ITO/玻璃、LaNiO3/Si(111)衬底以及Pt/Ti/SiO2/Si等衬底上制备了450℃至600℃退火的纯BiFeO3薄膜,并在500℃退火条件下通过工艺的控制在LaNiO3/SiO2/Si衬底上制备(101)择优取向和随机取向的BiFeO3薄膜。
在ITO/玻璃衬底上制备的BiFeO3薄膜根据不同的退火温度呈现不同的取向,500℃和600℃退火的薄膜分别呈(110)择优取向和完全的随机取向。两薄膜的剩余极化强度分别为2.0μC/cm2和1.76μC/cm2。通过介电性测试观察到了两薄膜中的德拜型介电弛豫。其中强的介电色散来自不同惯性的偶极子极化的贡献,而介电损耗峰则来自偶极共振。另外,在77 k观察到了弱的磁性。
而在450℃到600℃晶化的条件下沉积在Pt/Ti/SiO2/Si衬底上的BiFeO3薄膜的双剩余极化强度分别为0.51μC/cm2,0.72μC/cm2,4.54μC/cm2和5.94μC/cm2。相对ITO/玻璃衬底,Pt/Ti/SiO2/Si衬底上的BiFeO3薄膜表现出小的介电色散。主要因为BiFeO3薄膜与Pt电极之间具有良好的界面。在相同退火温度下制备的BiFeO3薄膜中,以LaNiO3/Si(111)衬底上的铁电性最强,450℃到600℃退火的薄膜的双剩余极化强度分别为6.4μC/cm2,8.2μC/cm2, 12.8μC/cm2和2.3μC/cm2。通过剖面透射电镜分析观察到了600℃退火的薄膜中存在严重的界面反应,界面反映生成的界面层分担了大部分测试电压应致使加在薄膜层上的测试电场降低,介电性质恶化。另外,薄膜的漏电流随退火温度的升高而增加,主要因为高的退火温度导致晶粒边界连通所致。
对于LaNiO3/SiO2/Si衬底上的BiFeO3薄膜,其取向决定于退火工艺。逐层退火有利于薄膜外延生长而获得择优取向,而一次性退火则不利于薄膜的择优取向生长。两薄膜的铁电性和介电性质由薄膜的结晶程度和取向共同决定。漏电导特性分析表明两薄膜由欧姆型电导向空间电荷限制的电导转化的电压相同。
(2)加入La(NO3) .6H2O,参照纯BiFeO3薄膜的制备工艺制备了A位La替代的Bi1-xLaxFeO3薄膜,替代前后的容忍因子分别为0.8479和0.8486,因此能够形成稳定的钙钛矿结构。XRD测试表明直到x=0.20,所有的薄膜都呈现良好的R3m结构,没有结构失调的现象出现,并且所有的薄膜均呈完全的随机取向。其中x=0.10的薄膜具有最强的铁电性,相应的2Pr为4.40μC/cm2。通过A位的10%的La替代,薄膜的铁电性得到了增强,双剩余极化强度从3.35μC/cm2提高到了4.40μC/cm2,增强的铁电性来自La替代导致的薄膜的取向变化和晶格形变。同时介电性质得到了提高,10 kHz的测试频率下的相对介电常数由未替代的120增加到10%替代的165。此外,漏电流得到了显著的限制,相同测试电压下20%La替代的薄膜的漏电流比未替代的薄膜减少了3～5个数量级。
(3)B位Zr、Ti替代
加入Zr(NO3)3.5H2O和Ti(C4H9O)4,参照纯BiFeO3薄膜的制备工艺,在LaNiO3/SiO2/Si,ITO/glass等衬底上制备了BiFe1-xTixO3+δ薄膜和BiFe1-xZrxO3+δ薄膜。
在LaNiO3/Si衬底上制备的600℃退火的BiFe1-xTixO3+δ薄膜,x=0时呈完全的随机取向,当x=0.05时呈(101)择优取向,而x增大到0.10时则重新变为随机取向。通过Ti的替代使薄膜的晶粒变小,薄膜的致密度增高,铁电性测试的测试电场也得到相应增加。在不同测试电场下得到x=0.00,0.05和0.10的BiFe1-xTixO3+δ薄膜的Pr值分别为1.2μC/cm2,0.58μC/cm2,和3.8μC/cm2。通过Ti替代的BFO薄膜表现出增强的介电性质和特定的损耗机构。同时薄膜的漏电流得到限制。磁性的测量表明所有的薄膜均表现出弱的反铁磁性,薄膜的磁性决定于替代导致的薄膜的反铁磁结构的破坏和单位体积净磁矩的减少。
在ITO/glass衬底上制备的600℃退火的BiFe1-xTixO3+δ(x取0.00到0.20)薄膜都呈现随机取向,但是可以观察到(012)和(110)峰有最大的强度。x=0.20的薄膜能够观察到(012)和(110)双峰的明显展宽,说明该薄膜具有较差的晶化。通过XRD精细结构分析表明Ti的替代导致晶格常数的增加。x=0.05的薄膜具有最大的双剩余极化强度8.30μC/cm2,而x= 0.0, 0.1和0.2的薄膜的剩余极化强度分别为2.12μC/cm2, 2.63μC/cm2和0.44μC/cm2。另外通过Ti的替代,ITO衬底上的BiFe1-xTixO3+δ薄膜表现出显著增强的介电性质,在0.1-100kHz的频率范围内,10%Ti替代的薄膜的介电常数相对BiFeO3增加了150。x=0.20的薄膜具有与其他薄膜不同的介电色散机构,主要因为过量的Ti替代导致的丰富的空间电荷所致。另外,通过Ti的替代,x=0.10和0.20的薄膜的线性欧姆电导到空间电荷限制电导的转化电场显著增加。
