Properties of PEDOT:PEG/ZnO/p-Si heterojunction diode

ContentslistsavailableatSciVerseScienceDirect

Materials Science and Engineering B

j o ur n a l ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m s e b

Properties of PEDOT:PEG/ZnO/p-Si heterojunction diode

Murat Soylu

, Mihaela Girtan

, Fahrettin Yakuphanoglu

aDepartmentofPhysics,FacultyofArtsandSciences,BingolUniversity,Bingol,Turkey

bPhotonicsLaboratory,AngersUniversity,2,Bd.Lavoisier,49045Angers,France

cDepartmentofPhysics,FacultyofArtsandSciences,FiratUniversity,Elazig,Turkey

a r t i c l e i n f o

Articlehistory:

Received7September2011

Receivedinrevisedform13February2012 Accepted5March2012

Available online 18 March 2012

Keywords:

Heterojunction Semiconductors Electricalproperties

Atomicforcemicroscopy(AFM)

a b s t r a c t

Thezincoxide(ZnO)andpoly(3,4-ethylenedioxythiophene)bis-poly(ethyleneglycol)(PEDOT:PEG)films weredepositedonp-Sisubstratebysputterandspincoatingmethods,respectively.Anorganic/inorganic heterojunctiondiodehavingPEDOT:PEG/ZnOonp-Sisubstratewasfabricated.Thebarrierheight(BH) andtheidealityfactorvaluesforthedevicewerefoundtobe0.82±0.01eVand1.9±0.01,respec- tively.IthasbeenseenthatthevalueofBHissignificantlylargerthanthoseofconventionalAu/p-Si metal–semiconductorcontacts.ThePEDOT:PEG/ZnO/p-Siheterostructureexhibitsanon-idealI–Vbehav- iorwiththeidealityfactorgreaterthanunitythatcouldbeascribedtotheinterfaciallayer,interfacestates andseriesresistance.ThemodifiedNorde’sfunctioncombinedwithconventionalforwardI–Vmethod wasusedtoextracttheparametersincludingthebarrierheightandseriesresistance.Atthesametime, thephysicalpropertiesofZnOandPEDOT:PEGfilmsdepositedbysputterandspincoatingtechnique, respectively,wereinvestigatedatroomtemperature.Theobtainedresultsindicatethattheelectrical parametersofthediodeareaffectedbystructuralpropertiesofZnOfilmandPEDOT:PEGorganicfilm.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Inorganic/organicheterojunctiondeviceshaveattractedconsid- erableinterestinrecentyearsduetotheirpotentialapplicationin electronics,suchastransistors,memoryelementsandrectifying devices[1–3].Therectifyingdevicecanbeformedasaresultof potentialbarrierinasemiconductor/semiconductorp–njunction orametal/semiconductorSchottkycontact.ASchottkycontactis formedbetweenametalandasemiconductor,creatingaSchot- tkybarrier.SomeauthorshavestudiedthefabricationofSchottky contactsusinghighworkfunctionmetalsandn-typeZnO[4–9].

Generally,undopedZnOthinfilmshaven-typeconductionandhigh resistivityduetolowcarrierconcentration.However,itsconduc- tivitycanbetailoredbyappropriatedopingfromahighlyresistive toahighlyconductivematerial.Thepresenceofhighdonorcon- centrationinthesurfaceregion,whichiscausedbynativedefects such as oxygen vacancies/zinc interstitials quite often reveals problemsin theformationofhighqualitymetal/semiconductor Schottkycontacts.Analternativeapproachsuchasthedeposition ofp-typepolymerforfabricatingp–njunctionsofZnOhasbeen attempted[10–12].Manyusefulmethodshavebeenusedtopre- parehighqualityZnOthinfilms, suchasmagnetronsputtering, metal–organicchemicalvapordeposition(MOCVD),pulsed-laser

∗Correspondingauthor.Tel.:+904262132550;fax:+904262132580.

E-mailaddress:soylum74@yahoo.com(M.Soylu).

deposition(PLD),molecularbeamepitaxy(MBE)andsol–gelpro- cess[13–18].

