Journal of Applied Research and Technology
www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology15(2017)278–282
Original
Fully solution-processed zinc oxide MIS capacitors by ultrasonic spray pyrolysis in air ambient
Miguel A. Dominguez
a,∗, Abdu Ordu˜na-Diaz
baCentrodeInvestigacionesenDispositivosSemiconductores,InstitutodeCiencias,BenemeritaUniversidadAutónomadePuebla(BUAP),Puebla72570,Mexico
bCentrodeInvestigaciónenBiotecnologíaAplicada(CIBA),InstitutoPolitécnicoNacional,Tlaxcala,Tlaxcala72197,Mexico Received10August2016;accepted9January2017
Availableonline1June2017
Abstract
Inthiswork,thefabricationandcharacterizationoffullysolution-processedzincoxidemetal–insulator–semiconductor(MIS)capacitorsby ultrasonicspraypyrolysis(USP)arepresented.FluorinetinoxidebyUSPwasusedastransparentelectrode,whilespin-onglassbyspin-coating wasusedasdielectricandzincoxidebyUSPwasusedasactivelayer.Also,thezincoxidefilmwascharacterizedusingphotoluminescence spectroscopy,X-raydiffractionandFouriertransforminfraredspectroscopy.TheMIScapacitorswerefabricatedoverglassslidesandwerehighly transparentinthevisiblerange,whichmakestheirusefeasibleintransparentelectronics.Employingcapacitance–voltageandcurrent–voltage measurements,theMIScapacitorswerecharacterized.
©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunder theCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: MIScapacitors;Solution-processes;Transparentelectronics
1. Introduction
Inrecentyears, thedevelopmentoftransparent electronics hasachievedconsiderableprogresssincewearableelectronics, transparentcircuitry,e-paper,solarcellsandmore(Bashiretal., 2009;Langleyetal.,2013;Zhu,Fang,Preston,Li,&Hu,2014).
Althoughtherecentdevelopmentinthin-filmtransistors(TFTs) hasbeenveryimportanttoenablethistechnology,itisnecessary todevelopotherbasicdevicessuch as transparentcapacitors.
Thesebasicdevicescouldworkassensors,energystoragefrom solarcells,memorydevicesandcharge–dischargecapacitorsin active-matrixdisplayscouldalsobeveryusefultoanalyzethe qualityofthedielectric-semiconductorinterfaceinfieldeffect devices (Khan, Bhansali, & Alshareef, 2011; Xian & Yoon, 2009;Yu,Li,Zhang,Sun,&Zheng,2015;Zhangetal.,2015).
Theuseoftransparentelectrodesiskeyfordevelopingtrans- parent electronic devices, in which indium tin oxide (ITO)
∗Correspondingauthor.
E-mailaddress:[email protected](M.A.Dominguez).
PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.
hasbeenmostcommonlyused.However,becauseofthehigh demandof indium,theuseofITOwillbeeconomicallyunvi- able. Therefore,much researchis under developmentto find alternatives to ITO. On the other hand, in addition to the unviability of ITO, the typically used deposition techniques involve highvacuum, present incompatibility with large-area substrates and high-cost. For these reasons, solution-process techniques havebecome veryattractive becauseof their low- cost, simplicity, compatibility with large-area substrates and the filmdeposition underambient conditions. It is important to mention that the consecutive solution-deposition of thin filmstofabricatesemiconductordeviceshasbeenagreatchal- lenge (Hunter et al., 2015). In this work, we present fully solution-processed zincoxide metal–insulator–semiconductor (MIS)capacitorsemployingtheultrasonicspraypyrolysisand spin-coatingtechniques.Fluorinetinoxideandspin-onglassare usedastransparentelectrodeanddielectricfilm,respectively.
