Annealing Effects on the Structural, Optical and … No. 2 2018...Keywords: Cds Thin Film, Chemical...
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International Journal of Nanoelectronics and Materials Volume 11, No. 2, Apr 2018 [221‐232] Annealing Effects on the Structural, Optical and Electrical Properties of Chemically Deposited CdS Thin Films using NH 4 Cl Complexing Agent 1 Rummana Matin, 1 M. S. Bashar*, 1 Munira Sultana, 2 Aninda Nafis Ahmed and 2 A. Gafur. 1 Institute of Fuel Research and Development (IFRD), Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka, Bangladesh. 2 Pilot Plant and Process Development Center (PP&PDC), Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka, Bangladesh. Received 14 September 2017; Revised 26 October 2017; Accepted 1 January 2018 ABSTRACT This study aims to deposit different thicknesses of CdS thin films with a variation of deposition time by using chemical bath deposition technique. The aqueous solution of cadmium chloride, ammonium chloride, ammonium hydroxide and thiourea were used During the deposition process. The analysis was done to observe the effect of annealing on structural, optical and electrical properties of the CdS thin films. From the x‐ray diffraction, this helps to confirm the cubic CdS phase formation, with a preferred orientation along (111). The finding indicates that the crystallite size is larger in films when it is deposited with low deposition rate. The crystallite size increased with increasing thickness and decreased after annealing. After annealing, the mobility and carrier concentration of the CdS thin films of different thicknesses is increasing and the resistivity decreasing. This study showed that the crystal structure, band gap, Urbach energy and electrical properties of CdS thin films can be changed when Cds thin films experienced annealing. Keywords: Cds Thin Film, Chemical Bath Deposition, Annealing Effect, XRD, Band Gap. 1. INTRODUCTION As a buffer layer in a solar cell, heterostructure cadmium sulfide (CdS) plays a significant role because of its properties as direct n‐type semiconductor with a band gap of about 2.4 eV and large absorption coefficient of 4×10 4 cm −1 [1]. Among various other promising cadmium‐free buffer layer; indium (III) sulfide (In 2 S 3 ), Zinc sulfide (ZnS), tin sulfide (SnS), tin oxide (SnO), etc, CdS showed highest efficiency to be used in solar cells [2]. Among nonvacuum or sol‐gel techniques; spray pyrolysis, spin coating, and dip coating, the chemical bath deposition (CBD) is known as most simple and low‐cost method and produces uniform, adherent and reproducible films. In addition, the CBD technique is a low temperature technique and can be used for CdS deposition into a wide range of substrates. Generally, the formation of CdS thin films by CBD is governed by two mechanisms, heterogeneous growth, i.e. ion by ion condensation of Cd +2 and S −2 ions on the substrate and homogeneous growth i.e. adsorption of colloidal particles on the substrate. At the earlier stage of deposition, the Cd +2 ions in the solution will form complexes with a complexing agent and NH 3 . At this stage, more free Cd +2 ions are available in the solution and these ions react with S −2 ions and produce a cluster type of CdS particles. Thereafter when the concentration of Cd +2 *Corresponding Author: [email protected]
Transcript of Annealing Effects on the Structural, Optical and … No. 2 2018...Keywords: Cds Thin Film, Chemical...
InternationalJournalofNanoelectronicsandMaterialsVolume11,No.2,Apr2018[221‐232]
AnnealingEffectsontheStructural,OpticalandElectricalPropertiesofChemicallyDepositedCdSThinFilmsusingNH4ClComplexing
Agent
1RummanaMatin,1M.S.Bashar*,1MuniraSultana,2AnindaNafisAhmedand2A.Gafur.
1InstituteofFuelResearchandDevelopment(IFRD),BangladeshCouncilofScientificandIndustrialResearch(BCSIR),Dhaka,Bangladesh.
2PilotPlantandProcessDevelopmentCenter(PP&PDC),BangladeshCouncilofScientificandIndustrialResearch(BCSIR),Dhaka,Bangladesh.
