Structure optical and electrical properties of ind (2)

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Structure, optical and electrical properties of indium tin oxide ultra thin films prepared by jet nebulizer spray pyrolysis technique

M. Thirumoorthi & J. Thomas Joseph Prakash

To cite this article: M. Thirumoorthi & J. Thomas Joseph Prakash (2016) Structure, optical and electrical properties of indium tin oxide ultra thin films prepared by jet nebulizer spray pyrolysis technique, Journal of Asian Ceramic Societies, 4:1, 124-132, DOI: 10.1016/j.jascer.2016.01.001 To link to this article: https://doi.org/10.1016/j.jascer.2016.01.001

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Structure, optical and electrical properties of indium tin oxide ultra thin films prepared by jet nebulizer spray pyrolysis technique

M. Thirumoorthi

^a

, J. Thomas Joseph Prakash

^b,∗

aDepartmentofPhysics,H.H.TheRajah’sCollege(AffiliatedtoBharathidasanUniversity),Pudukkottai622001,India

bDepartmentofPhysics,GovernmentArtsCollege(AffiliatedtoBharathidasanUniversity),Trichy620022,India

a r t i c l e i n f o

Articlehistory:

Received3November2015 Receivedinrevisedform 27December2015 Accepted5January2016 Availableonline20January2016

Keywords:

ITOthinfilms

Nebulizerspraypyrolysis X-raymethods Surfacesroughness Electricalproperties

a b s t r a c t

Indiumtinoxide(ITO)thinfilmshavebeenpreparedbyjetnebulizerspraypyrolysistechniquefor differentSnconcentrationsonglasssubstrates.X-raydiffractionpatternsrevealthatallthefilmsare polycrystallineofcubicstructurewithpreferentiallyorientedalong(222)plane.SEMimagesshowthat filmsexhibituniformsurfacemorphologywithwell-definedsphericalparticles.TheEDXspectrumcon- firmsthepresenceofIn,SnandOelementsinpreparedfilms.AFMresultindicatesthatthesurface roughnessofthefilmsisreducedasSndoping.TheopticaltransmittanceofITOthinfilmsisimproved from77%to87%invisibleregionandopticalbandgapisincreasedfrom3.59to4.07eV.Photolumi- nescencespectrashowmainlythreeemissionspeaks(UV,blueandgreen)andashiftobservedinUV emissionpeak.ThepresenceoffunctionalgroupsandchemicalbondingwasanalyzedbyFTIR.Halleffect measurementsshowpreparedfilmshavingn-typeconductivitywithlowresistivity(3.9×10⁻⁴-cm) andhighcarrierconcentrations(6.1×10²⁰cm⁻³).

©2016TheCeramicSocietyofJapanandtheKoreanCeramicSociety.Productionandhostingby ElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

Indiumtinoxide(ITO)isawellknownn-typetransparentcon- ductingoxidematerial.Heretinactsasacationicdopantinthe In₂O₃latticeandasasubstituteontheindiumsitestobindwith theinterstitialoxygen.Duetoitshighopticaltransmittance,electri- calconductivityandwidebandgap(>3.5eV),ITOhasbeenwidely appliedinvariousoptoelectronicdevicessuchasphotovoltaiccells [1],liquidcrystaldisplays[2]andgassensors[3].TheITOthinfilms arecommonlyfabricatedbyemployingdifferenttechniquessuch asmagnetron sputtering [4–8],sol–gelprocess [9–11],thermal evaporation[12],pulsedlaserdeposition[13,14],chemicalvapor deposition[15,16], spray pyrolysis[17–22]and nebulizer spray pyrolysis(NSP)[23].Allofthesemethods haveadvantages and disadvantages,butjetnebulizerspraypyrolysishasanoticeable advantage;itisa low-costandnon-vacuumtechniqueforlarge areaapplicationsandcanproducehighqualityfilmwithlowpre- cursorvolume.TheworkingofNSPmethodisbasedontheBernoulli principle;i.e.,whenapressurizedflowofairisdirectedthrougha

∗Correspondingauthor.Tel.:+919842470521.

