薄膜制备。。。

Deepultravioletandnearinfraredphotodiodebasedonn-ZnO/p-siliconnanowireheterojunctionfabricatedatlowtemperature

HaiZhou,1,2GuojiaFang,1,2,a͒LongyanYuan,1ChongWang,1XiaoxiaYang,1HuihuiHuang,1ConghuaZhou,1andXingzhongZhao1

1

DepartmentofElectronicScienceandTechnologyandKeyLaboratoryofAcousticandPhotonicMaterialsandDevicesofMinistryofEducation,SchoolofPhysicalScienceandTechnology,WuhanUniversity,Wuhan430072,People’sRepublicofChina2

StateKeyLaboratoryofTransducerTechnology,ChineseAcademyofSciences,Beijing100080,People’sRepublicofChina

͑Received8November2008;accepted10December2008;publishedonline6January2009͒n-ZnO/p-siliconnanowire͑SiNW͒photodiodeswerepreparedbyradiofrequencyreactivemagnetronsputteringofn-typezincoxideatroomtemperatureonphotoresistfilledp-SiNWs,whichwerefabricatedbyelectrolessmetaldepositionmethodat323K.Ourn-ZnO/p-SiNWphotodiodesshowedgoodtemperature-andlight-intensitydependence.Theyexhibitedstrongresponsivitiesof19.2and2.5A/Wfor254and1000nmphotons,respectively,underareversebiasof2Vwithastrongpeakofresponsivitynear390nm,thewavelengthcorrespondingtothebandgapofZnO.Theseresultspresentpotentialapplicationsofn-ZnO/p-SiNWphotodetectorsindeepultravioletandnearinfraredregions.©2009AmericanInstituteofPhysics.͓DOI:10.1063/1.30161͔

Thetechnologybasedonsiliconhasbeendevelopedmanyyearsagoandalwaysgottenmuchattention.However,thedevicesmadebysiliconbulkmaterialhavemuchlimita-tioninmanyfields,whichlimitmoredevelopmentofthesociety.Quasi-one-dimensionalsemiconductornanostruc-tures,especiallynanotubesandnanowires,offermoreoppor-tunitiesforaddressingtheefficiencysincenanotubesandnanowirescanfacilitatelightabsorptionandradialcollec-tionofphotogeneratedcarriers,thusgivingrisetohighefficiency.1–5Therefore,theyhavegreatpotentialtosubsti-tutesiliconbulkmaterialastheidealelectronicmaterial.Uptonow,thequasi-one-dimensionalmaterialshavebeenusedtofabricateavarietyofmolecular-scaledevices,suchasfieldeffecttransistors,diodes,andinverters.6–8

Recently,siliconnanowires͑SiNWs͒havebeenfabri-catedbasedonvapor-liquid-solidmethodtobesolarcells,9,10andPengetal.11synthesizedphotoelectrochemicalsolarcellsbasedonSiNWsbyelectrolessetching.Thep-poly͑9,9-diethylfluorene͒/n-SiNWheterojunctionhasalsobeenreportedinourpreviouswork.12However,ap-nhet-erojunctionappliedwithSiNWsandZnOhadfewreports,especiallyfabricatedatverylowtemperature.Herein,ap-nheterojunctionwasassembledusingp-SiNWsfabricatedbyelectrolessmetaldeposition͑EMD͒at323Kandn-ZnOsynthesizedbyradiofrequencyreactivemagnetronsputter-ingatroomtemperature͑RT͒.Electricalpropertiesofn-ZnO/p-SiNWheterojunctionshavebeeninvestigatedthroughtemperature-dependentandlight-intensity-dependentcurrentdensity-voltage͑J-V͒measurements,whichdemon-strategoodrectifyingbehaviorwithhighrectificationratio.Responsivitiesof19.2and2.5A/Wundera2Vreversebiasproveditspotentialapplicationsinthefuturedeeplevelul-traviolet͑UV͒andnearinfrarednanophotodiodes.