Received14September2017;Revised26October2017;Accepted1January2018
ABSTRACTThis study aims to deposit different thicknesses of CdS thin films with a variation ofdeposition time by using chemical bath deposition technique. The aqueous solution ofcadmium chloride, ammonium chloride, ammonium hydroxide and thioureawere usedDuringthedepositionprocess.Theanalysiswasdonetoobservetheeffectofannealingonstructural,opticalandelectricalpropertiesoftheCdSthinfilms.Fromthex‐raydiffraction,thishelps to confirm the cubicCdSphase formation,withapreferredorientationalong(111).Thefinding indicatesthatthecrystallitesize is larger in filmswhen it isdepositedwith low deposition rate. The crystallite size increased with increasing thickness anddecreasedafterannealing.Afterannealing,themobilityandcarrierconcentrationoftheCdS thin films of different thicknesses is increasing and the resistivity decreasing. Thisstudyshowedthatthecrystalstructure,bandgap,UrbachenergyandelectricalpropertiesofCdSthinfilmscanbechangedwhenCdsthinfilmsexperiencedannealing.Keywords:CdsThinFilm,ChemicalBathDeposition,AnnealingEffect,XRD,BandGap.
1. INTRODUCTION
Asabufferlayerinasolarcell,heterostructurecadmiumsulfide(CdS)playsasignificantrolebecauseofitspropertiesasdirectn‐typesemiconductorwithabandgapofabout2.4eVandlargeabsorptioncoefficientof4×104cm−1[1].Amongvariousotherpromisingcadmium‐freebuffer layer; indium(III) sulfide (In2S3),Zinc sulfide (ZnS), tin sulfide (SnS), tinoxide (SnO),etc,CdSshowedhighestefficiencytobeused insolarcells [2].Amongnonvacuumorsol‐geltechniques;spraypyrolysis,spincoating,anddipcoating,thechemicalbathdeposition(CBD)is known as most simple and low‐cost method and produces uniform, adherent andreproduciblefilms.Inaddition,theCBDtechniqueisalowtemperaturetechniqueandcanbeusedforCdSdepositionintoawiderangeofsubstrates.Generally, the formation of CdS thin films by CBD is governed by two mechanisms,heterogeneousgrowth, i.e. ionby ioncondensationofCd+2andS−2 ionsonthesubstrateandhomogeneousgrowthi.e.adsorptionofcolloidalparticlesonthesubstrate.Attheearlierstageofdeposition,theCd+2 ionsinthesolutionwill formcomplexeswithacomplexingagentandNH3.Atthisstage,morefreeCd+2ionsareavailableinthesolutionandtheseionsreactwithS−2ionsandproduceaclustertypeofCdSparticles.ThereafterwhentheconcentrationofCd+2*CorrespondingAuthor:[email protected]
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ionsgraduallydecreasesathigherNH3concentration,Cd+2ionsarestronglycomplexedbyNH3ions.Duetothis,thereisnofreeCd+2ionsareformedinthesolutionsothatfurtherformationofCdSparticlesisnotpossible.AtthisstageheterogeneousmechanismgovernsthedepositionofauniformthinlayerofCdS[3,4].PreviousstudiesfocusonobtaininghighqualityCdSthinfilmsbymanipulatingtheparametersof CBD method, such as deposition time, temperature, pH value and by varying theconcentrationsofvariousreagentsaswellastheircomplexingagents[4–9].ThehomogeneityandgrowthrateofCdSthinfilmsdependsontheuseofcomplexingagentduringchemicalbathdeposition.ItisreportedthatgoodqualityCdSthinfilmscanbegrownbyusingammoniaandammonium chloride as the complexing agent [5]. The anions from the cadmium andammoniumsaltsareclaimedtoplayasignificantrolewhengrowingtheCdSfilms.Thisroleissuspectedtobethatofacomplementarycomplexingagent[10,11].
