ZnOGaSiO2Si模板法制备用于CO传感器的纳米线ZnO.doc

ZnOGaSiO2Si模板法制备用于CO传感器的纳米线ZnO Sensors andActuators B125 (xx)498503ZnO nanowire-based CO sensors preparedonpatterned ZnO:Ga/SiO2/Si templatesTing-Jen Hsueha,Yi-WenChen b,Shoou-Jinn Changa,Sea-Fue Wangb,Cheng-Liang Hsuc,Yan-Ru Lind,Tzer-Shen Lind,I-Cherng Chene,?abInstitute ofMicroelectronics&Department ofElectrical Engineering,National Cheng Kung University,Tainan70101,Taiwan,ROCDepartment of Materials and Mineral Resources Engineering,National Taipei University of Technology,Taipei10608,Taiwan,ROCcDepartment ofElectronic Engineering,National University of Tainan,Tainan700,Taiwan,ROCdIndustrial TechnologyResearch Institute,Chutung,Hsinchu31040,Taiwan,ROCeMicro SystemsTechnology Center,Industrial TechnologyResearch InstituteSouth,Tainan709,Taiwan,ROCReceived23Novemberxx;received inrevised form24Februaryxx;aepted26FebruaryxxAvailable online12MarchxxAbstractThis investigationdiscusses the growth of high-density singlecrystalline ZnO nanowires onpatterned ZnO:Ga/SiO2/Si templatesand thefabrication of ZnO nanowire-based CO gas sensors.The ZnO nanowires grown on a sputtered ZnO:Ga layer were verticallyaligned whilethosegrown directlyon aSiO2layer were randomly oriented.Additionally,the average length of the nanowires increased and the average diameter ofthe nanowires decreasedas the amount of zinc metal powder in the quartz tube wasincreased from0.1to0.25g.As the amount of zinc metalpowerincreased to0.3g,the nanowiresbecame markedlyshorter.Measuring the resistivity changeof the samples at320?C indicatedthat thesensor responses(R a?R b)/R a)100%,R ais the resistivity in air andR bin CO gas)of the ZnO nanowireCO sensorsprepared with0.1,0.15,0.2,0.25and0.3g zinc metal powderwere5%,8%,35%,57%and29%,respectively.?xxElsevier B.V.All rightsreserved.Keywords:ZnO nanowire;CO gas sensor;CL;VLS1.IntroductionAs aninteresting chemicallyand thermallystable n-typesemiconductor,ZnO has a largeexciton bindingenergy of60meV anda largebandgap energyof3.37eV atroom tem-perature1.With theseproperties,ZnO iswidely appliedinvarious applications,such aspyroelectric devices2,transistors3,piezoelectric devices,mechanical devices4,5and surfaceacoustic wave devices6.ZnO isalso sensitiveto toxicand -bustible gases7.Toour knowledge,ZnO gas sensors ofvariousforms,including thick f i lms8,thin f i lms9,heterojunc-tions10,nanoparticles11and nanowires12,have allbeendemonstrated.Oxygen-related gas sensing generallyinvolvesthe chemisorptionof oxygenon theoxide surface,followed bychargetransfer during the reactionbetween chemisorbedoxy-gen andtarget gasmolecules,changing thesurface resistance?of the sensor element13.Aordingly,one-dimensional(1D)ZnO nanowireshave attractedconsiderable attentionsince theyprovidemuch largerlength-to-diameter andsurface-to-volumeratios thanbulk ZnOand ZnOf i lms.One-dimensional ZnO nanowires can be synthesizedby var-ious methods1420.We recentlyreported on the growth ofZnO nanowires on ZnO:Ga/glass templatesby theself-catalyzedvaporliquidsolid(VLS)method21.This studydiscussesthe growth ofhigh-density singlecrystalline ZnO nanowireson patterned ZnO:Ga/SiO2/Si templatesby the two-step oxygeninjectionmethod withoutcatalysts.Notably,various amounts ofZn powderwere usedduring growth.The sensingproperties ofthe ZnO nanowiresto carbonmonoxide(CO)gas willalso bediscussed.2.ExperimentsPrior to the growth of ZnO nanowires,an Sisubstrate wasthermallyoxidized toform a500nm-thick SiO2f i lm.A100nm-Corresponding author.Tel.:+88635915332;fax:+88635820386.E-mail address:EugeneChenitri.tw(I.