Physical properties of amorphous Mo-doped In-Ga-Zn-O films grown by magnetron co-sputtering technique

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Physical

properties

of

amorphous

Mo-doped

In–Ga–Zn–O

ﬁlms

grown

by

magnetron

co-sputtering

technique

Shiu-Jen

Liu

a,

*

,

Hau-Wei

Fang

b

,

Jang-Hsing

Hsieh

c

,

Jenh-Yih

Juang

d

a_Department_of_Mathematics_and_Science_{(Precollege),}_National_Taiwan_Normal_University,_Linkou_Dist.,_New_Taipei_City_24449,_Taiwan b

DepartmentofMaterialsScienceandEngineering,NationalChiaoTungUniversity,Hsinchu30010,Taiwan

c

DepartmentofMaterialsEngineering,MingchiUniversityofTechnology,TaishanDist.,NewTaipeiCity24301,Taiwan

d

DepartmentofElectrophysics,NationalChiaoTungUniversity,Hsinchu30010,Taiwan

1. Introduction

Amorphoustransparentconductingoxides(a-TCOs)are prom-ising materials owing to their applications on optoelectronic devices such as thin film transistors (TFTs) used in flat panel displays[1,2]. One of the most attractive a-TCO is amorphous InGaZnO4(a-IGZO) which hasbeendemonstratedtobeable to workastheactivelayersinthehigh-performanceTFTsfabricated onflexiblesubstratesatroomtemperature[1]andusedin large-sizepanels[3].IGZOhasacomplexstructurewithalternatingInO2 andGaZnO2layers[4].Duetotheuniqueelectronicstructurein whichtheconductionpathsofcarriersinIGZOarecomposedof extendedsphericalsorbitalsofheavymetalcations,thecarrier transportisnotaffectedbythechemicalbonddistortionandIGZO exhibitslargeelectronmobilityeveninamorphousstructure.

Manygroupsaredevotedtotheresearchanddevelopmentof a-IGZO based TFTs in recent years. Theoretical calculation of electronicstructure [4]and fundamentalstudieson opticaland electricalpropertiesofcrystallineandamorphousIGZOﬁlmshave also attracted much attention [5,6]. However, the physical propertiesofa-IGZOﬁlmsdopedwithtransition-metalimpurities

have rarely investigated [7,8]. In the meanwhile, the carrier mobility of In2O3 films has been reported to be drastically enhanced by Mo doping [9,10]. A theoretical model based on magneticinteractionswasproposedtoexplaintheenhancement ofcarriermobilityinMo-dopedIn2O3films[11].Theexperimental andtheoreticalresultsofMo-dopedIn2O3urgeustoexplorethe effectsofModopingonphysicalpropertiesofa-IGZOfilms.The enhancement of carrier mobility and room-temperature ferro-magnetisminducedbyModopingwereobserved.

2. Experimental

The Mo-doped In–Ga–Zn–O films used in this study were prepared by using magnetron co-sputtering technique. The sputtering wasconducted with a 75W rf power appliedon a InGaZnO4targetandavariousdcpowerrangingbetween2and 5Wappliedon a metallicMotarget.Bothtargetswere2in.in diameterand99.99%inpurity.Corning1737glasseswereusedas substratesandplaced458tothetargetssurfacewitha substrate-to-target distance of 10cm. During sputtering, the substrate temperaturewaskeptat1508Candtheflowratesofargonand oxygenwere17.5and10.5sccm,respectively.Thepressureofthe chamber duringsputtering was 1.2102_Torr._After _a _40-min sputtering,thefilmswerecooleddowntoroomtemperature.The thickness of the sputtered films was determined to be about

MaterialsResearchBulletin47(2012)1568–1571

ARTICLE INFO

Articlehistory:

Received12October2011

Receivedinrevisedform10February2012 Accepted15February2012

Availableonline24February2012

Keywords: A.Amorphousmaterials A.Semiconductors B.Sputtering D.Electricalproperties D.Magneticproperties D.Opticalproperties ABSTRACT

AmorphousthinfilmsofIn–Ga–Zn–O(a-IGZO)dopedwithMohavebeenfabricatedbyusingmagnetron co-sputteringtechnique.TheMoconcentrationina-IGZOfilmswasmodulatedbyvaryingthesputtering powerappliedontheMotarget.Theelectrical,opticalandmagneticpropertiesofMo-dopeda-IGZOfilms grownonglasseswereinvestigated.Thecarrierdensityandmobilityofa-IGZOfilmscanberemarkably enhancedbylowconcentrationModoping.Ontheotherhand,theopticalbandgapofa-IGZOfilmsisnot significantlyaffectedbyModoping.However,thetransmissionisdecreasedwithincreasingtheMo doping.Moreover,allMo-dopedfilmsexhibitroom-temperatureferromagnetism.

