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EFFECT OF THE NATURE OF BOTH TOP ANDEFFECT OF THE NATURE OF BOTH TOP ANDBOTTOM ELETRODES ON THE DIELECTRIC ANDBOTTOM ELETRODES ON THE DIELECTRIC AND

FERROELECTRIC PROPERTIES OF PZT THIN FILMFERROELECTRIC PROPERTIES OF PZT THIN FILM

CAPACITORSCAPACITORSNossikpendou Sama *Nossikpendou Sama * a)a) , Rachid Bouregba, Rachid Bouregba ++ , Caroline Soyer *, Denis Remiens *, Caroline Soyer *, Denis Remiens *

* IEMN, DOAE, MIMM group, Cite Scientifique, 59655 Villeneuve d Ascq Cedex, France.

++Laboratoire CRISMAT-ENSICAEN, CNRS UMR 6508,Boulevard du Marchal Juin, 14050 Caen Cedex France.

a) nossikpendou.sama@univa) nossikpendou.sama@univ- -valenciennes.frvalenciennes.fr

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IntroductionIntroduction

Films growthFilms growth

Model of capacitor and experimental workModel of capacitor and experimental work

ResultsResults

Summary and conclusionsSummary and conclusions

OutlineOutline

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I ntroductionI ntroduction

ManyMany studiesstudies havehave beenbeen mademade onon PZTPZT thinthin filmfilm capacitorscapacitors totoinvestigateinvestigate thethe degradationdegradation of of dielectricdielectric andand ferroelectricferroelectricpropertiesproperties whenwhen thethe thicknessthickness is is reducedreduced. .

TheThe aimaim ofof thisthis studystudy isis toto showshow thatthat thisthis degradationdegradationresultsresults fromfrom interfaceinterface effectseffects. . ThisThis isis achievedachieved byby usingusingelectrodeselectrodes withwith differentdifferent naturenature: : mm etaletal (Pt)(Pt) andand oxideoxide(LaNiO(LaNiO 33))..

ThisThis studystudy alsoalso aimsaims atat discriminatingdiscriminating whichwhich ofof thethe toptop ororthethe bottombottom interfaceinterface mainlymainly contributescontributes to to thisthis degradationdegradation. .

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I ntroductionI ntroduction

ToTo thisthis end,end, dielectricdielectric andand ferroelectricferroelectric measurementsmeasurements werewerecarriedcarried outout onon PZTPZT5454/ /4646 filmsfilms inin fourfour differentdifferent structures,structures,eacheach oneone withwith fourfour differentdifferent thicknessesthicknesses ((200200, , 400400, , 600600 andand800800 nm)nm). .

PZTPZT PZTPZTPZTPZT PZTPZT

substratesubstrate substratesubstrate substratesubstrate substratesubstrate

PtPt LNOLNO LNOLNO

LNOLNOPtPt

PtPt

LNOLNOPtPt

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BottomBottom electrodeselectrodes areare Pt/TiOPt/TiO xx /SiO /SiO 22 /Si /Si andand LNO/SiOLNO/SiO 22 /Si /Sidepositeddeposited byby sputtering,sputtering, Pt/TiOPt/TiO xx electrodeelectrode is is 120120 nmnm thickthickandand LNOLNO one,one, 250250 nmnm..

ThenThen PZTPZT isis depositeddeposited onon themthem byby sputteringsputtering andand annealedannealed at at625625CC..

Finally,Finally, PtPt andand LNOLNO toptop electrodes,electrodes, withwith respectivelyrespectively 120120 nmnmandand 200200 nmnm thicknesses,thicknesses, areare depositeddeposited onon PZTPZT byby sputteringsputtering. .

FourFour differentdifferent kindkind ofof structuresstructures werewere grown,grown, eacheach oneone withwith 44differentdifferent thicknessesthicknesses : : 200200, , 400400, , 600600 andand 800800 nmnm (a(a totaltotal ofof1616 structuresstructures available)available). .

F ilms growthF ilms growth

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F ilms growthF ilms growth

X RD spectra of PZT/Pt X RD spectra of PZT/LNO

PZT/PtPZT/Pt showsshows a a preferentialpreferential orientationorientation whilewhile PZT/LNOPZT/LNO hashas orientationorientation. .InIn bothboth casescases thethe intensityintensity of of thethe peakspeaks increasesincreases withwith thethe thicknessthickness of of thethefilm,film, withoutwithout changingchanging itsits texturetexture. .

PZT/Pt has larger grains than PZT/LNOPZT/Pt has larger grains than PZT/LNO

PZT/PtPZT/Pt PZT/LNO

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xperimental work E xperimental work

TheThe PZTPZT capacitorscapacitors areare modeledmodeled asas stackedstacked structurestructure withwith aa bulkbulk ferroelectricferroelectriclayerlayer sandwichedsandwiched betweenbetween twotwo nonferroelectricnonferroelectric spacespace chargecharge layerlayer atat eacheachsurfacesurface [ [11]]..

