Effects of Cr doping on physical properties of amorphous In-Ga-Zn-O films

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AppliedSurfaceScience257 (2011) 10018–10021

ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me p age :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Effects

of

Cr

doping

on

physical

properties

of

amorphous

In–Ga–Zn–O

ﬁlms

Shiu-Jen

Liu

a,∗

, Shih-Hao

Su

b

_{, Hau-Wei}

_Fang

c

_{, Jang-Hsing}

_Hsieh

d

_,

_Jenh-Yih

_Juang

b

a_Department_of_Mathematics_and_Science_{(Precollege),}_National_Taiwan_Normal_University,_Linkou,_Taipei_244,_Taiwan b_Department_of_{Electrophysics,}_National_Chiao_Tung_University,_Hsinchu_300,_Taiwan

c_Department_of_Materials_Science_and_Engineering,_National_Chiao_Tung_University,_Hsinchu_300,_Taiwan d_Department_of_Materials_Engineering,_Mingchi_University_of_Technology,_Taishan,_Taipei_243,_Taiwan

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received3February2011

Receivedinrevisedform21June2011 Accepted23June2011

Available online 1 July 2011 Keywords:

Amorphoustransparentconductingoxide Dilutedmagneticsemiconductor Indiumgalliumzincoxideﬁlms Crdoping

a

b

s

t

r

a

c

t

AmorphousthinﬁlmsofInGaZnO4(a-IGZO)dopedwithCrhavebeenfabricatedbyusingpulsed-laser

deposition(PLD).Theelectrical,opticalandmagneticpropertiesofCr-dopeda-IGZOﬁlmsgrownat25◦C and150◦_C_were_{investigated.}_The_{conductivity,}_optical_transmission_and_band_gap_of_ﬁlms_are

remark-ablyenhancedbyincreasingthegrowthtemperature.Conductivity,carrierconcentrationandmobility decreasewithincreasingtheCrcontent.However,theopticaltransmissionandbandgaparenot signif-icantlyaffectedbyCrdoping.Moreover,allCr-dopedﬁlmsexhibitroom-temperatureferromagnetism.

1. Introduction

Amorphoustransparentconductingoxides(a-TCOs)are promis-ingmaterialssince theirapplicationsonlarge-areaand flexible optoelectronicdevices suchas thin filmtransistors (TFTs)used in flat panel displays [1,2]. The most attractive a-TCO should beamorphous InGaZnO4 (a-IGZO) because it hasbeen demon-stratedthefeasibilityofusinga-IGZOastheactivelayersinthe high-performanceTFTsfabricatedonflexiblesubstratesatroom temperature[1] and used in large-size panels [3]. IGZO has a complexstructurewithalternatingInO2 andGaZnO2 layers[4]. CrystallineIGZOisarhombohedralcrystalandbelongstothespace groupof ¯R3m.Duetotheuniqueelectronicstructureinwhichthe conductionpathsofcarriers inIGZOarecomposedof extended sphericalsorbitalsofheavymetalcations,thecarriertransport isalmostnotaffectedbythechemicalbonddistortionandIGZO exhibitslargeelectronmobilityeveninamorphousstructure.The field-effectmobilityofTFTsusingamorphousIGZOastheactive channelcanachieve100cm2_V−1_s−1_[5].

Inthemeanwhile,dilutedmagneticsemiconductors/dielectrics synthesizedbyintroducingtransition-metal(TM)ionsinto con-ventionalIII–VandII–VIsemiconductorshaverecentlyattracted interestduetothepotentialapplicationofbothchargeandspinof electrons[6–8].Inaddition,widebandgapoxidesemiconductors

∗ Correspondingauthor.Tel.:+886277148400;fax:+886226022617. E-mailaddress:[email protected](S.-J.Liu).

suchasTiO2,ZnO,andSnO2dopedwithtransitionmetal includ-ing Fe,Co, Ni,Mn,and Crwereextensively investigated owing tothetheoreticalpredictionofroom-temperatureferromagnetism [9,10].Actually,anumberofstudiesonTM-dopedoxide semicon-ductors/dielectricsexhibitingroom-temperatureferromagnetism havebeenreported,suchasCo-dopedTiO2[11,12],Co-dopedSnO2 [13],Fe-dopedSnO2[14],Co-dopedZnO[15,16]andMn-dopedZnO [17].

