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Characterization of P-type zinc oxide films

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Characterization of P-type zinc oxide films
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Oleti Kalki Rajan, Madhavi
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Wideband gap semiconductors
RF Sputtering
Four point probe measurement
Hall Measurement
Absorption/Transmission Spectroscopy
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ABSTRACT: Zinc Oxide falls under the classification of transparent conductive oxides. They typical optical transmittance of Zinc Oxide is 90% in the visible wavelength region. Though stoichiometric ZnO is an insulator, due to the presence of internal defects such as Zn interstitials and Oxygen vacancies, it exists as a n-type conductor. The other important property of ZnO which could be used by the optical field is its widebandgap. ZnO has a wide bandgap of 3.2eV -3.3eV. The additional advantage of being a direct bandgap semiconductor has increased the probability of using ZnO for short wavelength applications. These practical applications are directly related to the fabrication of homostructural p-n junctions. ZnO can be readily doped n-type. Doping ZnO P-type is very difficult due to its native defects and the self-compensation that occurs during doping. But when P-type doping is obtained in ZnO it could be used in various optical applications such as light emitting diodes and laser diodes. This provided the motivation for this research. Theoretical studies have proposed nitrogen as a suitable material to achieve p-type ZnO. Literature provides a set of conditions that could be used to improve the doping in ZnO films. In this research, a set of these conditions were used to implement p-type doping in ZnO films. A sputtering system with a setup to support two Torus - 5M guns was used to deposit the ZnO films. A codoping technique using an aluminium doped zinc oxide target was the first method. Though an improvement in the nitrogen incorporation was found in this method in the beginning, a further increase in the nitrogen pressure did not show further improvement. A co-sputtering technique of a 99.999% pure ZnO target and a 99.99% pure Zn metal target was the second method. The ZnO target was rf sputtered while the Zn target was dc sputtered using the two guns provided in the deposition chamber. The extra Zinc obtained from sputtering the metallic Zn target was used to improve the incorporation of nitrogen. The films were later deposited in an oxygen ambient where the excess oxygen was used to suppress the oxygen vacancies that act as hole killers during the doping process. Four point probe measurement and Keithley 900 series Hall equipment were used for the electrical characterization of the films. An ORIEL monochromator was used to optically characterize the films. Hitachi S-800 T EDAX analysis system was used to measure the atomic weight % of nitrogen incorporated in the ZnO:N films. Deposition at an oxygen partial pressure of 0.3mT and 0.8mT of nitrogen produced p-type ZnO films. These films showed a carrier absorption in the short wavelength region. The carrier concentration and the mobility obtained for these films were 4.0x10¹⁶ cm⁻³ and 0.12 cm²/V-s respectively.
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Thesis (M.S.E.E.)--University of South Florida, 2004.
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by Madhavi Oleti Kalki Rajan.
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CHARACTERIZATIONOFP-TYPEZINCOXIDEFILMSbyMADHAVIOLETIKALKIRAJANAthesissubmittedinpartialfulllmentoftherequirementsforthedegreeofMasterofScienceinElectricalEngineeringDepartmentofElectricalEngineeringCollegeofEngineeringUniversityofSouthFloridaMajorProfessor:DonMorel,Ph.D.ChristosFerekides,Ph.D.AndrewM.Hoff,Ph.D.DateofApproval:July6,2004Keywords:Widebandgapsemiconductors,RFSputtering,Fourpointprobemeasurement,HallMeasurement,Absorption/TransmissionSpectroscopycCopyright2004,MadhaviOletiKalkiRajan

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DEDICATIONTomyMother,FatherandBrother

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ACKNOWLEDGEMENTSIamgreatfultoDr.DonMorelforgivingmethiswonderfulopportunitytobeapartofhisresearchteam.IalsowouldliketothankDr.ChristosS.FerekidesandDr.AndrewM.Hoffforbeingapartofmycommittee.MysincerethankstoHarishSankaranarayanforhisguidancethroughoutthiswork.Igainedarealdealofexperiencewhileworkinginthegroupalongwiththefunandsupport.

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TABLEOFCONTENTSLISTOFTABLESiiiLISTOFFIGURESivABSTRACTvCHAPTER1INTRODUCTION11.1ThinFilms11.1.1EvolutionofThinFilms11.2SemiconductingMaterials21.3WideBandgapSemiconductors31.3.1II-VISemiconductorCompounds41.3.2OverviewofOrganizationoftheThesis5CHAPTER2BACKGROUND72.1HistoryofZincOxide72.2PropertiesofZincOxide82.2.1CrystalStructure82.2.2ThermalProperties102.2.3ElectricalProperties102.2.3.1EffectofSubstrateTemperature102.2.3.2EffectofSputteringParameters112.2.4OpticalProperties122.3WhatMakesZincOxideMoreInteresting?122.3.1ZincOxideasaBetterTCO132.3.2ZincOxideinOptoelectronics142.4DopingTechniqueinZincOxide152.4.1DopingZincOxideN-type152.4.2IssuesinDopingZincOxideP-type192.4.2.1AvoidingFermi-Level-InducedCompensationEf-fectsbySpontaneousGenerationofNativeKillerDefects202.4.2.2EnhancingDopantSolubility212.4.2.3DopingRulesPertainingtoLocalDefectBondingEffects212.4.3LiteratureReview22i

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CHAPTER3PROCESSDESCRIPTION253.1DepositionChamber253.2ProcessTechnique273.2.1SputteringPrinciples273.2.2MagnetronSputtering283.2.3PlanarMagnetronSputtering293.2.3.1SystemConguration293.2.3.2DisadvantageoftheTechnique293.2.3.3AdvantagesoftheTechnique303.2.3.4ComparisonofDCandRFPMSputtering303.2.3.5Applications303.2.4TargetandSputteringGunAssembly313.3SubstrateProcessing313.4SampleLoading313.5PumpingSystem323.6ParametersInvolvedDuringDeposition323.6.1Temperature323.6.2Pressure333.6.3GasFlow333.6.4RFPower333.6.5DCPower343.7ShutDownProcedure343.8SafetyPrecautions34CHAPTER4RESULTSANDDISCUSSION364.1UndopedZincOxideFilms364.2CodopingTechniqueUsingAluminium374.2.1EffectofNitrogenonFilmThickness384.3DopingUnderZincRichEnvironment384.3.1EffectofZninDoping424.4DopinginanOxygen-andZinc-RichEnvironment454.4.1InuenceofOxygenPressureonCarrierConcentration464.4.2OpticalCharacteristics464.4.3InuenceofOxygenPressureonFilmThickness474.4.4EffectofIncreaseinNitrogen484.4.4.1OpticalCharacteristics49CHAPTER5CONCLUSION515.1FutureWork53REFERENCES54ii

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LISTOFTABLESTable1.1PropertiesofSemiconductorMaterials4Table4.1ElectricalPropertiesofAZO:NFilms39Table4.2ElectricalPropertiesofZnOFilms41Table4.3EDSDataforZnOFilms41Table4.4ComparisonTableforZnOFilmsatDifferentSputteringPressuresforZn43Table4.5ElectricalPropertiesofZnOFilmsDepositedUnderVariousPartialPressuresofOxygen45iii

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LISTOFFIGURESFigure2.1ConstructionoftheWurtziteStructureofZincOxide[1]9Figure2.2PlotofBandgapVersusCarrierConcentration13Figure2.3ConductivityofZnO:AlFilmsatVariousSputteringVoltages16Figure2.4VariationoftheAbsorptionCoefcientwiththeSputteringVoltagefortheZnO:AlFilms[2]17Figure2.5VariationinResistivityoftheFilmwiththeDepositionTemperature18Figure2.6VariationinResidualStressintheFilmwithVariationintheOxygenFraction19Figure2.7Then-typePinningEnergyandp-typePinningEnergyRelativetotheAbsoluteBand-EdgeEnergiesofIII-VandII-VISemiconductors20Figure3.1DepositionChamber26Figure3.2SimpliedCrossSectionofaSputteringSystem[3]27Figure4.1TransmissionResponseofanUndopedZnOFilm37Figure4.2ThicknessoftheAZO:NatDifferentPartialPressuresofNitrogen39Figure4.3Cross-SectionalViewoftheTargetPlacementInsidetheSputteringSystem43Figure4.4TransmissionResponseatShorterWavelengthsforSamplesDepositedatDifferentZnConcentration44Figure4.5InuenceofOxygenPartialPressureonCarrierConcentration47Figure4.6StudyontheShortWavelengthAbsorptionatDifferentDepositionConditions48Figure4.7InuenceofOxygenPartialPressureonFilmThickness49Figure4.8TransmissionResponseofZnO:NDepositedat0.3mTofOxygenand0.8mTofNitrogen50iv

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CHARACTERIZATIONOFP-TYPEZINCOXIDEFILMSMadhaviOletiKalkiRajanABSTRACTZincOxidefallsundertheclassicationoftransparentconductiveoxides.ThetypicalopticaltransmittanceofZincOxideis90inthevisiblewavelengthregion.ThoughstoichiometricZnOisaninsulator,duetothepresenceofinternaldefectssuchasZninterstitialsandOxygenvacancies,itexistsasan-typeconductor.TheotherimportantpropertyofZnOwhichcouldbeusedbytheopticaleldisitswidebandgap.ZnOhasawidebandgapof3.2eV-3.3eV.TheadditionaladvantageofbeingadirectbandgapsemiconductorhasincreasedtheprobabilityofusingZnOforshortwavelengthopticalapplications.Thesepracticalapplicationsaredirectlyrelatedtothefabricationofhomostructuralp-njunctions.ZnOcanbereadilydopedn-type.DopingZnOP-typeisverydifcultduetoitsnativedefectsandtheself-compensationthatoccursduringdoping.ButwhenP-typedopingisobtainedinZnOitcouldbeusedinvariousopticalapplicationssuchaslightemittingdiodesandlaserdiodes.Thisprovidedthemoti-vationforthisresearch.Theoreticalstudieshaveproposednitrogenasasuitablematerialtoachievep-typeZnO.LiteratureprovidesasetofconditionsthatcouldbeusedtoimprovethedopinginZnOlms.Inthisresearch,asetoftheseconditionswereusedtoimplementp-typedopinginZnOlms.ASputteringsystemwithasetuptosupporttwoTorus-5MgunswasusedtodeposittheZnOlms.Acodopingtechniqueusinganaluminiumdopedzincoxidetargetwastherstmethod.Thoughanimprovementinthenitrogenincorporationwasfoundinthismethodinthebeginning,afurtherincreaseinthenitrogenpressuredidnotshowfur-v