Inrecentyears,heterojunctionsbetweenn-andp-typesemi- conductorshave beendeveloped for variousgases and showed costeffectivetooltowardsgassensorsatlowconcentrationwith room temperature operation [19]. Many research groups have developedp–nheterojunctionswitha p-typepolymerpoly(3,4- ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS), whichisoneofthepromisingmaterialsforrealizingorganicbased p–nheterojunction.PEDOT:PSSisusedasanelectroactivepolymer duetoitshighconductivity,transparencyandgoodphoto-stability [20,21].Bangeetal.[22]haveusedPEDOT:PSSasaholeinjection layerinablue-emittingpolymerlight-emittingdiode.Theyhave reporteda significantbarrier fortheinjectionof holesinto the polymer.

In thisstudy,PEDOT:PEG/ZnO/p-Siheterojunctiondiode was fabricated by spin coating the PEDOT:PEG on ZnO film. The current–voltage(I–V)measurementsforthediodewereexecuted todeterminetheirelectricalparameters.

2. Experimentaldetails

The substrate used in this study is p-Si (100)-doped with boron. The substrate was chemically cleaned using the RCA cleaning procedure i.e., a 10min boiling in NH₄+H₂O₂+6H₂O followedbya10minboilinginHCl+H2O230%.Thesampleswere dippedindilute HFabout30storemove anynativethin oxide 0921-5107/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.

doi:10.1016/j.mseb.2012.03.025

786 M.Soyluetal./MaterialsScienceandEngineeringB177 (2012) 785–790

Fig.1.Aschematiccross-sectionofthePEDOT:PEG/ZnO/p-Sistructure.

layeronthesurface;finallythewaferwasrinsedwithdeionized water.ZnOonthep-typeSisamplewasformedbysputtersystem ofhigh purity (99.999%)Znmetal under vacuum2×10⁻⁵Torr.

ThePEDOT:PEG solution wasmixed witha magnetic stirrer at 60^◦C for 2hand then placed in air for 2hageing resulting in aclear andhomogeneoussol. Thefilm ofPEDOT:PEG wasspin coatedonthefrontsurfaceofthesubstratewithZnObyspinning (vacuumspin-coating)at1000rpm,for30s.ThePEDOT:PEGfilm thicknesswascontrolledbycoatingspeed-time.Thethicknesses of ZnO and PEDOT:PEG filmsweredetermined as 24±0.02nm and37.3±0.02nmwithanaccuracyof0.02nmusingaDektak6M Stylusprofiler,respectively. Golduppercontactof 90nmthick- nesswasformedonPEDOT:PEG/ZnO/p-Sibysputter technique.

Thediode contact areawas foundtobe 3.14×10⁻²cm². Thus, PEDOT:PEG/ZnO/p-Si heterojunction diodes were fabricated. A schematiccross-sectionof thePEDOT:PEG/ZnO/p-Sistructure is shownin Fig.1. Thecurrent–voltage measurementsweredone usingKEITHLEY236sourcemeasurementunit.

3. Resultsanddiscussion

3.1. PhysicalpropertiesofthePEDOT:PEG/ZnO/p-Si heterojunctiondiode

IUPAC (The InternationalUnion of Pure and Applied Chem- istry) name of PEDOT:PEG is poly(3,4-ethylenedioxythiophene) bis-poly(ethyleneglycol). The PEDOT:PEG/ZnO/p-Si structure shows organic-on-inorganic semiconductor heterojunction (OI- HJ)behavior.Forreference,themolecularstructureofPEDOT:PEG is given in Fig. 2. The structural properties of the films were investigated by tapping-mode atomic force microscopy (AFM) usinganLPResearchThermomicroscopeAutoprobe.Fig.3shows twoandthreedimensional(2Dand3D)AFMimagesoftheZnO and PEDOT:PEG films. AFM images indicate that the ZnO and PEDOT:PEGfilmsareformedfromthenanoparticles.Nanoparticle formation originatesfrom thestructural properties ofZnO and organicmaterial.ZnObelongs tothehexagonalwurtzitecrystal type. Sputtered ZnO films are polycrystalline, the individual crystalsgrovingpreferentiallywiththeircrystallographiccaxis perpendiculartothesubstrate[23].Therootmeansquare(rms) surfaceroughnessofthePEDOT:PEGfilmwasfoundas17.452nm.

