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Full length article

Investigation of structural, optical and electrical properties of ZnS thin ﬁlms prepared by ultrasonic spray technique for photovoltaic applications

A. Derbali

^a

, A. Attaf

^a,∗

, H. Saidi

^a

, H. Benamra

^a

, M. Nouadji

^a

, M.S. Aida

^b

, N. Attaf

^c

, H. Ezzaouia

^d

aLaboratoryofThinFilmsandApplicationsLPCMA,UniversityofBiskra,Algeria,BP145RP,07000Biskra,Algeria

bDepartementofPhysicsFacultyofSciences,KingAbdulazizUniversity,Djeddah,KSA,SaudiArabia

cLaboratoiredeCouchesMincesetInterfacesFacultédesSciencesUniversitédeConstantine,Algeria

dLaboratoiredesSemi-Conducteurs,NanostructuresetTechnologieAvancée,ResearchandTechnologyCentreofEnergy,Borj-Cedria ScienceandTechnologyPark,BP95,2050Hammam-Lif,Tunisia

a r t i c l e i n f o

Articlehistory:

Received25March2017 Receivedinrevisedform 26September2017 Accepted6October2017

Keywords:

Zincsulphide Thinﬁlms Ultrasonicspray Depositiontime XRD

Opticalandelectricalproperties

a b s t r a c t

ZnSﬁlmshaveimportantapplicationsinphotovoltaicdevices.Inthecurrentstudy,theZnS ﬁlmsweredepositedbyultrasonicspraymethodonheatedglasssubstrateattemperature equalto450^◦C.

Inthisarticle,wereporttheeffectofdepositiontimeonthedifferentstructural,optical andelectricalpropertiesofZnS.AllthesamplesobtainedweretreatedusingX-raydiffrac- tion(XRD),opticaltransmittancespectroscopy(UV-V)andfour-pointmethod.Theresults ofX-raysdiffractionshowedthatthedepositedmaterialwaspurezincsulfidehavinga cubicsphaleritestructurewithpreferentialorientationalongthe(111)direction.Aswell thegrainsizewasaround38–102nm,whenthedepositiontimeincreases.Thetransmit- tancemeasurementsexhibitanaverageopticaltransparencybetween35and75%inthe visiblerange(400–750nm)fordifferentdepositiontimes.Thedirectbandgapenergyforall filmswascalculatedandfoundtobefrom3.48to3.92eV.Thefilms’thicknessisincreased withdepositiontimefrom195nmto1756nm.Theelectricresistivityofthedepositedfilm variesalsowiththedepositiontimebetween1.51×10⁵and20.84×10⁵.cm.

1. Introduction

ZnScompoundisoneofthemostimportantsemiconductormaterial,andithasbeenlargelyinvestigatedintherecent years[1].Duetothewidebandgap(3.7eV),nontoxicity,safetytoenvironmentandhightransparency[2]ofZnS,itcan beusefulforextensivelyapplicationsinoptoelectronicdevices,suchaslight-emittingdiodeandlaserdiodefromblueto ultravioletband[3],fluorescenceandelectroluminescencethinfilmdevices[4]andn-typewindowmaterialinsolarcell [5].AswellincomparisonwithCdS,theZnSthinfilmsarepresentbetterlatticematchingtoCIGSowingtotheprecedent advantages.

∗ Correspondingauthor.

E-mailaddress:ab.attaf@gmail.com(A.Attaf).

https://doi.org/10.1016/j.ijleo.2017.10.034

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Fig.1. Theschematicexperimentaltousedofdepositionsystem.

Table1

ThedepositionconditionusedtodepositedofZnSthinﬁlm.

