Synthesis and characterization of ZnO micro-rods and temperature-dependent characterizations of heterojunction of ZnO microrods/CdTe and ZnO microrods/ZnTe structures

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SensorsandActuatorsA261(2017)56–65

ContentslistsavailableatScienceDirect

Sensors

and

Actuators

A:

Physical

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / s n a

Synthesis

and

characterization

of

ZnO

micro-rods

and

temperature-dependent

characterizations

of

heterojunction

of

ZnO

microrods/CdTe

and

ZnO

microrods/ZnTe

structures

M.A.

Olgar

a

_,

_Y.

_Atasoy

a

_,

_E.

_Bacaksız

a

_,

_S¸

_akir

_Aydo˘gan

b,c,∗

a_Department_of_Physics,_Faculty_of_Sciences,_Karadeniz_Technical_University,₆₁₀₈₀_Trabzon,_Turkey

b_Department_of_Physics,_Faculty_of_Sciences,_Atatürk_University,_25240,_Erzurum,_Turkey

c_Department_of_{Environmental}_Engineering,_Faculty_of_Engineering,_Ardahan_University,_Ardahan,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received30September2016

Receivedinrevisedform27April2017

Accepted28April2017

Availableonline3May2017

Keywords: Microrods Heterojunction XRD SEM

a

b

s

t

r

a

c

t

ZnOmicrorodswerefabricatedonZnO-coatedSnO2 glasssubstratesbyspraypyrolysismethod.To obtainp-nheterojunction,ptypeCdTeandZnTelayersweredepositedonZnOmicrorods.Thestructural characterizationsdemonstratedthatZnOmicrorodshaveahexagonalwurtzitestructurewithvertically alignedrodmorphology.Additionally,hexagonalrodgeometrywascompressedbycoatingCdTelayer onmicro-sizedZnOrods.ThediodetyperectifyingbehaviourofZnOmicrorods/CdTeandZnO micro-rods/ZnTeheterojunctionshavebeencarriedoutandtheelectricalcharacteristicsofbothdeviceshave beenanalyzedwithcurrent-voltagemeasurementsasafunctionoftemperature.Althoughthemismatch betweenZnOandZnTe,theZnOmicrorods/ZnTeheterojunctionshowedgoodrectifyingbehaviouratall temperatures.Accordingtotheourﬁndings,bothidealityfactornandbarrierheightbare temperature-dependentduetothedeviationfrompurethermionicemissiontheoryandinhomogeneityattheinterface ofZnO-CdTeandZnO-ZnTe.

1. Introduction

Zincoxide(ZnO)isaversatilematerialwhichhasagreat poten-tialforuseinmanybasicapplicationsofsemiconductingdevice technology.ZnOisan-typesemiconductorcompoundwithband gapof3.2eVatroomtemperatureanditisconsideredoneofthe mostpromisingalternativematerialtoGaNinmanyapplications; lightemittingdiodes,solarcells,andtransparentconductingoxides [1–4].ComparedwithGaN, ZnOhasa considerablelarger bind-ingenergyatroomtemperature(∼60meV)thatmakesitanideal candidateforoptical devicessuchasUVlasers[5].ZnOreveals differentmorphologiessuchasnanotubes[6],nanocones[7],and nanowires[8,9]withrespecttofabricationmethods.Various appli-cationﬁeldsavailableforZnOmaterialaccordingtoitsmorphology. ButregularlymorphologycontrollableZnOnanometarialprocess isstillunderdebate.Therefore,inthisstudyZnOmicrorodswere producedbylow-costnon-vacuummethod,spraypyrolysis.

∗ Correspondingauthorat:DepartmentofPhysics,FacultyofSciences,Atatürk

University,Erzurum,25240,Turkey.

E-mailaddress:saydogan@atauni.edu.tr(S¸.Aydo˘gan).