The surface morphology of the PEDOT:PEG film is reasonably smooth,withuniformgraindistribution.

Fig.3.AFMimagesofZnO(a)andPEDOT:PEG(b)thinfilmswiththethicknessesof 24±0.02nmand37.3±0.02nm,respectively.

Fig.4showsanenergybanddiagramofthePEDOT:PEG/ZnO/p- Sidevice.TheworkfunctionsofAuandPEDOT:PEGclosetoeach other.ThesmalldifferenceintheworkfunctionbetweenAuand PEDOT:PEGmakestheohmiccontact.ConcerningtheZnO/p-Sicon- tact,theelectronaffinitiesofsiliconandzincoxideare4.05eVand 4.35eV,respectively.Inthiscase,Schottkybarriercouldbeformed atZnO/p-Sicontact.However,abarriereffectcouldbenegligible sincetherearemanyimperfectionsactingaselectronpathswhich enabletheohmicbehaviorattheinterfacebetweenZnOandp-Siin non-annealeddevice[10,24,25].Duetodifferenceinenergylevels ofPEDOT:PEGandZnO,abarrierisformedattheinterfacewhich leadstodiode-likecharacteristics.Theelectronsinforwardbias willhavetoovercomethisbarrierinordertoflowtotheopposite electrode.

InFigs.5–7,plotsshowthespectrophotometricallymeasured transmission,absorptionandreflectionvs.wavelengthforfilmlay- ersdepositedatroomtemperature.Itisseenfromtheplotsthat boththefilmsarehighlytransparentoverthevisibleandinfrared regionsbecauseofthefundamentalabsorptionedge[26–28].As seeninfigures,thereisasharpabsorptionat±385nmindicating itsbandgap.Furthermore,thereflectancespectrashowastrong decreaseafter1009nm.Thisdecreaseisrelatedtoopticaltransi- tionsoccurringinopticalbandgap.Analysisofopticalabsorption spectraisoneofthemostproductivetoolsfordeterminingopti- calbandgapofthefilms.Fromthesespectraldata,theabsorption coefficient˛wascalculatedusingtherelationship[29]:

˛h=A(h−Eg)^m (1)

Fig.2.ThemolecularstructureofPEDOT:PEG.

Fig.4.Energybanddiagramforthedevice(a)beforeand(b)afterFermilevelalignment.

2000 1600 1200 800 400 0

Wavelenght (nm) 0

20 40 60 80

T (%)

0 0.2 0.4 0.6 0.8 1 1.2

Absorption (a.u.)

PEDOT:PEG

T

A

Fig.5. TransmittanceandabsorptionspectraofPEDOT:PEGthinfilm.

2000 1600 1200 800 400

Wavelenght (nm) 0

2 4 6 8 10

Reflectance (%)

0 1 2 3 4

Reflectance (%)

PEDOT:PEG ZnO

→

←

Fig.6.SpectrophotometricallymeasuredreflectanceforZnOandPEDOT:PEGthin filmswiththethicknessesof24±0.02nmand73.1±0.02nm,respectively.

where A is an energy-independent constant, E=h is thepho- ton energy and Eg is theoptical bandgapwith alloweddirect transitions. Itisevaluatedthat theopticalbandgapoftheZnO filmhasa directopticaltransition.Theexponentmdependson the nature of the transition, m=1/2, 2, 3/2, or 3 for allowed direct,allowednon-direct,forbiddendirectorforbiddennon-direct transitions,respectively.Theopticalabsorptionorabsorptioncoef- ficient(␣≥10⁴cm⁻¹)isrelatedtodirectbandtransitions[30,31].