Therearemanygroupsdevotedtotheresearchand develop-mentofa-IGZObasedTFTsinrecentyears.Theoreticalcalculation of electronic structure [4] and fundamental studies on optical andelectricalpropertiesofcrystallineandamorphousIGZOfilms have also attracted much attention [18,19]. Since the feasibil-ityof fabricatinghighperformance TFTsusinga-IGZOhasbeen demonstrated,ferromagnetica-IGZOcouldbeusedtoconstruct spin-electronicdevices.However,thephysical propertiesof fer-romagnetica-IGZOfilmsdopedwithtransition-metalimpurities haverarelyinvestigated[20].Inthisarticle,wereportthe room-temperatureferromagnetisminducedbyCrdopingina-IGZOfilms. TheeffectsofgrowthtemperatureandCrdopingontheelectrical, opticalandmagneticpropertiesarealsoinvestigated.

2. Experiments

TheundopedandCr-dopedIGZOﬁlmsusedinthisworkwere grownondouble-sidepolishedc-cutsapphiresbyusing pulsed-laserdeposition(PLD)withCr-dopedIGZOceramicoxideformed intopelletsasthetargetspreparedbyconventionalsolidstate reac-0169-4332/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

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S.-J.Liuetal./AppliedSurfaceScience257 (2011) 10018–10021 10019 tionsfromstoichiometricamountsofIn2O3,Ga2O3,ZnOandCr2O3

powdersof99.99% purity.TheCrcontentratio([Cr]/[In])ofthe filmsisdenotedasxat.%andx=0,2.5and5.0inthisstudy.The temperaturesof substrateswerekeptatroomtemperature(i.e. 25◦C)and150◦C.Theoxygenpressurewas10−2TorrduringPLD. KrFexcimerlaserwasusedtoablatethetargetwithenergy den-sityof1.5J/cm2_per_pulse_and_repetition_rate_of₅_Hz._The_distance betweenthetargetsandsubstrateswas4–5cm.Thegrowthrateis about0.05nmperpulseandthethicknessofthefilmsusedinthis studyisabout130–150nm.Thestructurepropertiesoffilmswere examinedby high-resolution transmission electron microscopy (HRTEM)andselected-areaelectrondiffraction(SAED)usingthe JEOLJEM-2100Fsystem.Theelectricalpropertiesincluding resis-tivity,carrierconcentrationandcarriermobilitywerecarriedout using thefour-probe vander Pauw method. The valence state ofCrionswasinvestigatedbyX-rayphotoelectronspectroscopy (XPS)analysisusingtheThermoVGScientificESCALAB250 sys-temwithaAlK␣ X-raysource(1486.6eV).Theanalysischamber isequippedwithafloodgunusedforchargecompensationwhen necessary.TheXPSspectraarereferencedtotheC1s photoemis-sionlineof284.8eV.Theopticalmeasurementsofthefilmswere recordedusingaUV–visdoublebeamspectrometer(JASCOV-570) inthewavelengthof200–900nm.Theroom-temperature magneti-zationvsmagneticfieldM(H)curveswereperformedonaQuantum Designsuperconductingquantuminterferencedevice magnetome-ter.

3. Resultsanddiscussion

TheHRTEMimagesandSAEDpatternsofundopedand5.0at.% Cr-dopedIGZOfilmsgrownat150◦CareshowninFig.1(a)and (b),respectively.Thereisnoclearcrystallinestructureobservedin theHRTEMimagesofbothfilms.Moreover,exceptthediffraction spotsofsapphiresubstrates,theSAEDpatternsshownodiffraction signalfromfilms.Theresultsindicateamorphousstructuresofboth films.Sincetheundopedand5.0at.%Cr-dopedIGZOfilmsgrownat 150◦Careamorphous,thefilmsgrownatroomtemperatureand 2.5at.%Cr-dopedIGZOfilmsarebelievedtobeamorphous.