2. Experiment
Thefilmsweredepositedusingatypicalhome-madeultra- sonic spray pyrolysis (USP) deposition system adapted from
http://dx.doi.org/10.1016/j.jart.2017.01.015
1665-6423/©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
a b FTO
ZnO SOG/DI
FTO
Glass
Fig.1.ZnOMIScapacitorstructure.(a)Schematiccrosssectionand(b)photographofthefabricateddevices.
anultrasonichumidifier(HeavenFresh).Thetransparentelec- trodeswereobtainedusingfluorinetinoxide(FTO)from0.2M precursorsolutionof tintetrachloridepentahydrate inethanol withammoniumfluoride dilutedindeionizedwater prepared withF/Sn ratio of 0.52. The ZnO was obtained using 0.2M precursorsolutionofzincacetateinmethanol.Thetransparent dielectricwasobtainedusingaspin-onglasssolution(SOG700B Filmtronics) diluted 3:1 with deionized water (DI) and spin coatedontothesamples(Dominguez,Rosales,Torres,Moreno,
&Ordu˜na,2012;Dominguez,Obregon,&Luna-Lopez,2016).
TheorientationofthefilmwasinvestigatedusinganX-ray diffractometer(XRD)(DiscoverD8-BrukerAXS)at2θrange between20◦and70◦and0.002◦step.Forthephotoluminesce- ncespectroscopy,alaserofHe-Cdwith325nmlinewasusedas excitingsource.Thephotoluminescencemeasurementwasper- formedusingasiliconPINThorlabs(DET-210)detectorwith aspectralresponseof200–1100nm.Theconventionallock-in techniquewasemployedseekingtominimizetheeffectofnoise.
ThevoltagesignalwasacquiredwithaComputerBoardsCIO- DAS08 data acquisition card, which operates in conjunction withasteppermotorcontrollertopositionthemonochromator Sciencetech9040totheappropriatewavelength.
TheIRabsorptionspectraofthefilmsweremeasuredwith aBruckerFTIRVertex-70spectrometer.TheIRspectrumwas observedforwavenumbersbetween3500and400cm−1.
TofabricatethetransparentMIScapacitors,glassslideswere usedassubstrates.Theglasssubstrateswerechemicallycleaned.
First,theywerecleanedinacetonefor10min,followedwith isopropylalcohol,alsofor10min,inanultrasonicbath.Finally, thesubstrateswererinsedwithdeionizedwater.Afterthat,FTO deposited by USP was used as bottom transparent electrode.
Then,theSOG/DIdielectricfilmwasspin-coatedontothesam- plesatroomtemperatureat3000rpmfor30s.Thesampleswere annealedat100◦Cfor15mintoreducehumidityandevaporate mostofthesolvents,tofinallybeannealedfor5hat200◦Cin airambient.Afterwards,asactivelayer,ZnOwasdepositedby USPusingairascarriergasatflowrateof467sccmonahot- plateat200◦C.Finally,FTOwasdepositedbyUSPasthetop transparentelectrodethroughashadowmask.TheMIScapaci- torshaveanareaof0.002cm2(1mm×0.2mm).Thethickness oftheZnOandSOG/DIfilmswas350±15nmand44±5nm, respectively.
50
40
30
(002) (102) (110) (103) (112)
(100) (101)
20
10
0
20 30 40 50 60 70
ZnO 200 ºC
2 theta (deg)
Intensity (a.u.)
Fig.2.XRDpatternoftheas-depositedZnOfilmsbyultrasonicspraypyrolysis.
The capacitance–voltage characteristics were measured usingtheKeithley-4200semiconductorcharacterizationsystem equippedwiththe4200-CVUintegratedC–V,underdarkcondi- tions,airambientandroomtemperature.Themeasurementshere reportedarerepresentativeof9capacitors.Themeasurements presentafluctuationlowerthanthe±10%ontheaccumulation capacitanceandbreakdownvoltage.
3. Resultsanddiscussion
Fig.1 shows theschematic crosssection of theZnO MIS capacitorsand atop-viewphotograph of the MIScapacitors.