Depositioncondition Correspondingvalues

Amountofsolution 30ml

Substrate–nozzledistance 50mm

Substratetemperature 450^◦C

Molarityofsolution 0.1M

Depositiontime 2min,4min,6min,8min,10min

Sprayingﬂowrate 50ml/h

Accordingtotheliterature,Zincsulphideﬁlmscanbedepositbyseveralmethodsincludingreactivesputtering[6], electro-deposition[7],pulsed-laserdeposition[8],chemicalvapordeposition(CVD)[9],molecularbeamepitaxy(MBE) [10],spraypyrolysis[11],chemicalbathdeposition(CBD)[12]andsol-gelprocess[13].Amongthosemethods,ultrasonic sprayisthebestonesuitedforthepreparationofZincsulphide(ZnS)thinﬁlmsbecauseitissimplicityanddonotcostan experimentalrequirement,easeofaddingvariousdopingmaterials,reproducibility,speedgrowthrateandmassproduction capabilityforuniformlargeareacoatings[14].

Inthispaper,weattendedzincsulphide(ZnS)thinﬁlmsontheglasssubstratebyultrasonicspraytechnique.Theaimof thisworkistoinvestigatethestructural,electricalandopticalpropertiesofZnSﬁlmsasafunctionofdepositiontime.The obtainedresultsarediscussedandcomparedwithotherresearchresultsreportedintheliterature.

2. Experimentaldetails

Thedepositionsystemhasbeenpreparedinthelaboratorybythesimplesetupshowedin(Fig.1).Zincsulphidethinfilms weredepositedonglasssubstratesofthedimension(25×15)mm²usingsprayultrasonictechnique.TheseZnSthinfilms exhibitedgoodadherencetothesubstratesurfaces.Theglasssubstrateswerecleanedinacetone,ethanol,anddistilledwater respectivelyfor15min,andthenblowingdrywithacompressedair.Thestartingsolutionwaspreparedbydissolving0.1M ofZincChloride(ZnCl2)andthiourea(SC(NH2)2)inmethanol.Alltheparameterswerekeptconstantsuchas:thesubstrate temperature(450^◦C),theflowratesolution(50ml/h),thedistancenozzle-substrate(50mm).While,thedepositiontime changedfrom2,4,6,8,10min.IntheTable1,wesummarizedallthedepositioncondition.

Commonlywhenthesolutiondropletsreachtotheheatedsubstratesurface,thefollowingchemicalreactionoccurs:

ZnCl₂+CS(NH₂)₂+CH₃OH+3

2O₂→ZnS+2NH₄Cl(gas)+2CO₂(gas)

Accordingtothisreaction,aZnSthinﬁlmshouldbeformedontheglasssubstratesurfaceandtheNH₄Cl,CO₂leavesthe systemingasesform.

Thestructural,opticalandelectricalcharacteristicsoftheseﬁlmshavebeenstudiedextensivelythroughseveraltech- niques.ForthecrystallinestructurepropertiesoftheﬁlmwasanalyzedusingX-raydiffracto-meter(D8ADVANCEDBRUKER) withCu-K␣radiation(␭=1.5418Å)in2␪rangefrom10^◦–90^◦.

Theopticalpropertieshavebeenrecordedbyusinganultraviolet-visible(UV-VIS)spectrophotometer(PerkinElmer LAMBDA25)betweenthewavelengthsof300and1100nm.Moreovertheelectricalresistivitywasdeterminedusingthe four-pointmethodatroomtemperature.

3. Resultsanddiscussion 3.1. Structuralproperties

Accordingtothedepositionprocessandexperimentalfactors,Zincsulphidefilmscanbecrystallizedintwoforms,cubic (Zincblend)and hexagonal.TheFig.2showsXRDpatternsofZnSthinfilmsdepositedbyultrasonicsprayatdifferent depositiontimes.Itwasobservedatdifferentdepositiontimethatthereisasingleonepeakforallfilmswiththepreferred orientedgrowthalong(111)planeatthediffractionanglesof28.7^◦ andthisiscompatiblewithexhibitedazincblende

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Fig.2. XDRpatternofZnSﬁlmsdepositedatvariousofdepositiontime.