E-mailaddress:armyjpr1@yahoo.co.in(J.ThomasJosephPrakash).

PeerreviewunderresponsibilityofTheCeramicSocietyofJapanandtheKorean CeramicSociety.

constrictedorifice,thevelocityoftheairflowisincreasedtocre- ateajetstream.Theimpactofajetstreamwithliquidproduces aerosolparticles(particlesize∼2.5␮m)[24].Themistformofsolu- tionishelpingtoimprovethequalityoffilmandtoobtainauniform growthduetogradualnucleationwithminimumwastage.Inthe presentstudy,thetindopedindiumoxidethinfilmswereprepared byasimpleandlow-costjetnebulizerspraypyrolysistechnique.

Thestructure,surfaces,optical,photoluminescenceandelectrical propertiesofpreparedfilmswereinvestigatedindetail.

2. Experiment

A jet nebulizer spray pyrolysisapparatus (Fig.1)is used in this work,which consistsofa jet nebulizerspraying unit, sub- strateholderwithheaterandaircompressor.Topreparetindoped indium oxidethin films, theindium(III)chloride (InCl₃)isdis- solvedin 100mLdouble distilled water tomake 0.4Mstarting solution.Tindopingwasachievedbyaddingtin(II)chloridedihy- drates(SnCl₂·2H₂O)tothestartingsolution.Afewdropsofacetic acidwereaddedtoobtainaclearandhomogeneoussolution.The dopinglevelinthesolutionwasvariedfrom0to30wt%insteps of10wt%.Themixturewasstirredunderconstantspeed for1h witha magneticstirrer.Priortothedeposition,glasssubstrates (1in.²) were cleaned with acetone, isopropyl alcohol, and dis- tilledwatersuccessivelyfor15mininultrasonicator.Thesubstrate http://dx.doi.org/10.1016/j.jascer.2016.01.001

2187-0764©2016TheCeramicSocietyofJapanandtheKoreanCeramicSociety.ProductionandhostingbyElsevierB.V.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Nomenclature

D Crystallitesize(nm) k Scherrer’sconstant d Latticespacing(Å) h,k,l Millerindices TC Texturecoefficient I_(hkl) Peakintensity N Numberofpeaks T Transmittance(%) h Photonenergy(eV) E_g Opticalbandgap(eV) n Carrierconcentrations(cm⁻³) ˇ Fullwidthathalfmaximum(rad) WavelengthofX-ray(Å)

Bragg’sangle(deg)

ε Strain

˛ Absorptioncoefficient Resistivity(-cm) Mobility(cm/(Vs))

temperatureforeachdepositionwaskeptat500^◦Cintheairatmo- sphere.Thepreparedsolutionwassprayed witha jetnebulizer (HUDSONRCImicromist,dropletsizeis∼2.7␮m)ontheheated substratewithsprayrate0.5mL/minusingcompressedairasacar- riergas.Thenebulizerwaskeptatadistanceof5cmfromsubstrate surface.

ThestructuralparametersofspraycoatedITOfilmswereana- lyzedbyX-raydiffractometer(XRD)usingthePANalyticalsystem withCu K␣1 radiation (k=1.54056 ˚A). Surface morphology and topographywerecarriedoutbythescanningelectronmicroscope (ESEMQUANTA200,FEI-Netherlands)andatomicforcemicroscopy (Agilent5500)respectively.Elementalanalysiswasmadebyusing energydispersiveX-rayspectroscopy(attachedtoSEM).Thewater contact angle measurement was made by using a protractor frommicrophotograph.Theopticalpropertiesofthefilmswere examinedwithadoublebeamspectrophotometer(Oceansoptics HR2000-USA)intheUV–visregions.Thefilmthicknesswasmea- suredbyaprofilometer(SJ-301Mitutoyo).Thephotoluminescence (PL)spectrawererecordedusingaspectrofluorometer(CaryEclipse EL08083851)withxenonarclamp.TheIRspectrumwasrecorded

Fig.1.Schematicdiagramofjetnebulizerapparatus.