There are several post deposition treatments such as annealing inN2/Air/H2 atmosphere atdifferent temperatures have been carried out to see their effect on structural and opticalpropertiesofCdSthinfilm.Maticiucetal.[12]foundthatdifferentpropertiesofCdSfilmaredependingon theneutral, reducingor oxidizing annealing gas.Oxygen in phases CdSO3 andCdOareobservedfromXRDpatternsforair‐andN2‐annealedCdSfilmsat400ºC.LongestN2annealing generated pure CdS layers, similarlywith the H2‐ annealed ones. Lichimura etal.[13]annealedCdSthinfilmsattemperaturesupto500°C.Thecubicphaseoftheas‐depositedfilm remains dominant until the annealing temperature reach higher than 400°C.When theannealingachieves450°C,theXRDpatternturnstothatofhexagonalphase.Thebandgapisdecreased by annealing done below 400°C and then abruptly increased by the annealing at450°C. Islam etal. [5] annealed the CBD processed CdS thin films for 30min at 400ºC and500ºC, respectively in a vacuum furnace with pressure 500mTorr. The cryatallinity of thefilmsincreasedwithincreasingannealingtemperature.Inthisstudy,thelow‐costCBDtechniqueisusedforthedepositionofCdSthinfilms.Ammoniaand ammoniumchlorideareused as the complexingagent. TheCdS thin filmsare grownatdifferentdepositiontimetoanalyzethefilmgrowth.Theas‐depositedfilmswereannealedinN2environmentat430ºCtoseetheannealingeffect.Theresultsfoundfromtheaboveanalysisare compared for as‐prepared and annealed CdS thin films of different thicknesses withrespecttothepreparationprocesses.2. EXPERIMENTALDETAILS
The CBD processed CdS thin films were grown onto sodalime glass substrates from thedecomposition of thiourea in an alkaline solution containing a cadmium salt and suitablecomplexingagentammonia(NH3)andammoniumchloride(NH4Cl).WhenCdCl2isusedasCdsource, NH4Cl is implemented as a buffer that controls the emission of ions during thedepositionprocess.Thecombinationhelpstoobtainastablecomplex.The substrateswere cleaned successively using ultrasonic bath and degreased inmethanol‐acetone‐methanol‐deionized (DI) water for 10 minutes. Following to this, the degreasedsubstrates is dried on a hotplate. CdS precursors were prepared with 0.0199 M of CdCl2,0.019835MofNH4Cland0.1423MofThioureainDIwaterforalltypesofCdSthinfilms.First,abeakercontainingDIwaterwasimmersedintheChemicalbath.TheprocedurecontinuesbyaddingCdCl2withDIwaterandwhilecontinuouslystirring,theNH4Clwasaddeddropbydropintothemixture.Thecleanedsubstrateswereimmersedinthebeakersvertically;thebeakerswerethencoveredandheated.At50ºCammoniawasmixedwiththesolution.TheThioureawasmixedwiththesolutionatthetemperatureof80ºC.Thebathtemperaturewasfixedat80ºCduringdepositionbecause,ithasbeendeducedthat,atlowsolutiontemperatureconditions
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(Ts < 60 ºC), the deposition mechanism can be achieved only with a lower activationtemperatureprocessi.e.theionbyionone.Whereasathighertemperatures(Ts>60ºC),thedeposition rate encompasses of larger activation energy, which lead the cluster by clusterprocessasthedominantgrowthmechanism.AnincreaseinthesolutiontemperatureinduceshigherhydroxideclusterCd(OH)2andsulfide ionconcentrations [6]. Inorder toensure thatthe pH is remained at 10, ammonia solution (28% of NH3 solution)was then added to thesolution.Afterthedepositionprocess,sampleswerewashedandultrasonicatedwithDIwaterfor1mintoremovethesurfaceimpurities,andthendriedonahotplate.Thedepositiontimewas varied from 20, 30 and 40minutes to obtain different thicknesses. The deposited thinfilms were annealed in an annealing chamber (MTI corporation, GSL– 1100X) in N2environmentunderadynamicvacuumofaround2Torrandat430ºCtemperaturefor1hour.The Surface Profilometer (DAKTAK‐150) is used to measure the