在ITO/glass上制备的600℃退火的BiFe1-xZrxO3+δ薄膜,x取0.00到0.40都呈现随机取向,x=0.20的薄膜开始观察到烧绿石相。通过XRD精细结构分析表明Zr的替代也导致晶格常数的增加。x=0.20的薄膜具有最大的剩余极化强度3.9μC/cm2。而x=0.10的薄膜具有最大的介电常数。所有的薄膜的漏电流表现出正负偏压下的对称性。由于烧绿石相的出现使的薄膜的漏电流得到了极大的限制。在相同的测试电场下,x=0.20的薄膜的漏电流比x=0的小5个数量级。
(4)研究了BiFeO3-PbTiO3, PZT-BiFeO3固熔薄膜的溶胶凝胶制备工艺。同时对薄膜的结构,铁电性,介电性,漏电流和磁性进行了测试和研究。
采用Pb(CH3COO)2.3H2O、Zr(NO3)3.5H2O和Ti(C4H9O)4为原料配制了Pb(Zr0.5Ti0.5)O3和PbTiO3前驱体溶液,然后与BiFeO3的前驱体溶液混合,加入适当的稳定剂获得了稳定的BiFeO3-PbTiO3和PZT-BiFeO3前驱体溶液。在LaNiO3底电极上制备了BiFeO3-PbTiO3, BiFeO3-PZT固熔薄膜。XRD分析表明通过与PbTiO3的固熔,BiFeO3在PbTiO3的比例为10%时发生了结构相变,由三方结构转为立方结构。根据Bi3+,Pb2+,Fe3+,Ti4+,Zr4+等离子的半分析,在以上两种固熔薄膜中,Bi3+,Pb2+占据A位,而Fe3+,Ti4+,Zr4+占据B位,通过固熔实际上实现了A位与B位的双替代。通过固熔,BiFeO3的铁电性,介电性得到显著增强,磁性也得到增强。PZT-BiFeO3薄膜中BiFeO3所占的比例为0%到20%,BiFeO3的固熔使PZT的铁电性先增强后减弱。x=10%的薄膜具有最强的铁电性,而x=20%的薄膜则具有最大的矫顽场。
Mutiferroics are also called magneto-electric materials, which shows coexistent ferroelectric order and magnetic order and was considered to have important potential application in ferroelectric and magnetic devices. The coexistence of magnetism and ferroelectricity in this kind of materials supplies them magneto-electric effects and the potential for applications as sensors, waveguides, switches, and modulators. The coexistence also deduces magneto-electric couple. The property is suggested to be applied to magnetically recording ferroelectric memories. For the above reasons, multiferroics are extensively focused in recent years.
BiFeO3 is a perovskite typed mutiferriocs(TC=1103 K,TN=643 K), which shows coexistent ferroelectricity and magnetism at room temperature. In recent years, epitaxial BiFeO3 films have been prepared by pulsed laser deposition method and galactic remnant polarization was observed, which attracted extraordinary attentions. In this paper, we studied the sol–gel preparation method of BiFeO3 film, and the improvements in physic properties by the substitutions in A and B site have been studied.