80 70 60

50 40

30 20 0 500 1000 1500 2000 2500 3000 3500

(800)

(444)(622)

(611)

(440)(521)

(431)

(411)(400)

(222)

(211)

I n t e n s i t y ( a.u )

2 θθ ( d e g r e e )

0 wt%

10 wt%

20 wt%

30 wt%

Fig.2. X-raydiffractionpatternsofITOthinfilmsfordifferentSnconcentrations.

32.0 31.5 31.0 30.5 30.0 29.5 29.0 0 500 1000 1500 2000 2500 3000 3500

(222)

I n t e n s i t y ( a.u )

2 θθ ( d e g r e e )

0 wt%

10 wt%

20 wt%

30 wt%

Fig.3.ShiftofpeakpositionofITOthinfilmsalong(222)planefordifferentSn concentrations.

usingFTIRspectrophotometer(PerkinElmer–RXI)intherange of400–4000cm⁻¹.Theelectricalparameterswerecollectedfrom roomtemperatureHalleffectmeasurements(RH2035 PhysTech GmbH)system.

3. Resultsanddiscussion 3.1. Structuralanalysis

X-raydiffractionpatternsareusedtostudythecrystalstruc- tureofpreparedITOthinfilms.Fig.2showstheX-raydiffraction patternsofindiumtinoxidethinfilmsfordifferenttinconcen- trationsonglasssubstrates.Itcanbeseenthatallthefilmsare polycrystallineinnatureandcrystallizeinacubicstructure(JCPDS:

71-2194)withpredominant(222)peak.Aswitchingintheprefer- entialgrowthfromthe(400)to(222)planeswasobservedwhen tindopedwithindiumoxide.Theincreasingintensityofthe(222) planeisattributedtotheincreaseinthedegreeofpreferentialcrys- talorientation.ItisevidentfromtheXRDspectrathatnodiffraction peaksofSnorotherimpurityphasesaredetectedintheprepared samples.AsshowninFig.3shiftofthe(222)peaktowardsmaller2

Table1

StructuralparametersofpreparedITOthinfilmsfordifferentSnconcentrations.

Sample 2(deg) hkl FWHM(deg) d-space(Å) Crystallitesize (nm)

Dislocation density,ı (×10¹⁴lines/m²)

Strain,ε (×10⁻⁴)

Lattice constant(Å)