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therimprovement.Aco-sputteringtechniqueofa99.999pureZnOtargetanda99.99pureZnmetaltargetwasthesecondmethod.TheZnOtargetwasrfsputteredwhiletheZntargetwasdcsputteredusingthetwogunsprovidedinthedepositionchamber.TheextraZincobtainedfromsputteringthemetallicZntargetwasusedtoimprovetheincorpora-tionofnitrogen.Thelmswerelaterdepositedinanoxygenambientwheretheexcessoxygenwasusedtosuppresstheoxygenvacanciesthatactasholekillersduringthedop-ingprocess.FourpointprobemeasurementandKeithley900seriesHallequipmentwereusedfortheelectricalcharacterizationofthelms.AnORIELmonochromatorwasusedtoopticallycharacterizethelms.HitachiS-800TEDAXanalysissystemwasusedtomeasuretheatomicweightofnitrogenincorporatedintheZnO:Nlms.DepositionatanOxygenpartialpressureof0.3mTand0.8mTofNitrogenproducedp-typeZnOlms.Theselmsshowedacarrierabsorptionintheshortwavelengthregion.Thecarriercon-centrationandthemobilityobtainedfortheselmswere4.0x10cmand0.12cm/V-srespectively.vi

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CHAPTER1INTRODUCTION1.1ThinFilmsEverymaterialiscomposedofatomswhichdecidetheirbasicproperties.Thespecial-ityofthethinlmsliesinthefactthatamaterialcouldbedepositedinitsatomicscaleusingthistechnology.Theatomsandfreeradicalsofthesematerialsintheirgasphasereactwiththemselvesorwithothersubstancesathightemperaturetocondenseordepositonasurface(substrate)relievingtheirhighenergies.Thematerialcoatingthusformedifintherangeofmicrometersorlessiscalledathinlm.Theselmspossesspropertiesthatarecompletelydifferentfromthebulkmaterialandvaryaccordingtothedepositioncon-ditionsandthethicknessofthelm.Theseuniquepropertiesofthinlmshaveenabledthemtobeusedinvariousapplicationssuchashardcoatingsandwearresistantlms,opticaldevicesandvariousotherapplications.1.1.1EvolutionofThinFilmsThehistoryofthinlmtechnologybeganwiththeinventionoftheon-chipresistorsbyJohnHallintheyearof1962[4].Sincethenthinlmshavehadamajorinuenceintheeldofelectronics.Themajoradvantageofusingthinlmsinelectricalcomponentsisthereductioninoneofthedimensions.Thiscouldbeclearlyexplainedusinganexample.Accordingtothedenitionofresistance,resistanceofamaterialisinverselyproportionaltoitscross-sectionalarea.Thuswhencompactnessisanissue,theresistorcanbemadeusingathinlmtoreplacetheirbulkcounterparts.1

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TheinventionofthinlmsleadtoanewtrendintheelectronicseldcalledTheMi-croelectronics.Microelectronicsprovedtobeaverygoodsolutionformanyresearchers.TheequipmentusedtomanufacturetheelectroniccomponentsinMicroelectronicswasreliableinoperationandrequiredlessmaintenance.Thisreducedthemaintenancecostaswellastheinitialcostoftheequipment.AccordingtoD.Bois[5],forthepast50years,sincetheinventionofthetransistor,theexponentialprogressofmicroelectronicshasbeenfueledbya15%yearlyminiaturizationrate,inducingalmosta30%costdecreaseanda50%performanceimprovementinallelectronicfunctionseachyear.Inturn,thisprogressofthetechnologyhasbeenthekeydrivingforcefora15%growthrateofthesemiconduc-tormarket.1.2SemiconductingMaterialsSemiconductorsareaninterestinggroupofmaterials.Thesematerialshavetheircon-ductivitythatliesbetweenaconductorandaninsulator.Studieshaveprovedthatthesematerialspossessverygoodelectricalandopticalpropertiesthatcouldbeusedforprac-ticalapplications.Theconductivityofthesematerialsisfurtherimprovedbyexternallydopingthemusingmaterialswhichhavecomparativelyhigherorlowervalenceelectronsresultingineithern-typeorp-typematerialaccordingly.Thesematerialscontributealottotheelectricaleldofminiaturization.Silicon,Germanium,Carbon(diamond)aretheelementalsemiconductorswithfourvalenceelectronsbelongingtogroupIVoftheperiodictable.AnextensiveresearchonGewasperformedintheyears1947-1958.TheinconsistentbehaviourofGeasatran-sistordivertedtheinterestofresearcherstowardsSiwhichhasabandgapof1.12eVatroomtemperature.Siliconbeingabundantinnature,wasverycheap.Duetoitsverygoodphysicalandopticalproperties,itevolvedintoadominatingmaterialintheeldofelectronics.Thoughmanymaterialshadbeendiscoveredaftersilicon,siliconproductionintheworldisstillgrowing.Theyear1962markedthebeginningofthesiliconerawith2

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1,130,000tonsastheworldproductionofsilicon.Thisnumberhasincreasedto3,500,000tonsintheyear2000[6].Extensiveresearchfoundthatbeinganindirectbandgapmate-rial,Sihadopticalpropertieswhichwereinsufcientforsomeoftheapplications.BinarysemiconductorslikeGalliumArsenide,GalliumPhosphide,GalliumStannate,IndiumAr-senide,IndiumPhosphide,IndiumStannateandSiliconGermaniumbecamethematerialsofinterestforresearchers.GalliumArsenidewithabandgapof1.42eVseemedtohaveinterestingproperties.BeingahighspeedandlowpowerdissipationmaterialGaAshadamajorinuenceonthecommunicationsindustry.Itshigherradiationhardnessandahigherrangeofoperatingtemperature(196Cto300C)[7]madeitattractivetospaceandmilitaryapplications.1.3WideBandgapSemiconductorsSiliconandGalliumArsenidehavehadamajoreffectonthesemiconductoreldwiththeabovementionedapplications.Siliconwasfoundsuitableforapplicationswithanop-eratingtemperaturenotexceeding300C.GalliumArsenidewasnotmuchdifferentfromsiliconasthemaximumoperatingtemperatureofGaAswasfoundtobe400C.There-quirementformaterialsreliableoverawiderangeoftemperaturefrom-20Cto400Candtheneedforextracoolingsystems[8]leadtothedevelopmentofwidebandgapsemi-conductors.Materialswithabandgapof2eVnEn6.5eV[9],[10]wereclassiedasWideBandgapMaterials.ThoughthesematerialscannotreplacethealreadyexistingefcientandmuchcheaperSiliconandGaAssemiconductortechnology,theywerefoundtobemoreadvantageousduetothesuperiorpropertiestheypossess.Thisincludedtheirlowerintrinsiccarrierconcentration,higherelectricbreakdowneld,higherthermalconductiv-ityandlargersaturatedelectrondriftvelocity.Alistingofthemajorphysicalpropertiesofimportantsemiconductingandwidebandgapmaterialsisincludedintable1.1[8]forabetterunderstandingofthesematerials.3

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Table1.1.PropertiesofSemiconductorMaterials Properties Silicon GaAs 3C-SiC 4H-SiC 6H-SiC GaN latticeconstant(A)(RT) 5.430 5.65 4.3596 3.073 3.0806 4.51 Density(g/cm) 2.328 5.32 3.210 3.211 MeltingPoint(C) 1420 1238 2830r 2830r 2830r Bandgap(eV) 1.1 1.43 2.39 3.26 3.02 3.45 Saturatedelectron velocity(x10cm/s) 1.0 1.0 2.2 2.0 2.0 2.2 Electron CarrierMobility(cm/V-s) 1500 8500 1000 1000 370 1250 Hole CarrierMobility(cm/V-s) 600 400 50 50 90 250 Dielectricconstant 11.8 12.5 9.7 9.6-10 11 Resistivity(-cm) 10 10 10 Refractiveindex 3.5 3.4 2.7 2.712 2.7 Hardness(kg/mm) 1000 600 3980 2130a Researchonthewidebandgapsemiconductorsledthemtohaveamajoreffectinvar-iouselds.Intheeldofpowerelectronics,duetotheirsuperiorbreakdownvoltagesandhighthermalconductivitiestheycanbeusedforhigh-voltageandhigh-powerelectronics[8].Theirexceptionallyhighthermalconductivity,lowdielectricconstantalongwiththeirhighchargecarriermobilitieshadmadethemsuitableforvariouselectronicapplicationslikeFETs,MESFETsandHEMTs[11],[8].Theirdirectbandgap(unlikesilicon)andtheirwiderbandgapprovedthemvitalinopticalapplicationssuchaslasers.Thustheycreatedamajorrevolutionintheeldoflasersandotheropticaldeviceslikelightemittingdiodes[12].Abriefdescriptionofthemajorclassicationofwidebandgapsemiconductorsisdis-cussedinthefollowingsections.1.3.1II-VISemiconductorCompoundsII-VIcompoundsemiconductorshadalwaysbeenaninterestingclassicationofsemi-conductors.Thesecompoundsmostlycrystallizeinthecubic(zincblende)orhexagonal(wurtzite)structure.Theyoccurinawiderangeofbandgapsandlatticeconstants.The4

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bandgapanditstypehasamajorinuenceonthepropertiesofthematerial.Thisin-cludespropertieslikeopticalabsorption,electricalconductivityandindexofrefraction.Materialsareclassiedaccordingtothetypeofbandgapasdirectandindirectbandgapsemiconductors.Thedirectbandgapsemiconductorsarefoundtobeadvantageousoverindirectbandgapsemiconductorsastheydonotrequirephononstosatisfywavevectorconservation.MostoftheII-VIcompoundsarefoundtoexistasdirectbandgapsemi-conductors.Allthesepropertiestakenintoconsideration,theyhavebeendominatingtheopticaleldforshortwavelengthapplications.Theyareusedinvariousapplicationssuchasinfraredlasersanddetectors,blue-greenlasersandlightemittingdiodes,non-linearop-ticalmaterials,magneto-opticaldevicesandradiationdetectors[13].ZincSelenide,oneoftheII-VIsemiconductorshasbeenasignicantmaterialinblue-greenlasers.Theystillaretheonlymaterialswithwhichgreenlaserscouldbeobtained[12].ZincSulphidewithabandgapof3.7eVatroomtemperaturehasproventobeoneofthepromisingmaterialsforultra-violet(UV)opticaldevices[12].ZincOxideisoneoftheII-VIcompoundswithsomeexceptionalandcomparablepropertiestotheirIII-Vcounterparts.Abriefdescriptionofthismaterialandacomparisonofitwithothermaterialsisdiscussedinthenextchapter.1.3.2OverviewofOrganizationoftheThesisThisthesiswillreportonthevariousmethodsusedtofabricatep-typeZnOlms.Theoretically,nitrogenispredictedtobeagoodcandidatefordopingZnOp-type.Re-searchershavebeenworkingsince1996onthistopicusingvarioustechniquestoachievethispractically.Itswidebandgap,crystalstructureandhighexcitonenergysuggestthatZnOcouldreplaceGaNinthefutureoptoelectronicseld[14].ManyoftheapplicationsinwhichZnOcouldbeusedinvolvep-njunctions.Thisprovokedtheinterestforthisresearch.5