TheMIScapacitorsarehighlytransparent,whichmakestheir usefeasibleintransparentelectronics.
Fig.2shows theXRDpatternof theZnOfilm.The peaks relatedtotheplanes(100),(002),(101),(102),(110),(103)and (112) can be identified according with the JCPDS Card No.
36-1451, wherethe strongest peak is associated tothe (100) plane.TheXRDdiffractogramshowsapolycrystallinenature regardlessofthelowdepositiontemperature.
Fig.3showstheroomtemperaturephotoluminescence(PL) spectra ofthe ZnOfilm.It showsaPL spectrumwithapeak
0,6
0,5
0,4
0,3
0,2
0,1
0,0
300 400 500 600 700 800
ZnO 200 ºC
Intensity (a.u.)
Wavelength (nm)
Fig.3.Photoluminescenceoftheas-depositedZnOfilmsbyultrasonicspray pyrolysis.
Zn-O
C-O
O-H
C-H, O-H ZnO 200 ºC
Wavenumber (cm-1)
Absorbance (a.u.)
0,25
0,20
0,15
0,10
0,05
0,00
500 1000 1500 2000 2500 3000 3500 C=O
Fig.4.FTIRspectraoftheas-depositedZnOfilmsbyultrasonicspraypyrolysis.
centeredat390nmandahighintensitybroadbandfrom450to 700nm.TheUVemissionpeakat390nmisassociatedwiththe nearband-edge(NBE)emissionof thewidegapattributedto therecombinationofthefreeexcitons(Padilla-Rueda,Vadillo,
&Laserna,2012;Sepulveda-Guzmanetal.,2010;Xiao,Zhao, Liu,&Lian,2013).Thevisibleemissionbroadbandfrom450to 700nmincludesthedefectsandimpurities,whichareconsider- ableinourcase.Theoriginofthisvisibleemissionbandmaybe attributedtooxygenvacancies,zincvacancies,oxygenantisites, zincantisites,oxygeninterstitialandzincinterstitial(Janotti&
VandeWalle,2007;Lin,Fu,&Jia,2001;Wangetal.,2006).
Fig.4 showsthe FTIR spectra of theZnO film. Thepeak canbeobservedat415cm−1relatedtoZn Ostretchingmodes (Adamopouloset al.,2011; Gayen,Sarkar, Hussain,Bhar, &
Pal,2011).Thepeakat1627cm−1isattributedtoO Hbending modes.Also,thepeaksat2500–3500cm−1areduetoC Hand O Hstretchingmodes(Adamopoulosetal.,2011;Gayenetal., 2011).Thepeaksat1413cm−1and1530cm−1areattributedto
Accumulation Cox
Depletion ZnO MIS capacitor 1,4×10-10
1,2×10-10 1,0×10-10 8,0×10-11 6,0×10-11 4,0×10-11 2,0×10-11
0,0
-6 -5 -4 -3 -2 -1 0 1
Capacitance (F)
Voltage (V)
Fig.5.Forwardandreversecapacitance–voltagecurvesoftheZnOMIScapac- itors using FTOand SOG/DIfilms astransparent electrodeand dielectric, respectively.
C Ostretchingmodes,whileat1750cm−1theyareattributedto C Obonds(Adamopoulosetal.,2011;Gayenetal.,2011).The presenceofC Obondssuggestsanincompleteprecursorpyrol- ysisduetothelowdepositiontemperature(Adamopoulosetal., 2011).However,thecharacteristicZn Oband,approximatelyat 415cm−1,indicatestheZnOformation.Ontheotherhand,itis importanttomentionthatbondsrelatedtoO Hstretchinghave beenpreviouslyreportedinZnOfilms,indicatingthepresence ofO Hcomplexesthatareassociatedwithdifferentdefectsand increasedfree-carrierconcentration(Adamopoulosetal.,2011;
Gayen et al., 2011; McCluskey, Jokela, Zhuravlev,Simpson,
& Lynn, 2002; Shi et al., 2005). Thisagrees with what was observedinthephotoluminescencespectra.AlthoughtheZnO filmsneedtobeimproved,thefilmshavebeensuccessfullyused inTFTsfabricatedinsiliconwafers(Dominguezetal.,2015;
Dominguez-Jimenezetal.,2015).Furtherresearchisnecessary tooptimizetheZnOfilm.