ThisThis modelmodel enablesenables toto determinedetermine somesome datadata relatedrelated toto thethe interfacesinterfaces andand thethebulkbulk. .

BasedBased onon thisthis model,model, dielectricdielectric andand ferroelectricferroelectric measurementsmeasurements werewere performedperformedinin orderorder toto characterizecharacterize thethe propertiesproperties of of thethe filmsfilms withwith differentdifferent electrodeselectrodes andandwithwith variousvarious thicknessesthicknesses. .

[1] R. Bouregba, G. L e Rhun, G. P oullain, and G. L eclerc, J. Appl. P hys. 99, 034102 (2006).

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R esultsR esultsDielectric measurements (Dielectric measurements ( ))

11851185815815928928709709800800

10871087803803797797606606600600

10781078651651755755476476400400

10281028445445717717422422200200

LNOLNO--PZTPZT--LNOLNO

PtPt--PZTPZT--LNO(bottom)LNO(bottom)

LNOLNO--PZTPZT--Pt(bottom)Pt(bottom)

PtPt--PZTPZT--PtPt

d (nm)d (nm)

always increases with the thickness of the PZT film but this trend is less pronouncedfor the LNO/PZT/LNO structure.

substitution of Pt by LNO as top electrode systematically leads to a significantimprovement of the dielectric constant. Such substitution appears less drastic whenperformed at the bottom electrode.

For a same top electrode, (100) oriented PZT on LNO has better thanthan ((110110))orientedoriented PZTPZT onon PtPt.. Finally, for a given thickness, is higher for theLNO/PZT/LNO/SiO2/Si structure and weaker for that Pt/PZT/Pt/TiOx/SiO2/Si one.

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R esultsR esults1/Cmeas (d) for all the structures1/Cmeas (d) for all the structuresInIn thethe frameworkframework of of thethe stackedstacked capacitorcapacitor modelmodel withwith depresseddepressed interfaceinterface layerslayers : :11/C /C

measmeas== 11/C /C

ff++ 11/C /C

ii== (d/(d/

0 / /

ff /A) /A) ++ 11/C /C

ii, CC

iiisis thethe capacitancecapacitance relatedrelated toto bothboth

interfacesinterfaces. .[remarque[remarque : : t est es paspas obligoblig dede l crirel crire sisi tutu lele dis!]dis!]

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R esultsR esults

Confirma

tion of

the firs

tobserva

tions :

- S t rong con t ribu t ion of t he Pt t op elec t rode in t he degrada t ion of t he dielec t ricproper t ies (ma t erialized by t he smalles t in t erface capaci t ance)

- F or a given bo tt om elec t rode, t he same value of f f isis foundfound wha t ever wha t ever t het he na t urena t ure of of t het het opt op elec t rodeelec t rode : : consis t encyconsis t ency of of t het he modelmodel asas f f isis mainlymainly de t erminedde t ermined byby t het hecrys t allographiccrys t allographic orien t a t ionorien t a t ion of of t het he filmfilm

-- PZTPZT onon LNOLNO exhibi t sexhibi t s larger larger f f t hant han PZTPZT onon PtPt :: consis t en tconsis t en t wi t hwi t h t het he fac tfac t t ha tt ha t ((100100))orien t a t ionorien t a t ion isis preferablepreferable t ot o achieveachieve be tt er be tt er dielec t ricdielec t ric proper t iesproper t ies

-- Moreover,Moreover, valuesvalues of of f f (#(#10001000 andand 16001600)) areare consis t en tconsis t en t wi t hwi t h t hoset hose of of bulkbulk PZTPZT for for aasimilar similar composi t ioncomposi t ion

16001600160016001070107010701070ff

4.104.10 --664.104.10 --666.106.10 --6666..1010 --66Slope 1/Slope 1/ 00 ** ff * S [(pF.nm)* S [(pF.nm) --11]]

LNOLNO--LNOLNOPtPt--PZTPZT--LNO(Bottom)LNO(Bottom)

LNOLNO--PZTPZT--Pt(Bottom)Pt(Bottom)

PtPt--PtPt

Determination of bulk permittivity and interface capacitance for all the structuresDetermination of bulk permittivity and interface capacitance for all the structures

3,3333,3330,3330,3330,910,910,4350,435CCii (nF)=1/y(0)(nF)=1/y(0)

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R esultsR esults

FerroelectricFerroelectric loopsloops asas aa functionfunction ofof thicknessthickness forfor aa givengiven electrodeelectrodeconfigurationconfiguration

All the ferroelectric loops have been measured for a constant maximumpolarisation (30 C/cm 2) in accordance with the model.

