溶胶—凝胶方法制备的BiFeO_3薄膜铁电改性研究
摘要
铁电薄膜在存储器、非制冷红外热释电探测器、压电MEMS等器件中已经获得了广泛的应用，而且被预测铁电薄膜器件的需求量将逐年提高。但是高性能铁电薄膜的低温、低成本、大规模量产技术是制约其在IC和MEMS领域快速发展的主要技术障碍。在多种薄膜制备技术中物理气相沉积（PVD）仍是占支配地位，但化学溶液沉积（如Sol-Gel）技术因其更好的材料沉积效果及性能将会在制备技术中取得一席之地。
与其他无铅铁电材料相比，BiFeO3具有优异的铁电及压电性、较低的制作成本、较高的居里温度，在IC和MEMS器件等领域具有诱人的应用前景。但BiFeO3薄膜的漏电和老化现象是其实用化进程中迫切需要解决的关键问题。漏电和老化都与薄膜中的缺陷有关，大量的研究工作表明通过掺杂或提高BiFeO3薄膜晶化程度来降低薄膜中的缺陷含量是改善铁电薄膜性质的有效手段。有文献报道已利用BiFe0.95Mn0.05O3薄膜制作了256M铁电存储器，为进一步提高其铁电性质，本论文针对BiFe0.95Mn0.05O3铁电薄膜进行了铁电改性研究。
根据缺陷化学理论，高价离子掺杂会降低氧空位浓度。本论文研究了Zr4+掺杂对BiFe0.95Mn0.05O3铁电薄膜老化行为的影响。将BiFe0.95Mn0.05O3中掺杂不同浓度的Zr4+，并对施加循环高电场前后薄膜的电滞回线变化进行了对比。BiFe0.93Mn0.05Zr0.02O3的薄膜表现出最大的饱和电滞回线、最大的剩余极化率、最强的抗老化效果及电荷保持能力，说明对Fe格位进行高价Zr4+的掺杂是一种提高材料铁电老化性能的行之有效的方法。具体结论如下：
（1） Zr4+的掺杂将铁电薄膜的双电滞回线转变为单电滞形、最大的剩余极化率、最小的Ec、最大的电荷保存能力，说明2%的Zr4+掺杂能够有效地起到去老化的作用。
（2）电滞回线的缺口反映BiFe1-0.95-xMn0.05ZrxO3薄膜中铁电畴的翻转驱动力的大小。Zr掺杂对氧空位以及缺陷对的抑制能力相对有限。
（3）施加高交流电场之后，矫顽电场非对称程度的提高源自BiFe1-0.95-xMn0.05ZrxO3薄膜中缺陷对断开后释放出的氧空位在非对称电场作用下重新分布和与二价铁离子的复合。
（4）施加高交流电场之后，矫顽电场非对称程度随着测试电场是先升高后降低。拐点出现对应的测试电场的高低也能反映薄膜的老化程度。
Bi在退火过程中会挥发，过量铋是提高BiFeO3薄膜晶化程度的一种常见方法。但过量氧化铋对BiFeO3薄膜和Pt电极界面的影响还未见报道。因此本论文研究了Bi2O3缓冲层对沉积在Pt/Ti/SiO2/Si衬底上的BiFe0.95Mn0.05O3薄膜结构和电学性能的影响。具体结论如下：
（1）采用Bi2O3缓冲层能够促使晶粒长大，并降低了薄膜内Bi的空位浓度、氧离子的空位浓度，降低薄膜的漏电流。
（2） Bi2O3缓冲层能够提高剩余极化率、降低矫顽场、提高P-E电滞回线的对称性、提高电荷保存能力、降低铁电疲劳性。文献报道和本论文的实验结果都表明BiFe0.95Mn0.05O3薄膜的抗击穿能力提高。阐明Mn掺杂对BiFeO3薄膜抗击穿能力提高的物理机制，对于提高薄膜的综合性质及相关类似的结果进行解释有着一定的指导意义。本论文就Mn掺杂对BiFeO3薄膜生长模式、结构以及性能的影响方面对BiFeO3薄膜抗击穿能力提高的现象进行了讨论，得到的具体结论为：
（1） Mn掺量从5%增加到20%，BiFe1-xMnxO3薄膜的生长模式从生长速率各向异性决定的(110)-取向择优生长转变为表面能最小化驱动的(100)-取向生长。
（2） Mn掺杂在一定程度上提高了BiFe1-xMnxO3薄膜的晶化能，却降低了（100）晶面的表面能。
（3）自极化BiFe0.95Mn0.05O3和BiFe0.90Mn0.10O3多晶薄膜的获得意味着不依靠外延应力也能提高薄膜的自极化程度。
（4） BiFe0.90Mn0.10O3薄膜比BiFe0.95Mn0.05O3薄膜的老化程度轻，更具应用价值。
Ferroelectric thin film has been widely applied in memory, uncooled infraredpyroelectric detectors, piezoelectric MEMS and other devices. And the need of theferroelectric thin film device will increase year by year. However, there are somemainly technical barriers to restrict its rapid development in the field of IC andMEMS, such as low-temperature, low-cost, and mass production techniques for thehigh-performance ferroelectric thin films. Among many technologies of the filmfabrication, physical vapor deposition (PVD) is still dominant, but the chemicalsolution deposition (such as Sol-Gel) technology will get a place in preparationtechnologies because of its better material deposition effects and performance.
