sublimate vapor effusion source Nanocolumnar CdS thin films grown by glancing angle deposition froma Technology Journal of Applied Researchand

Availableonlineatwww.sciencedirect.com

Journal of Applied Research and Technology

www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology15(2017)271–277

Original

Nanocolumnar CdS thin films grown by glancing angle deposition from a sublimate vapor effusion source

Luis Germán Daza

, Román Castro-Rodríguez

, Marco Cirerol-Carrillo

, Enrique Adrián Martín-Tovar

, José Méndez-Gamboa

, Rubén Medina-Esquivel

,

Ignacio Pérez-Quintana

, Augusto Iribarren

aDepartmentofAppliedPhysics,CINVESTAV-IPN,UnidadMérida,97310Mérida,Yucatán,Mexico

bYucatanAutonomousUniversity,FacultyofEngineering,AP150Cordemex,97310Mérida,Yucatán,Mexico

cInstitutodeCienciayTecnologíadeMateriales,UniversidaddeLaHabana,ZapatayG,Vedado,LaHabana10400,Cuba Received27October2016;accepted10February2017

Availableonline9June2017

Abstract

Theglancingangledeposition(GLAD)techniquewasusedtogrowcadmiumsulfide(CdS)thinfilmsonglassandindiumtinoxide(ITO)-coated glasssubstratesfromasublimatevaporeffusionsource.Thesampleswerepreparedunderdifferentincidentdepositionfluxangles(α)of0^◦,20^◦ and80^◦,whileboththesubstrateandthesourcewereunderrotation.Thetemperatureofthesourcewas923.15K.Scanningelectronmicroscopy imagesshowedthattheGLADmethodcombinedwiththesourceproduceddensenanocolumnarstructureswithheightanddiametersof∼200and

∼30nm,respectively.Thedepositedfilmsdisplayedahexagonalstructurewithpreferential(002)planeorientationandcrystallitessizesbetween

∼25nmand∼35nm.Amaximumsolarweightedtransmissionof∼92%wasobtainedforthesamplepreparedatα=80^◦,withasubstrate/source rotationvelocityratioof55/20inthewavelengthregionof400–900nm.Theaverageband-gapenergyofthefilmswas∼2.42eV.Refractive indexesbetween∼1.4and∼2.4ata550nmthewavelengthwasalsoobtained.

©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunder theCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Glancingangledeposition;Cadmiumsulfide;Nanostructures;Thinfilm;Structuralproperties;Columnarstructure

1. Introduction

Cadmiumsulfide(CdS)isaII–VI semiconductormaterial withanopticalbandgapof2.42eVandagreatdiversityofappli- cations,suchaswindowlayersforbothCdTeandCuInGaSe2

(CIGS)-basedsolarcellsdevices(Elbar,Tobbeche,&Merazga, 2015;Gerthofferetal.,2015;Hanetal.,2014;Leeetal.,2015;

Li,&Liu, 2015; Mohamed,2015; Yun,Cha,Ahn, Kwon, &

Al-Ammar,2014).TheCdS/CdTe andCdS/CIGSheterojunc- tionhasbeen activelyinvestigated asaphotovoltaic junction, moreover,theCdSwhichactsasawindowlayerintheafore- mentionedsolarcellshasbeendepositedbymanytechniques suchassputtering(Kim,Kim,Choi,Park,&Lee,2015),close

∗Correspondingauthor.

E-mailaddress:[email protected](E.A.Martín-Tovar).

PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.

spacesublimation(CSS)(Cruzetal.,2013),chemicalbathdepo- sition (Yücel & Kahraman, 2015; Lisco et al., 2015), spray pyrolysis(Yilmaz,2015),sol–gel,spincoating(Zhang,Huo,Li, Li,&Yang,2013),thermalevaporation(Baghchesara,Yousefi, Cheraghizade,Jamali-Sheini,&Saáedi,2016),ionbeamsput- tering(Liangetal.,2013)andmanyothers.TheCSStechnique consistsinasourceofanevaporatingmaterialandasubstrate which are separated by small distance ina controlled atmo- sphere.Ithastheadvantageofbeinganinexpensivedeposition methodbecauseofitssimpleconfigurationandcheapmaterials (Cruz-Campa&Zubia,2009),anditalsohasahighdeposition rate(Alamri,2003).