2 4 6 8 10

35 40 45 50 55 60 65 70 75 80 85 90 95 100 105

0,08 0,10 0,12 0,14 0,16 0,18 0,20 0,22

Fig.3.VariationofgrainsizeandFWHMof(111)peackwithdifferentdepositiontime.

structure(JCPDScardNô.55-0566)inthediffractionangle(2␪)rangefrom10^◦to90^◦,whichmeansthattheplaneswere paralleltothesubstratesurface.Thereisnopeakrelatedtotheoxidationphase(ZnO)evenatthemaximumdeposition time,whichprovesthatthestoichiometryofourfilmswasimprovedwithincreasingdepositiontimes.Thesameconclusion aboutgoodstoichiometrywerereportedbyotherauthor[15,16],forZnSthinfilmsobtainedbyspraypyrolysisatdifferent depositiontimeand(Zn:S)ratiointheprecursorsolution(A)respectively.

BothworksofHwangetal.[17]andAshratetal.[18]foundthesamecubicphasewiththepreferredorientatedalong (111)planeofZnSthinﬁlmspreparedbyRFmagnetronsputteringandclose-spacesublimation.Fig.2,alsorevealsthat theintensityof(111)peakincreaseswiththeincreasedepositiontimes;itbecomesnarrower,intensewhenthedeposition timeequals10minduetotheappearanceofthisorientation,itdependson:ﬁlmsandthesubstratesurfaceenergiesonone hand,thearrivingatomstosubstrateontheotherhand.Thegrowthisachievedalong(111)planeduetoitslowestsurface energy[19]

Thegrainsize(D)ofthedepositedﬁlmscanbedeterminedbyusingequationofScherer[20]:

D= K

ˇcos (1)

WhereKisapproximatelyequalto0.94,␭=1.54184Å,␪isthediffractionangleand␤isthewidthofdiffractionlineathalf maximumintensity.

ThegrainsizeresultsarepresentedinFig.3.Itcanbeobservedthereisaslightincreasewithincreasingindeposition timesbetween2and 6min(38–57nm)andthisresultisingoodagreementwiththereportedonesbyothersauthors (25–60nm)[21],becausegrainsizeenhancementyieldstotheﬁlmsopticaltransmissionimprovement.Thesoluteatoms comingtothesubstratesurfacecanbediffusedalongthesubstratesurfaceandformclusters(nuclei)whichcancontribute totheformationofcrystallites.Atlowdepositiontime(2–6min),thediffusionofsoluteatomsonsubstratesurfaceisnot

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Table2

StructuralparametersofZnSthinﬁlmsatdifferentdepositiontime.

Deposition time(min)

(hkl) 2␪(degree) FWHM(degree) D(nm) ␦x10¹⁴(Lines/m²) ␧x10⁻⁴

2 (111) 28.7488 0.216 38 6.93 9.13

4 (111) 28.7358 0.18 45.6 4.81 7.6

6 (111) 28.806 0.2 41 5.94 8.45

8 (111) 28.7785 0.144 57 3.08 6.08

10 (111) 28.6966 0.08 102 0.95 3.38

eminentwhichreducedtheclustersformation.Hencethedensityofnucleationcenterissmall,andthisleadtosmallergrain size.

Thensigniﬁcantincreasesbetween6and10mininthecrystallitesizefrom38to102nm.Similarresultshavebeen observedbyAbdietal.[22]andTouatiaetal.[23],ﬁndingthatthegrainsizewasvariedbetween64and104nmand 50–125nmbyelectronbeamdepositionandVacuumEvaporationMethod.Withincreasingthedepositiontime(6–10min) thenumberofsoluteatomsarrivingontothesubstratesurfaceisincreasing.Consequentlythenumberofnucleicenter formedonthesubstratesurfaceislargewhichleadstolargergrainsformation[24].

Theincreaseofthecrystallitesizewiththeincreaseofdepositiontimemaybealsoduetodecreasingthevalueoffull widthhalfmaximum(FWHM)correspondingto(111)peak(asseeninFig.3),resultingfromtheincreasingﬁlmsthickness.