Recently, ZnO based core-shell structureshave gained great attentionssincebandalignmentofthesestructuresseparatethe chargecarriers,electronsandholes,intodifferentregionsandthus may enhance thecarrier lifetimes. Vertically aligned core/shell micro-nanorodstructureswiththatkindofbandalignmenttakea wideapplicationspartinoptoelectronicdevicessuchassolarcells [10–12].Becauseitpresentshighsurface-to-volumeratioandmore preferablelighttrappingeffectcomparedtoplanarstructures[8]. Inaddition,ZnO-basedcore/shellstructureshaveaspecialinterest since ZnOshows perfect chemicalstability, nontoxic, and envi-ronmentallyfriendlyproperties. Remarkableoutputs havebeen obtained in ZnO/ZnS [13], ZnO/ZnSe [14], ZnO/ZnTe [15], and ZnO/CdTe[16]incore/shellstructuresduetoexcellentproperties ofthesestructures.Amongstthep-typesemiconductinglayerofthe abovementionedheterojunctions,ZnTeandCdTearewidelyused indevicetechnology.ZnTeisintrinsicallyp-type semiconductor withadirectbandgapof2.26eVatroomtemperature[17].And, CdTeisasemiconductingmaterialwhichhasabandgapof1.45eV and highabsorptioncoefﬁcient [18]. Severalstudieshave been managedonZnO/CdTeandZnO/ZnTebasedcore/shellstructures intheliterature[19,20].Butelectricalpropertiesofthese struc-turesatextendedlowtemperaturerange(150K(₋₁₂₃◦C)—300K (27◦C))havenotbeenexaminedinmoredetailed.Inthisstudy, http://dx.doi.org/10.1016/j.sna.2017.04.053

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M.A.Olgaretal./SensorsandActuatorsA261(2017)56–65 57

Fig.1.–Schematicdiagramofspraypyrolysissystem.

structuralandmorphologicalcharacterizationsofZnO/CdTeand ZnO/ZnTecore/shellstructureswerecarriedoutinadvancethen temperature-dependentelectrical characterizationof aforemen-tionedstructureswereperformed.

2. Experimental

ZnOmicrorodswerefabricatedonSnO2-coatedglasswithtwo

stagemethod;coatingZnOseedthinlayeronSnO2-coatedglass

anddepositionofZnOmicrorodsbyspray pyrolysis.Firstofall, thecoatedZnOseedlayerwasobtainedbydissolving5mMzinc acetatedihydrate(Zn(OOCCH3)2·2H2O)inethanol.Secondly,ZnO

microrodsweresynthesizedbyspraypyrolysismethodinair atmo-sphere.Thechemicalsolution(0.1M)waspreparedbydissolving zincchloride (ZnCl2,98+ %,AlfaAesar) indeionized water.The

schematicdiagramofspraypyrolysissystemwasshowninFig.1 [21].Priortodeposition,allglasssubstrates(10×15mm2₎_were

cleanedinacetone,ethanol,anddistilledwatersequentially.Then, cleanedsamplesweredried withairflow. Theflow rateofthe solutionwasadjusted2ml/min.Thedistancebetweennozzleand substrate waskeptat 5cm and thesubstrate temperaturewas maintainedto823K(550◦C).Inordertoobtainhomogenousfilms, thesubstrateswererotatedwithaspeedof10rpminatmospheric pressure[21–23].

ToobtainZnO/CdTeandZnO/ZnTemicrorodscore-shell struc-ture,CdTeandZnTelayersweredepositedseparatelyontheZnO microrodsbythermalevaporationsysteminaquasi-closed vol-umeatapressureof3×10−2Pa.Aquartzlinegraphitechamber wereusedasavacuumapparatusincludedaheatedsourcebottle, aquartzfilteroverthesourcebottleandsubstrateholdermounted 8cmabovethequartzfilter.Theclamp-mountedsubstrateswere faceddownontothesampleholder.Thesourcetemperaturewas controlledviathermocouple[17,18].CdTeandZnTethinfilmswere grownonZnOmicrorodsatasubstratetemperatureof300K(27◦C) usinghighpurity (99.99%,Alfa Aesar) CdTe and ZnTe powders. Evaporation rateof thesourcematerialswas controlledbythe sourcetemperatureintherangeof873–923K(600–650◦C). Evap-orationrateofCdTeandZnTelayerswereabout1.5and3nm/sand