a͒

Thep-typeSiNWarraysweresynthesizedbyself-assemblyEMDnanoelectrochemistry.Thep-Si͑100͒͑3–5⍀cm͒substratesreceivedastandardwetcleaningprocedurebeforeetching.ThedetailedfabricationprocessofSiNWswasreportedinourpreviouswork.12AfterSiNWgrowth,photoresistwasfilledinthefreespacebetweenSiNWsbyspin-coatingprocess.InordertomakephotoresistfillinSiNWsenough,letthetipsofSiNWsout,wechosethatthespeedofspincoatingwas3500rpm;thespin-coatingtimedurationwas20s.ThenzincoxidewasdepositedonphotoresistbyradiofrequencyreactivemagnetronsputteringfromaZnOtargetatRT.Thefilmsweredepositedattotalpressureof0.4Pa,thepowerofsputteringwas88W,andtheflowratioofArtoO2was15:5.Herein,thethicknessoftheZnOfilmmeasuredbysurfaceprofilometer͑TalysurfSerlesII͒wasabout240nm.

ForOhmiccontactstothen-ZnOfilms,80-nm-thickAgelectrodeswerepatternedonthen-ZnOsurfacewithashadowmaskbythermalevaporationatRT.Inordertogetlarge-areaOhmiccontactstothebacksideofp-Si,In–Gaalloywasapplied.Figure1͑a͒showstheschematiccrosssectionofn-ZnO/p-SiNWheterojunctiondiode.Themor-

Authortowhomcorrespondenceshouldbeaddressed.Electronicmail:gjfang@whu.edu.cn.

FIG.1.͑Coloronline͒͑a͒Theschematicillustrationofcross-sectionalviewofn-ZnO/p-SiNWphotodiode.Photonwindowareais␲ϫ1.52mm2.͑b͒ThesideviewofSEMimageofSiNWarrays.

©2009AmericanInstituteofPhysics

0003-6951/2009/94͑1͒/013503/3/$23.0094,013503-1

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013503-2Zhouetal.Appl.Phys.Lett.94,013503͑2009͒

FIG.2.͑Coloronline͒͑a͒J-Vcharacteristicsofthen-ZnO/p-SiNWhetero-junctionphotodiodeunderdifferenttemperaturesfrom333to243K.Theinsetshowscurrentdensitiesandreversevoltagepropertiesunderdifferenttemperaturesfrom333to243K.͑b͒Log-logplotsofcurrentdensityvsforward-biasvoltageunder333and303K.

phologiesandstructuresofp-SiNWsandn-ZnO/p-SiNWswereexaminedbyscanningelectronmicroscopy͑SEM͒.Forthetemperature-dependentJ-Vmeasurements,thesamplewasputinaprogram-controllablecalorstat.Thephotosensi-tivitywasperformedbyusing66984XeArcsource͑300WOriel͒andOrielCornerstone™2601/4mmonochromator.Thesamplewasunderilluminationdirectly͑parallelwiththenanowires͒͑theilluminatedareais␲ϫ1.52mm2͒andtheopticalpoweroflightwasmeasuredbyaUV-enhancedSidetector.AlltheJ-VcharacteristicsweremeasuredbyaKeithley4200electrometer.

Accordingtoourexperiments,thenanowirelengthcanbecontrolledbyvaryingtheetchingtimeandthediameterofnanowiredoesnotshowobviouschangeduringetchingpro-cess.IntheSEMimageofFig.1͑b͒,itshowsthetypicalmorphologyoftheSiNWswithalengthabout5␮mandthediameterabout100nmat60minetching.

TheJ-Vcharacteristicsofann-ZnO/p-SiNWhetero-junctionweremeasuredwithtemperaturerangingfrom333to243Kinacalorstatunderthedarkcondition.InFig.2͑a͒,wecanseethatthethresholdvoltageincreasesfrom6.6to7.8Vwhenthetemperaturedecreasesfrom333to243K.TheinsetinFig.2showsthatthereversecurrentdensitydecreasesaboutoneorderofmagnitudewithtemperaturedecreasingper30K,andthereversecurrentdensityisre-ducedto10−9A/cm2whenthetemperaturedecreasesto243K.Wecandeducethatthebreakdownvoltagemaygetlargerwiththedecreasingtemperature,andtherectificationratio͑Iforward/Ireverse͒of104wasobtainedatabiasvoltageof5Vat303K.