TC

Observed Standard

0wt%

21.47 211 0.13 4.1354 4.1302 62.19 2.5855 5.573 10.1296 0.7347

30.52 222 0.11 2.9266 2.9205 74.85 1.7849 4.631 10.1381 0.6047

35.37 400 0.12 2.5356 2.5292 69.49 2.0708 4.988 10.1424 3.6168

37.67 411 0.16 2.3859 2.3846 52.45 3.6350 6.609 10.1225 1.0069

45.60 431 0.15 1.9877 1.9841 57.47 3.0298 6.034 10.1353 0.9044

49.22 521 0.30 1.8497 1.8471 29.12 11.793 11.90 10.1312 0.5721

50.94 440 0.12 1.7912 1.7884 73.34 1.8598 4.726 10.1325 1.4156

55.92 611 0.26 1.6429 1.6411 34.59 8.3579 10.02 10.1275 0.7312

60.57 622 0.16 1.5274 1.5252 57.48 3.0266 6.030 10.1316 0.1023

63.68 444 0.27 1.4601 1.4602 34.63 8.3386 10.01 10.1158 0.2796

75.02 800 0.19 1.2651 1.2646 52.71 3.5992 6.576 10.1201 1.0416

Average 54.3927

10wt%

30.47 222 0.11 2.9313 2.9205 74.84 1.7853 4.632 10.1543 1.5071

35.32 400 0.17 2.5391 2.5292 49.03 4.1592 7.070 10.1564 1.1216

50.87 440 0.19 1.7935 1.7884 46.28 4.6688 7.489 10.1455 0.6831

60.47 622 0.21 1.5297 1.5252 43.76 5.2221 7.920 10.1468 0.6883

Average 53.4775

20wt%

30.42 222 0.11 2.9360 2.9205 74.84 1.7853 4.631 10.1706 1.4303

35.42 400 0.13 2.5391 2.5292 64.13 2.4315 5.405 10.1564 1.3201

50.80 440 0.13 1.7958 1.7884 67.64 2.1857 5.124 10.1585 0.6223

60.35 622 0.17 1.5324 1.5252 54.11 3.4154 6.413 10.1647 0.6271

Average 65.18

30wt%

30.37 222 0.11 2.9407 2.9205 74.85 1.7849 4.631 10.1868 1.9836

35.25 400 0.11 2.5440 2.5292 75.78 1.7413 4.574 10.1761 1.0333

50.79 440 0.12 1.7961 1.7884 73.31 1.8606 4.728 10.1602 0.6046

60.38 622 0.13 1.5397 1.5252 70.72 1.9994 4.901 10.2132 0.4345

Average 73.665

valueisduetothereplacementofsmallerradiusSn⁴⁺ion(0.71 ˚A) inindiumsitesandalsorelatedtochangesofstraininthecrystal lattice[25].

ThecrystallitesizewascalculatedfromtheXRDpatternusing Debye-Scherrerformula[26]:

D= 0.9

ˇcos, (1)

whereisX-raywavelength(=1.54060 ˚A),ˇisfullwidthathalf maximuminradianandisBragg’sangle.

Thedislocationdensity(ı)andstrain(ε)werecalculatedusing thefollowingequations[26]:

ı= 1

D² (2)

ε= ˇcos

4 (3)

Texturecoefficient(TC)measurestherelative degreeof pre- ferredorientationamongcrystalplanes,whichisobtainedfrom followingexpression[27]:

TC_(hkl)=

I(hkl)/Io(hkl)

N⁻¹

N(I(hkl)/Io (hkl))

(4)

whereI_(hkl)isthemeasuredintensityoftheplane(hkl),I_o(hkl)isthe standardintensityoftherespectivediffractionplane,accordingto theJCPDSdatacard(71-2194)andNisthenumberofdiffraction peakspresented.

ThecalculatedstructuralparametersofpreparedITOthinfilms using the above equations are listed in Table 1. The observed improvement in average crystallite size is attributed to strain formedinthenanocrystal.Thedislocationdensity(ı)istomea- surethedisorderoflatticeplanesinthecrystalstructure.Thestrain arisesduetopointdefects(vacancies,sitedisorder),dislocations

andextendeddefectsinthecrystalstructures.Thecalculatedval- uesofthelatticeconstant‘a’areingoodagreementwithstandard values (71-2194). Obtained lattice constant values are slightly increasedfor(222)planewithincreaseddopinglevel,whichcan berelatedtouniform-strainofthegrains.Thetexturecoefficient TC(hkl)valuescalculatedforthedifferentplanesofthefilmsare showninTable1.Theresultsindicatethatthepreferentialorienta- tionforpureindiumoxidefilmalong(400)plane,isshiftedto(222) planeforSndopedindiumoxidefilms.Thechangeofpreferredori- entationmaybeduetooccupancyofadditionalindiumvacancy sitesbytinatomswhichareunoccupiedpreviously.Infact,itis wellknownthatIn₂O₃thinfilmshaveseveraldefectlevelssuchas indiuminterstitialoxygenandindiumvacancies[28].Theincrease inpreferredorientationisassociatedwiththeincreaseofcrystallite growthalongthatplane.Thechangeinpeakintensity,d-value,lat- ticeconstantsandstrainconfirmsthesubstitutionofSnintoIn–O lattice.