The room-temperature resistivity, carrier concentration and mobilityofamorphousfilmsusedinthisstudyarelistedinTable1. Theresistivitiesoffilmsgrownat25◦Caremuchhigherthanthose offilmsgrownat150◦C.Moreover,Crdopingreducesthecarrier concentrationandmobility,andthusincreasesresistivityoffilms. The5.0at.%Cr-dopeda-IGZOfilmgrownat25◦Cisinsulating.

Thetemperaturedependentresistivities,(T),offilmsgrown at150◦C werecarriedouttoinvestigatetheeffectofCrdoping ontheelectricaltransportpropertiesofa-IGZOfilms.Asshownin Fig.2(a),nearlydegenerateconductionisobservedintheundoped a-IGZOfilm.TheresistivitiesofCr-dopedfilmsincrease monotoni-callywithdecreasingtemperature.Furthermore,(T)curveofthe filmdopedwith5.0at.%Crobeys the(T)∼exp(T−1/4) relation-shipwhichisgenerallyattributedtothevariable-range-hopping (VRH)conduction,ratherthanthethermallyactivatedconduction,

Table1

Electricalpropertiesofﬁlmsusedinthisstudy.Thex,TS,,nandaretheCr contentratio([Cr]/[In]),growthtemperature,resistivity,carrierconcentrationand carriermobilityoftheﬁlms,respectively.N/Ameansthatthevalueisnotavailable owingtolimitsoftheequipment.

Sampleno. x(at.%) TS(◦C) (cm) n(cm−3) (cm2V−1s−1)

1 0 25 1.8×102 _N/A _N/A

2 2.5 25 2.4×102 _N/A _N/A

3 5.0 25 N/A N/A N/A

4 0 150 6.8×10−3 _3.1_×₁₀19 _29.0

5 2.5 150 11.9×10−3 _2.6_×₁₀19 _21.6

6 5.0 150 45.6×10−3 _1.9_×₁₀19 _7.1

Fig.1.HRTEMimagesandSAEDpatterns(inset)of(a)undopedand(b)5.0at.% Cr-dopedIGZOﬁlmsgrownat150◦C.

(T)∼ exp(1/T),asshowninFig.2(b).Takagietal.reportedthat

VRHconductionisobservedina-IGZOfilmswithcarrier concentra-tion<1015_cm−3_and_{room-temperature}_resistivity_>104_cm_[18]. However,inthisstudy,thecarrierconcentrationofthe5.0at.% Cr-dopeda-IGZOfilmis∼1.9× 1019_cm−3_which_seems_to_be_too_high fortheVRHmechanism.AlthoughthehighconcentrationofCrions mayresultintheVRHconduction,itneedsmorestudiesto deter-minetheexacttransportmechanismintheCr-dopeda-IGZOfilms. Nevertheless,itcanbededucedthatthemajoreffectofCrdoping ontheelectricalpropertiesofa-IGZOfilmsistoreducethecarrier mobility.Ontheotherhand,Oritaetal.calculatedtheelectronic structureofIGZO[4]andconcludedthattheInO2layersserveasthe transportpathsinIGZO.SincethemoststableoxideofCrisCr2O3, thereductionofcarriermobilityimpliesthatCrionswithvalence of3+dopedina-IGZOfilmshavemainlybeenincorporatedintothe In3+_sites_and_the_scattering_or_localization_of_carriers_is_increased. Asaresult,thecarriermobilityisreduced.

InordertodeterminethevalencestateofCrions,XPS measure-mentswereconductedonallCr-dopedsamples(Samples2,3,5, and6).ThemeasuredCr2pspectraareshowninFig.3.Possibly becauseoflowintensityofspectraresultedfromlow concentra-tionofCrdoping,thecurvesareveryrough.Nevertheless,peaks ataround576.7eVcanbeobviouslyobservedandareattributed totheCr2p3/2level.Moreover,theintensityoftheCr2p3/2peaks ofboth5.0at.%Cr-dopedfilms(Samples3and6)arehigherthan thoseof2.5at.%Cr-dopedfilms(Samples2and5).Sincethebinding energyofCr3+_and_Cr6+_are_576.5_eV_and_579.3_eV,_respectively_[21], thevalenceofCrdopedinthea-IGZOfilmsisconfirmedtobe3+ bytheCr2p3/2peakslocatedataround576.7eVobservedinXPS.