theirfinalthicknesswerenominally2␮m.Thicknessesofthefilms weredeterminedfromscanningelectronmicroscopy(SEM,JEOL JST-6400)cross-sectionimages(notshown).Afterdepositionof CdTeandZnTe,thecompletestructureofZnO/CdTeandZnO/ZnTe wereannealedat673K(400◦C)for30mininairandAratmosphere, respectively. The reason for this anneal step wasin general to improvethecompositional,structuralandelectricalcharacteristics oftheroom-temperature-depositedlayers.Oneadditionalreason for theCdTeanneal inair wastoassurep-typeconductivityin thisfilm,whichcanbeobtainedbyannealinginoxygencontaining atmosphereataround673K(400◦C)[24].Annealingtemperature andthepresenceofoxygenintheannealingatmosphereis essen-tialforobtainingp-typeCdTefilmsiftheas-grownlayersarenot intentionallydoped[24].Ouras-depositedCdTefilmsshowedhigh resistivityn-typebehaviour.TheconversionofCdTefilmsfromn toptypewasachievedinmanypaststudieswithannealing treat-mentsincludingoxygen.Thisphenomenonwasattributedtosome reasonssuchasp-typenatureofoxygenasadopantand/or forma-tionofCdvacanciesandvacancycomplexes,whichareacceptors inCdTefilms[25]ismajormechanismresponsiblefortype con-versioninCdTefilms[26].ThenZnO/CdTestructurewasetchedin (Br+methanol)solutiontoremovethepossibleoxidelayeronthe surfaceoftheCdTethinfilm.TheAufrontcontactwascreatedby thermalevaporationandInGapastewasusedasbackcontact.The frontandbackcontactswereconstructedasadotwithasizeof 1mmdiameter.ElectricalcharacterizationoftheZnO/CdTe struc-turewas performedby I–Vmeasurements using Keithley2400 source-meter.

The X-ray diffraction (XRD) measurements were performed byRigakuD/Max-IIICdiffractometerwithCuK_␣radiationsource (␭=1.5405Å) inthe range of2=20–60◦ atroom temperature. The morphological analyses of core-shell structures were per-formedbySEM.InadditiontoSEMimagesofZnOmicrorods,the surfaceroughnesswasinvestigatedbyAtomicForceMicroscopy (AFM,Hitachi5100N).TemperaturedependentI–Vmeasurements ofZnO/CdTeandZnO/ZnTeheterostructureswerecarriedoutin ahome-madeHecryostatintherangeof150K(−123◦_C)—300_K

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58 M.A.Olgaretal./SensorsandActuatorsA261(2017)56–65

Fig.2.–XRDpatternsofannealedZnO/CdTe(a)andZnO/ZnTe(b)structures.

3. Resultsanddiscussion

XRDpatternsofZnO/CdTeandZnO/ZnTeafterannealing treat-mentareshowninFig.2.AscanbeseeninFig.2(a–b),astrong preferentialorientationinthe(002)planedirectionwhichbelongs towurtzitestructure ofZnO isobserved inallpatterns. Promi-nentdiffractionpeaksof(100),(101),(102)and(110)planesof ZnOweresymbolizedwith‘x’.The(002)preferentialorientation peakofZnOmicrorodremainedafterobtainingtheZnO/CdTeand ZnO/ZnTecore-shellstructures.Furthermore,thediffractionpeaks of(111),(220)and(311)planeorientationsofCdTeandZnTeare demonstratedinFig.2(a,b).Itmeansthatbothofthetwocore/shell structuresexhibitedthezincblendstructureofCdTeandZnTe.The calculatedlatticeparameterofthe(111)peakofCdTeandZnTeis foundtobe0.647nmand0.609nm,respectively.Thesevalues are inagoodagreementwiththescientiﬁcreports[17,18].