The main contents of this paper are as the following:
(1) Bi(NO3)3.5H2O and Fe(NO3)3.9H2O are adopted as starting materials, 2-methoxyethanol as solvent, ethanolamine was used to adjust the viscosity to preparation the precursor solutions and phase pure BiFeO3 films are successfully deposited on ITO/glass, LaNiO3/Si(111) and Pt/Ti/SiO2/Si substrates at different annealing temperatures from 450℃to 600℃. Moreover, (110) preferred oriented and randomly oriented BiFeO3 films were prepared on LaNiO3/SiO2/Si substrates by controlling the annealing technology at annealing temperature of 500℃.
The BiFeO3 films deposited on ITO/glass substrates adopt different orientation according to the annealing temperatures. The film annealed at 500℃and 600℃adopt (110) preferred orientation and random orientation, respectively. The remnant polarizations of the two films are 2.0μC/cm2 and 1.76μC/cm2 respectively. Debye typed relaxation was observed in the films by measuring the dielectric property. The intense dielectric dispersion is thought to be originated from the inertia of dipole, while the loss peaks are thought to be from the resonance of dipole.
The remnant polarizations of the BiFeO3 films deposited on Pt/Ti/SiO2/Si substrates are 0.51μC/cm2, 0.72μC/cm2, 4.54μC/cm2 and 5.94μC/cm2 respectively under annealing temperatures of 450℃, 500℃, 550℃, 600℃. Compared with the films on ITO/glass substrates, the films on Pt/Ti/SiO2/Si substrates show small dielectric dispersion and is attributed to better interface between the BiFeO3 film and Pt bottom electrode. The films on LaNiO3/Si substrates show largest remnant polarizations under the same annealing temperatures, the films annealed at 450℃to 600℃show double remnant polarizations of 6.4μC/cm2, 8.2μC/cm2, 12.8μC/cm2 and 2.3μC/cm2. Interface reaction was observed by the cross-section TEM measuring. The interface layer bore large measuring field, which reduced measuring fields supplied to the BiFeO3 film and deteriorated the dielectric property. In addition, the leakage current increased with annealing temperature and which is suggested to be caused by opening of grain boundary at higher annealing temperature.
As for the films deposited on LaNiO3/SiO2/Si substrates, the orientation is determined by the annealing technology. Annealing layer by layer makes for eptaxial growth and thus getting preferred orientation, while annealing by one time is in favor of random orientation. The ferroelectricity and dielectric property are determined by the crystallization the preferred orientation. The analysis of leakage conduction property showed that the films have the same transition field from ohmic conduction to space charge limited conduction.
(2)A site La substituted Bi1-xLaxFeO3 films was prepared by adding La(NO3).6H2O as starting material and referring to the preparation technology of pure BiFeO3 film. The tolerance factors are 0.8479 and 0.8486 before and after La substitution, respectively, therefore we expect a stable pervoskite structure through the La substitution. It has been found that all films show well R3m structure until x increases to 0.20 and no structural disorder are observed, moreover, all films adopt random orientation. The measurement of ferroelectricity shows that the film with x = 0.10 has most intense ferroelectricity, the 2Pr of it was 4.40μC/cm2. The ferroelectricity is enhanced through the La substitution on A site since the double remnant polarizations for the film with x=0.10 was enhanced to 4.40μC/cm2 from 3.35μC/cm2 for the film with x=0. The enhancement of ferroelectric is suggested originated from the change of orientation and distortion of lattice by the La substitution. More over, dielectric property was enhanced since the dielectric property for the film with x=0.10 was 165 while the film with x=0 was 120 under a measuring frequency 10 kHz. Furthermore, leakage currents were reduced substantially since the leakage current density for the film with x=0.20 is 3～5 magnitude orders lower than the undopted one.
(3)Zr and Ti substitution on B site
Zr(NO3)3.5H2O and Ti(C4H9O)4 were added as starting materials to prepared BiFe1-xTixO3 +δand BiFe1 - xZrxO3 +δfilms on LaNiO3/SiO2/Si, the detailed preparation technology referred to pure BiFeO3 films on ITO/glass substrates.
