【作者】 杨琼；

【导师】 周益春； 曹觉先；

【作者基本信息】 湘潭大学 ， 材料科学与工程， 2013， 博士

【摘要】 铁电薄膜存储器是一类公认的具有极大发展潜力的非挥发性存储器件。而要使铁电薄膜存储器实现广泛的应用，则必须要更好地解决由铁电失效导致的可靠性问题。铁电薄膜的电学失效主要包括疲劳、印记、保持性损失等。造成铁电失效的因素主要有：退极化、晶格缺陷、界面层、漏电流等。随着薄膜制备技术的发展以及铁电器件的日趋小型化，铁电薄膜的界面和应变效应变得相当突出，这给铁电薄膜的应用带来了巨大的挑战，也为铁电性能的调控提供了巨大的空间。本论文中，我们采用第一性原理的计算方法，研究了铁电薄膜的界面和应变效应与铁电失效之间的关联，并探索了铁电性能的界面和应变调控规律。我们的研究为铁电薄膜的界面和应变工程提供了理论基础。本论文取得的创新性研究成果包括以下几个方面：（1）合适的外延应变可以有效地调控铁电薄膜的极化性质。以铁电存储应用中最有前景的几类铁电体PbTiO₃（PTO）、SrBi₂Ta₂O₉（SBT）、Bi₄Ti₃O₁₂（BiT）为研究对象，采用第一性原理和Berry-phase方法我们研究了铁电薄膜自发极化的应变效应。研究表明，合适地引入外延应变，可以有效地调控铁电薄膜的极化性质。特别地，对于简单钙钛矿铁电体PTO，在3%的ab轴压应变下，其自发极化可提高40%，同时也有效提高了PTO的极化稳定性。而对于铋层状钙钛矿的SBT，当ab双轴张应变达到2.4%时，其自发极化可以提高45%。对于铋层状钙钛矿的BiT，单斜的B1a1相比正交的B2cb相更稳定，2.4%的c轴压应变和ab双轴张应变分别可使BiT的极化主分量Pa提高10%和21%。铁电薄膜自发极化应变效应的研究，对铁电薄膜极化性能的外延应变调控具有重要指导意义；（2） ab轴压应变可以抑制氧空位造成的铁电性能失效。以PbTiO₃（PTO）和BaTiO₃（BTO）为研究对象，我们研究了钙钛矿铁电体中氧空位对极化和畴的影响，同时研究了外延应变与氧空位的结构和演化规律的关联。众所周知，钙钛矿铁电体中V_c（沿极化方向B-O链上的氧空位）能够造成tail-to-tail的极化花样，从而钉扎电畴翻转，造成铁电疲劳。我们发现，适当的ab平面压应变能有效地使钙钛矿铁电体PTO中引起tail-to-tail畴结构的V_c^ud转化成为head-to-tail畴结构的Vswc。而另一类对极化无害、但是在无应变的时候能量较高的氧空位Vab（垂直于极化方向的BO₂平面内的氧空位）能够在压应变下变得比V_c更稳定。这表明，压应变能够将氧空位由缺陷钉扎中心转变为非钉扎中心。另外，压应变能够提高氧空位的形成能和迁移势垒，从而抑制了氧空位的形成几率和流动性，这可以阻止氧空位由V_ab向V_c转变，同时氧空位的聚集现象也可以得到缓解。氧空位应变效应的研究表明，钙钛矿铁电体中氧空位对铁电极化的影响能够被ab轴压应变所抑制，因此应变调控是克服铁电薄膜失效问题的有效方法；（3）合理地构造界面能有效地克服死层效应并改善极化性质。以PTO、BTO钙钛矿铁电薄膜与Pt、LaNiO₃（LNO）和SrRuO₃（SRO）电极构成的电极/铁电/电极结构的铁电电容为研究对象，我们研究了界面效应对铁电薄膜极化性质的影响，并且探索了铁电薄膜极化性质的界面改善和应变改善。我们发现氧化物电极与铁电薄膜之间具有较强的界面结合性能，这可能是氧化物电极覆盖的铁电薄膜具有较强的抗疲劳性能的原因之一。然而，具有氧化物电极的铁电薄膜，特别是TiO₂终端的情况，其极化强度和极化稳定性较差。我们发现在TiO₂终端LNO/BTO/LNO电容的界面处，甚至会产生极化反向的死层效应。我们分析这可能是由于界面电场与TiO₂终端铁电薄膜本身的极化不稳定性共同导致的。我们发现引入LaXO₃（X=Fe, Co）和YNiO₃（Y=Sr, Ba）缓冲层能够有效地消除极化死层效应，极大地改善铁电薄膜的极化性质。同时我们发现，在界面效应的影响下，氧化物电极LNO覆盖的铁电薄膜以及TiO₂终端的铁电薄膜具有较大的面内极化分量，即铁电薄膜r相结构在能量上要比c相结构稳定许多。通过施加一定的压应变，薄膜的面内极化分量得到抑制，垂直于界面的极化分量极大地提高。通过以上研究，我们对铁电薄膜的界面效应的微观机理有了更深刻的认识，这也为铁电薄膜电性能的界面和应变调控提供了有力的指导；（4）合适的掺杂可以抑制氧空位的界面聚集和漏电流。以Pt/PTO/Pt铁电电容为对象，我们研究了氧空位在界面附近的演化规律及其对铁电薄膜漏电流的影响，同时探索了掺杂的方法对漏电流的调控能力。我们发现氧空位在Pt/PTO界面附近的形成能要远低于PTO薄膜内部的形成能，最大可相差1.6eV以上。这表明氧空位容易在界面处产生或者从薄膜内部迁移到界面处，从而形成所谓的界面死层。通过对PTO薄膜输运性质的计算，我们发现氧空位的出现使PTO铁电薄膜的漏电流为原来的数倍以上，而且氧空位越靠近界面，铁电薄膜的漏电流越大。我们研究了离子掺杂对氧空位以及漏电流的调控能力。我们发现Cu和V掺杂能够俘获氧空位，从而有效地抑制铁电薄膜的漏电流。通过对含有氧空位的铁电电容的研究，我们更加清晰地认识了氧空位所造成的界面失效、漏电流等的微观机理。同时漏电流掺杂调控的研究，有助于我们更好地实现铁电薄膜性能的优化。更多还原

【Abstract】 Ferroelectric film memory is believed to be the most promising nonvolatilememory technology. For the wide application of ferroelectric film memory, itsreliability due to the ferroelectric failure should be better solved. The electricalfailure of ferroelectric film includes fatigue, imprint and retention loss. There aremany factors which result in the ferroelectric failure, such as depolarization, latticedefects, interface layer and leakage current. With the development of filmpreparation technology and the miniaturization trend of ferroelectric device, theinterface and strain effects in ferroelectric film become increasingly prominent. Theinterface and strain effects make the practical application of ferroelectric filmchallenging and meanwhile bring about a considerable adjustable range offerroelectricity. In this thesis, we studied the interface and strain effects on theferroelctricity and its failure mechanism by first principles calculation