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Thisthesisworkisdividedintofourchapters.Thesecondchapterwilldiscussindetailthehistoryandpropertiesofzincoxide,thetheoreticalproblemsthatarepredictedtobethecauseforachievingp-typeZnOandaliteraturesurveyonthevarioustechniquesfollowedbyvariousresearchersonthistopicwillalsobediscussed.Thethirdchapterwillexplaintheprocessingtechniqueandthedescriptionoftheproceduresinvolvedinfabricatingzincoxidelms.Chapterfourwilldiscusstheresultsobtainedinthiswork.Andnally,chaptervewillsummarizeconlcusionsdrawnfromthisresearchandgivesomeideasthatcouldbeusedinthefuturetoimproveonthisresearch.6

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CHAPTER2BACKGROUND2.1HistoryofZincOxideThehistoryofZincOxidebacksup100yearswhenitwasusedasamajorwhitepigmentandindustrialchemical.Ithadbeenfamousindiverseeldsasanexperimentalmaterialduetoitsuniqueproperties.Forexample,withitswidespectrumofsolid-statepropertiesitprovidesnumerousfertileeldsforinvestigation,exhibitsuniquepropertiesinferritesandthephotochemicaleld,andithascontributedquitealotinanalysingtherelationshipbetweenthecrystalstructureandpropertiesofmaterials.Itssimplercrys-tallinestructureandthereasonthatitcanbedepositedindifferentcrystaltypeshashelpedintheseinvestigations.Further,itsdrasticchangeinpropertieswiththeintroductionofimperfectionhasaddedadvantagetothisstudy[1].Itsgrowthintheindustrialeldhasoccuredduetoitsremarkablesolid-statepropertieswhichhadleadtoadvancesinnumer-ouseldssuchascatalysis,ferromagnetism,semiconductors,luminescence,photocon-ductivityandphotochemicaleffects.Someoftheachievementsinvariouseldsduetozincoxidearelistedbelow[1].ByintroducingZincOxideintonickel-ironferrites,therstferriteswithhighmag-neticpropertieswerediscovered.ByinvestigatingcertainluminescentpropertiesofZincOxideandZincSulphide,anewandsignicantmethodofillumination,Electroluminescencewasdiscovered.ThephotoconductivitypropertiesofZincOxideleadtothedevelopmentofanew,dry,photocopyingprocess.7

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Itisusedinconvertingsunlightintochemicalenergy,asitissupposedtobetheonlyinorganicmaterialknowntohavephotochemicalpropertiessimilartochlorophyll.AdvancementinthinlmsandfurtherstudieshavemadeZincOxidetobeapartofanendlesslistofapplications.Someofthemincludeacousticwavedevices[15],trans-parentconductingelectrodematerialsforelectronicdevicessuchassolarcells[16],[17]andelectroluminescencedisplays[18],[19],electromechanicaldevicesandoptoelectronicdevices[20],[17],gratingcoupledwaveguidelters[21],heatmirrors[22],multilayerphotothermalconversionsystems[23],gassensors[24].2.2PropertiesofZincOxideZincOxideisawidebandgapsemiconductorwithabandgapof3.2eV-3.3eV.Itexhibitsawiderangeofproperties.SomeoftheimportantandrelevantpropertiesofZnOtothisthesisarediscussedinfollowingsections.2.2.1CrystalStructureCrystalstructureisoneofthecriticalpropertiesofawidebandgapsemiconductorfordevicessuchaslightemittingdiodesandlaserdiodes.Improvementinthecrystallinityofthelmsincreasesthelifetimeofthedevice.Inthecaseofthelaserstructure,thediffer-enceinthechemicalandstructuralpropertiesofthematerialsinvolvedintheheterostruc-turecausesdefectssuchaspointedorextendedwhichsometimespropogateduringdeviceoperationcausingthedevicetofail.Toavoidthis,adetailedstudyofthestructuretoimproveandoptimizethesedeviceshasbecomeaneccessity.ZincOxidehasahexagonalclose-packedlatticestructurecalledtheWurtziteStructure.ItisreportedintheworkofY.R.Ryu[14]thatthehighlyc-axisoriented,self-texturedZnOlmscanbesynthesizedonanysubstratesuchasSiandGaAsandthuscontributetotheproductionofoptoelectronicdevices.Themannerinwhichthisstructureisformed8

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isdepictedin2.1.Tobeginwith,fouroxygenatomsarejoinedtoonezincatomtoformazinctetrahedron(A),and(B)illustratesthejoiningoffourzincatomstooneoxygenatomtoformanoxygentetrahedron.Duringthesetwosteps,eachzincatomdonatestwoelectronsinitsoutershelltoanoxygenatom,thusshrinkinginsizefrom1.33to0.74whiletheoxygenatomexpandsinitssizefrom0.64to1.40[1].Thesetwotetrahedrajointogetheratacommonzincatom(C).Thisisthebasicclosepackedstructureofzincoxide.Zincatomsarequitesmallerinsizecomparedtotheoxygenatoms.Thestructureisthusestimatedtobeoccupiedbyzincandoxygenatomstoonly44%ofthewholevolume,andtheremainingislledwithopenspaces[1].Thustheydonotformacompleteclosepackedstructure. Figure2.1.ConstructionoftheWurtziteStructureofZincOxide[1]9

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2.2.2ThermalPropertiesZincOxideisaverygoodthermalconductorwithahighheatcapacity.ThispropertyofZnOhasleadtodiscoveritsrelevancetothetruckindustry.Itisamajorcomponentofthetiresalongwithrubberwhichledtomanyinventionsintherubberindustry.TheothermajoreldthatisaffectedbythehighlythermallyconductiveZnOistheceramicindustry.Ithasaco-efcientofexpansionof3.16x/Candameltingpointof2000C.Thesevaluesareclosetothevaluesofsilicawhichisamajorcomponentofthiseld[1].ThemajordebatablepropertyofZnOisthatitsvaporpressurereaches760mm(normalatmosphericpressure)ataround1700Cwhichisalmost300Cbelowitsmeltingpoint.AccordingtoLeverenz[25],thispropertyisrelatedtothedifcultyinbreakingtheZn-ObondsalongtheC-axiswhicharesaidtopossessdirectionalstrengthunlikethenon-directionalstrengthofZn-Obondsintheotherdirections.2.2.3ElectricalPropertiesAlthoughstoichiometricZnOisaninsulator,itexistsasann-typeconductorduetothedeviationsinitsstoichiometry.TheimportantreasonthatcanbeattributedtothiscrystaldefectisthedifferenceinsizeofoxygenandzincresultinginalotoffreespaceinthestructureofZnOasdiscussedearlierinthischapter.Thefreechargecarriersthatresultfromtheshallowdonorlevelsassociatedwithoxygenvacanciesandinterstitialzincistheotherimportantfactor.TheelectricalpropertiesofZnOlmsarestronglyinuencedbytheirdepositionmethod,thermaltreatmentandoxygenchemisorption.Abriefdiscussionoftheseparametersisprovidedbelow.2.2.3.1EffectofSubstrateTemperatureUndopedZnOexhibitsavariationinitselectricalpropertiesdependingontherangeofsubstratetemperaturesatwhichitisdeposited.WhenZnOisdepositedbyreactivesputtering,thecarrierconcentrationincreasescausinganincreaseintheconductivityof10

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thelmuntilacriticaltemperature.Athighertemperatures,thecarrierconcentrationisfoundtobedecreasing.Thereasonforthiscouldbeduetotheincreaseintheoxygenincorporationattheoxygenvacancysitesofthelmyieldingabetterstoichiometriclm.Thiscriticaltemperatureisreportedtobe350Cinref[26].Themobilityisalsoreportedtoincreasewiththeincreaseinsubstratetemperaturebelow350C.Thereasonsforthisarea)bettercrystallizationofthelmwiththeincreaseintemperatureandb)decreaseinthegrainboundarybarrierpotentialofZnOlms[27].2.2.3.2EffectofSputteringParametersThedepositionenvironmenthasamajoreffectonthepropertiesofZnOlms.Zincox-idelmswhenreactivelysputteredusingaZntargetinoxygenatmosphere,atlowoxygenconcentrationsproducemetalliczincandzincoxidelmswhichareopaqueandconduc-tive.Athighoxygenpartialpressure,theextraoxygeneliminatestheoxygenvacanciesresultinginahighlystoichiometriclmthatistransparentandnon-conductive.ThusthepartialpressureatwhichaZnOlmisdepositedhastobeoptimisedtoliebetweenthesetwoextremes.Thelmthusobtainedisnon-stoichiometricduetotheinterstitialzincatomsand/ordecientoxygensitesandisassumedtopossessverygoodelectricalconduc-tivitywithahightransmissionresponse.ManyresearchershaveworkedonthisparameterofZnOlmsandhavereportedavalueof//cm[26].Workontheeffectofhydro-genpressurewasperformedbyWebbetal[28].Itisreportedinthisworkthatatpressureslowerthan1xtorr,resistivityoftheZnOlmdecreaseswiththeincreaseinhygrogenpartialpressure.Atthesepressures,hydrogenremovesoxygenformingoxygenvacanciesandzincinterstitialsinthelm.Thecarrierconcentrationthusincreasesdecreasingtheresistivityofthelm.Athighhydrogenpartialpressures,increaseincompensationfromanincreaseinthedensityofacceptorlevelsdecreasesthecarrierconcentrationandformslmswithcomparativelyhigherresistivity.11