ß2012ElsevierLtd.Allrightsreserved.

*Correspondingauthor.Tel.:þ886277148405;fax:þ886226022617. E-mailaddress:[email protected](S.-J.Liu).

ContentslistsavailableatSciVerseScienceDirect

Materials

Research

Bulletin

j our na l ho me pa g e : w ww . e l se v i e r . com / l oca t e / m a tr e sbu

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100nmbyusingasurfaceprofilometer(AlphaStep500,Tencor Instruments).TheMocontentratio[Mo]/[In]ofthedopedIn–Ga– Zn–O films was measured by using energy dispersive X-ray spectroscopyandisdenotedasxat.%(aslistedin Table1).The crystal structure of the films was examined by using X-ray diffraction(XRD)scanswithCuK

a

radiation(PANalyticalX’Pert PROMPD).Theelectricalpropertiesincludingresistivity,carrier concentrationandcarriermobilitywerecarriedoutusingthe four-probevanderPauwmethod.Thevalence stateofMoionswas investigatedbyX-rayphotoelectronspectroscopy(XPS)analysis usingtheThermoVGScientiﬁcESCALAB250systemwithaAlK

a

X-raysource(1486.6eV).Theanalysischamberisequippedwitha floodgunusedforchargecompensationwhennecessary.TheXPS spectraarereferencedtotheC1sphotoemissionlineof284.8eV. TheopticalmeasurementsofthefilmswererecordedusingaUV– Visdoublebeamspectrometer(JASCOV570)inthewavelengthof 200–900nm.Theroom-temperaturemagnetizationversus mag-netic field M(H) curves wereperformed on a Quantum Design superconductingquantuminterferencedevicemagnetometer. 3. Resultsanddiscussion

TheamorphousstructureofallﬁlmswasconﬁrmedbytheXRD scansshowninFig.1.ThereisnosharppeakobservedinallXRD curves.Thebroadpeakslocatedataround2

u

=228arecontributed bytheglasssubstrates.

XPSmeasurementswereconducted onall samplesto deter-minethevalencestateofMoionsinthefilms.ThemeasuredMo3d spectraareshowninFig.2.Atthefirstglance,thereisnopeak observedintheMo3dregionforsample#1,theundopeda-IGZO film.Twopeaksareobservedforsample#2andthreepeaksare observedforsamples#3,#4and#5.ThesepeaksarelabeledasG1, G2andG3accordingtotheirlocations,i.e.,bindingenergy.The exactvaluesofbindingenergyoftheseMo3dpeaksandpossible contributionstothesepeaksarelistedinTable2.TheG1peaksare contributedbytheMo6+3d3/2level[12].TheG2peakscouldbea combinationofMo6+_3d

5/2andMo4+3d3/2peaks[13].TheG3peaks

areassignedtotheMo4+_3d

5/2level.Itcanbededucedfromthe datalistedinTable2,thevalenceofMoionsdopedinsample#2is 6þ,andthevalencesofMoionsdopedinsamples#3,#4and#5 are6þand4þ.Moreover,sincethebindingenergyofMo4+3d3/2is higherthanthatofMo6+_3d

5/2,asshowninTable2,theshiftofthe G2peaksofsamples#3,#4and#5towardhigherenergyindicates theincreaseofthe[Mo4+]/[Mo6+]contentratiowithincreasingthe Modopingconcentration.Theincreaseof[Mo4+_]/[Mo6+_]_content ratiocanalsobeconﬁrmedbytheincreaseoftheintensityofthe G3peakswithincreasingtheMoconcentration.

Fig.3displaysthedependenceofelectricalpropertiesincluding resistivity,carrierconcentration,andmobilityontheMocontent

10 20 30 40 50 60 70 80 90

#5

#4

#3

#2

Intensity (a.u.) 2θ (degrees)

#1

Fig.1.XRDpatternsofallﬁlms.Thebroadpeakslocatedatabout2u=228come fromtheglasssubstrates.Theﬁguresdenotethesamplenumber(listedinTable1). Table1

Thedcpower(P)appliedontheMotargetandMocontentratios(x=[Mo]/[In]at.%) ofthin-ﬁlmsamplesusedinthisstudy.Thesample#1istheundopedIn–Ga–Zn–O ﬁlm. Sampleno. #1 #2 #3 #4 #5 P(W) 0 2 3 4 5 x(at.%) 0 3.8 7.6 9.9 18.1 Table2

ThebindingenergyofG1,G2andG3peaksobservedinXPScurvesaswellasthose ofMo4+

3dandMo6+

3dlevelsadoptedfromRefs.[12,13].