thickness of the cadmiumsulfide(CdS)thinfilms.Later,theX‐raydiffractionanalysis(XRD)wasperformed,byaBrukerD8XRDwithcopperkαradiationtoinvestigatethecrystallinestructureofthematerials.Theabsorbance and transmittancewere recorded using a duel beam UV‐vis spectrophotometer(Shimadzu,UV‐1601V).Thisspectrometerwasusedtomeasuretherelativetransmittanceandabsorbance of CdS thin films. The sheet resistance was measured by a four‐point probemeasurementset‐upandthemobilityandcarrierconcentrationwasmeasuredbyaHalleffectmeasurementsetup(ECOPIA,HMS‐3000,SK).3. RESULTSANDDISCUSSIONS
3.1 DepositionRate
Figure1showsthechangeinthicknessofCdSthinfilmswithincreasingdepositiontime.Itisobserved that the thickness increases faster following slower increase with increasingdeposition time. It has been reported that heterogeneous process is highly desirable ratherthanhomogeneousdepositionforuniformthinfilms.Addingtothat,thehomogeneousprocessformsprecipitationofCdSinthesolutionresultingnon‐adherentpowderyfilms.Thereactionsforbothprocessesaregivenasfollows:Cd2++S2‐→CdS(heterogeneousnucleation)Cd(NH3)n2+↔Cd2++nNH3(homogeneousnucleation)nCd2++2n(OH)‐→[Cd(OH)2]n[Cd(OH)2]n+nS2‐→nCdS+2nOH‐[7]Inourcase,theprecipitationofCdSisobservedafter30minutesofdeposition.Thisisbecauseattheearlierstageofdeposition,theCdSfilmsaredominantlygovernedbytheheterogeneousmechanismandshowsauniformthinlayer.However,thisthinlayerisgraduallygrowingwiththe homogeneousmechanism. Ouachtari etal. reported that increasing the concentration ofthe cadmium ions in the CBD process produces a higher final film thickness; also, longerdeposition time is needed to attain the final film thickness. In addition, after a certaindeposition time, the absorption and/or dissolution process predominates over theheterogeneousandhomogeneousprecipitation[11].
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Figure1.Variationofthicknessatdifferentdepositiontime.3.2.CrystalStructureIt is reported that cadmium sulfide can exist in both sphalerite cubic and hexagonal forms.CubicCdSgrows in thezincblendestructurebyconsideringmetastablephase [14].TheXRDpatternsofCdSthinfilmsareshowninFigure2.ThreediffractionpeaksmatchedwithJCPDS01‐074‐9663 correspond to (111), (220) and (311) reflections at 2Θ of 26.5°, 44°, and 52°,respectivelyindicatingthattheobtainedCdSfilmshaveacubicstructurewiththepreferentialorientation normal to the (111) direction. A similar film structure has been reportedpreviouslybyotherresearchers[15,16].AlltheXRDpatternsexhibitabroadhumpnearthe(002)peakat2θ=26.7˚,whichisduetotheglasssubstrate.Acomparisonbetweenthespectraofthefilmsshowsthatthepeakintensityincreaseswhenthedepositiontimeisincreasingi.e.with increasing thickness the thicker filmhasmore crystallinity than the thinner films. Thedegree of preferred orientation along (111) increasedwhen the thickness is increases. Thepeak intensity of each of the three films increased after annealing indicating an increase incrystallinity [16]. Annealing helps the atoms to rearrange themselves and eliminate defectdensityinthefilmresultingingoodcrystallinity.Tomasetal.[17]reportedthattheN2orArannealing brings reorientation of as‐deposited CdS films with a significant improvedcrystallinequalitybyincreasingthegrainsizeanddecreasingthenumberofgrainboundariesin CdS films.Also, such some annealing leads to phase transition from themetastable cubicphase to a stablehexagonalphaseofCdS [18],which isundesirablewhen stackingCdS filmwithacubicCdTeabsorber[12]. InourcaseCdS, thin filmsarenotgonethroughanyphasetransition but improved in structural orientation due to annealing at 430°C in the N2environment.ThecrystallinesizeoftheCdSthinfilmsfilmiscalculatedusingFWHMdataandDebye‐Schererformula[13].