Additionally,wechoosetwotemperaturesof333and303Ktoinvestigatethelog-logplotsofJversusV͓Fig.2͑b͔͒,whichshowtworegions.Foraverylowbiasvoltage,i.e.,VϽ1.3V͑regionI͒,withincreasingtemperaturefrom303to333K,thecurrentdensityincreasesaboutoneorderofmagnitude,whichcanbeinterpretedusingtheequation

J0ϰexp͑−qV/kT͒,

͑1͒

FIG.3.͑Coloronline͒Currentdensitiesreverse-biasvoltagecurvesunderdifferentlightintensities.Theinsetshowsforwardcurrentdensitiesandvoltagepropertiesunderdifferentlightintensities.

Currentdensitiesandreversebiaspropertiesunderdif-ferentlightintensitiesareshowninFig.3.Itshowsthatourdiodeshavelargephotocurrentunderlowopticalpoweroflight,andwithincreasinglightintensitiesfrom0.9to2.4mW/cm2,thephotocurrentrisesveryclearly.TheinsetinFig.3showsthecurrentdensity-forwardvoltagecharac-teristicsofthen-ZnO/p-SiNWheterojunction.Itcanbeob-servedthatthethresholdvoltagedecreaseswiththeincreas-inglightintensity,andthethresholdvoltageisabout7Vunderdark.However,itdecreasedto5Vwhenlightintensityincreasesto2.4mW/cm2.

Figure4presentsthespectralresponsivitycurveob-tainedfromourn-ZnO/p-SiNWheterojunctionsunderthereversebiasesof2V.Fromthespectrum,wecanseethatadistinctspectralresponsivitypeakisformednear390nm,whichcorrespondstothatofbandgapofZnO͑3.2eV͒.Atthiswavelength,theresponsivityofn-ZnO/p-SiNWhetero-junctionreachestoashighasϳ17.3A/W.Aweakpeakisalsoobservedatabout800nmintheinsetofFig.4,anditisformedduetothehigh-qualitySiNWswithphotonenergyof1.6eV,whichisclosetothatofbandgapofporousSi.15Itis

whereqistheelectroncharge,kistheBoltzmannconstant,andTisthetemperature.Fromthisequation,wecanseethattheheterojunctionistemperaturedependent.Thisbehaviorisinaccordancewiththethermionicemissionmechanism.ForVϾ1.3V͑regionII͒,theJ-VcharacteristicwasdeviatedfromtheidealthermionicemissionandbehavedasJϰexp͑␣V͒relation,whichisusuallyobservedinthewidebandgapp-nheterojunctionduetorecombination-tunnelingmechanism.13,14Here,theconstant␣wasevaluatedtobe4.98V−1byfittingtheexperimentaldatainFig.2͑b͒.

FIG.4.Photoresponsivityvslightwavelength.Theinsetshowsphotore-sponsivityinthewavelengthrangeof600–1000nm.

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013503-3Zhouetal.Appl.Phys.Lett.94,013503͑2009͒

understoodthatthevisibleandinfraredlighthavehightransmittance͑exceed80%͒inZnOfilms,cantransmitZnOfilmseasily,andabsorbedbySiNWscreatingmostelectron-holepairsthatgeneratethephotocurrentunderre-versebiasatabout800nm.Atthiswavelength,therespon-sivityreaches0.8A/Wat2V.DuetothepresenceofSibulk,somelightabsorbedbySibulkcancreatemoreelectron-holepairswhenthewavelengthisabove900nm,andtherespon-sivityreaches2.5A/Wat1000nmunderareversebiasof2V.Interestingly,below330nm,theresponsivityofn-ZnO/p-SiNWheterojunctionincreaseswiththedecreasingwavelength.Itislikelythatthephotonswithmoreenergiescausemoreelectron-holepairsinthen-ZnO/p-SiNWhet-erojunctionleadingtohigherresponsivity.16For254nmUVphotons,ourdiodeshowshighresponsivityof19.2A/Wat2V.

FromFig.4,ourn-ZnO/p-SiNWphotodiodeshowsverylargeresponsivitythann-ZnO/p-SiphotodiodereportedbyJeongetal.,16whoreportedthebiggestresponsivitiesof0.5and0.3A/Wfor310and650nmphotons,respectively,undera30Vbias.Fromtheequation

R=where

Iph=Aq͑Ln+W+Lp͒G.

͑3͒

Iph

,SPopt

͑2͒

andhighquality,ourn-ZnO/p-SiNWheterojunctionshowsbetterperformancethanthatofn-ZnO/p-Siheterojunction.