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Avastresearchwasperformedtoestimatetheeffectofbiasvoltage,depositionpres-sureanddepositiontemperatureonthepropertiesofZincOxidelmsbyvariousre-searchers.Itisfoundthat,regardlessofthedepositionconditionsandthemethodofdeposition,theundopedzincoxidelmsareunstableinthelongterm.Thisunstablena-tureoftheZnOlmsisattributedtothechangeinsurfaceconductanceofZnOlmsunderoxygenchemisorptionanddesorption[29].2.2.4OpticalPropertiesZincOxideistransparenttolightinthevisibleregionwithasharpcut-offintheUVregion.Thisregioncorrespondstothewavelengthregionfrom0.3-2.5m[26].Thisindi-catesthatitistransparenttovisiblelightandabsorbsultra-violetrays.Thetypicalopticaltransmittancedepositedunderoptimumconditionsis90%.Thispropertyandarefractiveindexof2.0resultsintheuseofZnOasthewhitepigmentinthepaintindustry[1].ZincOxidecanbedopedwithelementsfromothergroupsofperiodictabletoimproveitselec-tricalandopticalproperties.FromtheworkofSarkaret.al[30],itisobservedthatthebandgapvarieslinearlywiththecarrierconcentration(N)duetothedoping.ThisvalueisobservedtobedirectlyproportionaltoNindicatingthattheBurstein-MossmodelcanbeusedtoexplainthiseffectinZnOlms[31].AplotshowingthevariationbetweencarrierconcentrationandthebandgapofthelmsisdepictedinFigure2.2.Furtherstud-ieshaveprovedthatthebandgapvariationduetodopingathighdopingconcentrationsN(thecarrierconcentrationatwhichsemiconductor-metaltransitionissaidtooccur)isproportionaltoN[32],[33].2.3WhatMakesZincOxideMoreInteresting?ZincOxidehasbeenusedinindustryforthepast3to4decades.Researchonthismaterialhasrevealedthatitspropertieslikecrystalstructure,bandgapandsomeofitsphysicalandelectricalpropertiesarefavourabletotheextentofreplacingsomeofthema-12

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Figure2.2.PlotofBandgapVersusCarrierConcentrationterialsthatarebeingusedaslasersandaswindowlayersinsolarcells.Abriefcomparisonofzincoxidewithothermaterialswithsimilarapplicationsisdescribedinthefollowingsection.2.3.1ZincOxideasaBetterTCOZincOxide,besidesbeingoneoftheimportantmaterialsintheclassicationofwidebandgapmaterials,isoneoftheimportantTransparentConductiveOxides(TCO).BeingabundantinnatureitisavailableatlowercostcomparedtoTinandIndium,theincreas-inglyusedmaterialsasTCOs.Ithastheaddedadvantageofbeingnon-toxicanditcanbedepositedatrelativelylowtemperatures.ZincOxidehasverygoodopticalandelectricalproperties.AccordingtoLeeetal.[34],thesevaluesforZnOcouldbeeitheramatchorexceedthevaluesofTinorIndiumTinOxide.OneofthemajorapplicationsofTCOsisasthefrontelectrodeofamorphoussiliconsolarcellswhereinthesubstrateissubjectedtohydrogenduringtheinitialstagesofthefabrication.Inthecaseofpolycrystallinesilicon,hydrogenisusedtopassivatethegrainboundaries[35].Duringthisprocesstinoxidere-ducesleadingtoadditionalabsorptionoflight.Thisisoneofthemajordrawbacksoftinoxidethoughitisoneofthebestmaterialstobeusedinsiliconsolarcellsintermsoftheir13

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opticalandelectricalproperties.Butstudieshaveshownthatzincoxideisverystableinhydrogenatmosphere[36],[37].Theothermajordisadvantageoftinoxideisthatithaslesslight-trappingabilityleadingtoalimitationincellperformance.Mulleretal.[38]intheirworkhavefoundoutthatapost-depositionchemicaletchingsteptexturesZnO:Allmsresultinginloweringtheresistanceoftheselms.Itisalsofoundthattheselmswhenincorporatedinamorphous-Sisolarcellsproducedaninitialefciencythatisalreadyhigherthancommerciallyavailableusingtinoxidesubstrates.Thisistheimportantreasonwhichpreventstinoxidefrombeingusedforlargeareamodulemanufacturing[38],[39].Withtheseadvantages,theusageofZnOandAl-dopedZnOisincreasingintheindustrydaybyday.2.3.2ZincOxideinOptoelectronicsGalliumNitride(GaN)isusedasthemajorsourceforbluelaserssincethemid1990s.Itisalsousedextensivelyinfabricatingdeviceslikepiezoelectricandwaveguidedevices,lightemittingdiodesandphotodetectors.ThestructureandthebandgapofZincOxideprovepromisingfortheseapplications.RecentstudieshaveprovedthatZnOcouldbeusedasthesubstrateforGaNlmsasboththematerialssharethesamestructure[14].Theexcitonbindingenergy,theimportantpropertyforanyopticaldevicelikeLEDsandlasersis60meVinZnOwhichis2.4timesthatofGaN.ThisisanotherreasonwhyZnOisaprospectivecandidateforthesedevices[40].ApartfromtheseopticalpropertiesZnOhassomeinterestingphysicalpropertiesthathavemadeitmoreattractiveinthiseld.WhencomparedtoGaN,itisharderandtheZn-ObondislargerthantheGa-Nbondwhichcouldmakethemajorissuep-typedoping,lesscomplicated[14].Withameltingpointof2000Citissufcientlystableathightemperatures.Thisisanimportantrequirementduringdopingandformationofohmiccontacts.Thehigherhardness,resistancetomechanicalstressandhighmeltingpointtemperatureofamaterialalsoexpandthelifetimeofLEDsandbluelaserdiodes[14].14

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Therefore,ZnOasapotentialwidebandgapmaterialcanbeusedinshort-wavelengthlightemittingdevicessuchaslightemittingdiodes,photodetectors,electroluminescencedevicesandthenextgenerationUVsemiconductorlasers[40].Thefactorthatp-typedopingofZnOhasaddedadvantagetoitinitiatedtheinterestforthisresearch.Abriefdescriptionofdoping,issuesinp-typedopingandtheworkbymanyresearchersonthistopicisfurtherdiscussedinthischapter.2.4DopingTechniqueinZincOxide2.4.1DopingZincOxideN-typeZincOxideisan-typesemiconductorbynatureduetoitszincinterstitialsandoxygenvacancies.Itisexternallydopedtoproducelmsofhigherconductivityandhighcarrierconcentrations.ZincOxidecouldbereadilydopedn-typeduetoitshighelectronafnity.ManyresearchershaveworkedondopingZnOn-typeusinggroupIIIelementslikealu-minium,gallium,indium,etc.[41].ThisprocessisfoundtoproduceZnOlmswithmoren-typeconductivityimprovingitstransparencyandconductivity.AstudybyIgasakietal[42]hasprovedthatdopinginZnOimprovesnotonlytheelectricalpropertiesofthelmbutalsoitsthermalstability.Inthisstudy,ithasbeenprovedthatZnOlmsdopedwith3at%Inarefoundtoexhibitthermalstabilityupto650Kinvacuumandupto450Kinoxygenambients.Amongallthen-typedopantsbeingused,Aluminumisfoundtobethebestdopantasitproduceslmswiththehighestconductivityandtransparencycomparedtoanyotherdopant[41].Aluminiumdopedzincoxide(AZO)isdepositedusingvarioustechniquessuchaspulsed-laserdeposition,rfmagnetronsputtering,chemicalvapordepo-sition,spraypyrolysisandthesol-gelprocess.Pulsedlaserdepositionandrfmagnetronsputteringproducelmswithgoodelectricalandopticalpropertiesatacomparativelylowerdepositiontemperature.Thesputteringtechniquehastheaddedadvantageofpro-ducingauniformwideareadeposition.Butthesetechniquesaredisadvantageouswhenit15

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comestothedepositionrateastheyhaveverylowdepositionrates.Theinitialcostoftheequipmentsusedinthesetechniquesisalsohigh[43].ThepropertiesofAZOlmsvariesagreatdealwithrespecttothedepositioncondi-tions.OneoftheimportantinferencesthatisdrawnbyvariousresearcherswhodepositedAZOusingvarioustechniquesisthatthoughAZOproducesaverygoodsetofopticalandelectricalproperties,thereisalwaysatrade-offbetweenthetwo.TheresistivityoftheAZOlmsincreaseswiththeincreaseindopingduetotheincreaseinthenumberofchargecarriers.Althoughathightemperatures,theresistivitygetssaturatedasaluminiumoxideprecipitatesaroundthegrainboundariesduetotheexcessivealuminiumatoms.Theopticalpropertiesareaffectedinatotaloppositemannerwiththeincreaseindoping.Theincreaseinthenumberofchargecarriersincreasesthefreecarrierabsorptionathigherwavelengths.Thisphenomenoniswellexplainedinref[2].Fromgure2.3,itisevidentthattheconductivityofthelmincreaseswiththeincreaseinthesputteringvoltageafter500V.TheopticalpropertiesinFigure2.4explainstousthatat420V,thereisnofreecarrierabsorptionasthereisnoincorporationofAlinthelms.Butasthesputteringvolt-ageincreases,interferenceeffectgetsloweredwhichconrmsthepresenceofaluminiuminthelm.ThisisaccompaniedbytheincreaseinthefreecarrierabsorptionintheIRregion. Figure2.3.ConductivityofZnO:AlFilmsatVariousSputteringVoltagesOneofthetechniquesusedtoachievehigherconductivitywithlesscompensationofelectricalconductivityinAZOisreportedbyC.Agasheetal.[44].Inthiswork,aset16

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Figure2.4.VariationoftheAbsorptionCoefcientwiththeSputteringVoltagefortheZnO:AlFilms[2]ofAZOlmsdepositedusingtargetswithdifferentAlO/ZnOorAl/ZnweightratiosuseRF,DCandDCreactivesputtering.Itisreportedintheirworkthatalowresistivityof3.7x cmwithoutcompensatingmuchontheopticalpropertiesofthelmisobtainedbyloweringthetargetdopingconcentration.Depositiontemperatureisoneothermajorissuewhichhasamajoreffectonthecrys-tallinityofthelms.Thisresultsinvariationsinthepropertiesofthedepositedlm.Changetal.[45]havedoneaextensiveresearchinthisaspectofthedepositedlm.Itisinferredfromtheirworkthatthelmsarecrystallinethroughoutthedepositiontem-peraturerange,i.e.from200Cto350C.TheroughnessofthelmfromthesurfacemorphologyanalysisusingAFMhasrevealedthatthelmsthataredepositedinthetem-peraturerangeof200Cto250Creducewiththeincreaseintheadatommobilityofthedepositingmaterialonthesubstrate.Theresistivityisfoundtobeminimumuntil250Cduetotheincreaseinthecarrierconcentrationwiththeincreaseintemperature.Butathighertemperatures,theresistivityisfoundtobeincreasingasseeninFigure2.5.TheXPSmeasurementsontheselmshaverevealedthepresenceofoxygenimpuritiesasthemajorcontributorforthiseffectathightemperatures.Thisimpliesthatthereisanincrease17