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respectively.Thus, it canbeconcluded that theparticlesize increasedwhenthe thickness isincreasedanddecreasedafterannealing.Moulkia etal. (2009) [6] explained the reduction of the grain size by the variation of thegrowthrate.Athighdepositionrates(thickerfilms),thenucleationstepisfast,thereforethenucleationsitesconcentrationislarge.Duetothis,thenucleisizeenlargementislimitedandblockedbythesurroundingnuclei.However,at lowdepositionrates(thinner films), the lowconcentration of the nuclei sites enables the nuclei to grow and attain a larger size.Consequently,thegrainsizeislargerinfilmsdepositedwithlowdepositionrate.
Figure2.SuperimposedXRDpatternofas‐depositedandannealedCdSthinfilmsdepositedfor20,
30and40minutes.3.3.OpticalPropertiesUV‐Visspectrometryisusedtoobservetheopticalpropertiessuchastransmission,absorptionandoptical band gapof the thin films.Ablank sodalimeglass slidewasplaced in thebeamdirection during the scanning process. Figure 3(a) and 3(b) shows the variation oftransmittanceT(%)withwavelengthλ(nm)inthewavelengthrangefrom300to800nmforas‐depositedCdSthinfilmsofvariousdepositiontimeand20mindepositedannealedCdSthinfilm.Themaximumtransmittanceobservedare82%,58%,and84%respectivelyinthevisibleregion (400 to700nm). Figure3(b) shows that the transmittanceT (%) isdecreasingafterannealingduetothedenserCdSformationduringtheprocess[5].ItisfoundthattheannealedCdSthinfilmsexhibitthemaximumtransmittance(between40%to75%)inthevisiblerangeandhavesharp fall at thebandedgewhich isgood foroptoelectronicdevices,especially forsolarcellwindowlayers,confirmingthepresenceofcrystallinityofCdSthinfilms[5,21].
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Figure3.Transmittancevs.wavelengthplotsfor(a)Differentdepositiontime(b)As‐depositedandannealedCdSthinfilms.
Figure4(a)showsthevariationofabsorbancemeasuredinthewavelengthrangingfrom300to800nmwiththevariationofdepositiontimei.e.20,30and40min.Duetothevariationofdepositiontimethethicknesseswerefoundabout100,200and250nmfor20,30and40minrespectively.Figure4(a)showsanincreaseintheabsorbancewhentheCdSfilmthicknessisincreasing.Figure4(b)representanincreaseofabsorbanceafterannealingwhichindicatestheformationofdenserCdSthin filmsduetoannealing.Thefinding issimilartothestudy fromIslametal. [5].Fromtheabsorbancespectra, it isalsoobservedthatforalltypesofCdSthinfilmsthemaximumabsorbanceofCdSthinfilmsisobservedataround300nm.Inthevisibleregiontheabsorbancerisesveryrapidlybefore300nm,attainsitsmaximumvalueandthendecreasesrapidlyupto600nmi.e.theabsorbancedecreasesrapidlyfromultraviolettovisibleregion.
Figure4.Absorbancevs.wavelengthplotsfor(a)Differentdepositiontime(b)As‐depositedandannealedCdSthinfilms.
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The absorption coefficient, α, [22] is calculated from the measured absorbance data fordifferentwavelengthscorrespondingtodifferentphotonenergiesatroomtemperatureusingequation(2).
d
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(2)
where )(log 010 I
IA is the absorbance and d is the thickness of the film. An idea of the band
structureandelectronictransitioninvolvedinabsorptionprocesscanbeobtainedbystudyingthedependenceofopticalabsorptioncoefficientonthephotonenergy.The spectral dependenceofα, on thephoton energy, h, for as‐deposited and annealedCdSthinfilmsisshowninFigure5.TheabsorptioncoefficientofallCdSthinfilmsincreaseswithincrease inphotonenergy.Theabsorptionco‐efficient increaseddue to annealing indicatingdenserfilms.
Figure5.Absorptioncoefficientvs.photonenergyplotsforas‐depositedandannealedCdSthinfilms.