SEMmicrographstakenfromthetopviewoftheZnOmicrorods, ZnO/CdTe,andZnO/ZnTeweredemonstratedinFig.3(a–c).ZnO microrodswithhexagonalandsmoothfacetsweregrownalmost verticallyalignedonthesurfaceofthesubstrate.Thegrowing direc-tionoftherodsisnotuniformthroughoutthecoatedstructure. TheZnOmicrorodsgrewentirelyoverthesurfaceofasubstrateof 1cmx1cmsizeanddiameterofthemicrorodsvariesfromabout 0.7–3␮m.ThecoatingofZnOrodschangedtherodsmorphology, especiallyforCdTecoatedsurface.ThecoatingofZnOrodswith CdTecausedformationofafusedstructureafterannealing treat-mentat673K(400◦C).Thestructurehasalsosomeplaceswhich connecttherodstoeachother.Moreimportantly,thehexagonal rodgeometrywascompressedordisappearedbycoatingCdTe.The structureofthesamplecoatedbyZnTeisdifferentfromthe sam-plecoatedbyCdTe.Nofusedstructureoccursandtherodsremain theiroriginalrodshapewithacoating.Itresemblesasabunchof grapes.Alsowell-distributedandroundedultra-ﬁnegrainsformed ontheZnOrods.

Fig.4displays5␮mx5␮mAFMimageforZnOmicrorods.We observedformation ofvertically aligned hexagonalshapedZnO microrodswithvarieddiametersizes.TheroughnessofZnO micro-rodsweredeterminedbyAFMmeasurements.Ascanbeseenin

theﬁgure,theroughnessofthemicrorodsmeasuredfromthree (3)differentregionsandultimateresultwastakenasaverageof 3differentmeasurements.Theroughness ofthemicrorods was obtainedaround190nm.

As stated in the Experimental section, the ZnO/CdTe struc-turewasannealedat673K(400◦C)inairatmospheretoassure p-typeconductivityintheCdTelayer.Annealingprocesshas sig-nificanteffectsonthepropertiesofCdTethinfilms.Bacaksızetal. showedthat400◦Ciscriticalannealingtemperaturesincesome changesinthesurfacestructureandgrainmorphologymayoccur. Theyconcludedthatannealtemperaturesat400◦Candabovemay contributetoobtaindevice-qualitymaterial[18].Inaddition,the ZnO/ZnTestructurewasannealedatthesametemperatureinAr atmosphere.Inthiscasetherewasnoneedforoxygencontaining environmentduringannealingsinceZnTeisintrinsicallyp-type. Besides,airannealingwouldformaZnOsurfacelayerontheZnTe film.AscanbeseenfromourdatatheZnO/CdTestructuresappear morefusedthantheZnO/ZnTestructuresaftertheheattreatment. Thisobservationmaybeexplainedasfollows:CdTehasalower meltingpoint(1314K(1041◦C))thanZnTe(1568K(1295◦C)),and smallgrainCdTematerialdepositedatlowtemperatures(suchis thecaseforourexperiments)hasatendencyforgraingrowthand fusingattemperaturesaboveabout623K(350◦C),whichisnot thecaseforZnTe.Furthermore,whenannealedinair,CdTesurface getsoxidizedandaverythinCdTeO3compoundforms.CdTeO3has

evenlowermeltingpointof1063K(790◦C)[27].Inverythin sur-facelayerform,thismeltingtemperatureisexpectedtobemuch lower.Asaconsequenceofthefactorsdescribedabovewhenour ZnO/CdTestructureswereannealedinairthemobilityofatoms wereenhancedbytheﬂuxingactionofthelowmelting temper-atureoxideaswellastheinherentstraininthesmallgrainCdTe layerdeposited atroomtemperature,and graingrowth, coales-cenceandfusingwereobservedﬁllingthenarrowgapsbetween thehexagonalZnOrods.