Glancingangle deposition isanovel andusefultechnique to grow nanostructuredmaterials at lowcost, whichconsists in tilting the substrate against the vapor flux. It is based on the oblique angle deposition (OAD) technique. In OAD the incidentvaporflux arrivesatnon-normalangles(i.e.,oblique angles) to astatic substrate; therefore,the vapor flux can be

http://dx.doi.org/10.1016/j.jart.2017.02.003

1665-6423/©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

treatedasavector, whichhastwocomponents,averticaland lateralcomponentwithrespecttothesubstrate,whichproduces adistinctanddirectionalcolumnar growthbecauseof atomic shadowing mechanisms occurring at the substrate’s surface.

Atthe initialstage ofthe deposition, adatomscondenseonto thesubstratecreatingindividualseparatedislandsofdeposited material,ornuclei(Kaiser,2002).Tiltingthesubstratecauses theincidentvaportoarriveatobliqueangles,whichresultsin theappearingof shadowed regionswherethe vaporflux can- notdirectly reachthe substrate.The nucleicapturethe vapor flux thatwould havelandedinthe aforementionedshadowed regions, leading to the formation of a columnar growth in the same direction as the vapor source, resulting in the cre- ationofanano-arraypattern.Thegladtechniquedifferswith respectfromtheOADinthatthesubstrateisnolongerstatic;

instead,itisazimuthallyrotatedbyasteppermotor.Controlling the rotation of the substrateandotherdeposition parameters, namelyvaporfluxincidentangle,substratetemperatureamong others, it is possible to obtain three-dimensional nanostruc- turessuchasnanocolumns,zigzag,nanohelix,nanospringsand many others (Barranco, Borras, Gonzalez-Elipe, & Palmero, 2016; Jie, Yu, Qin & Zhengjun, 2008; Messier et al., 1997;

Robbie&Brett,1997;Zhou,Li,Ni&Zhang,2011).Theafore- mentionednanostructuresenhancethecontactareaforcertain materials, which in turn changes their optical and structural properties;thiscouldbebeneficialforcertainapplicationssuch as optoelectronic andphotocatalyticdevices (Jieetal., 2008;

Zhouetal.,2012).Furthermore,thesestructurescouldbeinte- grated intoasolarcell, whichmakesthemveryattractive for awidevarietyofapplications(Leem&Yu,2011;Zhouetal., 2012).

Inthispaper,CdSthinfilmswerepreparedusingtheGLAD techniquealongwithasublimatevaporeffusionsource,which wasunderanazimuthalcounter-rotationwithrespecttothesub- straterotationdirection,theeffectsofthisdepositionmethodon themorphological,structuralandopticalpropertiesofthefilms isreported.

2. Experimentaldetails

Figure1showstheschematicrepresentationoftheevapora- tionsourceandtheglasssubstrateinGLADtechnique.Ωandω areusedtonametherotationvelocityofthesourceandthesub- strate,respectively.Theevaporationsourceconsistsofagraphite containeraxiallyrotatedatΩ=20rpmandheatedbetweentwo halogen lampsat923K.2g of aCdSpowder (99.99% pure) wereplacedinthecontainer.Thepressurewithinthecavitywas closeto∼7×10⁵Pa.Thiswascalculatedassumingandidealgas behavior(PV=nkBT),whereV=14.73cm³isthevolumeofthe container,T(923K)theevaporationtemperatureofthesource, kBistheBoltzmann’sconstantandnthenumbersofmoles.It mustbeclarifiedthatalthoughtheidealgasmodelmightnotbe totally accuratetomodel thepressureofthe container,itwas made as a first approximationfor the sake of simplicity,and

a b

Substrate

L

Halogen lamp

Stainless steel support

ω α

Ω

Ω CdS

powder Source

Vapor flux effusive holes

Substrate

Fig.1.ExperimentalsetupforGLADmethodcombinedwiththesublimatevaporeffusionsource.Theleftside(a)showstheGLADapparatusasitisimplemented intheeffusionsource.Substrateandsourcemovementsareaccomplishedbytwoindependentmotors.Onthebottomright(b),thesourcecontainerisshown.

fortheimmediateintentsandpurposesof thisworkitiscon- sideredsufficientlycorrect.Thecontainerhasatopcoverwith 20holesof∼1.5mmdiametersymmetricallydistributedasis showninFigure1(b);theseholeshaveavaporeffusionfunction.