Adislocationisknownasacrystallographicdefect,orirregularity,inacrystalstructure,anditspresenceinsidethe crystallitestructurestronglyaffectsmanyofthepropertiesofmaterials.Usingthevaluesofgrainsize,thedislocation density(␦),isdeﬁnedasthenumberofdislocationlinesperunitvolumeofthecrystal,andithasbeencalculateddepending ontheWilliamsonandSmallman’srelationship[25]:

ı= 1

D² (2)

Themicrostrains(␧)ofﬁlmswereestimatedusingtheequations[26]:

ε=ˇcos

4 (3)

Thestructuralparameterswerecharacterizedaccordingtothecalculatedvaluesofaveragegrainsize(D),microstrains (␧)anddislocationdensities(␦)forZnSthinﬁlmswithdifferentdepositiontimerepresentedinTable2

TheTable2showsthecalculatedmicrostrainanddislocationdensityalongtheorientation(111)crystallographicplane fordifferentdepositiontimes.Itcanbeseenthatthemaximumvalueofdislocationdensitywasobtainedforfilmsprayed at2minofthedepositiontime,thereasonisduetoinversecorrelationbetweendislocationdensityandthegrainsizeas confirmedinEq.(2).AsseenintheTable2theincreaseingrainsizeinZincsulphidethinfilmsledstoadecreaseingrain boundariesanditdecreaseddislocationdefectinsidethecrystallatticeofthesamplesandtheresultofallthiswasthestress reducedincrystalstructure,thislateristheresultsofinternalstrains.Rahuletal.[27]havereportedincreasesthegrain sizebetween36.12–43.82nmanddislocationdensitydecreasesbetween7.6×10¹⁴–5.2×10¹⁴line/m²forZnSthinfilms bythermalevaporationtechnique.

3.2. Opticalproperties

Thethicknessoftheﬁlmswascalculatedbyweightingdifferencemethodusingasensitivemicrobalanceaccordingto relation(Eq.(4)):

thickness(t)= m

×a (4)

Wherem=massofthedepositedﬁlm,␳=densityofZnS,equal4.1g/cm³forbulkZnScubicstructureanda=areaofﬁlm [28].

Thegrowthrateisestimatedbydivisionthicknessfilmsonthetimeofdeposition.Thevariationsofthefilmthickness andgrowthrateasafunctionofdepositiontimearerepresentedinFig.4.ItcanbeobservedinFig.4thatthefilmthickness andthegrowthratearechangingalmostlinearlywithdepositiontime.Thefilmgrowthiscontrolledbykinetics[29]of thereactionbetweenthehotsurfacesubstrateandtheamountofsolution.Whenthedepositiontimeincreases,themass transfertothesubstratesurfaceincreasesaswellandthatmeanstheincreasingthemobilityofthedropletsreachingthe substratesurfaceathightemperature(450^◦C)andincreasingthenumberofionizedparticles.Therefore,thisisduetothe increasesofboth;thereactionspeedsandthekineticsofthereactionbetweenthesubstratesurfaceandthequantityof dropletssprayedontosubstratesurfaceandbecauseofthatitincreasesthegrowthratewhichincreasesthethicknessofZnS thinfilms.Thesameincreasinginfilmsthicknesshavebeenreportedbyotherauthors[15,16]atdepositiontimebetween 10and25min.Whileindepositiontimeequalto10min,wehavenoticedadecreaseinthegrowthrate.Thisindicatesthat thedepositedfilmsbecomesapproximatelyinsaturationcasewithincreasingthedepositiontimesandthisisnotallowed

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2 4 6 8 10 0

500

Fig.4.Filmthicknessandgrowthrateasafunctionofdepositiontime.

300 400 500 600 700 800 900 1000 1100 0

10 20 30 40 50 60 70 80

2 min 4 min 6 min

8 min 10 min

Fig.5.TransmissionofZnSﬁlmsdepositedatvariousdepositiontimes.

toincomingspeciesonsurfacesubstratewhichhaveenoughappropriateplacestoformsmorematerials,whichcausethe decreaseofthegrowthrate.

