Work function dependence of Schottky barrier height and leakage mechanism in thin film capacitors using perovskite dielectrics

Work function dependence of Schottky barrier height and leakage mechanism in thin film capacitors using perovskite dielectrics

著者 Kim Taeyoung

著者別名 金, 兌映

journal or

publication title

博士学位論文要旨 論文内容の要旨および論文審査 結果の要旨／金沢大学大学院自然科学研究科

volume 平成21年6月

page range 33‑37

year 2009‑06‑01

URL http://hdl.handle.net/2297/26905

氏名 学位の種類 学位記番号 学位授与の日付 学位授与の要件 学位授与の題目

金免映

博士（学術）

博甲第１０５０号 平成２０年９月２６日

課程博士(学位規則第４条第１項）

WOrkfimctiondependenceofSchottkybarrierheightandleakag

mechamsmmthmmmcapacitorsusmgperovskitedielectrics

（ペロブスカイト誘電体薄膜キャパシタにおけるショットキー障壁高さの

仕事関数依存,性とリーク電流機構）

森本章治（理工研究域・教授）

久米田稔（理工研究域・教授)，黒堀利夫（人間社会研究域・教授)，

高橋光信（理工研究域・教授)，猪熊孝夫（理工研究域・准教授）

論文審査委員（主査）

論文審査委員（副査）

DoctoraMissertationabstract

WorkflmctiondependenceofSchottkybarrierheightandlealKagemechanisminthinfilm capacitorsusingPerovslKitedielectrics

Abstract

Theleakagemechanismofmetal-insulator-mctal(ＭＩＭ)thinfilmcapacitorusingPerovskite dielectricsandbottomnoblemetalelectrodeshasbecninvestigatedTheleakagebehavior througlltheinterfaceatthetopelectrode/STOfilmsdependsonthesurfaceoftｈｅＳＴＯｆｉｌｍｓ、

Ontheotherh、d,theleakagecurentthroughtheinterfaceattheSTO/bottomelectrodesshows typicalinterface-1imitedleakagebehavioLThemicrostmcturesofbottomclectrodesarealtered bycrystallmcorientation,under-1ayerlmddepositiontemperature､Thismicrostmcturevariation leadｓｔｏａｃｈａｎｇｅａｂout０．１６eVontheworkfUnctionofmctalelectrodesmeasuredbyPESA・A plotofSchottkybarrierheightsasafUnctionofworkfilnctionshowsagoodlinearrelationship、

ItsuggcststhathighworkfimctionofthemetalelcctrodeiseffectiveinincreasingSchottky barrierheiglltsinthedielectric/、etalinterface,1cadingeffectivereductionoftheleakagccurent inMIMcapacitors・Thetunedmicrostmctureofanelectrodeappearstobeanessentialfactorin theperfOnnanceofexcellentMIMthinfilmcapacitors．

１.Introduction

Thesilicon-basedmetal-oxide-semiconductordeviccsfOrthedevicescalingfO11owing Moore,slawarebeingreachedinthefimdamentallimits,andthusdcmandfOrrcplacingsilicon dioxideisdrivingstronginterestinhighdielectricconstantmaterials(highＫ).ThehighK materialshavcbeenwidelyinvestigatedforanalternativegateoxideandacapacitor,However，

thelargcleakagecurrentproblemisstillregardedasgreatobstacleintheseapplications・

LeakagecⅢentbehaviorisoneofthemajorfactorstodetenninethereliability,anditisknown thattheMIMcapacitorisreasonablestructure・Especially,muchattentionhasbeenpaidtoMIM capacitorwhichconsistsofPerovskite-typedielectricsandnoblemctalelcctrode・Ｉｎｓｐｉｔｃｏｆ