Thevaporfluxdensity(φ)foreachholewasestimatedusingthe cosineofKnudsenlaw(Herman&Sitter,2012):

φ=3.153×10²²· a² πL

√ρG TM



cosα (1)

wherea=1.77×10⁻²cm²istheareaofasinglehole,L=6.0cm isthedistancebetweenthesubstrateandtheevaporationsource, PG is the CdS vaporpartial pressure within the container, T (923K)istheevaporationtemperatureof thesource,Misthe CdSmolecularweightandαistheincidentdepositionfluxangle.

Thefluxdensityhasanangulardistributionineachhole;this producesaconfinedregionof CdSvapordistributedbetween thesubstrateandtheevaporationsource.

To grow vertical nanocolumnar structures of CdS thin films, the substrate was put in GLAD mode, as shown in Figure1(b),andheatedbyradiationusing twohalogenlamps as depicted in Figure 1(a). The substrate was azimuthally rotatedusingangularvelocitieswithvaluesω=20,30,40,55, 150,350, 700and1020rpm,andtilted usingα=0^◦,20^◦ and 80^◦,asshowninFigure1(b).The experimentalset-upproce- dure was as follows: Corning 2947 glasses and commercial ITO/Glass (Delta Technologies CB-40IN-0107) with dimen- sions of 25mm×25mm×0.7mm were used as substrates, whichweresequentiallycleanedpriortothedepositionbyan ultrasonic agitation bath in distilled water, acetone and iso- propylalcoholfor5.0mineach.Subsequently,theyweredried withN2and heatedatasubstrate temperature (Ts) of 623K.

AfterreachingTs,ashutterplacedabovetheevaporationsource wasremoved,thesubstratewasputinfrontofitandadjusted inGLAD modewithaseparationdistanceof L=6.0cm.The sourceandthesubstratewereputincounter-rotation;thismeans thatthesourcerotatedinadirectionoppositetothatofthesub- strate.The background pressureinthe vacuum chamber was

∼4Pa and the deposition time was 1.0–3.0min for different typesofsamples.

Thecross-sectionalmorphologies ofCdSfilms werechar- acterizedusing afieldemissionscanningelectronmicroscope (FESEM)JEOL7600Finstrument.TheX-raydiffraction(XRD) data was obtained by a D5000Siemens X-ray diffractometer withabeamunderofCuK␣filteredmonochromaticradiation (λ=0.15418nm)by40kVwith35mAandaperturediaphragm of0.2mm.Thediffractogramswereregisteredinthestepscan mode with a beam incidence angle of 1^◦ and recorded in 2θ=0.02^◦stepswithasteptimeof10sina2θrangeof20–55^◦. TheopticalmeasurementswerecarriedoutusingaUV–visAgi- lent8453spectrophotometerwith0.1nmresolution,intherange of 400–900nm. The optical band gaps were calculated with directtransitionsonly.

3. Resultsanddiscussion

The flux density value defined by Eq. (1) decreases as α increases. Particularly, for α=80^◦, φ was estimated to be

∼1×10²¹CdSatoms/cm²s.Atfirst glance,thefilmsshowed noevidenceofpin-holeformation;thiscouldbeexplainedby thecounter-rotationbetweenthesubstrateandtheevaporation source,whichinturncouldfavorauniformangulardistribution oftheCdSvaporflux.

The optimal ratio of ω/Ω was found to be 55/20, this valuewasselectedfromtheoptimizationofthesolarweighted transmission (SWT) calculation, while the other deposition parametersremainedconstant,thiswill bediscussed inmore detaillaterinthetext.

AfterthegrowthoftheCdSthinfilms,thetransversalmor- phology was observed by FESEM with a magnification of 50,000×and100,000×.InordertoobtainaSEMimagewith goodquality,whichwouldallowthecorrectvisualizationofthe nanocolumnarstructures,itwasnecessarytoemployadeposi- tiontimeof3and2minfortheglassandITO/glasssubstrate, respectively.