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inthechemisorbedoxygenwhichformselectrontrapsthusreducingtheconductivityofthelms.Opticalpropertiesshowthatthecarrierconcentrationismaximuminthelmsdepositedat250Castheyhavethemaximumopticalbandgapof3.5eV. Figure2.5.VariationinResistivityoftheFilmwiththeDepositionTemperatureThemechanicalpropertiesofAZOlmsdetermineitsdurability.Thisinturngovernsthemechanicalstabilityandelectricalpropertiesofthelm.ThemechanicalpropertiesofZnO:AllmsisanalyzedbyPuchertetal.[46],andtheyfoundthattheresidualstressintheselmscouldreachvaluesashighas"!Pa.J.F.Changetal.[41]haveinvestigatedtheeffectofsputteringparametersonthestructuralcharacteristicsandresidualstressofZnO:Allms.Theyfoundthatataloweroxygenfraction,theRFsputteredZnO:Allmshavealowergrowthrateduetotheresputteringeffect.ThisresultsinalmwithhighcompressivestressandpoorcrystallinityduetotheformationofanAlOphase.Thiseffectisindependentofthesputteringpower.Withtheincreaseintheoxygenfraction,thereisareductionintheresputteringeffectandthedepositedlmhasahighergrowthratewithlowinternalstress.Almwiththehighestgrowthrateandlowestcompressivestressissaidtobeformedat12%oxygenfraction.Figure2.6depictstheconclusionsdrawninthiswork.18

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Figure2.6.VariationinResidualStressintheFilmwithVariationintheOxygenFraction2.4.2IssuesinDopingZincOxideP-typeTheincreaseininterestinZnOlmsisduetotheirincrediblyhighelectricalconduc-tivityandopticaltransmission.Theexcitonenergy(theimportantpropertyrequiredforopto-electronicdevicessuchaslaserdiodes)forZnOishighwithavalueof60meVwhencomparedtothe25meVofGaNand20meVofZnSe[13].ManyoftheapplicationsthatcouldutilizethispropertyofZnOarep-njunctionsemiconductordevices.ThusifZnOcouldbemadep-type,astrainfreeinterfacecouldbeproducedinthehomojunction.Thisimprovestheperformanceofthedevices.Dopinghasalwaysbeenanissueinwidebandgapmaterialsbecauseasthebandgapofamaterialincreasesitbecomesdifculttodopetheminasymmetric(p-typeandn-type)fashion.Forexample,dopingdiamondp-typeisveryeasycomparedton-typewhichisverydifculttoobtain.Similarly,therearequiteafewissuesindopingGaNp-typewhileitiseasytomaken-typeGaN.Thevariouspracticaldopingprinciplesarediscussedvig-orouslybyAlexZunger[47]andhehasencodedabasicthree-termformulathatdescribestheformationenthalpyofdopantDofchargestateqinhostcrystal“H”.Itisgivenas,19

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#%$'&)(+*,-(,E.)=qE.+n(((-0/)+#E1,where(and2/arethechemicalpotentialsofthedopantsandhost,E.istheelectro(chemical)potential(Fermienergy),n(isthenumberofdopants,#E1=E(host+defect)-E(host)istheexcessenergyofthelocalchemicalbondsaroundthedopantandEisthetotalenergy.ThesetermsintermsofZnOcouldbeexplainedas,2.4.2.1AvoidingFermi-Level-InducedCompensationEffectsbySpontaneousGen-erationofNativeKillerDefectsEveryp-typematerialhasasaturationenergylevelcalledthePinningEnergyrepre-sentedasE3&45-.Dopingbeyondthisenergylevel,itisbelieved,resultsinformationofnativeholekillerssuchasananionvacancy(oxygenvacancyinZnO)687:9orcationin-terstitials(Zninterstitials);87=-levelforZnOisconsiderablyabovetheValenceBandMaximum(VBM)whichindicatesthatthedownwardmovementofthefermilevelduetodopingtoobtainp-typeconductivitywillreachthepinningenergybe-20

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forereachingtheVBMthusresultinginzincinterstitialsandoxygenvacancies.ThisproblemcanbecontrolledbydopingZincOxideunderNitricOxide(NO)orNitrogendiOxide(NO)gasthuscreatinginternaloxygenprecipitatesthateliminateoxygenvacancies[48],[49].2.4.2.2EnhancingDopantSolubilityThesolubilityofthedopantisamajorfactorthathelpsinenhancingthedopingef-ciency.Theunstable,highlyreactiveNitrousOxide(NO)orNitrogendiOxide(NO)gaswhenimpingingonthegrowingsurfaceimprovesthesolubilityofthedopantsmorethanastablegassuchasNitrogen(N)[48],[49].TheAnion-substitutingdopantsareexpectedtobemoresolubleunderhostanionpoorandcationrichconditions.Thus,ZnO:NlmsarebestgrownunderZn-richconditions[50].2.4.2.3DopingRulesPertainingtoLocalDefectBondingEffectsAlthoughtheaboveconditionsareproducedbysomemeans,thelocalbondingaroundthelocalchemicalbondsshouldbestable.InthecaseofZnO,thiseffectcouldbewhatiscalled“clusterdoping”.Forexample,theweakZn-NbondscouldbereplacedbydopingZnOsuchthatoneoftheZnObondsisreplacedbyastableZn-AlbondandthefouroxygenatomsbyfournitrogenatomsthuscreatingfourstrongAl-Nbonds.SinceAl-Nbondsarestable,theirdopingofratio4:1betweenacceptorsanddonorscouldfacilitatestablelocaldopantbondingandenhancedsolubility.ThisworkiscurrentlyperformedbyresearchersL.WangandA.Zungerandisexpectedtoproducemorestablebondsthantheco-dopingtechnique.21

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2.4.3LiteratureReviewAccordingtotheliterature,achievingp-typedopinginZnOisverydifcultforvariousreasonsasdescribedintheprevioussection.Athoroughtheoreticalresearchonthistopicusingrstprinciplesprovedthatnitrogencouldbeusedtoproduceshallowp-typedopantsinZnO[51].Nitrogenwithvevalenceelectronsreplacesoxygenwith6valenceelec-tronsintheZn:ObondandformsZn:NbondswithadeciencyofoneelectronperZn:Obondmakingitp-type.ManyresearchershavebeentryingtoachievethisusingvariousdepositiontechniquessuchasMolecularBeamEpitaxy,MetalOrganicChemicalVaporDeposition,PulsedLaserDeposition,DC,RFandReactiveSputteringtechnique,etc.un-derdifferentdepositionconditions.Theworkofsomeoftheseresearchersisdiscussedinthissection.Satoetal.[52]wereoneoftheinitialgroupstoperformdopingofZnOusingnitro-gen.Thelmsweredepositedonan?-AlOsubstrateinareactiveevaporationsystemintheambienceofoxygenandnitrogenmixtures.ThegaseswereexcitedusingaRF(13.56MHz)dischargeof80W.Zincmetalwasheatedusingatungstenheaterandwasevapo-ratedataconstantrateusingaquartzcrucible.Thedepositedlmswereannealedat450Cinairornitrogenfor1hour.TheN/N+Oratiowasvariedfrom0%to100%.XRD,electricalpropertiesandphotoluminescencespectraforthelmsdepositedunderdifferentdepositionconditionsweremeasured.Filmswitharesistivityof0.1to10-cmwereob-tainedwiththeincreaseinthenitrogenmixingratioandthesepropertieswerefoundtobechangingwiththeannealingprocessinthepresenceofair.Futsuharaetal.[53]depositedZnOlmsusinganrfmagnetronsputteringtechniqueonborosilicateglasssubstrates.Allthelmsweredepositedatatemperatureof423KinthepresenceofAr-Nmixture.Theambiencewasvariedfrom0%to75%.Opticalpropertiespredictedthepresenceofnitrogenincorporationastheopticalbandgapwasfoundtobedecreasingform3.26to2.30eVwiththeincreaseinthenitrogenconcentrationfrom0%to75%;thoughthelmswerereportedasn-type.22

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Josephetal.[54]depositedZnOlmsoncorning#7059glassandsaphhiresubstratesusingpulsedlaserdeposition.AZnOdiscdopedwithdifferentconcentrationsofgalliumwasusedasthetarget.AnArF(Argon-Fluorine)excimerlaseroperatedat1Hzatauenceafabout0.5J/@5Awasusedforthispurpose.AnecessitytoaccountfortheoxygenvacanciesinZnOwasconsideredinthisresearch.AsreportedbySatoetal.inthepreviouspartofthissection,usingoxygenandnitrogenmixtureswasnotverysuccessful.ThefactthatNOgaspossessessanionizationpotentialbetweenoxygenandnitrogenwasusedinthiswork.Hence,NOgaswithandwithoutanECRorRFplasmasourcewasused.P-typeZnOlmswitharoomtemperatureresistivityof0.5cmandacarrierconcentrationof5xB!@CADwasobtained.Itwasobservedthatp-typeconductivityimprovedbyusingGaasthedonordopantandECRorRFasthesourcetoexcitenitrogen.Z.-Z.Yeetal.[55]in2003hadcomeupwithatotallynewphenomenon.Inthiswork,azincmetaltargetwasusedtoprovideaZn-richenvironmentandthelmsweredepositedinaNH/NH-Oenvironment.SincetheelectronegativityofOismore,ZnreadilycombineswithOratherthanwithNitrogen.ThereactionsthatoccurintheNH-Oenvironmentare,Zn(v)+1/2O(v)EZnO(s)F(1)Zn(v)+1/2O(v)+NH(v)EZnNH(S)+HO(v)F(2)(v)and(s)representvaporandsolidphaserespectively.Asrepresentedinequation(2)nitrogeniselectricallyinactiveduetothehydrogenpassivation.TheseN-Hbondsaredissociatedinthesubsequentgrowthprocessastheoxygenintheambientreactswiththehydrogenandformssurfacehydroxyl-groupsofwater.Thelmsweredepositedon?-AlO(0001)substratesandthecharacteristicsofthelmsdepositedinatemperaturerangeof400-500CwithNH-Oenvironmentvaryingfrom0%to67%wereobserved.Itwasobservedthatlmsdepositedatasubstratetemperatureof500Candanammoniaconcentrationof50%showedthebestelectricalresistivityof35-cmwithacarrierdensityof3.2xG@5AD.23