BiFe1-xTixO3+δfilms was prepared on LaNiO3/SiO2/Si substrates at an annealing temperature of 600℃. The film with x = 0.00 show throughly random orientation, while the film with x= 0.05 show (101) preferred orientation. The film with x=0.10 show randomly orientation anew. The crystal grains were observed finer and the compaction of the films increased through the substitution and which caused higher measuring fields. The remnant polarization under different measuring electric fields for the films with x=0.00, 0.05 and 0.10 are 1.2μC/cm2 , 0.58μC/cm2 and 3.8μC/cm2. New loss mechanisms and enhanced dielectric property were also identified. The leakage currents were also reduced. The measurements of magnetism shows that all films indicate weak antimagneitsm and the magnetism are dominated by the destruction of spiral spin structure and the reduction ofμB in unit volume caused by the substitution.
The BiFe1-xTixO3+δ(x = 0.00 to 0.20) films deposited on ITO/glass substrates were annealed at 600℃, which were identified adopted random orientation. The (012) and (110) peaks were observed have the largest intensity and the (012) and (110) peaks were observed widened in the film with x=0.20, it means that the film has poor crystallization. XRD subtle structure shows that the lattice constant increased through the Ti substitution. The film with x=0.05 has the largest double remnant polarization of 8.30μC/cm2, while the double remnant polarization for the films with x=0.0, 0.01 and 0.2 are 2.12μC/cm2, 2.63μC/cm2 and 0.44μC/cm2. Comparing with the films on LaNiO3/Si substrates, their showed more intense ferroelectricity. More over, the BiFe1-xTixO3+δfilms on ITO/glass indicated enhanced dielectric property since compared with BiFeO3 film, the dielectric constant increased 150 for the film with x=0.10 in the frequency range from 0.1 kHz to 100 kHz. The film with x=0.20 has different dielectric and dispersion mechanism from the others and is attributed to the abundant space charges in it caused by the substitution. In addition, the VΩ-TFL were substantially promoted by the substitution of Ti in the films with x=0.10 and 0.20.
BiFe1-xZrxO3+δfilms were prepared on ITO/glass substrates at an annealing temperature 600℃, in which x=0.00 to 0.40 and all films showed random orientation. Pyrochlore phase was observed in the film with x≤0.20. XRD subtle structure shows that the lattice constant increased through the substitution of Zr. The film with x=0.20 showed largest remnant polarization of 3.9μC/cm2. While the film with x=0.10 has the largest dielectric constant under the same measuring electric field. The leakage currents of all films show well symmetry under positive and negative bias applied field. The leakage currents were substantially reduced because of the pyrochlore phase. The currents for the film with x=0.20 was five magnitude order smaller that the one with x=0.00.
(4) The preparation technology of BiFeO3-PbTiO3, PZT-BiFeO3 solid solution films was studied. The structure, ferroelectricity, dielectric property, and leakage current property were measured and studied.
Pb(CH3COO)2.3H2O、Zr(NO3)3.5H2O、Ti(C4H9O)4 were adopted to prepared Pb(Zr0.5Ti0.5)O3(PZT) and PbTiO3 precursor solutions. The BiFeO3 precursor solution was mixed with it and stablizer was added to obtain the final BiFeO3-PbTiO3 and PZT-BiFeO3 precursor solutions. The (BiFeO3)1-x-(PbTiO3)x and (PZT)1-x-(BiFeO3)x solid solution films were deposited on LaNiO3 bottom electrodes. In the above solid solution systems, x=0.0, 0.05, 0.10 and 0.20 for (BiFeO3)1-x-(PbTiO3)x films and x=0.05, 0.10, 0.20 and 0.30 for (PZT)1-x-(BiFeO3)x system. A structural phase transition was observed in the film with x=0.10 for the (BiFeO3)1-x-(PbTiO3)x films, through which, the rhombohedral structure transferred to cubic structure. The analyse to the radii of Bi3+, Pb2+, Fe3+, Ti4+ and Zr4+ showed that Bi3+, Pb2+ occupy A site, while Fe3+,Ti4+, Zr4+ occupy B site. A site and B site co-substitution was realized through the solid solution. Meanwhile, the ferroelectricity and dielectric property were enhanced through the substitution and magnetism was also enhanced. In PZT films, the ferroelectricity of it increased first and then decreased with the increasing of x. The film with x=10% showed largest ferroelecity while the film with x=20% show largest coercive.