The cross-sectionalimageof theCdSarraysonglasssub- stratesgrownatα=80^◦isshowninFigure2(a).Highlyuniform anddenselypackedarraysofCdSnanocolumnarstructureswere successfully formed; theydisplayed vertical alignmentswith great uniformity through all their thicknesses, which was of

∼300nm.Thediameterofthesestructureswasapproximately 30nm,anditwasestimatedfromthezoomontheFESEMcross- sectionimages.Thepresenceoffissureswithsmallgapswhich dividesomeofthesestructurescanalsobeobserved.

Figure 2(b) shows a transversal structure for the sample grown glass/ITO substrate at α=80^◦. A uniform and dense columnar distributionwasalsoobserved,andthethicknessof thethinfilmwasapproximatelyof∼200nm.However,thepres- enceoffissureswaslessincomparisontothesampleshownin Figure2(a),sincetheywerenotpresentthroughallthethickness ofthefilm.ThiswasevidencethattheCdSdepositionwasmore denselypackedontheglass/ITOsubstrate.

Figure3showstheXRDnormalizedpatternforthethinfilms grownatα=0^◦,20^◦and80^◦with1.0minofdepositiontime.The single-phasehexagonalCdSWurtzitestructure(Razik,1987)is observedwithapreferentialorientationinthe(002)plane.Low intensitypeaksforthe(100),(101),(110),(103)and(112)planes arealsoobserved.TheXRDdatashowedthatallthepeaksare shifted towardhigher 2θ values incomparison tothat of the standardpattern(2θ=26.507^◦);thisisevidenceofcompressive strain(Zak,Majid,Abrishami,&Yousefi,2011).Thecrystal- litesize(DXRD)wascalculatedfromtheplane(002)usingthe Scherrer’sequation(Wangetal.,2015):

D_XRD= 0.9λ β002cosθ002

(2)

whereβ002isthefull-widthathalf-maximumandλisthewave- lengthoftheX-rayradiationemployed(λ=0.15418nm).The valuesofDXRDaregiveninTable1.Thecrystallitesizesval- ueswerebetween∼25nmand∼35nm.Thelatticeparameter c002 was calculated employing the standard latticegeometry

a

b

c

d

100 nm

100 nm 100 nm

100 nm

Fig.2.FESEMimageof:verticalnanocolumnarCdSasnanostructuregrownatα=80^◦onaglasssubstratewithamagnificationof(a)100,000×and(b)50,000×.

VerticalnanocolumnarCdSasnanostructurelayergrownatα=80^◦onanindiumtinoxide(ITO)-coatedglasssubstratewith(c)100,000×and(d)50,000×.

Table1

Structuralparametersforsampleswithdifferentincidentdepositionfluxangles.

α(degree) 2θ002(degree) β002(radians)×10⁻³ c002(nm) ε(%) DXRD(nm)

0 26.54 4.6 0.6711 −0.119 ∼31

20 26.57 4.1 0.6705 −0.208 ∼35

80 26.63 5.7 0.6691 −1.61 ∼25

20 25 30 31 40 45 50 55

2θ (º)

80 20

0

α (º)

Normalized count (a.u.) (101)(100) (002) (110) (103) (112)

Fig.3. XRDnormalizedpatternforCdSthinfilmgrownwithdifferentincident depositionfluxangles.

equations for the hexagonal structure (Iribarren, Hernández- Rodríguez, & Maqueira, 2014; Kaur, Kumar, Sathiaraj, &

Thangaraj,2013).Asitwasmentionedbefore,theshiftingofthe 2θpositionstohighervalueswithrespecttotheCdSstandard

pattern isevidenceofanegativecompressivestrain,thissug- geststhatthe latticeparametersof theunitcellwouldshowa decrease,thisagreeswiththereportedc002latticeparameterval- uessummarizedinTable1,whichareingeneralsmallerthan thoseofthestandardCdS(c0=0.6719nm).