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Ohshimaetal.[56]attemptedtodepositp-typeZnOlmsbyco-dopingZnOandGaon?-AlOsubstrates.ThedepositiontechniqueemployedtodothisworkwasPulsedLaserDeposition.ThetargetwasablatedinNOgas.ItwassuggestedthatNOgashasalowerdissociationenergyof6.1eVcomparedtothatofNof9.1eV.ThisphenomenonwassupportedbythetheoreticalanalysisofYanetal[49].ThePLDdepositedlmsweren-typeconductivewitharesistivityof-HcmwithitscarrierdensityvaryingfromH@5ADtoBIJH@CAD.Itwasproposedinthisworkthatfurtherresearchinthistechniquewouldhelpinthefabricationofhigh-qualityp-typeZnO.24

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CHAPTER3PROCESSDESCRIPTION3.1DepositionChamberVacuumchambersusedtodepositthinlmsaremostlymadeupofstainlesssteelorpyrexglassasthesematerialsarenoncorrosible,non-magnetic,easytoweldandclean,highlymalleableandhavegoodoutgassingcharacteristics.Forthesamereason,thevac-uumchamberusedtodeposittheZincoxidelmsdiscussedinthisthesisismadeupofstainlesssteelpartsenclosedwithinapyrexbelljar.Arubbergasketisusedtoholdthevacuumbetweenthebelljarandthecollarplacedoverthebaseplate.Thebaseplateandthecollarcontainfeedthroughsforpower,waterlines(forthesputtergunandcrys-talmonitor),thermocouple,gaslinesandotherpurposes.RubberO-ringsactasvacuumsealantbetweenthefeedthroughsandthechamber.Thechambercontainsarotatablesub-strateholdersupportedbyanexternalmotorconnectedtothechamberthroughoneofthefeedthroughs.ThetypicalbasepressurethatcouldbemaintainedusingthissetupisinthelowKrange.TheVacuumchamberusedtodepositthelmsforourexperimentsisconstructedbyConsolidatedVacuumCorporation,NewYork.Figure3.1depictsthevacuumchamberusedtodeposittheZnOlms.VacuuminsidethechamberiscreatedusingacombinationofSeargent-WelchrotaryvanemechanicalpumpandCVC'sPMCdiffusionpump.Adescriptionofthepumpingsystemandothercomponentsinvolvedwiththisdepositionsystemisdiscussedinthefollowingsections.25

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Figure3.1.DepositionChamber26

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3.2ProcessTechniqueForsputteringfromaZnOtargetRFsputteringisused.Zinctargetisusedonasecondguntoproduceazincrichenviornmentduringdeposition.3.2.1SputteringPrinciplesSputteringisatypeofPhysicalVaporDepositionasitusestheprincipleofmomentumtransfertoremoveneutralatomsfromatarget.AsimpliedcrosssectionofasputteringsystemisdepictedinthefollowingFigure3.2. Figure3.2.SimpliedCrossSectionofaSputteringSystem[3]Thematerialtobedeposited(target)actsasthecathodeandisconnectedtoanegativevoltagesupplywhichcouldbeeitherDCorRF.Thesubstrateisplacedonasubstrateholderandcouldbegrounded,oating,biased,heated,cooled,orcouldbeacombinationofthese.Thesubstratesareplacedexactlyabovethetarget.Thissetupisplacedinsideavacuumsystemwhichispumpeddownandmaintainedathighvacuum.Aninertgaslike27

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Argonisintroducedintothesystemasthemediumforglowdischarge.ThisisbecauseaninertgaslikeArgonhasitsmetastableenergygreaterthanitsrstionizationpotentialwhichhelpsinproducingasufcientsupplyofionsforselfsputtering.Whenthisglowdischargeisinitiated,ionswithhighkineticenergystrikethecathodeandthesubsequentcollisionsknockloosetheneutralatomsfromthematerialbymomentumtransfer.Theseneutralatomsthencondenseonthesubstratetoformthinlms.Theprincipleofmomen-tumtransferusedtodepositmaterialshasmadethesputteringtechniqueveryattractive.Usingthistechniqueitiseasytodepositmaterialswhichcouldnotbeeasilydepositedusingothertechniques.Thisincludesevenrefractorymaterials.3.2.2MagnetronSputteringAlthoughtheglowdischargesputteringtechniqueseemstobeapowerfuldepositiontechnique,themajordisadvantageofthistechniqueisitslowionizationefciency.Furtherdevelopmentstoimprovetheionizationefciencyoftheglowdischargeisobtainedbyincludingothertechniqueslikeaxialandtransversemagneticelds,thermionicadditionsofelectrons,andrfcoils.InthecaseofMagnetronsputtering,theionizationefciencyisimprovedbyusingamagneticeldparalleltothecathodesurfaceandthusrestrainingtheprimaryelectronmotiontothevicinityofthecathode.Theseelectronsthustrappedmoveinsidetheorbitgainahighermeanfreepathandcollisionallyscatterbeforereachingtheanode.Conse-quentlymagnetronsputteringrequireslowergaspressurestosustaintheplasmacomparedtothatofthediodesputteringtechnique.Reducedscatteringandincreasedelectronusageefciencyleadstoabetterdepositionrateandreducedappliedvoltagetosustainaplasmainthistechnique.Alongwiththeseadvantages,thistechniquehasthedisadvantageofthedichargebeingswepttoonesidebytheExBforce.Thisisavoidedbyusingcylindricalcathodesthatallowthesedriftcurrentstocloseonthemselves.Onthebasisofthetypeofmagnetsused,magnetronsputteringisclassiedintodifferentcategorieslikecylindrical28

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magnetron,hollowcathodes,cylindrical-post,cylindrical-hollowandplanarmagnetrons.[3]3.2.3PlanarMagnetronSputteringThePlanarMagnetronSputteringtechniqueusesbothDCandRFsourcestodepositmaterialsdependingonthephysicalpropertiesofthematerial.3.2.3.1SystemCongurationAsetofPermanentmagnets,electromagnetsoracombinationofbothareplacedbe-neaththecathode(targetmaterial)insuchawaythatthereisatleastoneclosedpathofthemagneticeldlinesinfrontofthecathode.Ananodeinbetwenthecathodeandthedark-spaceshieldisgenerallypreferredtopreventelectronbombardmentonthesubstrate.Aplanarmagnetronsetupcouldbeeithercircularorrectangularinshape.TheTorus-5Mgunusedinthisworkhasacircularplanarmagnetroncongurationwithapermanentmagnetinthecenterbehindthetarget.Waterrunsthroughthecathodeassemblytopreventthetargetfromgettingheatedduetothehighsputteringpowersused.Inthisthesiswork,theRFsputteringtechniqueisusedtodepositzincoxide.ZincoxidebeingahighlyinsulatingceramictargetrequireshigherpowerlevelstoinitiateandmaintainaglowdischargeandisthuspoweredusingaRFsource.ZincisaconductivemetaltargetnotrequiringhighpowerlevelsforoperationandthusaDCpowersourceisusedforsputteringZinc.3.2.3.2DisadvantageoftheTechniqueInthecaseofaplanarmagnetronsputteringsystem,thethicknessofthedepositedmaterialisnon-uniformonthesubstrate.Thecathoderegionexactlyabovethemag-nethasahighplasmadensitywhichhasadirecteffectontheamountofmaterialgettingsputteredfromthetargetthusresultinginanon-uniformlm.Researchershavefoundthatthethicknessdistributionfromtheplanardiskmagnetronagrees29

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withthatcalculatedforaringsourcehavingacosineemissioncharacteristic.Ac-cordingtothat,thebestlmuniformityisobtainedwhenthesourceinnerradius“r”isabout0.7timesthesource-to-substratedistance“h”,andtheouterringradiusisabout0.8h[57].ThoughthePlanarmagnetronsputteringtechniquehasanappreciabledepositionrate,itislimitedbythemaximumpoweruxthatcanbeappliedtothetarget(cath-ode)withoutcracking,sublimatingormeltingit.3.2.3.3AdvantagesoftheTechniquePlanarMagnetronSputteringtechniqueisgoodforwideareadepositions.ThedepositionparametersthatgovernthepropertiesofthelmsuchasDC,RFpower,temperatureandpressurecanbepreciselycontrolled.3.2.3.4ComparisonofDCandRFPMSputteringRFPMSputteringhastheabilitytosputterinsulatorsanddielectriclmsthatcannotbesputteredusingaDCPMSputteringtechnique.However,RFsputteringisacomplexsystemthatrequiresanimpedancematchingnetworkasanadditionalcomponent.Forthisreason,theDCPMSputteringtechniqueisusedtodepositconductinglms.3.2.3.5ApplicationsIndustries-Itisusedtoreplaceelectron-beamdepositionforplasticlmmetal-lizationandforantireectionandthermalbarriercoatingofarchitecturalglass.Itisalsousedasadecorativeandfunctionalcoatingonplasticandanticorrosionorabrasion-resistantcoatingonmetal.30

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ThinFilmElectronics-Itisusedtodepositaluminiummetalforintegratedcircuitmetallization,Cr-Cu,Cr-AuCeramicmetallization,Ta-Al,TaNresistorlms,Cralloyplasticmetallization,insulatorsanddielectricsforvariousapplications.3.2.4TargetandSputteringGunAssemblyThesourceforthematerialtobedeposited,thesputteringtarget,isa1/4inchzincoxideceramictargetwhiletheZinctargetisametaltargetof1/8inchthickness.Torus-5Msputteringgunsareusedtosputterboththetargets.Thesegunsaredesignedtoholdboth1/8and1/4inchtargets.Toensureasecureseatingofthetargetsonthewatercooledblock,aretainerringisscrewedusingasetofstainlessscrewstothewaterblockwiththetargetinbetweenthem.Thedarkshieldisplacedoverthissetup.Oncethespacingbetweenthedarkshieldandtheretainerringischeckedforitsuniformitythroughout,thegunisreadyforoperation.3.3SubstrateProcessing7059Borosilicateglassisusedasthesubstratefordepositingthethinlms.Theglasspieceissubjectedtoasequentialcleaningprocessbeforebeingloadedfordeposition.Theglasspiecesarerinsedinowingde-ionizedwatertoremovedustparticles.Theyarethencleanedwitha5secondsdiluteHF(1:10)dip,followedbya3secondsdip.EverytimetheglasspieceisdippedinHFsolution,itisushedunderowingwatertoremovetheHFsolutionontheglasspieceandblowndryusingcompressednitrogen.3.4SampleLoadingThesubstratesthuspreparedareloadedontoagraphitesubstrateholderandareheldbyfourstainlesssteelscrews.Thissetupisthenmountedontherotarysampleholderassembly.Thisrotaryassemblyaidsinplacingthesubstrateexactlyoverthezincoxidetargetandundertheheatinglampassembly.Atype-K,hightemperaturequickdiscon-31