-C.Chen).0925-4005/$see frontmatter?xxElsevier B.V.All rightsreserved.doi:10.1016/j.snb.xx.02.059T.-J.Hsueh etal./Sensors andActuators B125 (xx)498503499Fig.1.Schematic diagramof patternedZnO:Ga/SiO2/Si templates.thick Ga-doped ZnO thin f i lmwas subsequentlydeposited ontotheSiO2f i lm byRF magronsputtering.X-ray diffraction(XRD)demonstrated thatthe sputteredZnO:Ga f i lmwas ori-ented in the (002)direction.Four-point resistivitymeasurementindicated thatthe sheetresistance of the sputteredZnO:Ga f i lmwasaround200?/sq.Standard photolithographywas thenper-formed topartially etchaway the ZnO:Ga f i lmand def i heb-like pattern.During wetetching,the templatewas dippedin2%HCl for3min toremove theexposed ZnO:Ga.As shownin Fig.1,the etchingmask wasdesigned tomake thef i ngersof theb-like pattern10?m wideand80?m longwith aspacingof10?m.Two smallpieces ofglass were used tocoverthe two electrodes of the patternedZnO:Ga f i lmso thatno ZnOnanowiregrows in these regions22,23.Various amounts of99.9%pure zinc metal powder(0.1,0.15,0.2,0.25and0.3g)wereusedas theZn sourceto growZnO nanowires.As presentedin Fig.2,the growthwas performedin aquartz tubelocatedin ahorizontal furnace.The samplewas placedwith differentamountsof Zn powder ontoan aluminaboat,and insertedtheminto thequartz tube.The growth of ZnO nanowires wasdividedinto?two steps.The f i rststep wasto rampthe temperatureup at30C/min.Initially,only Argas was introduced at a f l owrateof54.4sm into the furnace.When the temperature reachedthemelting pointof Zn(420?C),theZn vapor pressureinsidethe furnace?increased rapidly.When thetemperature reached450C,the oxygengas waspoured into the chamberatafl owrate of0.8sm tostart thegrowthof ZnO nanowires.When thetemperaturereached700?C,thetemperatureramping waster-minated and the chambertemperature wasmaintained at700?Cto growthe ZnO nanowires continuously.The totalgrowthtime was40min.After growth,standard photolithographyandliftoff wereused todeposit Au/Pd(80%:20%)onto theelectrodeFig.2.Schematic illustrationof thesystem used to growZnO nanowires.regions toserve as the contactpads.The sampleswere thenther-mally annealedin Arambience at350?C for15min toform goodohmic contacts betweenAu/Pd and the underlyingZnO:Ga fi lm.A JEOLJSM-6500F fi eldemission scanningelectron micro-scope(FESEM)at5keV wasthen usedto characterizethestructures of the as-grown ZnO nanowires.Cathodolumines-cence(CL)was alsousedtoevaluate thequality of the depositedZnO nanowires.During CLmeasurements,the electronbeampower wasmaintained at0.405W(aelerated at5kV with anemission currentof81?A).The samplewas placedina sealedchamber andthe resistivity of the sample inair was measuredwith the twoelectrodesof thepatternedZnO:Ga fi lmat320?C tomeasuregassensingproperties of the nanowires.Then,500ppmCO gas was injectedinto thechamber andthe resistivityof thesamplewas measuredagain at320?C inthe presenceof COgas.3.Results anddiscussionFig.3(a)shows atop-view FESEMimage of the ZnOnanowires grown with0.25g zinc metal powder inside theFig.3.(a)Top-view FESEMimage of ZnO nanowires grown with0.25g zincmetal powderinside aquartz tube.The insetshows anenlarged imageof thissam-ple and(b)IV characteristics between thetwo neighboring electrodes,bridgedby the ZnO nanowires,measured inair.500T.