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10020 S.-J.Liuetal./AppliedSurfaceScience257 (2011) 10018–10021 140 160 180 200 220 240 260 280 300 0.01 0.1

(a)

x = 0 x = 2.5 x = 5.0 ρ ( Ω -cm) T (K) 0.24 0.25 0.26 0.27 0.28 0.29 -3.2 -3.1 -3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3

(b) x = 5.0

T -1/4 (K -1/4)

ln(

ρ

)

3.5 4.0 4.5 5.0 5.5 6.0 6.5 -3.3 -3.2 -3.1 -3.0 -2.9 -2.8 -2.7 -2.6 -2.5 -2.4

ln(

ρ

)

1000/T (K-1)

Fig.2.(a)Temperature(T)dependenceofelectricalresistivity()inxat.%Cr-doped a-IGZOﬁlmsgrownat150◦Cand(b)ln()vsT−1/4andln()vs1000/Tplotsfor 5.0at.%Cr-dopeda-IGZOﬁlmsgrownat150◦C.

570 573 576 579 582 585 588 591 594

2 Cr 2p1/2

Cr 2p3/2

6

3

Intensity (a. u.)

Binding energy (eV)

5

Fig.3. XPSCr2pspectraoftheCr-dopeda-IGZOfilms.Thefiguresdenotethesample numbers(listedinTable1).Thepeakslocatedataround576.7eVareattributedtoCr 2p3/2level.Thesmoothinglines(dashlines)areobtainedbymeansofcurvefitting usingGaussianfunctionsandthebackgroundsarenotsubtracted.

TheXPSresultsexcludetheexistenceofCrO2,aroom-temperature ferromagneticmaterial,inourCr-dopedIGZOﬁlms.

ThetransmissionspectraofallfilmsareillustratedinFig.4(a). Thetransmissionoffilmsgrownat150◦Cexceeds80%inthe vis-iblerange (400–700nm),which aremuch higher thanthose of filmsgrownat25◦C.Theimprovementinthetransmissioncanbe attributedtothereductionofdefectstatesoffilmsduetoraising

thegrowthtemperature.Moreover,itisclearthattheabsorption edgesofa-IGZOfilmsgrownat25and150◦C arebothnot sig-nificantlyaffectedbyCrdoping.Theresultsfurtherindicatethe substitution of Cr3+ _ions_for _In3+_ions. _Furthermore,_the_optical bandgaps(Eg)ofthesefilmscanbeestimatedbytherelationship betweenabsorptioncoefficient(˛)andphotonenergy(h)ofthe form(˛h)∼(h−Eg)rwithr=2suggestedbyTaucforamorphous semiconductors[22,23].Theopticalbandgapsoffilmsgrownat25 and150◦Careabout2.0and3.1eV,respectively,obtainedbylinear extrapolationof(˛h)0.5_to_the_h_-axis,_as_depicted_in_Fig._4(b).