Incrystallineandamorphousmaterials, thephotonabsorption isobserved toobey theTaucrelation[23],
αh=B(h‐Eopt)n(3)where, h is the incident photon energy, h Planck constant, the frequency of incidentradiation, B an energy independent constant, Eopt the optical band gap and n is an indexdependingonthetypeofopticaltransitioncausedbyphotonabsorption.Theindexnequals½and2foraccepteddirectandindirecttransitionsrespectively.Therefore,thedirecttransitionenergygap(Eqd)canbeobtainedbyplotting(αh)2versushcurveandthenextrapolatingthelinearportionoftheplotstotheinterceptinthehνaxis.Figure6showsthechangeinbandgapduetoannealing.AnincreaseinthebandgapofCdSthinfilmsisobservedafterannealing.Thebandgapsobtainedforas‐depositedandannealedCdSthinfilmsofdifferentthicknessesarelisted inTable1.FromTable1 itcanbedepictedthatthebandgapenergy ischanging from2.66to2.82eVforas‐depositedCdSthinfilmsofdifferentthicknessesandforannealedthatare ranged from 3.60 to 3.87 eV for CdS thin films of different thicknesses. The increase ofbandgapafterannealingcanbeattributedduetotheformationofCdOduringannealing.Itisobservedthatthebandgapchangesslightlywiththevariationofdepositiontimei.e.thereisalittleincreaseofbandgapwithincreasingthicknessmaybeduetosomestructuralchangesas
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alsodepictedfromXRD.Butafterannealingthebandgapdecreasedwithincreasingthickness.Thismaybeduetothefactthat,asthethicknessisincreasing,therelativeamountofformationofCdOdecreases.ThishappensbecausetheCdOformsontothesurfaceoftheCdS.Butwhenthethicknessincreases,theO2cannotpenetratethethickerfilms.Ontheotherhand,itisfoundthat the band gap energy of the CdS thin films increased significantly after annealing. ThisoccurrencecanbeexplainedbasedontheO2 incorporation.SimilarexplanationonannealedCdS thin film has previously reported by Islam (2013) [5].Wu etal. (2004) studied on RF‐sputteredCdSdepositionusinganO2/AratmosphereandshowedthattheCdS:OfilmsbecometransparentinthewavelengthrangeabovethebandgapofCdSandsuggestedthatCdSfilmsgrownwithinclusionofoxygenduringdepositioncanbeabetterchoiceforthinfilmsolarcellsas oxygenation will further increase their band gap [21]. The changes of band gap withincreasingannealing temperaturearecausedbychanges in thedefects, thecompositionandthecrystallinepropertiesof theCdS thin films.Thespectraldependenceofαwasstudiedatphotonenergieslessthanthebandgapofthefilms.
Figure6.(αhν)2vs.hνplotsforas‐depositedandannealedCdSthinfilms.
TheUrbachenergyiscalculatedfromtheslopeoftheplotsof lnαvshν.Thisslopegivesthewidth of the localized states associated with amorphous states in the band gap at photonenergies less thanthebandgap.Theabsorptioncoefficientnear the fundamentalabsorptionedgeisexponentiallydependentontheincidentphotonenergy[24].
α=α0exp(hν/Eu) (4)whereα0 is a constant and Eu is the Urbach energy. Thus, the plots of lnα should be linearwhose slope gives Urbach energy. The lnα vs hν plots for as‐deposited CdS thin films ofdifferentthicknessesareshowninFigure7(a)andthatofannealedCdSthinfilmsareshowninFigure7(b).TheUrbachenergiesarecalculated fromtheslopesof theUrbachplotsandarelisted inTable1. TheUrbach energies are ranging from2.61 to 5.1 eV for as‐deposited and2.34 to3.4eV for annealedCdS thin filmsofdifferent thicknesses.FromTable1 thatas thethicknessincreasedthebandgapandtailwidthofCdSfilmsincreasedduetotheincreaseindisorder/impurityandfilmstoichiometrywiththicknessordepositiontime.Dingyuetal.[20]reportedthatthechangeofbandgapandtailwidtharerelatedtofilmstoichiometryinwhichitchangeswithdepositionoftheparameter.ItcanbenoticedthattheUrbachenergydecreasesafterannealingforallthethicknesses.Thisbehaviormaybeduetoadecreaseinthedegreeofdisorderanddecrease indensityofdefectstates(whichresults in thereductionof tailingofbands). Due to this, it showed an increase in crystallinity. It is also found that the Urbach
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energy increaseswith increasingthickness indicatingan increase indisorderordefectswiththicknessordepositiontime.