-J.Hsueh etal./Sensors andActuators B125 (xx)498503Fig.4.Cross-sectional FESEMimages of ZnO nanowires grown with(a)0.1g,(b)0.15g,(c)0.2g,(d)0.25g and(e)0.3g zinc metal powderinsideaquartz tube.quartz tube.The insetof Fig.3(a)shows anenlarged imageofthis sample.Clearly,the ZnO nanowires were grown onboththe conductingZnO:Ga fi ngerregions andthe insulat-ing SiO2spacer regions.The ZnOnanowires grownon theZnO:Ga fingerregions werewell alignedinthevertical direction.The verticalnanowires areobserved inthese regionsbecausethey weregrown alongthe columnargrains underthe sputteredZnO:Ga fi lm22.In contrast,theZnOnanowires grownonSiO2spacer regionswere randomly oriented.Notably,these ran-domly oriented ZnOnanowiresprovide electricalpaths betweentheneighboring fi ngers.When theseZnOnanowires wereran-domlyoriented,thetwoelectrodes wereno longerelectricallyopen.Hence,theresistivityof thesample couldthus bedeter-mined byapplying aconstant voltageacross thetwo electrodesandmeasuring thecorresponding current.Fig.3(b)plots thecurrentvoltage(IV)characteristicsbetween thetwoneigh-boringelectrodesbridged bytheZnOnanowiresinair.Themeasured currentincreased linearlywith increasingthe appliedbias.Such linearbehavior revealsthat goodohmiontactswereformed betweenthe nanowiresandtheelectrodes.In thisstudy,ZnOnanowires weregrownwith variousamountsof zincmetal powder.Fig.4(a)(e)show thecross-sectional FESEMimages of theZnOnanowiresgrownwith0.1,0.15,0.2,0.25and0.3g zincmetal powderinside thequartztube,respectively.The fi vefigures showthat high-density verti-cal ZnOnanowiresweregrownon the conductingZnO:Ga fi ngerregionswhile randomly orientedZnOnanowiresweregrown ontheinsulating SiO2spacer regions.Given thesame growthtem-perature profi leand growthtime,however,the averagediameterand length of the nanowires variedsignif i cantly.Notably,thescale barsin Fig.4(a)and(e)differed fromthose in(b)(d).Fig.5summarizes the averagelengthand diameterof the ZnOnanowires prepared with differentamountsofZnpowder.Asshown inFig.5,theaveragelength of the nanowiresincreasedwhile theaveragediameterof thenanowires decreasedas theamount of zincmetal powderinthequartz tubewas increasedfrom0.1to0.25g.The exactreasons for these observationsarenot clearyet.As the amount of zincmetal powder increased from0.1to0.25g,the numberof zincvapor particleson topof theZnvapor sourceincreased,so thatlonger ZnOnanowires wereT.-J.Hsueh etal./Sensors andActuators B125 (xx)498503501Fig.5.Average lengthand diameterof ZnOnanowiresgrownon ZnO:Ga/SiO2/Si templateswith various amountsof zincmetal powder.formed.At thesame time,the amount of oxygenmolecules thatpassedthrough theZnvaporsource becamesmaller.Tseng etal.demonstrated thatthe diameteroftheZnOnanowires decreasedas theoxygen flowrate decreased24.We believethat asim-ilar phenomenonourred astheamountof zincmetal powderincreasedfrom0.1to0.25g.Thus,thinner ZnOnanowires wereformed.As theamountof zincmetal powder increased to0.3g,however,these nanowiresbecame signifi cantlyshorter,probablybecause ofthe def i ciencyof oxygenmolecules.Fig.6shows theroom temperatureCL spectraof ZnOnanowires prepared withvariousamountsofzincmetalpow-der.A clearsharp strongpeak locatedat approximately380nmwas observedfor allfive samples.This sharpCL peakoriginatesfrom thenear band-edge emissionsof ZnO25.Deep levelemission(green-yellow band)was observedasabroad peakforthese samples.This deeplevel emissionis relatedto thesinglyionized oxygen vacancy inZnO26.The electricalproperties ofoxidenanowires aregoverned byoxygenvacancy-related donorstates,aording tothe ionizationprocesses13:V oV o+e? (1)Fig.6.Room temperatureCL spectraof ZnOnanowiresprepared with variousamountsofzincmetal powder.Fig.7.Sensor responses of aZnOnanowire-based COgas sensor measured at320?C.V o+V o2+e? (2)where Vo+and Vo2+are thesingly anddoubly ionizedvacancies,respectively.The spectrainFig.6yield calculatedUV-to-visibleCL intensityratios of37,13,7.5,0.5and0.3for theZnOnanowires