First-principles Study Of Interface And Strain Effects In Ferroelectric Film For Memory Application

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.