Figure7.Urbachplots(lnαvshνcurves)for(a)as‐depositedCdSthinfilmswithdifferentthicknessesand(b)annealedCdSthinfilmswithdifferentthickness.
Table1Variationofbandgapwiththedepositiontimeofas‐depositedandannealedCdSthinfilms
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3.4.ElectricalPropertiesTo characterize the electrical properties of the thin films, Four Point probe and Hall effectmeasurements are performed in air at room temperature. The as‐deposited films exhibitsemiconductingbehaviorswithresistivity in therangeof10‐2Ω.cm foras‐depositedCdS thinfilms and 10‐3Ω.cm for annealed CdS thin films. The resistivity decreased one order ofmagnitudeafterannealing.Thisdecreaseisattributedtotheimprovedcrystallinity,growthingrainsizeandtheimprovementinfilmstoichiometry,asisindicatedbytheXRDpatterns.n‐typeconductivity isobservedbytheHallmeasurements forbothtypesof theCdSthin films.Themobilityandcarrierconcentrationofas‐depositedandannealedCdSthinfilmsaregiveninTable2. It isobserved thatbothof themobilityandcarrierconcentrationof theCdS thinfilmsofdifferent thicknesses are increasing (and accordingly the resistivitydecreases) afterannealing. Generally, the carrier concentration and mobility have increased due to theinterstitialCd+2 ionsor sulfurvacancieswhichexist in the film.Thesecarriersactasdonorsandresultsinanincreaseinthecarrierconcentrationaswellasaconsequentdecreaseintheresistivity [10].Anysignificant change in carrier concentrationandmobility isnotobservedduetothicknessvariation.
Table2Electricalparametersforas‐depositedandannealedCdSthinfilms
4. CONCLUSION
In this study, the investigation is carried out to explore the structural, optical and electricalpropertiesofCBDdeposited films. It is found thatat theearlier stageofdeposition, theCdSfilmsaredominantlygovernedbytheheterogeneousmechanismbecauseitshowsauniformthin layer.However,gradually theCdS films finishedwithhomogeneousgrowth.Apart fromthis,fromtheXRDpattern,thegraphindicatedshowedthatthepeakintensityincreasedwhenthedepositiontimeisincreasing.Also,thepeakintensityofeachofthethreefilmsincreasedafterannealingindicatinganincreaseincrystallinity.Theopticalbandgapliesintherangeof2.66 to 2.82 eV for as‐deposited and 3.6 to 3.87 eV for annealed CdS thin films of differentthicknesses.TheUrbachenergiesarerangingfrom2.34to5.1eVforas‐depositedand2.61to3.4 eV for annealed CdS thin films of different thicknesses. Both of themobility and carrierconcentration of the CdS thin films of different thicknesses increase (and accordingly theresistivity decrease) after annealing. The structural, optical and electrical properties of CdSthinfilmsareimprovedafterannealing.TheseCdSthinfilmsdepositedbyCBDtechnique,withtheaddedadvantagesofcost‐effective,largeareadepositionandeasythicknesscontrol,canbeveryimportantforsolarcellapplication.
CdSthinfilm FilmThickness(nm)
Resistivity(Ohm.cm)
Mobility(cm2/Vs)
Carrierconcentration(cm‐3)
As‐deposited 100200250
4.08x10‐25.00x10‐26.43x10‐2
9.62x1011.46x1005.40x101
5.45x10112.80x10119.62x1011
Annealed 100200250
3.32x10‐32.21x10‐31.81x10‐3
2.96x1011.41x1021.68x102
1.60x10123.05x10126.12x1012
InternationalJournalofNanoelectronicsandMaterialsVolume11,No.2,Apr2018[223‐234]
231
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