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nectthermocoupleisinsertedintothegraphiteholdertomonitorthetemperatureofthesubstratesduringheatinganddeposition.3.5PumpingSystemThevacuuminsideavacuumchamberiscreatedandmaintainedusingapumpingassembly.Therearevariousfactorsthataretobeconsideredwhenchoosingapumpingsystem.Thepumpingrate,ultimatepressurethatcanbeachieved,abilitytohandlegasloadsandpotentialforoilcontaminationofthepumparesomeofthem.Thepumpingsystemusedforthedepositionchamberusedinthisworkisacombinationofarotaryvanepump(Sargent-Welch)forroughingthesystemtoapressureoftorrandaPMCCVCDiffusionpumpformaintainingahighvacuuminthesystem.Thispumphasthecapabilitytopumpthesystemdowntoapressureinthemicrotorrrange.Acoldtrapforliquidnitrogenimprovestheefciencyofpumping.Ittrapswatervaporandcondensesit.Hotoilinsidethediffusionpumpcollectsthesecondensedwatervaporparticlesandpumpsitouteventuallyusingtherotaryvanepump.Theliquidnitrogencontentinthecoldtrapismaintainedfullthroughouttheprocess.3.6ParametersInvolvedDuringDeposition3.6.1TemperatureThesubstratesareheatedusingtwo300W,120VOSRAMhalogenphotoopticlamps.Thetemperatureofthesamplesiscontrolledandmaintainedusinga3PN1010BNewarkInoneVariabletransformerandmonitoredusingak-series,hightemperaturequickdis-connectthermocouple.Thisthermocouplecanreadtemperaturesupto700Candisgroundedusingastainlesssheath.Thetemperatureisreadusingamultimeterintermsofvoltageandisconvertedtoitstemperatureequivalentusingavoltagetotemperature32

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chart.Acorrectionvalueof25C(roomtemperature)isaddedtoittoobtaintheactualtemperatureofthesubstrateholderandthesamples.3.6.2PressureDepositionpressureisoneoftheimportantparametersthatcouldaffectthepropertiesofthelm.Thepressureinsidethevacuumchamberisreadusinga122AA-00002AB(0-10VDCO/P)MKSBaratron.WhilethechamberisroughedusingthemechanicalpumpaKurtJ.Lesker6000seriesTCgaugeinserieswithaTC-10KJLreadoutisusedtoreadthepressureinsidethechamber.PortsforTCgaugesareprovidedatdifferentpartsofthechamberenablingacheckofpressureatdifferentsectionsofthechamber.Thiswayitiseasytolocalizetheleakinsideavacuumchamber.ThelowpressuresinsidethechamberaredetectedbytheG100Tkionizationgaugeandarereadusinga260seriesGrainville-Phillipsiongaugecontroller.ThiscontrollercanreadavacuumbetweenBandB!Torr.Goodcontrolofthepressureinsidethechamberisthusachievedusingthesepressurereadouts.3.6.3GasFlowThesputteringgas,argonandreactivegasesoxygenandnitrogenusedfordopingthatarepassedintothechambershouldbecontrolled.Thiswouldotherwiseaffectthepropertiesofthelm.Forexample,excessargongasincreasestheatomiccollisionsintheglowdischargeandreducesthedepositionrate.Goodcontrolofthesegasowsisachievedusingacombinationofneedlevalvesandmassowcontrollers.TheMFCcontrollershaveahighprecisionincontrollingthegasoweveninthemTorrrange.3.6.4RFPowerTheRFpowersuppliedtotheZnOsputteringgunisprovidedusinganexternal910seriesVeecoInstrumentsrfpowersupplyandaMWS-500EPlasma-Thermisusedforrf33

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tuningandasthematchingnetwork.Theamountofpowersuppliedtotheguncontrolstheamountofionsproducedduringthesputteringofthetargetmaterialandthustheirdensity.3.6.5DCPowerMDX1.5KbyAdvancedEnergySystemsisusedastheDCpowersupplytotheZnmetalsputteringgun.ThevalueoftheDCvoltageappliedtothegunispreciselycon-trolledusingasetpointcurrent.3.7ShutDownProcedureThechamberisallowedtocooldowntoroomtemperatureafterthedepositioniscom-pleted,andthepowersourcesforthegunassembliesandthelampsareswitchedoff.Thediffusionpumpisthenswitchedoffandallowedtocooldownforsufcientamountoftimeuntiltheoilinsidethepumpreachesroomtemperature.TheforelineandroughingvalvesarethenclosedandthechamberisbacklledusingultrahighpurityArgon.3.8SafetyPrecautionsOillevelinboththepumpsshouldbecheckedperiodically.Theoilinsidetheme-chanicalpumphastobechangedonceeverytwomonthsforefcientpumping.Thediffusionpumpshouldbeallowedtocooldowncompletelybeforeclosingtheforelinevalveofthechamber.Waterlinestothesystemshouldbecheckedforblockageperiodically.Always,thewaterlineshouldbeopenedbeforethebeginningoftheprocess.Absenceofwaterowmayresultinmajorproblemsliketargetmeltingandspoilingtheoperationofthediffusionpump.34

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ThefansconnectedtoboththeRFandDCpowersupplysystemsshouldbekeptONforsufcientamountoftimeafterthepowersuppliesareswitchedofftomakesurethatthesystemscooleddowncompletely.Thewaterlinetothechambershouldbeshutdownwhentheprocessiscompletedtoavoidanunexpectedwaterleakinsideandoutsidethedepositionchamber.Thedarkshieldspacingshouldbecheckedbeforethechamberispumpeddown.Theshieldshouldbetightenedproperlytothecathodeofthegunwhichmightotherwiseshortcircuitthegun.Whilecheckingforleaksinthechamberusingmethanol,careshouldbetakentoavoidmajorreaccidents.Theseaccidentscouldoccurwhenmethanolgetsspilledonhotplaceslikebeneaththepumpandcancatchreasmethanolishighlyammable.Theseaccidentscouldbeavoidedbyusingakimwipesprayedwithmethanol.35

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CHAPTER4RESULTSANDDISCUSSIONTheZincOxidelmsdiscussedinthischapterweredepositedinthesetupdescribedinchapter3.Thelmsweredepositedatsubstratetemperaturesrangingfrom200Cto400Cwiththebasepressurevaluesrangingfrom2to4Torr.Theotherdepositionconditionssuchasthepartialpressuresofoxygenandnitrogen,theconditionsusedforprovidingazincrichenvironmentarementionedintherespectivesubdivisions.Through-outtheresearch,aDektak3030STauto/surfacetextureprolerwasusedtomeasurethethicknessofthesamples.ThesheetresistanceoftheZnOlmswasdeterminedusingafourpointprobesetupinthelaboratory.Thevaluesforcarrierconcentrationandthemobil-itywereobtainedusingKeithley900seriesHallequipment.Theopticalcharacterizationwasperformedusingtransmission/absorptionspectroscopy.ThepresenceofnitrogeninthelmswassubstantiatedbythedataobtainedfromtheEDSanalysisusingaHitachiS-800TEDAXanalysissystem.4.1UndopedZincOxideFilmsUndopedzincoxidelmsaren-typeduetothepresenceofZninterstitialsandoxygenvacancies.A99.999%pureZnOtargetusedforthispurposewasRFsputtered.Thesub-stratetemperaturewasmaintainedat491C.Thesputteringgas,argon,wasmaintainedatapressureof5mtthroughouttheprocess.Thetypicalthicknessoftheselmswasaround1900A.Opticalmeasurementsshowthattheselmsarehighlytransparentwithatransmittanceof90%inthevisiblewavelengthregion(seeFigure4.1).Theabsorptionwavelengthwasobservedatawavelengthof340nm.Thebandgapoftheselmswas36

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3.2eV.Hallmeasurementsconrmthelmstoben-typeconductivewitharesistivityof1.9x10LM-cm.Thevaluesformobilityandcarrierconcentrationwerefoundtobe4.3x10Lcm/V-sand7.3x10cmrespectively.Thishighvalueofconductivityisduetonativedefects.Theresistivitycanbesignicantlyincreasedbyprovidingoxygensputteringambient. 400 600 800 1000 0 20 40 60 80 100 Wavelength (nm)Transmittance (in %) Figure4.1.TransmissionResponseofanUndopedZnOFilm4.2CodopingTechniqueUsingAluminiumCodopingistheprocessofaddingacceptorimpuritiesinthepresenceofdonorimpu-rities.Accordingtotheliterature,thepresenceofdonorimpuritiesdecreasesthebindingenergyofthedopantthusimprovingtheincorporationoftheacceptorimpurity[47].AnaluminiumdopedZnOtargetwith2wt%AlOwasusedforthispurpose.Thesubstratetemperaturewasmaintainedat411Candthetargetwassputteredatanrfpowerof60W.Thesputteringpressurewasconstantlymaintainedat5mTthroughouttheperiodofdeposition.Theseconditionsholdgoodforallthesamplesdiscussedinthissection.Table4.1showsthedataobtainedfromthefourpointprobeandhallmeasurementsforZnO:Al37