Thehightransmittanceoftheﬁlmsinthevisiblerangeisveryimportantforopticalapplications.TheFig.5,showsthe opticaltransmittanceofZnSﬁlmsdepositedatdifferentdepositiontimes,forincidentlightatwavelengthsfrom300to 1100nm.

Thewholeofthespectraoftransmissionobtainedinoursamplesarecomposedoftworegions:a regionofstrong absorption(␭<400nm),correspondingtofundamentalabsorptioninthinﬁlmsofZnS.Thisabsorptionisduetotheelectronic transitioninterband(thevalencebandandthebandofconduction).Thevariationofthetransmissioninthisregionis exploitedforthedeterminationoftheopticalbandgapenergyandthedisorder.Itwasalsoobservedthatdepositiontime hasaffectedthepositionofabsorptionedgewhichshiftedslightlytolongerwavelength.

Thesecondregionofstrongtransparency(␭>400nm):thevalueofthetransmissionisabout35%to75%,inthewavelength rangeof400–800nm.Thischangingintransmissionvaluesisingoodagreementwiththatobtainedvalues(40–70%)by Daranfedetal.[30]onZnSthinfilmspreparedbyultrasonicspraymethod.Itisnoticedthatthetransmittancedecreasewith theincreasingdepositiontime,canbeexplainedbythefilmthicknessincrease.Wenoticedthattransmittancespectrahave nointerferencefringeduetotheincidentlightscatteringinthematerialbecauseofinterfaceair/filmroughness.

TheopticalbandgapofZnSﬁlmscanbeobtainedbytheabsorptionrangeinthetransmittancecurveandtheTauc relationship[31](Eq.(5)):

˛h=K

h−Eg

n

(5) Where␣istheabsorptioncoefﬁcient,Kisaconstant,E_gistheopticalbandgapandnis½foradirect-bandgapsemiconductor.

Extrapolationofthelineportionofthecurveto(␣h␯)²=0givestheopticalbandgap.Thevariationofgapopticand extrapolationofthelineportionofthecurveto(␣h␯)²asafunctionofh␯wasshowninFig.6

MostofbandgapsenergyvaluesaresomewhatlowerthanthatofbulkvalueofZnS(3.7eV).AccordingtoFig.6,theband gapenergyofZnSthinfilmsincreasesfirstfrom3.48eVto3.92eVintherange2–6min.Thisresultisingoodagreement withthatobtainedbyElidrissietal.[32],thewidebandgapenergyisthereasonofwindowlayerabsorptiondecreasesloses andconsequentlythisyieldstothesolarcellshortcircuitcurrentimprovement.Theincreaseofbandgapenergyiscaused byfilmthicknessincreas[33]andBurnstein-Mosseffect[34].However,afterthat,thegapopticdecreasedfrom3.92eVto

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Fig.6.BandgapenergyoftheZnSﬁlmsasafunctionofdepositiontime.

Table3

RefractiveindexofZnSthinﬁlmatdifferentdepositiontimecalculatedbyHerve-Vandammeequation.

Depositiontime(min) BandgapEg(eV) CrystallitesizeD(nm) Refractiveindexn

2 3.48 38 2.21

4 3.62 45.6 2.18

6 3.92 41 2.11

8 3.66 57 2.17

10 3.56 102 2.19

3.56eVwiththeincreasingdepositiontime,itcanbeexplainedthisdecreaseinthegapopticenergytoquantumsizeeffect [35]owingtoincreasedcrystallitesize.SimilarresultshavebeenobservedbyAbduljabbar(3.7–3.3eV)[36],Yildrimetal.

(3.73–3.57eV)[37]andNadeemetal.(3.84–3.51eV)[38].