3.2. Surfacemorphologicalanalysis

Forthinfilmsthesurfacemorphologydependsonthedeposition techniqueanditsparameters.Thesurfacemorphologymayinflu- encethefilmpropertiessuchasmechanical,electricalandoptical properties.Fig.4showsthescanningelectronmicroscopicimages ofITOfilmsfordifferentSnconcentrationsdepositedonglasssub- strates.It canbeseenthat allthefilmshave auniformsurface morphologyconsistingofsphericalparticleswithoutanyvoidsand cracks.Theloweringgrainboundaryandlargeactivesurfacehelp toimprovetheelectricalconductivityandopticaltransmittance.

Energy dispersive X-ray spectroscopy (EDX) is an analytical techniqueusedfortheelementalanalysisofasample.TheEDX spectraofthepreparedITOthinfilmsareshowninFig.5.Thespec- trarevealthatthepresenceofIn,OandSnelementsinthedeposited

Fig.4. Scanningelectronmicroscopyimages(60,000×magnification)ofpreparedITOthinfilms.

films.Theweightpercentageisalmostnearlyequaltotheirnominal stoichiometrywithintheexperimentalerror.

Topography is one of the most important physical charac- teristics of surfaces, which influence theirsignificant technical properties.Fig.6showsthe3D-AFMimagesofpreparedITOthin filmsfordifferentSnconcentrations.Theun-dopedindiumoxide

filmhasalargedifferencebetweenpeakandvalleythanSndoped films. As shown in Table 2 the surface profile parameters are changedwithincreasingSnconcentration.Thesurfaceroughness (Sa)androotmeansquare(RMS)(Sq)valuesarereducedasincreas- ingSnconcentrations.TheobtainedRMSvalueofITOfilmsislower thanthepreparedonebyothertechniquessuchasink-jetprinting

Fig.5.EnergydispersiveX-rayspectraofITOthinfilms.

Fig.6. AFM3DimagesofpreparedITOthinfilmsfordifferentSnconcentrations.

[29],RFsputtering[30],chemicalvapordeposition[31]andspray pyrolysis[32].Theaverageroughnessisthemeanvalueofpeak andvalleyascalculatedoverthemeasuredentiresurfacearea.It isusefulfordetectinggeneralvariationsinoverallprofileheight characteristics.Rootmeansquareroughnessisthesquarerootof thedistributionofsurface heightandis consideredtobemore sensitivethantheaverageroughness.Itrepresentsthestandard deviationoftheprofileheightsandisusedinthecomputations ofskewnessandkurtosis.Surfaceskewness(Ssk)isusedtomea- surethesymmetryofthevariationsofsurfaceaboutthemeanline andismoresensitivetooccasionaldeepvalleysorhighpeaks.Sur- facekurtosis(Sku)isusedtomeasurethedistributionofthespikes aboveandbelowthemeanline.Theroughnessreductionofthe filmshelpstoreducethescatteringofincidentlightandleadingto increaseofopticaltransmittance.

Fig. 7 shows the microphotographs of water droplet on the ITO surface, and thus it can be perceived that the water contact angles (127^◦, 114^◦, 106^◦ and 92^◦ for 0, 10, 20 and

30wt%respectively)aredecreased.Thisresultindicatesthatthe hydrophilicities of ITO surface are increased with Sn doping concentrations,andsuchabehaviorisduetoreductionofsurface roughness.

3.3. Opticalstudies

The optical properties of thin films are known to depend stronglyonthefilmthickness,microstructures,levelsofimpurities anddepositionparameters.Fig.8showstheopticaltransmittance andtheabsorptionspectrumofpreparedITOthinfilmsfordifferent dopingconcentrations.Itisfoundthattheaveragetransmittance ofthepureindiumoxidefilmis77%andthatoftheSndopedfilms couldbeintherangeof82–87%,andsuchresultsmayberelatedto lowscatteringoflightandthickness.AsshowninFig.8(inset)the absorptionedgeisshiftedtolowerwavelengthforSndopedindium oxidefilms.Thetransmissionspectrashowjusttheoppositetrend oftheopticalabsorptionspectra.Theabsorptionofallthefilmsis

Table2

HeightparametersofITOthinfilmobtainedfromAFManalysis.