Compared to other lead-free ferroelectric material, BiFeO3has the excellentpiezoelectric and ferroelectric properties, the lower the production costs, and thehigher Curie temperature, and has an attractive prospect in the field of IC and MEMSdevices. But the leakage and aging of BiFeO3film are key issues to address in theprocess of actually using. The leakage and aging are all related to the defects in thefilm. A lot of research work has shown that the doping or enhancing the degree ofcrystallization of BiFeO3film is an effective method to improve the properties of theferroelectric film. The literature about fabricating256M ferroelectric memory usingBiFe0.95Mn0.05O3has been reported. In order to improve its ferroelectric properties, wecarried out the modification research for the ferroelectric film BiFe0.95Mn0.05O3.
According to defect chemistry, the high ion doping will reduce the oxygenvacancy concentration. We studied the effect the Zr4+doping on the aging behavior ofBiFe0.95Mn0.05O3ferroelectric thin films. Zr4+was doped in BiFe0.95Mn0.05O3filmswith different concentrations. And the hysteresis loop the loop before and afterapplying a high electric field was systematically studied. The film ofBiFe0.93Mn0.05Zr0.02O3shows the largest saturation hysteresis loops, the largestremaining polarization, the strongest antiaging effects, and the strongest charge retention capability, and these can illustrate that the Fe sites of conduct expensive Zr4+doping is a proven method to improve the aging properties of ferroelectric materials.The specific conclusions are as follows:
(1) The P-E hysteresis loops of thin films before and after applying with the loopelectric field around700KV/cm was compared. By doping Zr4+, the double hysteresisloop was changed into a single hysteresis loop, and the saturation polarization wasalso improved through the effect of the high loop electric field;
(2) P-E ferroelectric hysteresis loop of the BiFe0.95Mn0.05Zr0.02O3film has goodrectangular, the largest remaining polarization ratio, minimum ΔEc, and maximumcharge retention ability. These indicate that the Zr4+doping with2%concentration caneffectively serve to remove the aging effect. But the Zr-doping has a relatively limitedability to inhibit the oxygen vacancies and defects;
(3) Size of the backswitching driving force in the BiFe1-0.95-xMn0.05ZrxO3ferroelectric film was reflected by the gap of the hysteresis loop. Zr doping isrelatively limited for the suppression of oxygen vacancies and defects.
(4) After applying a high alternating electric field, the improvement of asymmetrydegree of the coercive field is mainly coming from the redistribution of the the releaseof oxygen vacancies in asymmetric electric field after disconnecting of defects inBiFe1-0.95-xMn0.05ZrxO3film, and its complex with divalent iron ions.