BiFeO3是一种具有钙钛矿结构的多铁材料,其居里温度为Tc=1103 K,尼尔温度TN=643 K,室温下表现出共存的铁电性和弱的反铁磁性。因为最近通过脉冲激光沉积方法实现了异质外延生长而获得了极大的铁电性和强的磁性而尤其受到关注。本文研究了BiFeO3薄膜的溶胶-凝胶制备方法,对其进行了A位和B位的替代改性,并对室温下的铁电性,磁性,介电性质以及漏导性质进行了研究。
本文研究的主要内容如下:
(1)采用溶胶-凝胶方法成功的在ITO/玻璃、LaNiO3/Si(111)衬底以及Pt/Ti/SiO2/Si等衬底上制备了450℃至600℃退火的纯BiFeO3薄膜,并在500℃退火条件下通过工艺的控制在LaNiO3/SiO2/Si衬底上制备(101)择优取向和随机取向的BiFeO3薄膜。
在ITO/玻璃衬底上制备的BiFeO3薄膜根据不同的退火温度呈现不同的取向,500℃和600℃退火的薄膜分别呈(110)择优取向和完全的随机取向。两薄膜的剩余极化强度分别为2.0μC/cm2和1.76μC/cm2。通过介电性测试观察到了两薄膜中的德拜型介电弛豫。其中强的介电色散来自不同惯性的偶极子极化的贡献,而介电损耗峰则来自偶极共振。另外,在77 k观察到了弱的磁性。
而在450℃到600℃晶化的条件下沉积在Pt/Ti/SiO2/Si衬底上的BiFeO3薄膜的双剩余极化强度分别为0.51μC/cm2,0.72μC/cm2,4.54μC/cm2和5.94μC/cm2。相对ITO/玻璃衬底,Pt/Ti/SiO2/Si衬底上的BiFeO3薄膜表现出小的介电色散。主要因为BiFeO3薄膜与Pt电极之间具有良好的界面。在相同退火温度下制备的BiFeO3薄膜中,以LaNiO3/Si(111)衬底上的铁电性最强,450℃到600℃退火的薄膜的双剩余极化强度分别为6.4μC/cm2,8.2μC/cm2, 12.8μC/cm2和2.3μC/cm2。通过剖面透射电镜分析观察到了600℃退火的薄膜中存在严重的界面反应,界面反映生成的界面层分担了大部分测试电压应致使加在薄膜层上的测试电场降低,介电性质恶化。另外,薄膜的漏电流随退火温度的升高而增加,主要因为高的退火温度导致晶粒边界连通所致。
对于LaNiO3/SiO2/Si衬底上的BiFeO3薄膜,其取向决定于退火工艺。逐层退火有利于薄膜外延生长而获得择优取向,而一次性退火则不利于薄膜的择优取向生长。两薄膜的铁电性和介电性质由薄膜的结晶程度和取向共同决定。漏电导特性分析表明两薄膜由欧姆型电导向空间电荷限制的电导转化的电压相同。
(2)加入La(NO3) .6H2O,参照纯BiFeO3薄膜的制备工艺制备了A位La替代的Bi1-xLaxFeO3薄膜,替代前后的容忍因子分别为0.8479和0.8486,因此能够形成稳定的钙钛矿结构。XRD测试表明直到x=0.20,所有的薄膜都呈现良好的R3m结构,没有结构失调的现象出现,并且所有的薄膜均呈完全的随机取向。其中x=0.10的薄膜具有最强的铁电性,相应的2Pr为4.40μC/cm2。通过A位的10%的La替代,薄膜的铁电性得到了增强,双剩余极化强度从3.35μC/cm2提高到了4.40μC/cm2,增强的铁电性来自La替代导致的薄膜的取向变化和晶格形变。同时介电性质得到了提高,10 kHz的测试频率下的相对介电常数由未替代的120增加到10%替代的165。此外,漏电流得到了显著的限制,相同测试电压下20%La替代的薄膜的漏电流比未替代的薄膜减少了3～5个数量级。
(3)B位Zr、Ti替代
加入Zr(NO3)3.5H2O和Ti(C4H9O)4,参照纯BiFeO3薄膜的制备工艺,在LaNiO3/SiO2/Si,ITO/glass等衬底上制备了BiFe1-xTixO3+δ薄膜和BiFe1-xZrxO3+δ薄膜。
在LaNiO3/Si衬底上制备的600℃退火的BiFe1-xTixO3+δ薄膜,x=0时呈完全的随机取向,当x=0.05时呈(101)择优取向,而x增大到0.10时则重新变为随机取向。通过Ti的替代使薄膜的晶粒变小,薄膜的致密度增高,铁电性测试的测试电场也得到相应增加。在不同测试电场下得到x=0.00,0.05和0.10的BiFe1-xTixO3+δ薄膜的Pr值分别为1.2μC/cm2,0.58μC/cm2,和3.8μC/cm2。通过Ti替代的BFO薄膜表现出增强的介电性质和特定的损耗机构。同时薄膜的漏电流得到限制。磁性的测量表明所有的薄膜均表现出弱的反铁磁性,薄膜的磁性决定于替代导致的薄膜的反铁磁结构的破坏和单位体积净磁矩的减少。