Energy-banddiagramof ZnO/ZnTeheterojunctionis givenin Fig.5(a) forbefore contactand (b) for aftercontact and under thermalequilibrium.Intheﬁgure,eVd2isthebarriertohole

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Fig.3.–SEMmicrographsofZnOmicrorod(a),ZnO/CdTeheterojunction(b)andZnO/ZnTeheterojunction(c).

Fig.4.–AFMimageofZnOmicrorods.

injectionfromZnOsidetoZnTeside.Ecisthedifferenceinthe electronafﬁnitiesbetweenZnOandZnTematerials.Similar dia-gramisdrawnforZnO/CdTeandgivenFig.5(c,d).

The conduction bandoffsetof ZnO/ZnTe and ZnO/CdTe het-erostructuresaredeterminedusingtheelectronafﬁnities(␹)of ZnO,ZnTeandCdTeas4.20,4.95and4.50eV,respectively.This determinationforZnO/ZnTeheterojunctionisgivenas:

EC= (ZnTe)− (ZnO)=4.95−4.20=0.75eV (1)

Ev=

Eg(ZnO)−Eg(ZnTe)

+EC= (3.37−2.26)+0.75=1.86eV (2) andalsotheconductionandvalancebandoffsetofZnO/CdTe arerespectivelygivenas;

EC= (CdTe)− (ZnO)=4.50−4.20=0.30eV (3)

Ev=

Eg(ZnO)−Eg(CdTe)

+EC= (3.37−1.45)+0.30=2.22eV (4)

Fig.6depictsthecurrent–voltage (I–V)characteristicsof the ZnO/CdTeheterojunctionasafunctionoftemperatureintherange of 150K (−123◦_C)—300_K ₍₂₇◦_C)._As _can _be _seen, _all _I–V_plots

present typical rectifying characteristics of the heterostructure diode.Theforwardturn-onvoltageoftheZnO/CdTeheterojunction wasdeterminedtobeabout0.62V,atroomtemperature. Further-more,asseenfromthetemperaturedependenceofI–Vcurvesat constantforwardbias,thecurrentincreaseswithincreasingthe temperatureandthisresponsecanbeexplainedbythethermionic emission(TE)theoryofthemaincurrenttransportmechanism.The

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Fig.5.–Energy-banddiagramofZnO/ZnTeheterostructure(a)presentationofbandalignment(bandoffset)(b)underthermalequilibriumconditions.Theenergyband

diagramofZnO/CdTeheterostructurebeforec)andaftercontact,underthermalequilibrium(d).

temperaturedependentidealityfactornandthebarrierheightb

oftheZnO/CdTeheterojunctionweredeterminedfromEqs.(5)and (6),respectively[28]: n= _kTq _d(nI)dV (5) q˚b=kTln

AA∗T2_/I 0

(6) whereqiselectroniccharge,kisBoltzmannconstant,Visthebias, TisthetemperatureinKelvin,I0isthesaturationcurrent,A*the

Richardsonconstant,andAeffectivediodearea.Experimental val-uesoftheidealityfactorandthebarrierheightsoftheZnO/CdTe heterojunctionshowtemperaturedependency(seeFigs.7and8). Thebarrierheightforelectrons(seeFig.13):