Thec002parameterdecreasesforhigherincidentdeposition fluxangles,thiscorrelationbetweentheaforementionedparam- etersisillustratedinFigure4.Thegeneralreductionofthec002

values indicates that there is shrinkage on the c-axisfor the CdSunitcellwithrespecttothepatternvalue.Furthermore,the strainεalongthec-axiswascalculatedbyusingtheexpression ε=(c−c0)/c0×100%(Ghosh,Basak,&Fujihara,2004).The absolutevaluesofthec-axisstrainincreaseasαincreases,thisis alsoshowninTable1.Thenegativevaluesofεconfirmshrink- ageinthec-axis.Thissuggeststhatahighincidentdeposition fluxangleleadstoalatticecompressioneffectalongthec-axis direction,asitwasalsostatedintheXRDanalysis.

In orderto find the optimal ω/Ω ratio the solar weighted transmission(SWT)wascalculatedforthethinfilmsgrownat differentsubstraterotationspeeds(ω).Figure5showstheSWT asafunctionω,fordifferentsamplesallgrownatα=80^◦and with 1.0min of deposition time. Basedon the SEM images,

0.673

0.672

0.671

0.670

0.669

0.668

0 20 40 60 80

α (degrees)

Parameter c (nm)

c_pattern = 0.6719 nm

Fig.4.c002parameterversusthetiltingangleofthesubstrate.

95

90

85

80

75

70 0 200 400 600 800 1000

SWT (%)

ω (rpm)

Fig.5.Solarweightedtransmission(SWT)measurementsofCdSthinfilmsas afunctionofsubstraterotationgrownwithdepositionfluxangleofα=80^◦.

thisvalue of αwas selected since it allowed observing with moreclaritythenano-columnardepositiononthesubstrate.The SWTformulaisgivenbyMendez-Gamboa,Castro-Rodriguez, Perez-Quintana,Medina-Esquivel,andMartel-Arbelo(2016):

SWT =

_λ2

λ1 T(λ)S(λ)dλ

_λ2

λ1 S(λ)dλ (3)

whereS(λ)istheAM1.5solarradiationspectrum,T(λ)isthe opticaltransmittance, andthenumerator ofEq.(3) represents the solar photon flux-weighted transmittancefor the sample.

TheSWTis theratio of theusable photons transmittedfrom thetotalavailablephotons,itcanbeestimatedthroughthenor- malizationofthe transmittancespectra, andthiswasdoneby integratingthe solarspectralphotonfluxover thewavelength rangeof400–900nm.Figure5showsthatSWTreachedamax- imumof∼92%atω=55rpm,andforgreaterωitdecreasedto

100

80

60

40

20

0

400 500 600 700 800 900

80º 20º

0º

Wavelength (nm)

Transmittance (%)

Fig.6.TransmittancespectraofCdSthinfilmsgrownwithincidentdeposition fluxangles.

2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9

hν (ev)

(αhν)2

20º

80º

Fig.7.TaucplotofCdSthinfilmsgrowthatα=0^◦,20^◦and80^◦.

∼71%.Inaccordancewiththecalculationsof SWTaratioof ω/Ω=55/20wasselectedastheoptimumvalue.

Figure6showsthetransmittancespectrumofsamplesgrown atα=0^◦,20^◦and80^◦with1.0mindepositiontime.Atransmit- tancevalueof∼95%wasobservedforthesamplewithα=80^◦. Theincreasinginthetransmittancevalueisduetothechange inthethickness,sincealargertiltingangleleadstoasmaller depositionrate(Nagiri,Yambem,Lin,Burn,&Meredith,2015), resultinginthinnersamples,whichinturnwillproducehigher transmittancevalues.Ascanbeseen,thetransmittanceforthe film grown at α=80^◦ at the 400nm wavelength is less than 80%, while it is below40% for the sampleswith α=0^◦ and 20^◦.The optical band-gap energy (EG) was calculatedusing theTaucformalism(Tauc,Grigorovici,Vancu,1966)asshown inFigure7,EGvaluesbetween∼2.43eVand∼2.45eVwere

2.8

2.4

2.0

1.6

1.2

500 600 700 800 900

Refractive index

Wavelength (nm) α=80º

α=20º α=0º

Fig.8.Refractiveindexversuswavelengthofthinfilmsgrownatdifferentvalues ofαfora500–900nmwavelengthrange.