在ITO/glass衬底上制备的600℃退火的BiFe1-xTixO3+δ(x取0.00到0.20)薄膜都呈现随机取向,但是可以观察到(012)和(110)峰有最大的强度。x=0.20的薄膜能够观察到(012)和(110)双峰的明显展宽,说明该薄膜具有较差的晶化。通过XRD精细结构分析表明Ti的替代导致晶格常数的增加。x=0.05的薄膜具有最大的双剩余极化强度8.30μC/cm2,而x= 0.0, 0.1和0.2的薄膜的剩余极化强度分别为2.12μC/cm2, 2.63μC/cm2和0.44μC/cm2。另外通过Ti的替代,ITO衬底上的BiFe1-xTixO3+δ薄膜表现出显著增强的介电性质,在0.1-100kHz的频率范围内,10%Ti替代的薄膜的介电常数相对BiFeO3增加了150。x=0.20的薄膜具有与其他薄膜不同的介电色散机构,主要因为过量的Ti替代导致的丰富的空间电荷所致。另外,通过Ti的替代,x=0.10和0.20的薄膜的线性欧姆电导到空间电荷限制电导的转化电场显著增加。
在ITO/glass上制备的600℃退火的BiFe1-xZrxO3+δ薄膜,x取0.00到0.40都呈现随机取向,x=0.20的薄膜开始观察到烧绿石相。通过XRD精细结构分析表明Zr的替代也导致晶格常数的增加。x=0.20的薄膜具有最大的剩余极化强度3.9μC/cm2。而x=0.10的薄膜具有最大的介电常数。所有的薄膜的漏电流表现出正负偏压下的对称性。由于烧绿石相的出现使的薄膜的漏电流得到了极大的限制。在相同的测试电场下,x=0.20的薄膜的漏电流比x=0的小5个数量级。
(4)研究了BiFeO3-PbTiO3, PZT-BiFeO3固熔薄膜的溶胶凝胶制备工艺。同时对薄膜的结构,铁电性,介电性,漏电流和磁性进行了测试和研究。
采用Pb(CH3COO)2.3H2O、Zr(NO3)3.5H2O和Ti(C4H9O)4为原料配制了Pb(Zr0.5Ti0.5)O3和PbTiO3前驱体溶液,然后与BiFeO3的前驱体溶液混合,加入适当的稳定剂获得了稳定的BiFeO3-PbTiO3和PZT-BiFeO3前驱体溶液。在LaNiO3底电极上制备了BiFeO3-PbTiO3, BiFeO3-PZT固熔薄膜。XRD分析表明通过与PbTiO3的固熔,BiFeO3在PbTiO3的比例为10%时发生了结构相变,由三方结构转为立方结构。根据Bi3+,Pb2+,Fe3+,Ti4+,Zr4+等离子的半分析,在以上两种固熔薄膜中,Bi3+,Pb2+占据A位,而Fe3+,Ti4+,Zr4+占据B位,通过固熔实际上实现了A位与B位的双替代。通过固熔,BiFeO3的铁电性,介电性得到显著增强,磁性也得到增强。PZT-BiFeO3薄膜中BiFeO3所占的比例为0%到20%,BiFeO3的固熔使PZT的铁电性先增强后减弱。x=10%的薄膜具有最强的铁电性,而x=20%的薄膜则具有最大的矫顽场。
Mutiferroics are also called magneto-electric materials, which shows coexistent ferroelectric order and magnetic order and was considered to have important potential application in ferroelectric and magnetic devices. The coexistence of magnetism and ferroelectricity in this kind of materials supplies them magneto-electric effects and the potential for applications as sensors, waveguides, switches, and modulators. The coexistence also deduces magneto-electric couple. The property is suggested to be applied to magnetically recording ferroelectric memories. For the above reasons, multiferroics are extensively focused in recent years.