˚b=Ecmax−EF (7)

whereEcmaxisconductionbandminimumofthesemiconductorin

thejunction.Ifanyinteractionbetweenjunctionmaterialswould neglect,thenbarrierheightwilldependonlyontheelectron afﬁn-ityofthejunctionmaterials(Schottky-Mottmodel).However,in practicethisisnotthecasesuchthatitisaffectedbythesurface

ofthesematerialduetothedanglingbondscauseinhomogeneity properties(Bardeenmodel).Oneresultofinhomogeneoussurfaces isthepresenceoflaterallyinhomogeneousbarrierheightasseenin Fig.13.Theanotheroneisbarrierheightinhomogeneity.According toinhomogeneitybarrier,barrierheightformedbetweencontact materialsmightbeaffectedbythenon-uniformityofthenature interfaciallayer.Similarly,inhomogeneousstructures,the ideal-ityfactorequals1.AccordingtoEqs.(5)and(6),forforwardbias, theidealityfactorandeffectivebarrierheightdependonbias(V) andtemperature,respectively.Namely,theeffectivebarrierheight ismeasuredashigherandidealityfactorisdeterminedlower,at highertemperatures.However,forpureTEconductionmechanism, theidealityfactorandbarrierheightvalueofidealcontactare inde-pendentoftemperature.

According to experimental results, both n and b are

temperature-dependentanditmaybeattributedtothe inhomo-geneityattheinterfaceof ZnOmicrorods and CdTe[29].n and b valuesof ZnO/CdTeheterojunctionvariedbetween5.98and

0.65eV at300K (27◦C) and 11 and 0.32eV at150K(−123◦_C),

respectively.Lowvalueofthebarrierheightisattributedtoimage forcelower.Forexample,Hussainreportedthiseffectinhisstudy

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Fig.6.–Thetemperaturedependentcurrent–voltage(I–V)characteristicofthe

ZnO/CdTeheterojunction.

Fig.7.–TemperaturedependentoftheidealityfactorsfortheZnO/CdTe

hetero-junction.

[30].Furthermore,Hazraetal.reportedtheimageforcelowering andinhomogeneitiesatp-siliconnanowire/n-ZnOthinﬁlm[31]. Sincethecarriersmaybefreeze-outatlowtemperatureitcanbe expectedthattheimageforceloweringtoo.Inaddition,thiseffect maybethenanostructurenatureofourdevicestoo.

The high values of ideality factor for ZnO/CdTe heterojunc-tioncanbeexplainedbythepresenceofhighdensityofinterface

Fig.8. –TemperaturedependentofthebarrierheightsfortheZnO/CdTe

hetero-junction.

states[32]inheterojunctiondiodes.Thepropertiesoftheinterface dependonthefabricationmethodandthusthenatureofinterface variesgreatlyfrommaterialtomaterial.So,thehighervaluesof theinterfacestatesdensityplayavitalroleinthecurrent mecha-nismofheterojunctionsuchthattheyactascarriertraps.Thetraps mayemptytowardthehighertemperatureduetotheionizationof traps[33].Afterionizationofthetraps,themobilityandthe con-ductivityincreaseandasaresultofthis idealityfactordecrease towardhighertemperatures.Asknownthatthehigherthevalue formobility,themoretheidealityfactorclosetounity.Contrary, recombinationof carriersviatrapsmayalsoincrease the ideal-ityfactortoosinceidealityfactoralsodependstherecombination processtoo[34].Otherreasonsofthehighidealityfactormaybe explainedasfollows:i)recombinationofelectronsandholesinthe depletionregion[35],ii)largeseriesresistanceassociatedwithZnO microrodsembeddedheterostructure[36].

Fig. 9 depicts experimental ideality factors versus barrier heightsforZnO/CdTeheterojunctiondeviceinrangeof temper-ature from 150K (−123◦_C) _to ₃₀₀_K ₍₂₇◦_C). _As _seen, _a _linear

variationisobservedbetweenbothparameters.Thislinear rela-tionshipbetweenthebarrierheightbandidealityfactornisan

evidenceoftheinhomogeneitiesofbarrierheights[37].The het-erojunctionparametersdeterminedfromexperimentalresultsmay giveinformationaboutthepresenceorexistenceofinhomogeneity asexplainedintextaboutthissubject.Namely,thisinhomogeneity mayoccuratthestageoffabricationofdevicesincetheproperties ofthedevicedependsonpreparationtechniquetoo.