Therefractiveindex(n)ofZnSthinﬁlmselaboratedatdifferentdepositiontimeiscalculatedusingthemodelofHerve- Vandamme.TheHerve-Vandammerelationship[39]is:

n²=1+

A

Eg+B

2

(6) WhereAandBareconstantsasA≈13.6eVandB≈3.4eVandE_gistheexperimentalvaluesofgapopticenergy.

TherefractiveindexofZnSthinﬁlmselaboratedatdifferentvaluesofdepositiontimeisillustratedinTable3.

AsobservedinTable3therefractiveindexdecreasesbetween2.11and2.21intherange2–6min,thenincreasedwith theincreasethedepositiontime.Abduljabbar[40]hadreportedinearlierwork,thevalueoftherefractiveindexofZnSthin ﬁlmswasaround2.13–2.19atroomtemperatureandatdifferenttemperatures(200^◦C,300^◦Cand400^◦C).Wecanattribute thisdecreaseasfollows:theincreaseofthegrainsizeduetodecreaseofgrainsboundaries,thismaybeduetoreducedofthe stressinthecrystallinenetworkoftheZnSthinﬁlms.Younghunetal.[41]havealsoreportedtheeffectsofbiaxialstresson therefractiveindexofZnO:Ga,sotheyhavefoundthattherefractiveindexdecreaseswithincreasingbiaxialtensilestress.

3.3. Electricalproperties

Forourstudytheresistivityiscalculatedbythefollowingequation[42]:

=Rd (7)

Whereistheresistivity,Risthesquareresistanceanddthesamplethickness.ThevariationsofZnSlayersresistivityand conductivityversusthedepositiontimearepresentedinFig.7.

Itcouldbeobservedthattheresistivityofthegrownﬁlmsincreasesfrom1.51×10⁵cmto20.84×10⁵cmwiththe increasingofthedepositiontime.TheseresultsarelowerthanTuranetal.ones[43],whostudiedtheelectricalpropertiesof ZnO/Au/ZnS/Auﬁlmsdepositedbyultrasonicspraypyrolysis.Hence,thegrainsizeenlargementleadstoadecreaseingrain boundaryeffects,becausetheboundaryhinderstheconductionmechanismofcarrierscharge,andthisisthereasonforan increaseinresistivity.

AsobservedthattheincreaseinthethicknessofthesamplesledalsototheincreaseoftheresistivityofZnSthinﬁlmsas showinginEq.(7).Therefore,theresistivitywasaffectedbycarrierconcentrationanddepositiontime.

4. Conclusion

Inthiswork,wehavebeenshownthatthedepositiontimeplaysafundamentalroleonthepropertiesofZnSthinﬁlms depositedbyultrasonicsprayontoglassat450^◦Csubstratetemperature.TheXRDmeasurementsrevealthattheﬁlms

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Fig.7.VariationoftheresistivityandconductivityofZnSthinﬁlmswithdifferentdepositiontime.

depositedat10minhaveastrongly(111)preferredorientationandareparalleltothesubstratesurface.ThesmallestFWHM valueof0.08^◦hasbeenalsobeenobservedfortheseﬁlms,indicatingthatthecrystallinityoftheﬁlmscanbeimproved byincreasingthedepositiontime.ThegrainsizesofZnScalculatedbySchererrelationshipwerefoundbetween38and 102nm.Ithasbeenfoundthatthetransmittancedecreaseswiththeincreaseofdepositiontime.Thecalculatedbandgap energyofZnShasgiventhevaluesintherangeof3.48–3.92eV.Furthermore,theelectricalresistivity(␳)increasedwhen thedepositiontimeincreases.

ItcanbeconcludedthattheperfectconditionforthebestapplicationphotovoltaiccellefficiencyofZnSthinfilmsisto increasethedepositiontime,owingtobandgapenergywidening,whichcanbeusedasawindowlayerinheterojunction photovoltaicsolarcellorcanbeusedasareflectoranddielectricfilterbecauseofitshighrefractiveindex(2.2)anditshigh transmittanceinthevisiblerange

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