－３３－

thcseeffbrts,1eakageproblcmremamsunsolved・Itisknownthattheleakagecurrentisnotonly govcmcdbythebulkferro-ordiclcctricfilms(bulk-1imited)butalsobytheSchottkyjunction betweenfilmsandmetalelectrodes(interface-limited)Herelwishtopointouttheimportantof barrierfOmationtoreduceleakagecurrcntefficientlysothatthemetalelectrodewithhigllwork fimctionwillbegreathelpinreducingthelcakagecurrentBasedonthisidea，Ihavebeen fOcusedontheinterfacephenomena,andinvestigatedtheeffectofe血ancedworkfimctionon theSchottkybarrierfOnnationofMIMcapacitorusingSrTiO3(STO)dielectricsandnoblc metalelectrode,Ihaveobtainedaninterestingresultandwishtoreportitherein．

2.Ｆｉｌｍｐ肥parationandcharacterization

ThefilmswerepreparedbyPIDmethod､ThePIDteclmiqueiswidelyusedfOrcither variousmetalfilmsoroxidecompoundssuchaslligh-temperaturcsuperconductors，

ferroclectrics,multiferroics,andelectro-optics,ＴｈｅoneofthcmostadvantagesfeaturesofPID techniqueistheabilitytoreproducethetargetcompositionwithrelativeeaseunderthe appropriateconditions、ItisanefficientmethodtoproducethelayerbylayergrowthfOrthin filmsinthcatomicrangc､TheX-raydiffraction(XRD）wasuscdfOrcrystallincstructurc analysis,andatonlicfOrcemicrospectroscopy(AFM)wasusedfOrmicrostructurcobservationof filmsurface・TheworMmctionwasmeasuredbyphotoelectronspectroscopyintheair(PESA）

basedonanelcctroncountingmecllanismbyanopencounter．

3.ConsiderationofleaM巴emechanismandworMimction

TheconsideratiOnofleakagemechanismsecmstobeworthwhilesubjectinorderto investigatehowtocontroltheleakagecurrent・TheavailableleakagemechanismsarefOllowing：

Scllottkyemission，Ohmiccontact，Fowler-Nordheimtunneling，PooleFrenkelemission，ａｎｄ Space-cllargc-limitedcurrent・Likewise，tlleworkfimctionisalsoessentialfactors・Thework fmctionisfimdalnentalphysicalquantitylanditisknownthattheworkflmctionisdepcndson thcmaterialtypes,However,theworkfUnctionisstronglyaffectedbysurfaceconditionbecausc

theworkfUnctiｏｎｃａｎｂｅｕｎｉｆｉｃｄａｓｓｕｍｎａｒｉｚｅｏｆｓｕｒfacetcnn△ｆａｎｄｂｕｌｋｔｅｎｎ②Bas fO11owing：

必－Ａ9,＋必 (1)

ItindicatesthewoIkflmctioncanbecontrolledbysurfacemodification,anditwillbeusefill fOrSchottkybarricrfOmation．

４LeaMgemeclnanismintlninfilmcapacitorusin＝PeIDvsMtedielectrics

ThedielectricconstantsfOramorphousSTOfilmdepositedbelow300oCareaｒｏｕｎｄＺＯａｓ ｓｈｏｗｎｉｎＦｉｇ・LIntheI-Vcharacterization，thecurrentdirectionisdefinedfromthetop electrodc(positivc)tothebottomelectrode(negative)asshowninFig､２.Thcleakagecurrent throughtheintｅｒｆａｃｅａｔｔｈｅＳＴＯ／bottomelectrodeshowstypicalinterfacc-1imitedleakage behavioLTheinterface-1imitedleakagebehavioriｓａｌｓｏｄｅｔｅｃｔｅｄｏｎｔｈｅｃｏｎｔａｃｔｏｆｔｏｐ