TheEgvaluescouldbeobtainedbytheextrapolatingmethodusing the(˛h)²vs.hplot.Fig.8showsplotsof(˛h)² vs.hforthe PEDOT:PEGandZnOfilms.TheEgvaluesforthefilmsweredeter- minedas3.81eVand3.22eV.TheopticalbandgapofZnOstudied islowerthanthatofundopedZnOmaterialsobtainedbyvarious methods[32,33].ThissuggeststhattheopticalbandgapofZnO semiconductorchangeswithrespecttosynthesismethodused.

1600 1200

800 400

0

Wavelength (nm) 0

1 2 3 4

Absorption (a.u.)

0 40 80

Transmittance (%)

ZnO

→

←

Fig.7.TransmittanceandabsorptionspectraofZnOthinfilm.

788 M.Soyluetal./MaterialsScienceandEngineeringB177 (2012) 785–790

4 3 2 1

hν (eV) 0

0.0005 0.001 0.0015 0.002 0.0025

(αhν)2(eV)2

0 0.01 0.02 0.03 0.04

(αhν)2(eV)2 Eg (ZnO)=3.22 eV

Eg (PEDOT:PEG)=3.81 eV

Fig.8. Plotsof(˛h)²vs.hfortheZnOandPEDOT:PEGthinfilms.

3.2. Thecurrent–voltagecharacteristicsofthe PEDOT:PEG/ZnO/p-Siheterojunctiondiode

Thecurrent–voltagecharacteristicswereanalyzedtodetermine theeffective values of the diode parameters. Fig.9 shows the current–voltagecharacteristicsofthePEDOT:PEG/ZnO/p-Sihetero- junction.AsseeninFig.9,theheterojunctionshowsarectifying behaviorwithrelatively lowreverse currentof 8.65×10⁻⁷A at reversebiasofVR=−1.0V.Thecurrentincreasedwhenthebias voltagewasdecreasedbelow −0.5V.Thissituationisrelatedto leakagecurrent.Leakagecurrentscanarisedue primarilytothe existenceofdiscrete,dislocation-relatedleakagepaths[34].How- ever,theleakagecurrentcanalsobeassociatedwiththeinterface oxidelayer.Thecurrent–voltage characteristic ofthediode, for V−IRs>3kT/q,isanalyzedbythefollowingrelation[35]:

I=I0exp

s) nkT

(2) whereI0isthereversesaturationcurrentgivenby:

I0=AA^∗T² exp

−q˚^I–V b

kT

, (3)

whereqistheelectroncharge,Visthedefiniteforwardbiasing voltage,Aistheareaofthecontact,A*istheeffectiveRichardson constant,kistheBoltzmannconstant,Tistheabsolutetemper- ature,˚^I–V

b isthebarrierheightandnistheidealityfactor.The I/[1−exp(−qV/kT)]vs.V(Fig.10)curveshowstwolinearregions, fromthefirstsegmenttheslopeandtheinterceptofthisplotonthe

1 0.5 0

-0.5 -1

Voltage (V) 0

4E-006 8E-006 1.2E-005 1.6E-005 2E-005

Current (A)

PEDOT:PEG/ZnO/p-Si

Fig.9.I–VcharacteristicsofthePEDOT:PEG/ZnO/p-Siheterojunctiondiode.

0.6 0.4

0.2 0

Voltage (V) 1E-009

1E-008 1E-007 1E-006

I/(1-exp(-qV/kT)) (A) ^ll

l

Fig.10.PlotofI/[1−exp(−qV/kT)]vs.VofthePEDOT:PEG/ZnO/p-Siheterojunction diode.

currentaxis(y-axis)yieldtheidealityfactornandthesaturation currentI0ofthedeviceat1.9±0.01and2.05×10⁻⁹A,respectively.

AsthecurveinFig.10wascorrectedfortheeffectofseriesresis- tance,thecurvaturecanbeattributedtoacontinuumofinterface states.BysubstitutingthevalueofI0intoEq.(3),wehaveobtained thebarrierheight˚^I–V

b whichisabout0.82±0.01eV.Theresul- tantvalueof idealityfactoris verylowtothereportednvalue for ZnO/PEDOT:PSSheterojunction diode thatit is necessaryto improvethequalityofthep–nheterojunction[36].Itisevaluated thatPEDOT:PEG/ZnO/p-Siheterojunctiondiodecanbeusedasa goodmaterialcombinationforpossibleapplicationscomparedto otherworks.