BiFeO3 is a perovskite typed mutiferriocs(TC=1103 K,TN=643 K), which shows coexistent ferroelectricity and magnetism at room temperature. In recent years, epitaxial BiFeO3 films have been prepared by pulsed laser deposition method and galactic remnant polarization was observed, which attracted extraordinary attentions. In this paper, we studied the sol–gel preparation method of BiFeO3 film, and the improvements in physic properties by the substitutions in A and B site have been studied.
The main contents of this paper are as the following:
(1) Bi(NO3)3.5H2O and Fe(NO3)3.9H2O are adopted as starting materials, 2-methoxyethanol as solvent, ethanolamine was used to adjust the viscosity to preparation the precursor solutions and phase pure BiFeO3 films are successfully deposited on ITO/glass, LaNiO3/Si(111) and Pt/Ti/SiO2/Si substrates at different annealing temperatures from 450℃to 600℃. Moreover, (110) preferred oriented and randomly oriented BiFeO3 films were prepared on LaNiO3/SiO2/Si substrates by controlling the annealing technology at annealing temperature of 500℃.
The BiFeO3 films deposited on ITO/glass substrates adopt different orientation according to the annealing temperatures. The film annealed at 500℃and 600℃adopt (110) preferred orientation and random orientation, respectively. The remnant polarizations of the two films are 2.0μC/cm2 and 1.76μC/cm2 respectively. Debye typed relaxation was observed in the films by measuring the dielectric property. The intense dielectric dispersion is thought to be originated from the inertia of dipole, while the loss peaks are thought to be from the resonance of dipole.
The remnant polarizations of the BiFeO3 films deposited on Pt/Ti/SiO2/Si substrates are 0.51μC/cm2, 0.72μC/cm2, 4.54μC/cm2 and 5.94μC/cm2 respectively under annealing temperatures of 450℃, 500℃, 550℃, 600℃. Compared with the films on ITO/glass substrates, the films on Pt/Ti/SiO2/Si substrates show small dielectric dispersion and is attributed to better interface between the BiFeO3 film and Pt bottom electrode. The films on LaNiO3/Si substrates show largest remnant polarizations under the same annealing temperatures, the films annealed at 450℃to 600℃show double remnant polarizations of 6.4μC/cm2, 8.2μC/cm2, 12.8μC/cm2 and 2.3μC/cm2. Interface reaction was observed by the cross-section TEM measuring. The interface layer bore large measuring field, which reduced measuring fields supplied to the BiFeO3 film and deteriorated the dielectric property. In addition, the leakage current increased with annealing temperature and which is suggested to be caused by opening of grain boundary at higher annealing temperature.
As for the films deposited on LaNiO3/SiO2/Si substrates, the orientation is determined by the annealing technology. Annealing layer by layer makes for eptaxial growth and thus getting preferred orientation, while annealing by one time is in favor of random orientation. The ferroelectricity and dielectric property are determined by the crystallization the preferred orientation. The analysis of leakage conduction property showed that the films have the same transition field from ohmic conduction to space charge limited conduction.
(2)A site La substituted Bi1-xLaxFeO3 films was prepared by adding La(NO3).6H2O as starting material and referring to the preparation technology of pure BiFeO3 film. The tolerance factors are 0.8479 and 0.8486 before and after La substitution, respectively, therefore we expect a stable pervoskite structure through the La substitution. It has been found that all films show well R3m structure until x increases to 0.20 and no structural disorder are observed, moreover, all films adopt random orientation. The measurement of ferroelectricity shows that the film with x = 0.10 has most intense ferroelectricity, the 2Pr of it was 4.40μC/cm2. The ferroelectricity is enhanced through the La substitution on A site since the double remnant polarizations for the film with x=0.10 was enhanced to 4.40μC/cm2 from 3.35μC/cm2 for the film with x=0. The enhancement of ferroelectric is suggested originated from the change of orientation and distortion of lattice by the La substitution. More over, dielectric property was enhanced since the dielectric property for the film with x=0.10 was 165 while the film with x=0 was 120 under a measuring frequency 10 kHz. Furthermore, leakage currents were reduced substantially since the leakage current density for the film with x=0.20 is 3～5 magnitude orders lower than the undopted one.
(3)Zr and Ti substitution on B site
Zr(NO3)3.5H2O and Ti(C4H9O)4 were added as starting materials to prepared BiFe1-xTixO3 +δand BiFe1 - xZrxO3 +δfilms on LaNiO3/SiO2/Si, the detailed preparation technology referred to pure BiFeO3 films on ITO/glass substrates.