－３４－

electrodc/STOfilmspreparedat300･Ｃａｎｄ５５０゜CWhereastheSTOfilmshasveryrougll surfaceatthedepositiontemperａｔｕｒｅｏｆ４５０ｏＣｗｈｉｃｈｉｓａｂｏｕｔｈａlfWaytemperaturefrom amorphoustocrystallizedphase､Thisroughsurfaceoriginatedfrommcta-stablephaseofSTO filmsleadstobulk-1imitedleakagebehavioronthecontactoftopelectrodc/STOfilmslnthc leakagebehaviorofMIMcapacitorsusingSTOfilms,thCinterfacephenomenaarepredominant colnponenttocontroltheleakagecurrent．

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｢二二:=、

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-１５０－１００－５００５０１００１５０

EIectricFieId(kV7Cm）

３００３５０４００４５０５００５５０

STOdepositiontemperature[。C］

FigureLThedielectricconstantofSTOfilmsat variousdepositiontemperature

Figure2・Theleakagecurrentdensityasa

fimctionofelectricfieldofSTOfilms

5.EffectofmicmstmctueofbottOmelectrOdesontlneworkfmction

TheworkfUnctionofthe（111)-orientedPt

５.O

electrodeisapproximately493eVwhereastheへ

wmMmciionoftho(100)cri．nMＭ…do書４， is478eVThisdifferenceofworkfUnction二

_g

dcpcndingonthecrystalorientationcanbc昌

_＝

cxplainedtheelectrondensitydistributionon芒４８

－－

themetalsurfaceduetosmoothingeffect､ｎｌｅ夢

ｗｏｒｋfUnctionofPt/Rubilayerelectrode４７

0３６９１２１５

increasesfrom475to490eVaccordingtothe

Rootme肌sqHre,RRMs(､、）

thicknessofthePttoplayerdecrcases、Ｔｈｃ

Ｆｉｇｕｒｅ３・TheworkfUnctionasafilnctionofroughness

workfimctionofRutoplayerelectrodcsalso

onthemetalsurface・Insetshowstllesmoothingeffect

incrcasesfrom487to４．９７ｅＶａｃcordingtothe

under-1ayerJtsupportstheassumptionthattheworkfimctionisrelatedwithmicrostmctureon thesurfacc・TheworkfimctionsareplottcdagainstroughnessofsurfaceasshowninFig.３．It showsthestronglinearrelationshipagainstroughnessofelectrodesJnotherwords，thework fimctiondecreasesatelectrodeswithroughsurface・Itcanbeexplainedbysmoothingeffect whichisatendencytosmoothoutthesurfaceasshownintheprcviouschapter6Thesmoothing effectlcadstheelectronflowfromthehillstothevalleysandfOrmsthcpositivepotentialbarrier tothevacuumasshｏｗｎｉｎｓｅｔｉｎＦｉｇ３・Itsuggeststhattheworkfimctioncaｎｂｅｅｎｈａｎｃｅｄｂｙ

－３５－

modificationofmicrostructuresuchascrystallineorientationorsurfacerougllness・The enhancedworkfUnctionbytunedmicrostructureofelcctrodesisexpectedtofOmhighSchottky barrieronthecontactofbottomelectrodeanddielectrics・Thisrelationshipwillbeinvestigated atnextchapter．

6.SchottlWbarrierenMncementbytlmedworMmction ThevariationofSchottkybarrierhcights