Parameters 0wt% 10wt% 20wt% 30wt%

Rootmeansquare(Sq)(nm) 20.2 15.3 11 12

SurfaceSkewness(Ssk) 0.455 0.244 0.468 0.106

Coefficientofkurtosis(Sku) 5.21 3.27 4.25 2.85

Maximumpeakheight(Sp)(nm) 137 63.5 55.7 47.2

Tenpointmeanheight(Sz)(nm) 431 119 99.5 82.8

Averagesurfaceroughness(Sa)(nm) 17.6 12.1 8.29 9.58

Filmthickness(nm) 96 43 37 32

Fig.7.MicrophotographsofwaterdropletonITOsurface.

1100 1000 900 800 700 600 500 400 300 0 10 20 30 40 50 60 70 80 90 100

0.0 0.5 1.0 1.5 2.0 2.5 3.0

450 440 430 420 410 25 400 30 35 40 45 50 55 60 65 70

T r a n s m i t t a n c e ( % )

W a v e l e n g t h ( n m )

T r a n s m i t t a n c e ( % )

W a v e l e n g t h ( n m )

0 wt%

10 wt%

20 wt%

30 wt%

A b s o r p t i o n ( % )

Fig.8. UV–vistransmittanceandabsorptionspectrumofITOthinfilms.Inserted imageshowstheblueshiftofabsorptionedges.

intherangeof0.04–1.10%inthevisibleregion,thelowabsorption ofincidentlightisessentialforTCO.

Theopticalbandgap(Eg)values calculated fromtheplotof (˛h)²versus(h)isshowninFig.9(a)–(d)forthepreparedfilms.

Theabsorptioncoefficient(˛)iscalculatedfromthefollowingrela- tion[26]:

˛=

₁

d

ln

₁

T

(5) wheredisthicknessofthefilmsandTistransmittance.Theabsorp- tioncoefficient(˛)andincidentphotonenergy(h)isrelatedbythe followingrelation[26]:

(˛h)²=A(h−Eg) (6)

whereAisaconstant,hisphotonenergyandEgisopticalbandgap.

ObtainedbandgapsareplottedasafunctionofSnconcentrations thatareshowninFig.9(e).Itisobservedthatbandgapincreases rapidlyfrom3.59eV(0wt%)to4.07eV(10wt%)andthendecreased slightlyto4.01eV(30wt%).Similarbehaviorofbandgapwidening andthennarrowingforhigherdopingoftinisreportedpreviously byBenamaretal.[20].Thewideningoftheopticalbandgapis relatedtoincreasedcarrierconcentrationandwhichisexplained byusingMoss–Bursteineffect[33].Thenarrowingofbandgapfor higher Sndopingmaybe duetomany-body interactioneffects eitherbetweenfreecarriersorbetweenfreecarriersandionized impurities[34].

3.4. Photoluminescencestudies

Theroomtemperaturephotoluminescence (PL)spectroscopy techniqueisaselectiveandextremelysensitiveprobeofdiscrete electronicstates. Fig.10 shows theroomtemperaturePL spec- tra of ITOthin filmsrecorded underthe excitationwavelength =310nm.Mainlythreeemissionpeaksareobservedasfollows:

a strongUVemission peak (P₁ at363nm),a strongblueemis- sionpeak(P₂at493nm)andaweakgreenemissionpeak(P₃at 521nm).TheUVemissionpeakisalsocallednearbandedge(NBE) emission,anditoriginatesduetotherecombinationofthefreeexci- tonthroughanexciton–excitoncollisionprocess[35].Asshownin Fig.10(inset)achangeobservedintheUVemissionspeakindicated thattherecombinationcentersaremodifiedbySndoping.Ablue emissionbandindicatedthatanewdefectlevelisintroducedinto thebandgapbytheSndoping[36].Theoriginofgreenemission isgenerallyascribedtodeepleveldefectssuchassurfacedefects andsinglyionizedoxygenvacancies[37].Arapiddecreaseinthe intensityofallpeaksandthechangeofUVemissionpeakposition confirmthesubstitutionofSnatominindiumsites.