obtained,whichagreesverywellwiththepreviousreportsofthe bandgapenergyfortheCdS(Mohamed,2015).Additionally, therefractiveindex(n)wascalculatedfromtheequation(Kim, Yim,Son,&Leem,2012):

n= 1+R 1−R+

 4R

(1−R)² −K² (4)

where R is the normal-incident light reflection, k is the extinctioncoefficientdefinedask=αabsλ/4π,whereαabsisthe absorptioncoefficientandλthewavelength.Intheregionbelow the band gap energy the absorption follows A→0.Then the expressionT+R+A=1canbereducedtoR=1−Tfromwhere itispossibletoestimatethereflectedlight.Thecalculatedrefrac- tiveindexatdifferentincidentdepositionfluxanglesaredepicted inFigure8,whichshowsthatitdecreasesasαincreases.For α=80^◦therefractiveindexwas∼1.6atwavelengthnear550nm, forα=0^◦and20^◦itwas∼2.4,whichisveryclosetotherefrac- tiveindexvaluethatisrequiredforathinfilmthatisdesiredtobe usedasanantireflectionlayerbetweenITOandCdTeaccording tothelaw(nITOnCdTe)^1/2(Prevo,Hwang,&Velev,2005)where nITOandnCdTearetherefractiveindexofITOandCdTelayers, respectively.

4. Conclusions

In this paper, an innovative source for vapor effusion is reported.Suchasourcewasemployedtoprovideavaporfluxin theGLADdepositionmodewhichledtotheobtainingofCdS verticalnanostructuredthinfilms.Thesourceandthesubstrate werecounter-rotatedwithrespect toeachother,atanoptimal ratio ω/Ω: 55/20. The effect caused on the morphological, structuralandopticalpropertiesfortheCdSthinfilmsgrownat differentincidentdepositionfluxangleswasinvestigated.The principalcharacteristicsoftheseCdSnanocolumnarthinfilms wereauniformdistributionoverthesubstratewithdimensions ofheightanddiametersof∼200nmand∼30nm,respectively.

Thefilmsshowedasinglehexagonalwurtzitestructure phase

with a preferential (002) plane orientation, band gap energy of∼2.42eVandarefractiveindexbetween∼1.4and∼2.4in wavelengthof550nm.

Conflictofinterest

Theauthorshavenoconflictsofinteresttodeclare.

Acknowledgments

Authors wish to thank Oswaldo Gómez, Mario Herrera, Daniel Aguilar, Dora Huerta and Wilian Cauich for techni- calassistanceandLourdesPineloforhersecretaryassistance.

ThisworkhasbeensupportedbyProjectNo.CB/2012/178748 CONACYT/México. One of the authors (A. Iribarren) also thanks CONACYT/México for sabbatical leaves for Mexi- canandforeignresearcherslivingabroad,underprogramNo.

260967.

References

Alamri,S.N.(2003).ThegrowthofCdTethinfilmbyclosespacesublimation system.PhysicaStatusSolidiA:ApplicationsandMaterialsScience,200(2), 352–360.

Baghchesara,M.A.,Yousefi,R.,Cheraghizade,M.,Jamali-Sheini,F.,&Saáedi, A.(2016).PhotocurrentapplicationofZn-dopedCdSnanostructuresgrown bythermalevaporationmethod.CeramicsInternational,42,1891–1896.

Barranco,A.,Borras,A.,Gonzalez-Elipe,A.R.,&Palmero,A.(2016).Per- spectivesonobliqueangledepositionofthinfilms:Fromfundamentalsto devices.ProgressinMaterialsScience,76,59–153.

Cruz,L.R.,Pinheiro,W.A.,Medeiro,R.A.,Ferreira,C.L.,Dhere,R.G.,

& Duenow, J.N.(2013).Influence ofheattreatment and backcontact processingontheperformanceofCdS/CdTethinfilmsolarcellsproduced inaCSSin-linesystem.Vacuum,87,45–49.