Fig.10showsthecurrent–voltage (I–V)characteristicsofthe ZnO/ZnTe heterojunction carried out between 150K (−123◦_C)

and 300K(27◦C). Asseen thatall I–Vcurves are temperature-dependentand allshowtypical rectifyingcharacteristicsof the heterostructurediodeasZnO/CdTeheterojunction.Itisexpected thatthemaincurrenttransportmechanismtakesplaceafter jump-ingthechargecarriersoverthebarrier.Thisphenomenonoccursby theappliedforwardbias-voltage.Highervoltageshouldbeapplied atlowtemperaturesforjumpingthecarriersoverthebarriersdue tolowenergeticallychargecarriers.Therefore,whileI–Vcurves ofbothZnO/CdTeandZnO/ZnTestructuresdonotshow remark-able differences at reverse-biased voltage, the turn-on voltage increasesatforward-biasedsituationswithdecreasing tempera-turesforoperatingtheheterojunctionstructure.

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Fig.9.–ExperimentalidealityfactorsagainstbarrierheightsforZnO/CdTe

hetero-junctiondeviceintemperaturerangefrom150K(–123◦C)to300K(27◦C).

Fig.10.–Thecurrent–voltage(I–V)characteristicsoftheZnO/ZnTeheterojunction

asafunctionoftemperature.

Temperaturedependentoftheidealityfactorandthebarrier heightsoftheZnO/ZnTeheterojunctionobtainedfrom tempera-turedependentI–VplotsaregiveninFigs.11and12,respectively. Themeasurementsofcurrent-voltagecharacteristicsforZnO/ZnTe heterojunctionwerecarriedoutatlowtemperaturesintherange from150K(−123◦_C)_to₃₀₀_K₍₂₇◦_C)._As_seen_in_both_ﬁgures,

sim-Fig.11.–TemperaturedependentoftheidealityfactorsfortheZnO/ZnTe

hetero-junction.

Fig.12.–TemperaturedependentofthebarrierheightsfortheZnO/ZnTe

hetero-junction.

ilartemperatureresponseofZnO/CdTe heterojunctionhasbeen determinedfortheZnO/ZnTeheterojunctiontoo.

n and b values of ZnO/ZnTe heterojunction varied from

4.83 and 0.44eVat 300K (27◦C) to 7.29and 0.24eV at 150K (−123◦_C),_{respectively.}_An_important_reason_of_lower_barrier_may

beattributedtoincreaseinimageforceloweringattheinterface of theheterojunction material [38,39].Whenimage force low-eringisefﬁcient,itmeansthatThermionicEmissionconduction mechanismmaybeaffectedfromthat.So,ourexperimentalresults indicatethattheeffectoftheimageforceisobservedinZnO/ZnTe heterojunctiondevicesuchthatalowerbarrierheighthasbeen determinedcomparedtotheZnO/CdTeheterojunction. Further-more,itisknownthatinlaterallyinhomogeneousbarriersthere arevariousbarrierheightsasseenbelowFigure(3-dimentional)at theinterfaceofjunction/diodematerials.Onereasonofthismay

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Fig.13.–Apresentationofalaterallyinhomogeneousbarrierheightofrectifyingdevices.

defectsorpatches.Namely,moreelectronspassoverthebarrierat highertemperatureduetohavinghigherthermalenergy,contrary atlowtemperatures,theelectronspassoveratlowerbarrier[40]. Fig.13showsapresentationofalaterallyinhomogeneousbarrier heightofrectifyingdevices.

Inaddition,thedecreaseofidealitywithincreasingtemperature issharplyupto150K(−123◦_C)_and_that_changes_are_weaker._The

seriesresistanceoftheZnO/CdTeheterojunctionwhichisthe resis-tanceofneutralregionofsemiconductorsdecreaseswithincrease intemperatureandthermionicﬁeldemissionmechanismmaybe effectiveatlowtemperatures.Namely,thevoltagedropsarehigher atlowertemperatureandthiswillincreasetheexperimental ide-alityfactor[41].