Sampleno. G1(eV) G2(eV) G3(eV)

#1 – – – #2 235.6 232.2 – #3 235.9 232.5 230.0 #4 235.5 232.7 230.4 #5 235.3 232.9 230.4 a Mo6+ 3d3/2 235.8 a_Mo6+_3d 5/2 232.6 b Mo4+ 3d3/2 233.2 b Mo4+ 3d5/2 230.1 a Ref.[12]. b Ref.[13]. 224 226 228 230 232 234 236 238

#1

#2

#3

#4

Intensity (a.u.)

Binding Energy (eV)

G3 G2 G1

#5

Mo 3d

Fig. 2.XPS spectra intheMo 3dregionof theMo-doped a-IGZOﬁlms. The smoothinglines(dashlines)areobtainedbymeansofcurveﬁttingusingGaussian functionsandthebackgroundsarenotsubtracted.G1,G2andG3denotedasthe locationsofGaussianpeakswhichcomprisethesmoothingcurvesintheMo3d region.

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ratio[Mo]/[In]ofMo-dopeda-IGZOfilm.AsshowninFig.3(a),the carrierdensity(n)ofsample#2isabout13.61019_cm3_,_which_is muchhigherthanthatoftheundopedfilmwhosevalueofnis 2.71019_cm3_._The_increase_of_carrier_density_can_be_attributed totheextraelectronsprovidedbyMo6+dopedina-IGZOfilms.On theotherhand,carrierdensitiesof samples#3,#4and #5are 3.71019_, _3.4₁₀19 _and _1.7₁₀19_cm3_, _{respectively,} _which decreasewithincreasingtheModoping.Thecarriermobilitiesof filmsareshowninFig.4(b).Itisnoteworthythat,incomparison withtheundopedfilm(sample#1),thecarriermobilityofsample #3isdistinctlyenhanced.Theenhancementofthecarriermobility similartothatobservedin Mo-dopedIn2O3 films[9,10]canbe explained by the magnetic interactions resulting in smaller effectivemass ofcarriersand largerfundamentalbandgap [11]. Exceptforthesample#3,thecarriermobilitydecreaseswiththe Modoping.ItcanbeattributedtotheincreaseofthedensityofMo ionswhichserveasstrongscatteringcentersandthussuppressthe transportofchargecarriers.Thelowcarriermobilityofsample#2, comparedtothatofsample #3,is attributedtothefree-carrier scattering,sincethecarrierdensityofsample#2ismuchhigher thanthatofothersamples.Theelectricalexperimentsrevealthat thecarrierdensityandmobilityofa-IGZOfilmscanbeenhancedby Modopingwithaproperconcentration. Ontheotherhand,the resistivitiesoflowconcentrationMo-dopedfilms(samples#2and #3) are lower than that of the undopedfilm (sample #1) but increasewithincreasingMoconcentration,asshowninFig.3(c), which is resulted from the decrease of carrier mobility with increasingtheMoconcentration(exceptforthesample#3).

ThetransmissionspectraofallﬁlmsareillustratedinFig.4(a). Thetransmissionoftheundopedﬁlm(sample#1)exceed80%in

the visible range (400–700nm); however, the transmission decreaseswithincreasingtheModoping.Moreover,the absorp-tionedgeofsample#2,thea-IGZOfilmdopedwith3.8at.%Mo, shiftstowardshorterwavelengthregion.Theopticalbandgaps(Eg) of these films can be estimated by the relationship between absorption coefficient (

a

) and photon energy (h

n

) of the form (

a

h

n

)(h

n

Eg)r with r=2 suggested by Tauc for amorphous semiconductors[14,15].TheEgofsamples#1,#3and#4isabout 2.95eVobtainedbylinearextrapolationof(