(5) After applying a high alternating electric field, the asymmetric extent of thecoercive field was increased first and then decreased with the testing electric field.The degree of aging of the film can also be reflected by the testing electric field whenthe inflection point appearing.
Bi atoms can volatilize during the annealing process, and excess bismuth is acommon method to enhance the crystallization degree of BiFeO3film. But the impactof excess bismuth oxide BiFeO3film and Pt electrode interface has not been reported.Thus the effect of Bi2O3buffer layer on film structure and electrical properties ofBiFe0.95Mn0.05O3film deposited on the substrate Pt/Ti/SiO2/Si was studied. Thespecific conclusions are as follows:
(1) The Bi2O3buffer layer can cause grain growth, and reducing the oxygen ion vacancy concentration and Bi space in the film. The generation of the defected ionwas also decreased. The polarization performance and the fatigue resistance of theferroelectric were improved.
(2) The Bi2O3buffer layer can be used to improve the residual polarization, reducethe coercive field, improve P-E hysteresis loop symmetry and the ability to save thecharge, reduce the ferroelectric fatigue.
The reported literatures and our experimental results all show that the fight againstwear ability of the BiFe0.95Mn0.05O3film was increased. The physical mechanism ofMn-doping for improving fight against wear ability of the BiFeO3films was clarified.This also has a certain significance for improving the overall results of a similarnature and relevant to explain the film. Mn doping in the BiFeO3thin film, impact ofthe growth mode, structure and performance to raise the fight against wear ability ofthe ability of the BiFeO3film were discussed, specific conclusions are as follows:
(1) When the content of Mn-doping changing from5%to20%, the growth patternsof BiFe1-xMnxO3film was changed from the (110)-oriented preferential growthdetermined from the anisotropic growth rate into (100)-oriented growth driven byminimized surface energy.
(2) Mn-doping can improve the crystallization energy of the BiFe1-xMnxO3film at acertain extent, but reduces the surface energy at (100) plane.
(3) The achievement of polarization BiFe0.95Mn0.05O3and BiFe0.90Mn0.10O3polycrystalline films means that the degree polarization of the film can be alsoincrease without self-epitaxial stress.
(4) The aging degree of BiFe0.90Mn0.10O3film is softer than that of theBiFe0.95Mn0.05O3film, and has a more application value.
与其他无铅铁电材料相比，BiFeO3具有优异的铁电及压电性、较低的制作成本、较高的居里温度，在IC和MEMS器件等领域具有诱人的应用前景。但BiFeO3薄膜的漏电和老化现象是其实用化进程中迫切需要解决的关键问题。漏电和老化都与薄膜中的缺陷有关，大量的研究工作表明通过掺杂或提高BiFeO3薄膜晶化程度来降低薄膜中的缺陷含量是改善铁电薄膜性质的有效手段。有文献报道已利用BiFe0.95Mn0.05O3薄膜制作了256M铁电存储器，为进一步提高其铁电性质，本论文针对BiFe0.95Mn0.05O3铁电薄膜进行了铁电改性研究。