Cruz-Campa,J.L.,&Zubia,D.(2009).CdTethinfilmgrowthmodelunderCSS conditions.SolarEnergyMaterialsandSolarCells,93,15–18.

Elbar,M.,Tobbeche,S.,&Merazga,A.(2015).Effectoftop-cellCGSthickness ontheperformanceofCGS/CIGStandemsolarcell.SolarEnergy,122, 104–112.

Gerthoffer,A.,Roux,F.,Emieux,F.,Faucherand,P.,Fournier,H.,Grenet,L., etal.(2015).CIGSsolarcellsonflexibleultra-thinglasssubstrates:Char- acterizationandbendingtest.ThinSolidFilms,592,99–104.

Ghosh,R.,Basak,D.,&Fujihara,S.(2004).Effectofsubstrate-inducedstrain onthestructural,electrical,andopticalpropertiesofpolycrystallineZnO thinfilms.JournalofAppliedPhysics,96(5),2689–2692.

Han,J.-F.,Liao,C.,Cha,L.-M.,Jiang,T.,Xie,H.M.,Zhao,K.,etal.(2014).

TEMandXPSstudiesonCdS/CIGSinterfaces.JournalofPhysicsand ChemistryofSolids,75(12),1279–1283.

Herman,M.,&Sitter,H.(2012).Molecularbeamepitaxy:Fundamentalsand currentstatus.Berlin,Germany:Ed.SpringerScienceyBusinessMedia.

Iribarren, A.,Hernández-Rodríguez,E., &Maqueira,L. (2014).Structural, chemicalandopticalevaluationofCu-dopedZnOnanoparticlessynthesized byanaqueoussolutionmethod.MaterialsResearchBulletin,60,376–381.

Jie,N.,Yu,Z.,Qin,Z.,&Zhengjun,Z.(2008).Morphologyin-designdeposi- tionofHfO2thinfilms.JournaloftheAmericanCeramicSociety,91(10), 3458–3460.

Kaiser,N.(2002).Reviewofthefundamentalsofthin-filmgrowth.Applied Optics,41,3053–3060.

Kaur,J.,Kumar,P.,Sathiaraj,T.S.,&Thangaraj,R.(2013).Structural,opti- calandfluorescencepropertiesofwetchemicallysynthesizedZnO:Pd²⁺

nanocrystals.InternationalNanoLetters,3(1),4.

Kim,M.S.,Yim,K.G.,Son,J.S.,&Leem,J.Y.(2012).EffectsofAlcon- centrationonstructuralandopticalpropertiesofAl-dopedZnOthinfilms.

BulletinoftheKoreanChemicalSociety,33(4),1235–1241.

Kim,D.,Kim,Y.P.,Choi,Y.,Park,Y.S.,&Lee,J.(2015).Opticalandstruc- turalpropertiesofsputteredCdSfilmsforthinfilmsolarcellapplications.

MaterialResearchBulletin,69,73–78.

Lee,S.,Lee,E.S.,Kim,T.Y.,Cho,J.S.,Eo,Y.J.,Yun,J.H.,etal.(2015).

EffectofannealingtreatmentonCdS/CIGSthinfilmsolarcellsdepending ondifferentCdSdepositiontemperatures.SolarEnergyMaterials&Solar Cells,141,299–308.

Leem,J.W.,&Yu,J.S.(2011).GlancingangledepositedITOfilmsforefficiency enhancementofa-Si:H/␮c-Si:Htandemthinfilmsolarcells.OpticsExpress, 19,258–269.

Li,H.,&Liu,X.(2015).ImprovedperformanceofCdTesolarCellswithCdS treatment.SolarEnergy,115,603–612.

Liang,G.-X.,Fana,P.,Zheng,Z.-H.,Luo,J.-T.,Zhanga,D.-P.,Chen,H.-M.,etal.

(2013).Room-temperaturepreparationandpropertiesofcadmiumsulfide thinfilmsbyion-beamsputteringdeposition.AppliedSurfaceScience,273, 491–495.

Lisco,F.,Kaminski,P.M.,Abbas,A.,Bass,K.,Bowers,J.W.,Claudio,G., etal.(2015).ThestructuralpropertiesofCdSdepositedbychemicalbath depositionandpulseddirectcurrentmagnetronsputtering.ThinSolidFilms, 582,323–327.