Fig.5showsthefielddependenceofmagnetizationM(H)curves measuredat300KfortheCr-dopeda-IGZOfilms.Allfilms obvi-ouslyexhibitroom-temperatureferromagnetism.Forfilmsgrown at25◦C,themagnetizationofthea-IGZOfilmdopedwith5.0at.% Crislargerthanthatofthe2.5at.%Cr-dopedfilm.Itseemsthat higherconcentrationofmagneticionsresultsinlarger magneti-zation.Sincethe2.5and5.0at.%Cr-dopeda-IGZOfilmsarehighly resistiveandinsulating,respectively,theboundmagneticpolarons mayplay animportant role intheferromagnetic interactionin thesefilms[24,25].Ontheotherhand,forfilmsgrownat150◦C, themagnetizationofthe2.5at.%Cr-dopedfilmislargerthanthatof thefilmdopedwith5.0at.%Cr.Itisnotedthatthecarrier concen-trationandmobilityoftheformerarelargerthanthoseofthelatter althoughbothofthemareconductive.Itimpliesthatthe delocal-izedcarriersmaybecrucialfortheferromagnetismoffilmsgrown at150◦C[9].Furthermore,thenearlynegligibledifferencebetween M(H)curves of 5.0at.%Cr-dopeda-IGZOfilmsgrownat25 and 150◦Crevealsthatmagneticpropertyofa-IGZOfilmsdopedwith highconcentrationofCrisalmostnotaffectedbythecarrier con-centrationandmobility.Onepossiblereasonisthatalthoughthe

200 300 400 500 600 700 800 900 0.0 0.2 0.4 0.6 0.8 1.0 T_S=150 oC x=0 x=2.5 x=5.0 TS=25 o C x=0 x=2.5 x=5.0

(a)

Transmission

Waveleng

th (nm)

1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 200 400 600 800 T_S=25 oC x=0 x=2.5 x=5.0 _T S=150 o C x=0 x=2.5 x=5.0

(b)

Pho

ton

ene

rgy (eV)

(

α

h

ν

)

0.5

(eV cm

-1

)

0.5

Fig.4. (a)Transmissionspectraand(b)(˛h)0.5_vs_h_plots_for_x_at.%_Cr-doped_a-IGZO ﬁlms.TSdenotesthegrowthtemperatureofﬁlms.

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S.-J.Liuetal./AppliedSurfaceScience257 (2011) 10018–10021 10021 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 -15 -10 -5 0 5 10 15

M (emu/cm

3

)

H (kO

e)

x=2.5 (T_S=25oC) x=2.5 (T_S=150oC) x=5.0 (T_S=25oC) x=5.0 (T S=150 o C)

Fig.5. Field(H)dependentmagnetization(M)ofxat.% Cr-dopeda-IGZOfilms measuredatroomtemperature.TSdenotesthegrowthtemperatureoffilms.The magneticfieldswereappliedparalleltotheplaneoffilms.

relativehighgrowthtemperature(i.e.150◦C)remarkablyincreases thecarrierconcentrationintheﬁlm,thescatteringofdelocalized electronsisincreasedbyCr3+_ions_doped_in_the_InO

2layerswhich aretheconduction paths.Therefore,carrier-mediated exchange exhibitsnoorfewcontributiontotheferromagnetismof5.0at.% Cr-dopeda-IGZOfilms.Anotherpossiblemechanismwhichexplains thatthemagnetizationof5.0at.%Cr-dopedfilmsissmallerthan thatof2.5at.%Cr-dopedfilmsgrownat150◦Cisthe antiferromag-neticdopant–dopantinteractionsatcloseproximity[26].However, duetotheamorphousstructure,themechanismofferromagnetism observedintheCr-dopeda-IZGOfilmsismorecomplexthanthat ofotherTM-dopedcrystallineoxidesemiconductorssuchasSnO2, ZnO,andTiO2.Theinterplayamongtheferromagneticinteraction, electronicstructureandlatticeorderinginamorphousdiluted mag-neticsemiconductorsisattractive.

4. Conclusions

Inconclusion, theelectrical, opticalandmagnetic properties of a-IGZO filmsdoped with Crprepared by using pulsed laser depositionwereinvestigated.Theelectricalpropertiesincluding conductivity,carrierconcentrationand mobilityofa-IGZOfilms arereducedbyCrdoping.Nevertheless,theopticalbandgapand transmissionofa-IGZOfilmsarenotsignificantlyaffectedbyCr

doping.Furthermore,allCr-dopedﬁlmsexhibitroom-temperature ferromagnetism.

Acknowledgment

ThisworkwassupportedbytheNationalScienceCouncilof Taiwan,undergrantno.NSC98-2112-M-003-005-MY3.

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