Thehighervalueoftheidealityfactor indicatesthatthefor- wardcurrentisgovernedbyrecombinationcurrentviatraps.The currentdue torecombination isimportanthereas canbeseen bythevalueoftheideality factor.Thisindicatesthatthediode exhibitsanon-idealbehaviorduetotheoxidelayer,thepresence ofsurfacestatesandthenanocrystallinenatureoftheZnOfilm.

Thesesurface statesprovideadditional energystateswhich are responsiblefortheexistenceofmultiplecurrentpathways[37–40].

Thehigheridealityfactorsuggeststhatthetransportpropertiesof PEDOT:PEG/ZnO/p-Siheterojunctiondiodecouldnotbedefinedby thermionicemission(TE)onlyandcouldbeduetopresenceofsec- ondarymechanismattheinterface.Furthermore,thesurfacestates wouldgenerateanupwardbandbendingandthenformapoten- tialbarrieronthesurface,whichwasalmostindependentonthe workfunctionofthemetalduetotheso-calledpinningofFermi energy.Whenthedensityofsurfacestateswashighenough,the barrierheightcontrolledbythesurfacestatesmustinfluencethe I–Vbehaviorleadingtothehighvalueofn[35,41].

Onasemi-logscaleandatlowforwardbiasvoltage,theforward biasI–Vcharacteristicsofthemetal–semiconductorcontactsdevi- atefromlinearityduetotheseriesresistanceandinterfaciallayer.

Thus,theseriesresistanceisaneffectiveparameterinI–Vcharac- teristicsanditcannotbeignored.Therefore,theseriesresistance Rsforthediodecanbedeterminedusingcurrent–voltagecharac- teristicsbymeansofRs=∂^V/∂^{Irelation.Theseriesresistanceand} diffusionpotentialvalueswerecalculatedas165kand0.696V, respectivelyfromtheforwardbiasI–VplotofFig.9.Furthermore, Nordeproposedanalternativemethodtodeterminethevalueof theseriesresistance.InNorde’smethod[42],thefollowingfunction isusedas:

F(V)= V0

−kT q

A^∗AT²

1.08 0.96 0.84 0.72 0.6 0.48 0.36 0.24 0.12 0

Voltage (V) 0.8

0.82 0.84 0.86 0.88 0.9 0.92 0.94 0.96 0.98 1 1.02 1.04 1.06 1.08

F(V) (V)

PEDOT:PEG/ZnO/p-Si

F(V0)=0.819 V V0=0.128 V I0=1.9x10-8 A Φb0(F-V)=0.857 eV Rs=136 kohm

−

← V0

Fig.11. PlotofF(V)vs.VofthePEDOT:PEG/ZnO/p-Siheterojunctiondiode.

where is the integer (dimensionless) greater than n. I(V) is thecurrentobtainedfromtheI–Vcharacteristic.n istheideal- ity factordetermined fromtheslope of thelogarithmic plotof I/[1−exp(−qV/kT)]vs.V.TheplotofF(V)vs.voltageforthediode isshowninFig.11.TheF(V)givesaminimumpointandthus,the barrierheightiscalculatedbytherelation:

˚b=F(V0)+V0

−kT

q, (5)

whereF(V0)istheminimumpointofF(V)functionandV0 isthe correspondingvoltage.FromtheF(V)–Vplot,thebarrierheightwas foundtobe0.85eV.ThebarrierheightofthePEDOT:PEG/ZnO/p-Si heterojunction diodeis higher than that of Au/ZnO/p-SiSchot- tky diodes [43]. This suggests that the PEDOT:PEG thin film hasa significanteffect onbarrier heightof Au/PEDOT:PEG and PEDOT:PEG/ZnO/p-Sijunctiondiodeinseries.Theinterlayerfilm appearstocauseasignificantmodificationofinterfacestateseven thoughtheZnO/PEDOT:PEGinterfacebecomesabruptandunreac- tive.ThebarrierheightofthePEDOT:PEG/ZnO/p-Siheterojunction diodeis lowerthanthat ofsomeZnO Schottkydiodes[44–46], whereasitishigherthanthatofAu/ZnO/n-Si/AuSb[14].Also,the idealityfactorishigherthanthoseofZnOdiodes[15,44,47,48].The discrepancyinbarrierheightsoftheZnOdiodesmaybeduetothe carrierconcentrationofZnOfilmusedindiodes.