PZT on PtPZT on Pt

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R esultsR esults

PZT on LNOPZT on LNO

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R esultsR esults

Comparison of the ferroelectric loops for a given thicknessComparison of the ferroelectric loops for a given thickness

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R esultsR esultsMain prediction of the modelMain prediction of the model

d

V C

AD

E E bi

i f

)(!

The model predicts a linear variation of Ec vs (1/d) which the slope andThe model predicts a linear variation of Ec vs (1/d) which the slope andthe ordinate origin give interface potential Vbi and the coercive field inthe ordinate origin give interface potential Vbi and the coercive field in

the bulk ferroelectric layer Efc, respectivelythe bulk ferroelectric layer Efc, respectively

TheThe modelmodel givesgives thethe followingfollowing relationrelation betweenbetween thethe electricelectric fieldfield appliedapplied totothethe wholewhole structure,structure, thethe electricelectric fieldfield inin thethe ferroelectricferroelectric layerlayer andand thetheelectricelectric displacementdisplacement : :

CaseCase ofof specialspecial interestinterest :: coercivecoercive fieldfield EcEc obtainedobtained whenwhen D=D=00

d V

E Ec bi fc!

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R esultsR esultsCoercive fields Ec (1/d)Coercive fields Ec (1/d)

PZT on PtPZT on Pt

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R esultsR esults

PZT on LNOPZT on LNO

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S ummary and conclusionsS ummary and conclusions

DielectricDielectric andand ferroelectricferroelectric propertiesproperties of PZTPZT thinthin filmfilm capacitorscapacitors withwithdifferentdifferent electrodeelectrode configurationconfiguration andand differentdifferent thicknessesthicknesses werewere investigatedinvestigated

11 -- ForFor aa givengiven bottombottom electrode,electrode, significantlysignificantly increasedincreased andand EcEc significantlysignificantlydecreaseddecreased whenwhen LNOLNO replacesreplaces PtPt asas toptop electrodeelectrode

44 -- TheThe naturenature ofof thethe bottombottom electrodeelectrode (LNO(LNO oror Pt)Pt) hashas onlyonly aa minorminor influenceinfluenceinin termsterms ofof thicknessthickness effecteffect. . BottomBottom electrodeelectrode seemsseems notnot toto degradedegrade thethequalityquality ofof thethe interface,interface, thoughthough it it stronglystrongly influencesinfluences thethe permittivitypermittivity in in termstermsofof crystallographiccrystallographic orientationorientation (better(better withwith orientedoriented PZTPZT thanthan withwith orientedoriented PZT)PZT)

22 -- ThicknessThickness dependencedependence of of muchmuch lessless significantsignificant withwith LNOLNO asas toptop electrodeelectrodebutbut notnot totallytotally absentabsent (finite(finite valuevalue ofof interfaceinterface capacitance)capacitance)

33 -- ThicknessThickness dependencedependence of of EcEc almostalmost nonnon--existentexistent withwith LNOLNO asas toptopelectrodeelectrode

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S ummary and conclusionsS ummary and conclusions

ExperimentalExperimental resultsresults werewere examinedexamined in in thethe frameworkframework of of thethe stackedstackedcapacitorcapacitor modelmodel withwith nonnon ferroelectricferroelectric chargedcharged interfaceinterface layerslayers

22 -- LNOLNO toptop electrodeelectrode InterfaceInterface capacitancecapacitance Ci Ci

andand interfaceinterface potentialpotential barrierbarrier Vbi Vbi comparedcompared to to PtPt

11 -- GeneralGeneral considerationconsideration : : accordingaccording to to thisthis modelmodel thethe thicknessthickness dependencedependenceofof maymay bebe interpretedinterpreted as as duedue toto presencepresence of of finitefinite interfaceinterface capacitancecapacitance Ci,Ci,butbut thethe thicknessthickness dependencedependence of of EcEc requiresrequires CiCi toto bebe chargedcharged (potential(potentialbarrierbarrier Vbi) Vbi) !! HenceHence thethe onlyonly notionnotion ofof passive passive or or dead dead layerlayer modelmodel isis notnotsufficientsufficient

3 - Oxyde electrode only at top surface of the film EliminationElimination of ofthicknessthickness dependencedependence of of bothboth dielectricdielectric andand ferroelectricferroelectric propertiesproperties

4 - MeasurementsMeasurements are are inin progressprogress to to investigateinvestigate thethe effecteffect ofof PtPt andand LNOLNO asastoptop andand bottombottom electrodeselectrodes uponupon thethe fatigue fatigue mechanismmechanism