grownwith0.1,0.15,0.2,0.25and0.3g zincmetalpowder,respectively.These numberssuggest thatthe numberof oxygen vacancies increased with theamountofzincmetalpowder.This factcan againbe attributedtothedeficiency ofoxygenmolecules24.Fig.7shows thedetector responseoftheZnOnanowire-basedCO gas sensormeasuredat320?C.To makethese measure-ments,500ppm COgaswasintroduced intoasealedchamberand theresistivityofthesamplewasmeasuredboth inair(R a)and inCOgas(R b).The performanceofthe sensor wasmea-sured asthesensorresponse,defined by(R a?R b)/R a)100%.As theCOgaswasintroducedinto thechamber,theresistiv-ityofthesampledecreased exponentiallywithatime constantof6s.As showninFig.7,thesensorresponses oftheZnOnanowireCO sensorspreparedwith0.1,0.15,0.2,0.25,and0.3g zincmetalpowderwere5%,8%,35%,57%and29%,respectively.A possiblemechanism bywhich ZnOnanowiressense COgas isas follows.First,reactive oxygenspeciessuch asO2?,O2?and O?are adsorbedon theZnO sur-face atelevated temperatures.Notably,the chemisorbedoxygenspecies dependstrongly?onthetemperature.At temperaturesoflower than130C,?O2?is monlychemisorbed.Athigher temperatures,O isnormally chemisorbedwhile O2?is rapidly27.The reactionkiics isdescribed asfollows28:O2(gas)?O2(adsorbed) (3)O2(adsorbed)+e?O2? (4)O2?+e?2O? (5)Thus,the conductanceoftheZnOnanowiresincreases asthereducing gas(CO)is introducedintothetest chamberbecauseof theexchange ofelectrons betweenthe ionosorbedspecies andtheZnO itself.The reactionbetweenthereducing gas andthe502T.-J.Hsueh etal./Sensors andActuators B125 (xx)498503surface ofoxide sensorcanbedescribed byRef.29,R+O?(ads)?RO+e? (6)where Ris thereducing gas.Notably,this reactioninvolvesa changeof surfacestate charge.Aordingly,the responseofthe fabricatedZnOnanowireCOsensordepends stronglyonthe numberof oxygen vacanciesinthenanowiresand thelength-to-diameter ratio.Fig.6shows thatthe numberofoxy-genvacanciesincreased with theamountofzincmetal powerusedduringthegrowth.Thus,the increase in detectorsensorresponse with the massofzincmetal powerfrom0.1to0.25g isattributabletotheincreaseinthe numberof oxygenvacancies inthenanowires.However,the smallersensorresponseoftheZnOnanowires grownwith0.3g zincmetalpowderis attributabletothe smalllength-to-diameter ratioofthenanowires.Further-more,the responseand recoverytimes ofthe fabricatedsensorswere around20and60s,respectively.These valueswere sim-ilar tothose reportedfor ZnOthick film-based CO sensors30.4.ConclusionsIn summary,this investigationreports onthegrowthofhigh-density singlecrystalline ZnOnanowiresonpatternedZnO:Ga/SiO2/Si templatesandthefabricationofa ZnOnanowire-based COgas sensor.The ZnOnanowires grownonasputteredZnO:Ga layerwere verticallyaligned whilethosegrown directlyontheSiO2layerwererandomlyori-ented.The averagelengthofthenanowiresincreased whiletheaverage diameterofthenanowiresdecreasedastheamountof zincmetalpowderinthequartztubeincreasedfrom0.1to0.25g.The nanowiresbecame significantlyshorteras theamountofzincmetalpowder furtherincreasedto0.3g.Measuring theresistivity changeofthesamples at320?C yieldedsensorresponsesof5%,8%,35%,57%and29%for theZnOnanowireCOsensorspreparedwith0.1,0.15,0.2,0.25and0.3g zincmetalpowder,respec-tively.References1N.Kumar,A.Dorfman,J.Hahm,Fabrication ofoptically enhancedZnOnanorods andmicrorods usingnovel biocatalysts,J.Nanosci.Nanotechnol.5 (xx)19151918.2C.S.Wei,Y.Y.Lin,Y.C.Hu,C.W.Wu,C.K.Shih,C.T.Huang,S.H.Chang,Partial-electroded ZnOpyroelectric sensorsfor responsivityimprovement,Sens.Actuators A128 (xx)1824.3H.H.Hsieh,C.C.Wu,Scaling behaviorof ZnOtransparent thin-film 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