and theninvestigated the interface and strain tuning of ferroelectric film. Our works offeruseful information and theoretical basis for improving the ferroelectricity byinterface and strain engineering. The primary coverage of this thesis is as follows:（1） The polarization properties of ferroelectric film can be effectivelyimproved by proper epitaxial strain. With the most promising types offerroelectrics for memory use, shch as PbTiO₃（PTO）, SrBi₂Ta₂O₉（SBT） andBi₄Ti₃O₁₂（BiT）, we studied the strain effects on the spontaneous polarization offerroelectric films by first principles calculation and Berry-phase method. Resultsshow that the polarization properties of ferroelectric films can be effectivelyimproved by introducing proper epitaxial strain. Particularly, the spontaneouspolarization of perovskite ferroelectric PTO is improved by40%under3%ab-biaxial compressive strain, and the polarization stability of PTO is also enhancedby the compressive strain. For the bismuth-layer-structured ferroelectric SBT, whenthe ab-biaxial tensile strain reaches2.4%, spontaneous polarization is improved by45%. For bismuth-layer-structured ferroelectric BiT, the monoclinic B1a1structureis more stable than the orthorhombic B2cb structure. Under2.4%c-uniaxialcompressive strain or2.4%ab-biaxial tensile strain, the principal component ofpolarization Pais increased by10%or21%, respectively. The spontaneous polarization-strain coupling study is instructive for the improvement of polarizationproperties of ferroelectric film by epitaxial strain tuning;（2） The oxygen vacancy induced ferroelectric failure can be suppressed byab-biaxial compressive strain. With typical perovskite ferreoectrics PbTiO₃（PTO）and BaTiO₃（BTO）, we investigated the influence of oxygen vacancy on theferroelectric polarization and domain, and the strain effects on the oxygen vacancyconfiguration and evolution properties were also studied. It is known that the Vc（oxygen vacancy located in the B-O-B chains along the polarization axis） inperovskite ferroelectrics can cause a tail-to-tail polarization pattern and pin thedomain switching, which is the important origin of electrical fatigue. We found thatthe tail-to-tail domain configuration of Vc（Vudc） in PTO can be tuned to be ahead-to-tail one （Vswc） by applying proper ab-biaxial compressive strain. And theoriginally polarization-harmless Vab（oxygen vacancy located in the BO₂-plane thepolarization axis）, with a higher energy than Vcin the strain-free case, can bestabilized by the compressive strain. This indicates that the compressive strain cantransform the oxygen vacancy from a domain pinning centre to be a non-pinningcentre. In addition, the formation and migration energies of oxygen vacancy can bothbe increased and thus the possibility and the mobility of oxygen vacancy can besuppressed by the compressive strain. It suggests that, under compressive strain, thetransformation of Vabto Vccan be impeded and oxygen vacancy clusteringphenomenon can be mitigated. The study of strain effects on the oxygen vacancyindicates that the influence of oxygen vacancy on the ferroelectric polarization canbe reduced by ab-biaxial compressive strain. Therefore, strain tuning is an effectiveway