Insummary,an-ZnO/p-SiNWheterojunctionwasfab-ricatedthroughEMDandthenradiofrequencyreactivemag-netronsputteringatverylowtemperature.Theheterojunc-tionshowedarectificationratioof104at5Vinthedarkandgoodtemperature-andlight-intensitydependence.Basedonspectralresponsivitymeasurement,ourphotodetectorsex-hibitstrongresponsivityforbothdeepUVandnearinfraredregions.Weconcludethatourn-ZnO/p-SiNWphotodiodemayreplacephotodetectorsbasedonbulksilicontodetectbothdeepUVandnearinfraredphotonsandmayhavepo-tentialapplicationsinthefuturenanoelectronicandphotonicdevices.

ThisworkwassupportedbytheSpecialFundofMinis-tryofEducationforDoctor’sConfermentPostunderGrantNo.20070486015.

B.Z.Tian,X.L.Zheng,T.J.Kempa,Y.Fang,N.F.Yu,G.H.Yu,J.L.Huang,andC.M.Lieber,Nature͑London͒449,885͑2007͒.2

M.Law,L.E.Greene,J.C.Johnson,R.Saykally,andP.D.Yang,NatureMater.4,455͑2005͒.3

W.U.Huynh,J.J.Dittmer,andA.P.Alivisatos,Science295,2425͑2002͒.4

K.Peng,Y.Xu,Y.Wu,Y.Yan,S.-T.Lee,andJ.Zhu,Small1,1062͑2005͒.5

L.Tsakalakos,J.Balch,J.Fronheiser,B.A.Korevaar,O.Sulima,andJ.Rand,Appl.Phys.Lett.91,233117͑2007͒.6

S.J.Tans,A.R.Verschueren,andC.Dekker,Nature͑London͒393,49͑1998͒.7

R.Martel,T.Schmidt,H.R.Shea,T.Hertel,andPh.Avouris,Appl.Phys.Lett.73,2447͑1998͒.8

Z.Yao,W.Ch.Postma,L.Balents,andC.Dekker,Nature͑London͒402,273͑1999͒.9

A.P.Goodey,S.M.Eichfeld,K.K.Lew,J.M.Redwing,andT.E.Mallouk,J.Am.Chem.Soc.129,12344͑2007͒.10

J.R.Maiolo,B.M.Kayes,M.A.Filler,M.C.Putnam,M.D.Kelzenberg,H.A.Atwater,andN.S.Lewis,J.Am.Chem.Soc.129,12346͑2007͒.11

K.Q.Peng,X.Wang,andS.T.Lee,Appl.Phys.Lett.92,163103͑2008͒.12

Y.Z.Cheng,G.J.Fang,C.Li,L.Y.Yuan,L.Ai,B.R.Chen,X.Z.Zhao,Z.Y.Chen,W.B.Bai,andC.M.Zhan,J.Appl.Phys.102,083516͑2007͒.13

R.GhoshandD.Basak,Appl.Phys.Lett.90,243106͑2007͒.14

M.DuttaandD.Basak,Appl.Phys.Lett.92,212112͑2008͒.15

R.Prabakaran,T.Monteiro,M.Peres,A.S.Viana,A.F.daCunha,H.Águas,A.Gonçalves,E.Fortunato,R.Martins,andI.Ferreira,ThinSolidFilms515,86͑2007͒.16

I.S.Jeong,J.H.Kim,andS.Im,Appl.Phys.Lett.83,2946͑2003͒.

1

Here,Ristheresponsivityofthephotodiode,Iphisthepho-tocurrent,Poptisthelightintensity,Sistheilluminatedarea,Aisthecontactareaofthep-njunction,LnandLparethediffusionlengthofelectronsandholes,respectively,Wisthedepletionlayerwidthofthephotodiode,andGisthegenerationrateoftheelectron-holepairsunderlightillumi-nation.

OwingtothenanometricdiameterofSiNWs,theyhaveaverylargespecificsurfaceareathanthatofbulksilicon.Thelargerspecificsurfaceareacanincreasethecontactareaofthep-njunctiongreatly,herein,ASiNWsϾAbulk.Asaresult,thephotovoltaiccurrentofn-ZnO/p-SiNWheterojunctionishigherthanthatofn-ZnO/p-Siheterojunctionandsoistheresponsivity,whichisaccordingtoourresults.Fromabove,wecanseethatsinceSiNWshavelargespecificsurfacearea

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