However,towardstohighertemperaturethermionicemission (TE)iseffectiveconductionmechanism.So,aboutandafter150K (−123◦_C)_the_main_mechanism_is_TE_conduction_mechanism_and

theexperimentalidealityfactorislowerandhasmorestablevalue. Fig. 14 depicts experimental ideality factors versus barrier heightsforZnO/ZnTeheterojunctiondeviceintemperaturerange from150K(−123◦_C)_to₃₀₀_K₍₂₇◦_C)._The_decrease_in_ideality

fac-torand increasein thebarrier heightwithtemperaturecanbe explainedwithdeviationfrompurethermionicemissiontheory and inhomogeneity barrier properties. Accordingto thermionic emission(TE) electronsare emittedacrossa barrier.So, at low temperatures,theyhavelowenergyandpassoveratlowbarriers orviceversaandthiscontributetothelargeidealityfactor. Con-trary,athighertemperaturesthecarriersgainsigniﬁcantenergy tocrosshigherbarrierandthisresultsinloweridealityfactorat higher temperatures[43]. Disagreement betweenthe values of barrierheightsandtheconductionbandoffsetEC ofZnO/ZnTe

andZnO/CdTeindicatestheinhomogeneousbarrierheightnature betweenheterojunctionmaterials[42].Indeed,itishardtoseethe sameofourstudyinliterature.However,itispossibletoseevarious studiesdealingwithZnOnanorodsornanowiresandingenerally, thebarrierheightandidealityfactorofthesenanostructuresyield lowerbarrierheightandhigheridealityfactorvalues.For exam-ple,K.BandopadhyayandJ.Mitra[44]havedeterminedthebarrier heightandidealityfactorofZnOnanostructureas0.37eVand8.5, respectively.

Fig.14. –ExperimentalidealityfactorsagainstbarrierheightsforZnO/ZnTe

het-erojunctiondeviceintemperaturerangefrom150K(–123◦_C)_to₃₀₀_K₍₂₇◦_C).

4. Conclusions

In this study, ZnO microrods werepreperad onZnO seeded SnO2-coatedglasssubstratesbyasimplespraypyrolysismethod.

CdTe and ZnTe layers were deposited separately on the ZnO microrodstoformZnO/CdTeandZnO/ZnTecoreshellstructures. Scanning electron microscopy and x-ray diffraction measure-ments revealed almost vertically aligned ZnO microrods with a hexagonal wurtzite structure. The electrical characteristicsof theZnOmicrorods/CdTeandZnOmicrorods/ZnTeheterojunctions were investigated by current–voltage (I–V) measurements as a function oftemperature in therange of 150K (−123◦_C)—300_K

(27◦C).Thecurrent–voltagecharacteristicsofbothZnO/CdTeand ZnO/ZnTeheterojunctionsshowedrectifyingproperties.Both ide-ality factors and the barrier heights have been found to be

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stronglytemperature-dependentforZnO/CdTeandZnO/ZnTe het-erojunctionsand this behaviourhasbeen attributedthelateral inhomogeneityofthebarrierheightforZnO/CdTeandZnO/ZnTe heterojunctions.

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Biographies

Mr.M.AliOlgarisaresearchassistantatDepartment

ofPhysics,ScienceFaculty,KaradenizTechnical

Univer-sity,inTurkey.Hisstudyareasmainlyare;1)Synthesis

ofinorganicmaterialssuchasZnO,ZnTe,CZTSetc.;(2)

developmentandapplicationsofnovelmaterialssuchas

heterojunctionsandSchottkydiodes;(3)Synthesisand

characterizationofnanoandmicromaterialsand(4)The

Synthesisandcharacterizationsofbulkandthinﬁlm

semi-conductorslikeIII-VandII-VI.

Mr.YavuzAtasoyisaresearchassistantatDepartment

ofPhysics,ScienceFaculty,KaradenizTechnical