to overcome the ferroelectric failure problems;（3） The dead layer effects of ferroelectric film can be effectively overcome andpolarization properties can be significantly improved by constructing properinterface structures. With the electrode/ferroelectric/electrode capacitors consistingof PTO and BTO perovskite ferroelectric films and Pt, LaNiO₃（LNO）, SrRuO₃（SRO）electrodes, the interface effects on the polarization properties of ferroelectric filmswere studied, and the improvement of polarization properties by interface and straintuning was also investigated. It is found that the oxide electrodes possess strongerinterface adhesion with ferroelectric film, which may be one of the reasons for thebetter endurance of ferroelectric film with oxide electrodes. However, theferroelectric film with oxide electrodes exhibits poorer polarization magnitude and stability, especially for the TiO₂-terminated film. In the TiO₂-terminatedLNO/BTO/LNO capacitor, we even found a reversed polarization dead layer nearthe top interface. This may be induced by the interface electric field and the inherentpolarization instability of TiO₂-terminated ferroelectric films. It is found that theLaXO₃（X=Fe, Co） and YNiO₃（Y=Sr, Ba） buffer layers can effectively eliminatethe dead layer effect and significantly improve the polarization properties. We alsofound that, under the interface effects, the ferroelectric films with oxide electrodes orTiO₂-termination show a large in-plane polarization component. It means that ther-phase is much more stable than the c-phase. By applying proper compressive strain,the in-plane polarization component is suppressed and out-plane component can belargely increased. The above studies lead to a more profound understanding for themicroscopic mechanism of interface effect and provide guidance for the interfacetuning of ferroelectricities;（4） The interface gathering of oxygen vacancy and its related leakage can byeffectively suppressed by proper doping. With Pt/PTO/Pt ferroelectric capacitors,the evolution of oxygen vacancy near the interface and its effects on the leakagecurrent were studied, and we also investigated the doping modification of theleakage. It is found that the formation energy of oxygen vacancy near the Pt/PTOinterface is much lower than that in the inner film. The maximum energy differenceis more than1.6eV. This indicates that the oxygen vacancy can readily form at theinterface or migrate from inner film to the interface, resulting in the so-calledinterface dead layer. By calculating the transport, we found that the appearance ofoxygen vacancy would increase the current by more than several times, and thenearer of the oxygen vacancy from the interface, the larger of the leakage caused byoxygen vacancy. The doping tuning of oxygen vacancy and leakage were studied. Itis found that the Cu and V doping could trap the oxygen vacancy and thereby greatlysuppress the leakage. Through the study of oxygen vacancy in the ferroelectriccapacitors, the mechanisms of oxygen vacancy induced interface problem andleakage current are clearer. And the doping modification study is meaningful for theimprovement of electrical properties of ferroelectric films.更多还原