a

h

n

)0.5_to_the_h

_n

_-axis, asdepictedinFig.4(b).TheEgofsample#2isabout3.05eVwhich isslightlyhigherthanthatoftheundopedsample#1andcanbe attributed to the Burstein–Moss (BM) effect, since the carrier densityofsample#2ismuchhigherthanthatofthesample#1. ThesameEgofsamples#1,#3and#4andBMeffectobservedfor sample #2 imply the incorporation of Mo atoms in the IGZO lattices.Ontheotherhand,theEgofsample#5is2.7eVwhichis believedtoberesultedfromthehighconcentrationofModoping whichmayresultinthepossibleexistenceofMooxideimpurities. Fig.5showsthefielddependenceofmagnetizationM(H)curves measuredat300KfortheundopedandMo-dopeda-IGZOfilms. Theundopedfilm(sample#1)showsalinearM(H)curvewhich indicates paramagnetic behavior. All Mo-doped filmsobviously exhibitroom-temperatureferromagnetism.Moreover,exceptthe sample#3,themagnetizationincreaseswithincreasingtheMo doping.AccordingtoMedvedeva’sstudy,theexchangesplittingof d states results in the magnetic moments of Mo atoms [11]. However,Medvedeva’scalculationssuggestaveryweakmagnetic couplingbetweenModopantsandferromagnetismwhichwould notbeobservedinMo-dopedIn2O3.Itisnotconsistentwithour experimental results and similar ferromagnetism in Mo-doped In2O3filmswasalsoobserved[16].Theferromagnetismobserved in this workcouldbe attributed tothe free-electron mediated ferromagneticinteraction[17,18]betweenthemagneticMoions.

0 2 4 6 8 10 12 14 16 18 1E-3 0.01 0.1 1

(c)

ρ

(

Ω

-cm)

x

(at. %)

0 2 4 6 8 10 12 14 16 18 0 5 10 15 20 25

_(b)

μ

(cm

2

/ Vs)

0 2 4 6 8 10 12 14 16 18 0 5 10 15

#4

#5

#3

#2

#1

n

(10

19

cm

-3

)

(a)

Fig.3.Electricalpropertiesincluding(a)carrierdensityn,(b)carriermobilitymand (c)resistivityrofxat.%Mo-dopeda-IGZOﬁlms.xat.%=[Mn]/[In].

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 500 1000 1500 #5 (x=18.1) #4 (x=9.9) #3 (x=7.6) #2 (x=3.8) #1 (x=0)

(

α

h

ν

)

0.5

(cm

-1

eV)

0.5

Pho

ton

Ene

rgy

(eV)

300 400 500 600 700 800 900 0.0 0.2 0.4 0.6 0.8 1.0 #4 #3 #1 #5 #2

Transmission

Waveleng

th (nm

)

Fig.4.(a)Transmissionspectraand(b)(ahn)0.5_vs_h_n_plots_for_x_at.%_Mo-doped

a-IGZOﬁlms.

S.-J.Liuetal./MaterialsResearchBulletin47(2012)1568–1571 1570

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Thecorrelationbetweenthehighmagnetizationandhighmobility observed for sample #3 supports the free-electron mediated model.

4. Conclusions

Inconclusion,thevalenceofMoionsis6þinlowconcentration Modopeda-IGZOﬁlmsandbecomesamixtureof6þand4þwith increasingtheModopinglevel.Thecarriermobilityand carrier densityof a-IGZO ﬁlmscanbeenhanced by Modopingwitha

properconcentration.Theopticaltransmissionofa-IGZOfilmsis decreasedbytheModoping.Nevertheless,theopticalbandgapis notsignificantlyaffectedbytheModoping.However,ablueshift in transmissionspectraresultedfromBurstein–Mosseffectwas observedinlow-concentrationModoping.Furthermore,all Mo-doped films exhibit room-temperature ferromagnetism. The carrier mobilityofthe Mo-dopedsample exhibiting thelargest magnetizationisremarkablyenhancedbyModoping.Theresult reveals thecorrelationbetween theenhancementofthecarrier mobilityandferromagneticcharacteristics.

Acknowledgment

ThisworkwassupportedbytheNationalScienceCouncilof Taiwan,underGrantNo.NSC98-2112-M-003-005-MY3. References

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[14]Tauc,J. AmorphousandLiquidSemiconductors, Plenum,NewYork,1979. [15]Jayaraj,M.K.Saji,K.J.Normura,K.Kamiya,T.Hosono,H. J.Vac.Sci.Technol.B26

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-5 -4 -3 -2 -1 0 1 2 3 4 5 -4 -3 -2 -1 0 1 2 3 4

#1

(x = 0)

#2

(x = 3.8)

#3

(x = 7.6)

#4

(x = 9.9)

#5

(x =18

.1)

Magnetization (emu/cm

3

)

Magneitc Field (kOe

)

Fig.5.Field(H)dependentmagnetization(M)ofxat.%Mo-dopeda-IGZOﬁlms measuredatroomtemperature.