0 5 10 15 20 25 30 3.6

3.7 3.8 3.9 4.0 4.1

(e)

E g ( e V )

Sn doping (wt%)

2.62.83.03.23.43.63.84.04.24.44.64.85.0 2.50E-022

5.00E-022 7.50E-022 1.00E-021 1.25E-021 1.50E-021 1.75E-021 2.00E-021 2.25E-021 2.50E-021

(d)

(

αα

h

νν

)

2

(e V /c m )

2

h

νν

(eV)

30 wt%

2.62.83.03.23.43.63.84.04.24.44.64.85.0 5.00E-022

7.50E-022 1.00E-021 1.25E-021 1.50E-021 1.75E-021 2.00E-021 2.25E-021 2.50E-021 2.75E-021

(c)

(

αα

h

νν

)

2

(e V /c m )

2

h

_νν

(e V)

20 wt%

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0 5.00E-022

7.50E-022 1.00E-021 1.25E-021 1.50E-021 1.75E-021 2.00E-021 2.25E-021

(b)

(

αα

h

νν

)

2

(e V /c m )

2

h

_νν

(eV)

10 wt%

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 2.00E-022

4.00E-022 6.00E-022 8.00E-022 1.00E-021 1.20E-021 1.40E-021 1.60E-021 1.80E-021

(a)

(

αα

h

νν

)

2

( eV /c m )

2

h

_νν

(eV)

0 wt%

Fig.9. (a–d)The(˛h)²versushplotsofITOthinfilmsfordifferentSnconcentrations;(e)variationsofopticalbandgapasfunctionofSnconcentrations.

550 500

450 400

350 1 2 3 4 5 6 7 8

390 385 380 375 370 365 360 355 350 345 340 335 330 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

5.5

P1

I n t e n s i t y (a.u)

W a v e l e n g t h (nm)

P3 P2 P1

0 wt%

10 wt%

20 wt%

30 wt%

I n t e n s i t y (a.u)

W a v e l e n g t h (nm)

Fig.10.Roomtemperaturephotoluminescence(PL)spectrumofITOthinfilms;

(insert)theobservedchangeofUVemissionspeaksposition.

3.5. FTIRanalysis

FTIRtechniqueisusedtoobtaininformationaboutthechem- icalbondingand thepresenceof certainfunctionalgroups in a

500 1000 1500 2000 2500 3000 3500 4000 20 30 40 50 60 70 80 90 100

433

592

885

1519 1827 2225 0 wt%

10 w t%

20 w t%

30 w t%

T r a n m i t t a n c e ( % )

W a v e n u m b e r ( cm^-1 )

Fig.11.FTIRspectraofITOthinfilmsfordifferentSnconcentrationsinwavenumber rangefrom400to4000cm⁻¹atroomtemperature.

material. Fig. 11 shows the FTIR spectra of prepared ITO thin films for different Sn concentrations. The broad band around 3500–1900cm⁻¹ is attributedto theO H stretchingvibrations ofhydroxylsfromabsorbingwater molecules[38].Wecanalso recognizethestrongabsorptionbandat1827cm⁻¹isrelatedto

0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 0.0009 0.0010

30 25 20 15 10 5

0 2.0

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

30 25 20 15 10 5 0

30 32 34 36 38 40 42 44 46 48 50 52

R e s i s i t i v i t y (ρ) (ohm-cm)

Sn d o p i n g ( wt% )

Carrier concentrations (n) (1020 cm-3 ) ρρ n μμ

M o b i l i t y (μ) (cm2 /Vs)

Fig.12.ElectricalparametersofpreparedITOthinfilmsasfunctionofSnconcen- trations.