Mendez-Gamboa,J.A.,Castro-Rodriguez,R.,Perez-Quintana,I.V.,Medina- Esquivel,R.A.,&Martel-Arbelo,A.(2016).Afigureofmerittoevaluate transparentconductoroxidesforsolarcellsusingphotonicfluxdensity.Thin SolidFilms,599,14–18.

Messier,R.,Gehrke,T.,Frankel,C.,Venugopal,V.C.,Ota˜no,W.,&Lakhtakia, A.(1997).Engineeredsculpturednematicthinfilms.JournalofVacuum Science&TechnologyA:Vacuum,Surfaces,andFilms,15,2148.

Mohamed,H.A.(2015).Optimizedconditionsfortheimprovementofthinfilms CdS/CdTesolarcells.ThinSolidFilms,589,72–78.

Nagiri,R.C.R.,Yambem,S.D.,Lin,Q.,Burn,P.L.,&Meredith,P.(2015).

Room-temperaturetilted-targetsputteringdepositionofhighlytransparent andlowsheetresistanceAldopedZnOelectrodes.JournalofMaterials ChemistryC,3(20),5322–5331.

Prevo, B.G.,Hwang, Y., &Velev, O.D.(2005). Convectiveassembly of antireflectivesilicacoatingswithcontrolledthicknessandrefractiveindex.

ChemistryofMaterials,17(14),3642–3651.

Razik,N.A.(1987).Useofastandardreferencematerialforpreciselattice parameterdeterminationofmaterialsofhexagonalcrystalstructure.Journal ofMaterialsScienceLetters,6(12),1443–1444.

Robbie,K.,&Brett,M.J.(1997).Sculpturedthinfilmsandglancingangle deposition:Growthmechanicsandapplications.JournalofVacuumScience

&TechnologyA:Vacuum,Surfaces,andFilms,15,1460.

Tauc,J.,Grigorovici,R.,&Vancu,A.(1966).Opticalpropertiesandelectronic structureofamorphousgermanium.PhysicaStatusSolidiB:BasicSolid StatePhysics,15,627–637.

Wang,L.W.,Wu,F.,Tian,D.X.,Li,W.J.,Fang,L.,Kong,C.Y.,etal.(2015).

EffectsofNacontentonstructuralandopticalpropertiesofNa-dopedZnO thinfilmspreparedbysol–gelmethod.JournalofAlloysandCompounds, 623,367–373.

Yilmaz,S.(2015).TheinvestigationofspraypyrolysisgrownCdSthinfilms dopedwithfluorineatoms.AppliedSurfaceScience,357,873–879.

Yücel,E.,&Kahraman,S.(2015).Theeffectsofcoumarinadditiveonthe propertiesofCdSthinfilmsgrownbychemicalbathdeposition.Ceramics Internationals,41,4726–4734.

Yun, J. H., Cha, E. S., Ahn, B. T., Kwon, H., & Al-Ammar, E. A.

(2014).PerformanceimprovementinCdTesolarcellsbymodifyingthe CdS/CdTeinterfacewithaCdtreatment.CurrentAppliedPhysics,14(4), 630–635.

Zak,A.K.,Majid,W.A.,Abrishami,M.E.,&Yousefi,R.(2011).X-rayanalysis ofZnOnanoparticlesbyWilliamson–Hallandsize–strainplotmethods.

SolidStateSciences,13(1),251–256.

Zhang,C.,Huo,H.,Li,Y.,Li,B.,&Yang,Y.(2013).PreparationofhelicalCdS nanotubesusingasol–geltranscriptionapproach.MaterialsLetters,102, 50–52.

Zhou, Q.,Li,Z., Ni,J.,& Zhang,Z. (2011).Asimplemodelto describe the rule of glancing angle deposition. Materials Transactions, 52(3), 469–473.

Zhou, Y., Taima,T., Miyadera, T., Yamanari, T., Kitamura, M., Nakatsu, K., et al. (2012). Glancing angle deposition of copper iodide nanocrystals for efficient organic photovoltaics. Nano Letters, 12(8), 4146–4152.