InNordemethod,theseriesresistanceisdeterminedas:

Rs= kT(−n)

qI0 , (6)

TheRsvalueforthediodewasdeterminedusingEq.(6)andwas foundtobe136.0k.ThereisagreementbetweenthevaluesofRs

obtainedfromtheNordefunctionsandRs=∂^V/∂^{Irelation.TheR}s

valueisconsiderablyhigherduetotheinterfaciallayerandresis- tanceoftheSisemiconductor.Thiscausesanon-linearbehavior forthediode.Thevalueofseriesresistanceindicatesthattheseries resistanceisacurrent-limitingfactorforthisstructure.Theeffectof theseriesresistanceisusuallymodeledwithseriescombinationof adiodeandaresistorwithresistanceRsthroughwhichthecurrent Iflows.Thevoltagedropacrossarectifyingcontactisexpressedin termsofthetotalvoltagedropacrossthediodeandtheresistance R_s[49].

In order to determine the charge transport mechanism of thePEDOT:PEG/ZnO/p-Siheterojunctiondiode,I–Vcharacteristics wereanalyzedusingI∝V^mrelation.Forthis,thecurvewasshownin formoflnI−lnV(Fig.12).AsseeninFig.12,theplotindicatesdiffer- entthreecurrentregions.Withtheappliedvoltageincreasing,the

1 0.1

Voltage (V) 1E-009

1E-008 1E-007 1E-006 1E-005 0.0001

Current (A)

ll. region

l. region

lll. region PEDOT:PEG/ZnO/p-Si

Fig.12.I–VcharacteristicinlogarithmicscaleofthePEDOT:PEG/ZnO/p-Sihetero- junctiondiode.

slopeoftheI–Vcurvechangesfrom1.09to4.2.TheregionIshows twoohmicregion.Theobtainedmvalueforthesecondregionofthe curvesuggestsaSCLCmechanismcharacterizedbyanexponential distributionoftrappinglevels[50].InregionIII,theinjectedfree chargecarriersaremuchgreaterthanthethermallygeneratedfree carriersandthusthechargetransportmechanismisgovernedby thetrap-charge-limitedspacecharge-limitedconductivity(TCLC) mechanism.

4. Conclusions

The morphology, optical and electrical characteristics of PEDOT:PEG/ZnO/p-Siheterostructurefabricatedbyvacuumspin- coatingtechniqueandsputtersystemwereidentifiedusingAFM, optic spectroscopy and semiconductor characterization system.

The prepared heterojunction diode exhibits a good rectifying behaviorwithanidealityfactorof1.9±0.01andrectificationratio of1.11×10²at±1V.TheAFMresultsindicatethatthesurfacemor- phologiesofthefilmsarereasonablysmooth,withuniformgrain distribution.TheopticalbandgapoftheZnOfilmwasfoundto be3.22eV.Fromtheopticalabsorptionspectrum,ithasbeenseen thatPEDOT:PEGisasemiconductormaterialwithwideopticalband energygapof3.81eV.Theelectricalconductionintheheterojunc- tionwasfoundtotakeplacebythermionicemissionatlowvoltages (V≤0.2V)andbyspacechargelimitedconductionathighvoltages (V>0.2V).TheobtainedresultsshowthatthePEDOT:PEG/ZnO/p-Si heterojunctionisagoodcandidateforthethinfilmstypeelectronic deviceapplications.

Acknowledgements

ThisworkwassupportedbytheManagementUnitofScientific ResearchprojectsofBingolUniversity(BÜBAP)(ProjectNumber:

2010/08).OneofauthorswishestothankBÜBAPandUniversité d’Angers,France.

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