根据缺陷化学理论，高价离子掺杂会降低氧空位浓度。本论文研究了Zr4+掺杂对BiFe0.95Mn0.05O3铁电薄膜老化行为的影响。将BiFe0.95Mn0.05O3中掺杂不同浓度的Zr4+，并对施加循环高电场前后薄膜的电滞回线变化进行了对比。BiFe0.93Mn0.05Zr0.02O3的薄膜表现出最大的饱和电滞回线、最大的剩余极化率、最强的抗老化效果及电荷保持能力，说明对Fe格位进行高价Zr4+的掺杂是一种提高材料铁电老化性能的行之有效的方法。具体结论如下：
（1） Zr4+的掺杂将铁电薄膜的双电滞回线转变为单电滞形、最大的剩余极化率、最小的Ec、最大的电荷保存能力，说明2%的Zr4+掺杂能够有效地起到去老化的作用。
（2）电滞回线的缺口反映BiFe1-0.95-xMn0.05ZrxO3薄膜中铁电畴的翻转驱动力的大小。Zr掺杂对氧空位以及缺陷对的抑制能力相对有限。
（3）施加高交流电场之后，矫顽电场非对称程度的提高源自BiFe1-0.95-xMn0.05ZrxO3薄膜中缺陷对断开后释放出的氧空位在非对称电场作用下重新分布和与二价铁离子的复合。
（4）施加高交流电场之后，矫顽电场非对称程度随着测试电场是先升高后降低。拐点出现对应的测试电场的高低也能反映薄膜的老化程度。
Bi在退火过程中会挥发，过量铋是提高BiFeO3薄膜晶化程度的一种常见方法。但过量氧化铋对BiFeO3薄膜和Pt电极界面的影响还未见报道。因此本论文研究了Bi2O3缓冲层对沉积在Pt/Ti/SiO2/Si衬底上的BiFe0.95Mn0.05O3薄膜结构和电学性能的影响。具体结论如下：
（1）采用Bi2O3缓冲层能够促使晶粒长大，并降低了薄膜内Bi的空位浓度、氧离子的空位浓度，降低薄膜的漏电流。
（2） Bi2O3缓冲层能够提高剩余极化率、降低矫顽场、提高P-E电滞回线的对称性、提高电荷保存能力、降低铁电疲劳性。文献报道和本论文的实验结果都表明BiFe0.95Mn0.05O3薄膜的抗击穿能力提高。阐明Mn掺杂对BiFeO3薄膜抗击穿能力提高的物理机制，对于提高薄膜的综合性质及相关类似的结果进行解释有着一定的指导意义。本论文就Mn掺杂对BiFeO3薄膜生长模式、结构以及性能的影响方面对BiFeO3薄膜抗击穿能力提高的现象进行了讨论，得到的具体结论为：
（1） Mn掺量从5%增加到20%，BiFe1-xMnxO3薄膜的生长模式从生长速率各向异性决定的(110)-取向择优生长转变为表面能最小化驱动的(100)-取向生长。
（2） Mn掺杂在一定程度上提高了BiFe1-xMnxO3薄膜的晶化能，却降低了（100）晶面的表面能。
（3）自极化BiFe0.95Mn0.05O3和BiFe0.90Mn0.10O3多晶薄膜的获得意味着不依靠外延应力也能提高薄膜的自极化程度。
（4） BiFe0.90Mn0.10O3薄膜比BiFe0.95Mn0.05O3薄膜的老化程度轻，更具应用价值。
Ferroelectric thin film has been widely applied in memory, uncooled infraredpyroelectric detectors, piezoelectric MEMS and other devices. And the need of theferroelectric thin film device will increase year by year. However, there are somemainly technical barriers to restrict its rapid development in the field of IC andMEMS, such as low-temperature, low-cost, and mass production techniques for thehigh-performance ferroelectric thin films. Among many technologies of the filmfabrication, physical vapor deposition (PVD) is still dominant, but the chemicalsolution deposition (such as Sol-Gel) technology will get a place in preparationtechnologies because of its better material deposition effects and performance.
Compared to other lead-free ferroelectric material, BiFeO3has the excellentpiezoelectric and ferroelectric properties, the lower the production costs, and thehigher Curie temperature, and has an attractive prospect in the field of IC and MEMSdevices. But the leakage and aging of BiFeO3film are key issues to address in theprocess of actually using. The leakage and aging are all related to the defects in thefilm. A lot of research work has shown that the doping or enhancing the degree ofcrystallization of BiFeO3film is an effective method to improve the properties of theferroelectric film. The literature about fabricating256M ferroelectric memory usingBiFe0.95Mn0.05O3has been reported. In order to improve its ferroelectric properties, wecarried out the modification research for the ferroelectric film BiFe0.95Mn0.05O3.