BiFe1-xTixO3+δfilms was prepared on LaNiO3/SiO2/Si substrates at an annealing temperature of 600℃. The film with x = 0.00 show throughly random orientation, while the film with x= 0.05 show (101) preferred orientation. The film with x=0.10 show randomly orientation anew. The crystal grains were observed finer and the compaction of the films increased through the substitution and which caused higher measuring fields. The remnant polarization under different measuring electric fields for the films with x=0.00, 0.05 and 0.10 are 1.2μC/cm2 , 0.58μC/cm2 and 3.8μC/cm2. New loss mechanisms and enhanced dielectric property were also identified. The leakage currents were also reduced. The measurements of magnetism shows that all films indicate weak antimagneitsm and the magnetism are dominated by the destruction of spiral spin structure and the reduction ofμB in unit volume caused by the substitution.
The BiFe1-xTixO3+δ(x = 0.00 to 0.20) films deposited on ITO/glass substrates were annealed at 600℃, which were identified adopted random orientation. The (012) and (110) peaks were observed have the largest intensity and the (012) and (110) peaks were observed widened in the film with x=0.20, it means that the film has poor crystallization. XRD subtle structure shows that the lattice constant increased through the Ti substitution. The film with x=0.05 has the largest double remnant polarization of 8.30μC/cm2, while the double remnant polarization for the films with x=0.0, 0.01 and 0.2 are 2.12μC/cm2, 2.63μC/cm2 and 0.44μC/cm2. Comparing with the films on LaNiO3/Si substrates, their showed more intense ferroelectricity. More over, the BiFe1-xTixO3+δfilms on ITO/glass indicated enhanced dielectric property since compared with BiFeO3 film, the dielectric constant increased 150 for the film with x=0.10 in the frequency range from 0.1 kHz to 100 kHz. The film with x=0.20 has different dielectric and dispersion mechanism from the others and is attributed to the abundant space charges in it caused by the substitution. In addition, the VΩ-TFL were substantially promoted by the substitution of Ti in the films with x=0.10 and 0.20.
BiFe1-xZrxO3+δfilms were prepared on ITO/glass substrates at an annealing temperature 600℃, in which x=0.00 to 0.40 and all films showed random orientation. Pyrochlore phase was observed in the film with x≤0.20. XRD subtle structure shows that the lattice constant increased through the substitution of Zr. The film with x=0.20 showed largest remnant polarization of 3.9μC/cm2. While the film with x=0.10 has the largest dielectric constant under the same measuring electric field. The leakage currents of all films show well symmetry under positive and negative bias applied field. The leakage currents were substantially reduced because of the pyrochlore phase. The currents for the film with x=0.20 was five magnitude order smaller that the one with x=0.00.
(4) The preparation technology of BiFeO3-PbTiO3, PZT-BiFeO3 solid solution films was studied. The structure, ferroelectricity, dielectric property, and leakage current property were measured and studied.
Pb(CH3COO)2.3H2O、Zr(NO3)3.5H2O、Ti(C4H9O)4 were adopted to prepared Pb(Zr0.5Ti0.5)O3(PZT) and PbTiO3 precursor solutions. The BiFeO3 precursor solution was mixed with it and stablizer was added to obtain the final BiFeO3-PbTiO3 and PZT-BiFeO3 precursor solutions. The (BiFeO3)1-x-(PbTiO3)x and (PZT)1-x-(BiFeO3)x solid solution films were deposited on LaNiO3 bottom electrodes. In the above solid solution systems, x=0.0, 0.05, 0.10 and 0.20 for (BiFeO3)1-x-(PbTiO3)x films and x=0.05, 0.10, 0.20 and 0.30 for (PZT)1-x-(BiFeO3)x system. A structural phase transition was observed in the film with x=0.10 for the (BiFeO3)1-x-(PbTiO3)x films, through which, the rhombohedral structure transferred to cubic structure. The analyse to the radii of Bi3+, Pb2+, Fe3+, Ti4+ and Zr4+ showed that Bi3+, Pb2+ occupy A site, while Fe3+,Ti4+, Zr4+ occupy B site. A site and B site co-substitution was realized through the solid solution. Meanwhile, the ferroelectricity and dielectric property were enhanced through the substitution and magnetism was also enhanced. In PZT films, the ferroelectricity of it increased first and then decreased with the increasing of x. The film with x=10% showed largest ferroelecity while the film with x=20% show largest coercive.
引文
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