Fig. 5 shows the capacitance–voltage characteristics at 10kHz for the MIS capacitors. When a negative voltage is applied atthe top contact, there is an accumulation layer of electrons inthe ZnO filmat theSOG/DI-ZnO interface,as a result,thecapacitance–voltagecharacteristicsshowthedielec- tric capacitance (Cox). On the other hand, when a positive voltageisappliedatthetopcontact,thecharacteristicsshowa minimumcapacitanceduetothedepletionregionintheZnOfilm at the SOG/DI-ZnO interface. From the capacitance–voltage characteristics, it can be observed that there is a very low hysteresiswhenthecharacteristicsareforwardandreversemea- sured.Nevertheless,atnegativevoltagevalues,theaccumulation regionpresentseffectsofinterfacestates(Neamen,2003).This isattributedtothedefectsintheZnOfilmnearthedielectric- semiconductor interface,as the photoluminescence andFTIR spectroscopies show.Despitethis,theMIScapacitorsdisplay asimilarbehaviorcomparedtothecapacitance–voltagecurves inZnOTFTsfabricatedinsiliconwaferswiththermallygrown siliconoxideasgatedielectric(Bangetal.,2011).
ZnO MIS capacitor 10-1
10-2
10-3 10-4 10-5 10-6 10-7
0 10 20 30 40 50
Current density (A/cm2)
Voltage (V)
Fig.6.CurrentdensityoftheZnOMIS capacitorsindicatingthecapacitor breakdownvoltageat36V.
Fig.6showsthecurrentdensityoftheZnOMIScapacitors.
Typically,insolution-processeddevices,theevaporationofsol- ventsduringthedepositionofthefilmsinducesporesandlower densityinthefilmwhichaffecttheelectricalcharacteristicsof thedevices (Yoo etal.,2013).The breakdownvoltageof the MIScapacitorcanbeobservedat36Vbecauseofthedielectric breakdown.Althoughahighercapacitorbreakdownvoltagewas expected,thevaluesofcurrentdensityandcapacitorbreakdown voltageobtainedarereliableforelectronicdevicesapplications (Parketal.,2013;Yooetal.,2013).
Fromtheoverallcharacteristicspresented,ZnOMIScapac- itorscanbeexploredtobeusedintransparentelectronics.ZnO MIScapacitorswerefabricatedemployingsimpleandlow-cost solution-processtechniqueswithno-vacuumneeded,usingfea- sibleandeasily-preparedprecursorsolutions.
4. Conclusions
In summary, the development of fully solution-processed zincoxidemetal–insulator–semiconductorcapacitorsemploy- ingtheultrasonicspraypyrolysisandspin-coatingtechniques was successfully presented. Fluorine tin oxide and spin-on glass were used as transparent electrode and dielectric film, respectively. The MIS capacitors were highly transparent, whichtheirusemakesfeasibleintransparentelectronics.The capacitance–voltagecharacteristicsshowedaverylowhystere- siswhenthecharacteristicswereforwardandreversemeasured.
The accumulation region presents effects of interface states attributed to the defects in the ZnO film near the dielectric- semiconductorinterface,as the photoluminescence andFTIR spectroscopiesshowed.
Conflictofinterest
Theauthorshavenoconflictsofinteresttodeclare.
Acknowledgment
The authorswouldlike tothank NPTC-PRODEPbySEP- MexicoandCienciaBasicaSEP-CONACYTgrantno.256513 forfundingforthisproject.
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