According to defect chemistry, the high ion doping will reduce the oxygenvacancy concentration. We studied the effect the Zr4+doping on the aging behavior ofBiFe0.95Mn0.05O3ferroelectric thin films. Zr4+was doped in BiFe0.95Mn0.05O3filmswith different concentrations. And the hysteresis loop the loop before and afterapplying a high electric field was systematically studied. The film ofBiFe0.93Mn0.05Zr0.02O3shows the largest saturation hysteresis loops, the largestremaining polarization, the strongest antiaging effects, and the strongest charge retention capability, and these can illustrate that the Fe sites of conduct expensive Zr4+doping is a proven method to improve the aging properties of ferroelectric materials.The specific conclusions are as follows:
(1) The P-E hysteresis loops of thin films before and after applying with the loopelectric field around700KV/cm was compared. By doping Zr4+, the double hysteresisloop was changed into a single hysteresis loop, and the saturation polarization wasalso improved through the effect of the high loop electric field;
(2) P-E ferroelectric hysteresis loop of the BiFe0.95Mn0.05Zr0.02O3film has goodrectangular, the largest remaining polarization ratio, minimum ΔEc, and maximumcharge retention ability. These indicate that the Zr4+doping with2%concentration caneffectively serve to remove the aging effect. But the Zr-doping has a relatively limitedability to inhibit the oxygen vacancies and defects;
(3) Size of the backswitching driving force in the BiFe1-0.95-xMn0.05ZrxO3ferroelectric film was reflected by the gap of the hysteresis loop. Zr doping isrelatively limited for the suppression of oxygen vacancies and defects.
(4) After applying a high alternating electric field, the improvement of asymmetrydegree of the coercive field is mainly coming from the redistribution of the the releaseof oxygen vacancies in asymmetric electric field after disconnecting of defects inBiFe1-0.95-xMn0.05ZrxO3film, and its complex with divalent iron ions.
(5) After applying a high alternating electric field, the asymmetric extent of thecoercive field was increased first and then decreased with the testing electric field.The degree of aging of the film can also be reflected by the testing electric field whenthe inflection point appearing.
Bi atoms can volatilize during the annealing process, and excess bismuth is acommon method to enhance the crystallization degree of BiFeO3film. But the impactof excess bismuth oxide BiFeO3film and Pt electrode interface has not been reported.Thus the effect of Bi2O3buffer layer on film structure and electrical properties ofBiFe0.95Mn0.05O3film deposited on the substrate Pt/Ti/SiO2/Si was studied. Thespecific conclusions are as follows:
(1) The Bi2O3buffer layer can cause grain growth, and reducing the oxygen ion vacancy concentration and Bi space in the film. The generation of the defected ionwas also decreased. The polarization performance and the fatigue resistance of theferroelectric were improved.
(2) The Bi2O3buffer layer can be used to improve the residual polarization, reducethe coercive field, improve P-E hysteresis loop symmetry and the ability to save thecharge, reduce the ferroelectric fatigue.
The reported literatures and our experimental results all show that the fight againstwear ability of the BiFe0.95Mn0.05O3film was increased. The physical mechanism ofMn-doping for improving fight against wear ability of the BiFeO3films was clarified.This also has a certain significance for improving the overall results of a similarnature and relevant to explain the film. Mn doping in the BiFeO3thin film, impact ofthe growth mode, structure and performance to raise the fight against wear ability ofthe ability of the BiFeO3film were discussed, specific conclusions are as follows:
(1) When the content of Mn-doping changing from5%to20%, the growth patternsof BiFe1-xMnxO3film was changed from the (110)-oriented preferential growthdetermined from the anisotropic growth rate into (100)-oriented growth driven byminimized surface energy.
(2) Mn-doping can improve the crystallization energy of the BiFe1-xMnxO3film at acertain extent, but reduces the surface energy at (100) plane.
(3) The achievement of polarization BiFe0.95Mn0.05O3and BiFe0.90Mn0.10O3polycrystalline films means that the degree polarization of the film can be alsoincrease without self-epitaxial stress.
(4) The aging degree of BiFe0.90Mn0.10O3film is softer than that of theBiFe0.95Mn0.05O3film, and has a more application value.
引文
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