ａｓａｆｉｍｃｔｉｏｎｏｆｗｏｒｋｆｉｍｃｔｉｏｎｏｎｔｈｅmetalへ_{＞１．００}

elcctrodcsurfaccshowsastronglinear－

^出一ダニ･

・晋og5 relationaInongthesamecrystalorientation

_出

ofmetalbothPtandRuelectrodesasｓｈｏｗｎ _{・臼０．９０}^凶

ｉｎＦｉｇ．４．ItsuggeststhattheSchottkycd

_国目

barricrenhancementismightbedominated〉、ｏ･８５

^塁

一一

byenlargementofworkfilnctiononｍｅｔａｌｏ

_ＳｏＢＯ

electrode，hotherwords，theleakage

の

●Ｐｔ(111)/Ru/A1203

鰯Ｐｔ(111yA1203 ^霧 (111ysio2

(111yGGG (100MV190

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●。

▼ 蕊

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４．７５４．８０４．８５４．９０４．９５５.OO

currentofdielectricthinfilmswithmetal

WOrkFimction(eV）

electrodesmaybecontro11edbytuningofFigme4TheSchottkybarrierheightsasafUnctionof

theroughnessonthemetalelectrode・ＴｈｅworkfUnctionofthemetalsurface、

tunedmicrostructureofelectrodesappears

tobeanessentialfactorintheperfOmanceofexcellentMIMthinfilmcapacitors．

7.Leaka窪ebel1avioraffbctedbytopandbottominterhce

Figure5showsAFMimagesofRuelectrodesaccordingtothedepositiontemperature・The amorphousSTOfilmsurfacerelatestothesurfaceofRubottomelectrodeasshownintheinset ofFig､５.ThissurfacevariationofalnolphouｓＳＴＯｆｉｌｍｓｌｅａｄｓｔotheleakagecurrentvariation assｈｏｗｎｉｎＦｉｇ６・TheleakagecurrentcurvesaffCctedbybothtopandbottomelectrodes resembleeacllother､ItindicatesthattheleakagecurrentisalsoaffectedbymorphologyofＳＴＯ

ｆｉｌｍｓ

(b)ＲｕｇＯＯｏＣ (a)Ｒｕ４００°Ｃ _1.00

0.75 0.50 0.25

1.00 0.75 0.50 0.26

－●－STO300oC/Ｒｕ400.0.-..ﾑｰ…STO300oC/Ru500oC STO300。C/Ru600oC-O-S｢0300℃/Ru700oC

２４６８０００００列ごＩ■Ｉイー凸Ｉ（Ｕ■Ｉ

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(のＲｕ７００°Ｃ に)Ｒｕ６００°Ｃ _1.00

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－２５０‐200‐1５０－１００５００５０１００１５０２００２５０

ElectricFieId(kⅥc、）

Figure6,I-VcurvesfbramorphousSTOfilms・Leakage currentatnegativebiasshowscurrcntoriginatedby interfacｅｏｆＡ１ｔｏｐｅｌｅｃｔｒｏｄｅａｎｄＳＴＯｆｉｌｍｓ

－３６－

Ｏ(nｍ）

FigureaAFMimagesfbrRuelectrodesdepositedat varioustemperature・ＩｎｓｅｔｓｈｏｗｓｔｈｅａｍｏｒｐｈｏｕｓＳＴＯ

ｆｉｌｍｓontheseRuelectrodes

8.ＳｕｍｍａＩｙ

IYleeffectofworkfimctionbymicrostructureofelectrodeontheleakagebehaviorhasbecn investigatedontheMIMthinfilmcapacitors・ＴｈｅＳＴＯｆｉｌｍｓｄｅｐｏｓｉｔｅｄｏｎｔｈｅＰｔａｎｄRumetal electrodesshowatypicalmterface-1imitedleakagebehavioronthecontactofSTOfilms/bottom electrodes、Ｏｎｔｈｅｃｏ､traryltherougllsurfaceoriginatcdfrolnmeta-stablephaseofSTOｆｉｌｍｓ leadsabulk-1ilnitedleakagebehavioronthecontactoftopelectrode/STOfilms・Theleakage currentdensitycanbesuppressedbySchottkybarrierheightfOmation、TheSchottkybalTier canbefOnncdbyenhancementofworkfmctiononthebottomelectrodes・Moreover,thework fUnctioncanbeincreascdbymodificationofmicrostructuresuchascrystallincorientationor surfacerouglmess・Inotherwords,theleakagecurrentontheMIMcapacitorcanbecontrolled bytuningofrougllnessonthemetalelectrode､Thetunedmicrostructureofelectrodesappearsto beanessentialfactorintheperfOnnanceofexcellentMIMthinfilmcapacitors．

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