study_on_substitution_effect_of_bi4ti3o12_ferroele

7StudyonSubstitutionEffectofBi4Ti3O12FerroelectricThinFilmsJianjunLi1,2,PingLi1andJunYu21UniversityofElectronicScienceandTechnologyofChina2HuazhongUniversityofScienceandTechnologyChina1.IntroductionNowadays,ferroelectricthinfilmshaveattractedconsiderableattentionbecauseoftheirpotentialusesindeviceapplications,suchassensors,microelectro-mechanicalsystem(MEMS)andnonvolatileferroelectricrandomaccessmemory(NvFRAM)especially(Scott&PazDeAraujo,1989;PazDeAraujoetal.,1995;Parketal.1999).Leadzirconatetitanate[PbZrxTi1-xO3(PZT)]ferroelectricthinfilmisanearlymaterialforNvFRAM.PZTandrelatedferroelectricthinfilms,whicharemostwidelyinvestigated,usuallyhavehighremanentpolarization(Pr).However,theyaregenerallysufferedfromaseriousdegradationofferroelectricpropertieswithpolarityswitching,whentheyaredepositedonplatinumelectrodes.Bismuth-layeredperovskiteferroelectricthinfilms,withthecharacteristicsoffastswitchingspeed,highfatigueresistancewithmetalelectrodes,andgoodretention,haveattractedmuchattention.Bismuthtitanate[Bi4Ti3O12(BIT)]isknowntobeatypicalkindoflayer-structuredferroelectricswithageneralformula(Bi2O2)2+(Am-1BmO3m+1)2-.ItscrystalstructureischaracterizedbythreelayersofTiO6octahedronsregularyinterleavedby(Bi2O2)2+layers.AtroomtemperaturethesymmetryofBITismonoclinicstructurewiththespacegroupB1a1,whileitcanbeconsideredasorthorhombicstructurewiththelatticeconstantofthecaxis(c=3.2843nm),whichisconsiderablylargerthanthatoftheothertwoaxis(a=0.5445nm,b=0.5411nm).TheBIThasaspontaneouspolarizationinthea-cplaneandexhibitstwoindependentlyreversiblecomponentsalongthecandaaxis(Takenaka&Sanaka,1980;Rameshetal.,1990).Itshowsspontaneouspolarizationvaluesof4and50µC/cm2alongthecandaaxisrespectively.Theferroelectricpropertiesofthesebismuthlayer-structuredthinfilmsaremostlyinfluencedbytheorientationofthefilms(Simoesetal.,2006).TheBITthinfilmishighlyc-axisoriented,thusitsspontaneouspolarizationismuchlowerthanthatfora-axisoriented(Fuierer&Li,2002).ForapplicationsinNvFRAMdevices,ferroelectricmaterialsshouldhavehighremanentpolarization,lowcoercivefield(Ec),lowfatiguerateandlowleakagecurrentdensity.However,BITthinfilmhasmuchlowervaluesofswitchingpolarizationandsuffersfrompoorfatigueenduranceandhighleakagecurrentasaresultoftheinternaldefects(Uchidaetal.,2002).NumerousworkshavebeenmadetosubstituteBITthinfilmwithproperionstooptimizetheferroelectricproperties.Inrecentyears,itwasreportedthatsomeA-siteorB-sitesubstitutedBITshowedlargeremanentpolarizations.InthecaseofA-sitesubstitutioninBIT,La-substitutedBITFerroelectrics120[Bi3.25La0.75Ti3O12(BLT)]filmsexhibitedenhancedProf12µC/cm2withhighfatigueresistance,whichmakethemapplicabletodirectcommercialization(Chonetal.,2002).Otherlanthanidesions,suchasNd3+,Pr3+,Sm3+,etc.resultinsimilarresults(Watanabeetal.,2005;Chonetal.,2003;Chenetal.,2004).InthecaseofB-sitesubstitutioninBIT,somedonorionssuchasV5+,Nd5+,W6+,couldeffectivelydecreasethespacechargedensityresultingintheimprovementoftheferroelectricproperties(Kimetal.,2002;Wang&Ishiwara,2003).Forfurtherimprovementoftheferroelectricproperties,AandB-sitecosubstitutionbyvariousionsshouldbeconsideredbecausethepropertiesofBITbasedmaterialsstronglydependonspeciesofthesubstituentions.Inthischapter,wefirstsummarizedtheresearchesontheeffectofA-siteor/andB-sitesubstitutiononmicrostructuresandpropertiesofBi4Ti3O12ferroelectricthinfilms.ThenLa/VsubstitutedBITthinfilmsweredepositedbysol-gelmethod,andtheeffectofsubstitutionofLa3+andV5+onstructuralandelectricalpropertiesoftheBITthinfilmwasinvestigated.2.A-siteorB-sitesubstitution2.1A-sitesubstitutionThepropertiesofdifferentA-sitesubstitutedBITthinfilmsweresummarizedinTable1.La-substitutedBIT(BLT)filmsexhibitedlargePrandlowEcwithhighfatigueresistance,andBLTthinfilmhasbeenalreadyappliedincommercialNvFRAMproductinallA-sitesubstitutedBITthinfilms.SubstitutionIonContentOrientation2Pr(µC/cm2)2Ec(kV/cm)FatigueEnduranceReferencesLa0.75random241003×1010Parketal.1999random63.62603×1010Houetal.2005c1031906.5×1010Chonetal.2002Nd0.85(104)40100109Gargetal.20030.3a92100-Matsudaetal.20030.85c40-4.5×1010Chometal.2003Pr0.9random60104-Chenetal.2004Sm0.85c491134.5×1010Chonetal.2001Gd0.6random49.62491.45×1010Kimetal.2005Table1.SummaryofthepropertiesofdifferentA-sitesubstitutedBITthinfilmsStudyonSubstitutionEffectofBi4Ti3O12FerroelectricThinFilms121Yauetal.(Yauetal.2005)reportedthemechanismofpolarizationenhancementinLa-substitutedBITthinfilms.IndependentlycontrollingprocessingtemperatureorLasubstitutioncouldadjusttheorientationandenhancethePr.IncreasingLasubstitutiondecreasedcorientationbutincreasedPr.Thelatticeparamentsa,bandcofBi4-xLaxTi3O12filmsincreasemonotonouslywithLacontentxfromx=0-0.6.Thefittedlines(assumedforx0.6),withdifferentpositiveslopes,indicateddifferentenlargementsina,bandcandanincreaseincellvolume.Theincreaseinthea,bandclatticeparamentersapproachedsinglecrystalvalueswithincreasingx,whichalsoloweredtheorthorhombicdistortion,i.e.,2(a-b)/(a+b).ThisstronglysuggestedthattherelaxationsofstructuraldistortionandstrainarisefromtheLasub