C Ostretchingvibrationof carboxylicacids,amediumbandat 1519cm⁻¹ isrelatedtoC Cstretchingvibrationofarenesanda mediumbandat885cm⁻¹isascribedtoC Hbendingvibration.

Theweakbandsat592cm⁻¹and433cm⁻¹areattributedtoIn O bond[39].

3.6. Electricalstudies

Fig.12showsthemeasuredelectricalparametersofprepared ITOthinfilmsasafunctionofdifferentSnconcentrations.Itcan beseenthattheresistivityisdecreased,carrierconcentrationis increasedandmobility is increasedasincreasingSnconcentra- tions, and this behavioris desirable for transparentconducting oxide.Theresultsindicatethat thepreparedITOthin filmsare highlydegenerating n-typeconductivity.Theincrease ofcarrier concentrationvalueofITOfilmisduetothevalence difference betweenSn⁴⁺andIn³⁺ions,whichgeneratesextraonefreecar- rierperatomicsubstitutionandleadingtodecreaseofresistivity.

TheincorporationofSndopantinindiumoxidefilmschangesthe overallelectricalpropertiessignificantly.Weobtainedaminimum electricalresistivity(3.9×10⁻⁴-cm)andmaximumcarriercon- centration(6.1×10²⁰cm⁻³)for20wt%oftindopedindiumoxide thinfilm.Similarresultswerereportedinpreviousliteratureby Sekietal.[40].Theincreaseofmobilityisrelatedtoreductionof grainboundaryscattering.Theelectronictransportpropertyforthe ultrathinfilmsisentirelyrelatedtothemicrostructureofthefilms anddopingconcentration.

4. Conclusions

Transparentconductingindiumtinoxide(ITO)thinfilmswere successfullypreparedbyjetnebulizerspraypyrolysistechnique.

Effects of Sn dopingconcentrations onstructure, surface, opti- cal,photoluminescenceandelectricalpropertieswereinvestigated.

Thefollowingconclusionsarederivedaftertheseinvestigations:

•TheXRDpatternsrevealedthatpreparedITOfilmsarepolycrys- tallinein naturewithcubic structure. We observeda shiftof preferentialgrowthfrom(400)planeforpureindiumoxideto (222)planeforSndopedindium oxidethinfilms. Thechange observed in the peak position of (222) plane and structural parametersconfirmsthesubstitutionsofSninIn–Olattice.

•SEMimagesshowtheuniformdistributionofsphericalparticles andthesurfacemorphologyalsochangedwithSndoping.

•TheEDXspectrumrevealsthepresenceofIn,OandSnelements inthedepositedfilmswiththeirnominalpercentage.

•Accordingtothe3D-AFMimagesthesurfacetopographyofthe filmsisbetterthanundopedfilms.Theroughnessandrootmean squarevaluesarereducedasincreasedSnconcentrations.

•Opticaltransmissionsofthefilmshaveimprovedfrom77%to87%

andshowanoppositetrendoftheopticalabsorptionresults.The observedinitialblueshiftinenergybandgapfrom3.59to4.07eV canbeexplainedbytheBurstein–Mosseffect.

•Thedecreasingphotoluminescencepeakintensityandobserved shiftinUVemissionspeakpositionsisindicatingthesubstitution ofSninIn–Osystem.

•Thepresence offunctional groups andchemical bonding was confirmedbyFTIR.

•HalleffectmeasurementsshowthatSndopinginindiumoxide filmseffectivelyincreasesthecarrierconcentrationandreduces itsresistivitywithanimprovementinthemobility.

Fromthesefindings,weconcludethatITOthinfilmsaresuitable foroptoelectronicapplicationsandthejetnebulizerspraypyrolysis techniqueissuitableforproducinguniformthinfilmswithgood quality.

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