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lmsdepositedatvariouspartialpressuresofnitrogen.Fromthetable,itcanbeseenthattheresistivityoftheZnO:Allmsincreasedwiththeintroductionof1mTofnitrogengasintheambient.Afurtherincreaseinthepartialpressureofnitrogengasto1.7mTproducedahighlyinsulatinglm.ThisindicatesthatnitrogenhascompensatedtheAlandsuggeststhatafurtherincreaseinthenitrogenpartialpressuremightresultinashiftinthecarriertype.Inanefforttoobtainthis,sampleno.4wasdepositedat2mTofnitrogenpartialpressure.Nolmwasobtainedonthesubstratedepositedunderthiscondition.Atrendinthethicknessofthelmwhichwasobservedwiththeincreaseinthepartialpressureofnitrogenisdiscussedinthefollowingsection.4.2.1EffectofNitrogenonFilmThicknessFigure4.2depictstheeffectofnitrogenpressureonthethicknessofthelm.Thethicknessofthelmreducedwiththecorrespondingincreaseinthenitrogenpressure.Thegradualdecreaseinthicknessfrom1150insample2to500insample3withanincreaseinthepartialpressureofnitrogenfrom1mTto1.7mTwasobserved.Theothersputteringparameterssuchassputteringpower,substratetemperatureandthetotaldepositionpressureweremaintainedconstantforthissetofexperiments.Inthecaseofsampleno.4,therewasnodeposition.Whencomparedtothedepositionconditionsofthissamplewiththepreviousones,theincreaseinthepartialpressureofnitrogen(2mT)istheonlydifference.Thoughthisvalueishigh,theArpartialpressurewas3mTandthuscomprisedmorethan50%oftheambientgasmixture.ThisindicatesthattheplasmawasstableduringthedepositionprocessandshouldhavecontinuedsputteringZnO.Thisreasonsuggeststhatnitrogenhadetchedthelmasitgrew.4.3DopingUnderZincRichEnvironmentLiterature[47]suggeststhatanion-substitutingdopantswillbemoresolubleunderhostanionpoorgrowthconditions.Theanionpoorconditionotherwisemeansthelow38

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Table4.1.ElectricalPropertiesofAZO:NFilms Sample Partialpressure Substrate Thickness Resistivity Carrier No. pressure Temperature (A) (-cm) type ofNitrogen(mT) (C) 1 0 411 1500 4.22x10 n 2 1 411 1150 0.97x10L n 3 1.7 411 500 NotConductive 4 2 411 NOFILM 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 500 1000 1500 Partial Pressure of Nitrogen (mT)Thickness (in Angstroms) Figure4.2.ThicknessoftheAZO:NatDifferentPartialPressuresofNitrogenchemicalpotentialfortheanionformation.Oneofthewaystoobtainthisistodopethehostmaterialandattainacation-richenvironment.ThusdopingZnOwithnitrogeninazinc-richenvironmentisexpectedtoincreaseitsincorporationintheZnOlm.ThisconditionisincorporatedintheworkbyZ.-Z.Yeetal.[55]inwhichZnOlmsweredepositedusingreactivesputteringofametallicZntargetinanoxygenambiencewithammoniausedfordopingpurposes.Theyobtainedp-typelmswithacoductivityof35cm.39

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Inourexperiments,weattainedacation-richenvironmentasfollows.A99.999pureZnOtargetwassputteredatanrfpowerof70W.Thetemperatureofthesubstrateswasvariedfrom200Cto491Cwhilethesputteringpressurewasmaintainedat5mT.TheinitialexperimentsusedazincchunkontheZnOtargettoprovidetheZincrichenvironmentduringthedepositionofthelms.Followingthis,excessZnwassuppliedbycosputteringfromanadjacentZntarget.Furtherdetailsofthearrangementwillbeprovidedbelow.Theeffectonthelmpropertiesbythevariationinsubstratetemperatureandpartialpressureofnitrogenwasstudied.Table4.2summarizestheelectricalpropertiessuchasconductivityandcarriertypeobtainedfromthehallmeasurements.Samplenos.1and2aretheZnOlmsobtainedwhentheZnchunkwasusedtoprovidetheZn-richenvironment,whilesamplenos.3,4,5and6representtheresultsobtainedusingthedcsputteringoftheZntarget.Fromthetableitisseenthattheincreaseinthenitrogenpressuredidnothaveanyeffectonthethicknessofthelminsamplenos.1and2.Table4.3summarizesthedataobtainedfromtheEDSanalysisperformedonthesamesamples.FromTable4.2,itcanalsobeobservedthattheincreaseinthenitrogenpressureinsampleno.2ascomparedtosampleno.1didnotresultinmeasurableconductivity.Thoughthefour-pointprobeandhallmeasurementsdidnotshowanyconductivityofthesesamples,theEDSanalysisindicatedthepresenceofnitrogen.TheEDSdatashowedthattheincreaseinnitrogenpartialpressureresultedinadecreaseintheatomic%ofnitrogenwhichisacontradictoryresultasanincreaseinthenitrogenpartialpressureisexpectedtoproduceanincreaseinthenitrogenincorporationinthelm.Thenon-uniformdistributionoftheZincrichenvironmentprovidedbytheZnchunkmightcontributetothisresult.Inordertoimprovethedepositionparameters,thezincchunkwasremovedfromthetargetandreplacedbysputteringfromasecondsputtergun.TheDCpowerfortheZnmetalgunwasmaintainedatlowervaluesof3or4WinordertoreducetheZnleveltoacceptablevalues.ThisproducedaZndepositionrateofapproximately0.03ONs.The40

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Table4.2.ElectricalPropertiesofZnOFilms Sample Partial Sub Dep no. pressure Temp time Thickness Resistivity Carrier ofN (C) (Hrs) (A) (-cm) type (mT) 1 0.5 200 4 3300 2 1 200 4 3450 3 0.5 200 4 4000 4 0.5 466 4 1400 1.6x10L n 5 0.5 466 8 1800 1.8x10L n/p 6 0.5 491 4 4500 3.74x10 n Table4.3.EDSDataforZnOFilms Sample Partial Sub EDSdataforN no. pressure Temp ofN (C) atomic (mT) 1 0.5 200 4.54 2 1 200 0.48 3 0.5 200 1.10 4 0.5 466 1.28 5 0.5 466 0.72 6 0.5 491 1.28 orientationofthisgunwithrespecttothesubstrateisshowninFigure4.3.FromTable4.2,itcanbeseenforsampleno.3thattheZnOlmstillremainednon-conductivethoughdepositedatthesamesubstratetemperatureof200C(assamplenos.1and2)andat0.5mTofnitrogenpartialpressure.Theprolometermeasurementsshowedanimprovementinthethicknessdistributionoftheselms.Nitrogenwassuccessfullyincorporated(accordingtoTable4.3),butitwasnotelectronicallyactive.Tohelpactivateit,thesubstratetemperaturewasincreasedfrom200Cto466C.AsseeninTable4.2,anincreaseinsubstratetemperaturereducedthethicknessofthelms.ThisisinagreementwiththeresultsobtainedbyS.S.Linetal.[58]andK.B.Sundarametal.[59].Intheirwork,itwasstatedthatatlowsubstratetemperatures,theadatommobilityoftheatomsislowresultinginalowdensityandhighlyporouslmwithroughsurfaces.This41

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indicatesthepossibilityofnitrogengettingincorporatedinthedefectivesitesinsamplenos.1,2and3whichresultedinnitrogenbeingelectronicallyinactive.Accordingtothesameresearchgroups,whenthesubstratetemperaturewasincreased,theadatommobilitywasincreasedandre-evaporatesthepoorlycombinedstructure.Thusanincreaseinthediffusiveabilityofatomsormoleculeswiththeincreaseinsubstratetemperatureresultedintheloweringofthedepositionrate.Thoughthedepositionrateisaffected,thelmsobtainedinthesesubstrateshadbettercrystallinity.Butthedeciencyofoxygenintheselmsleadtoanon-stoichiometriclm.TheseeffectsofhightemperatureonZnOexplainthen-typeconductivityofthelmsdepositedathightemperatures(samplenos.4,5and6).Sampleno.5wasdepositedunderthesameconditionsassampleno.4exceptforthedepositiontimewhichwasdoubled.Thissampleshowedanunstablecarriertype.Thethicknessofthelmincreasedbyjust400andthenitrogenincorporationdecreased.Thisindicatedthatmaintainingthesamplesathightemperaturesforalongertimecouldhaveannealedthesamplesresultinginafurtherimprovementinthestoichiometryofthelm.Furtherdevelopmenttoimprovetheconductivityofthesamplesdepositedunderthisconditionwasnotperformedfortworeasons.First,theunstablenatureofthecarriertypeindicatedthattheamountofnitrogenmightnotbecompensatingforalltheoxygenvacancies.Andsecond,depositingZnOlmsfor8hourswasnotafeasibleoption.Themethodusedtoovercomethisproblemisdiscussedinthenextsection.4.3.1EffectofZninDopingTobetterunderstandtheeffectofZnondoping,samplenos.5and6fromTable4.2weredepositedunderthesamepartialpressuresofnitrogenandargonbutatdifferentsputteringpowersofthemetallicZntarget.Table4.4indicatestheeffectofZnonthegrowthoftheselmsandprovidesthedataobtainedfromhallmeasurements.Note:PPofN-PartialPressureofN,CC-CarrierConcentration,DCPWR-DCPower,T-Thickness,CT-CarrierType42

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SHIELD SEPARATING THE TWO TARGETSTO HOLD SUBSTRATESCIRCULAR MOUNTINGSUBSTRATE HOLDERPLACING SLOT FOR Zn TARGET HOLDERSUBSTRATETARGETZnO Figure4.3.Cross-SectionalViewoftheTargetPlacementInsidetheSputteringSystemTable4.4.ComparisonTableforZnOFilmsatDifferentSputteringPressuresforZn Sub PP DC Resistivity Temp of PWR from from S.No. N for T CC (cm Hall 4ptprobe CT (C) (mT) Zn (A) cm V-s) (-cm) (-cm) (W) 5 466 0.5 4 1800 5.67E18 3.2 1.8E-1 1.8E-1 n/p 6 491 0.5 3 4500 1.83E20 7.4 4.5E-3 4.1E-3 n FromtheTable4.4,itisseenthatthereisasignicantdifferenceinthethicknessofthelms.ComparingthisdatawiththedatafromTable4.2,itcanbesaidthattheincreaseinthethicknessofthelmsisduetothedecreaseintheconcentrationofZnratherthantheincreaseintemperature.Fromthehallmeasurements,itwasfoundthattherewasanincreaseinthecarriercon-centrationwhenthedcpowerwasdecreasedfrom4Wto3W.Theconductivityofthelmsincreasedby2ordersofmagnitude.FromtheopticalmeasurementsinFigure4.4,itwasfoundthattheabsorptionedgeshiftedfrom340nm,obtainedforundopedZnO,toavalueof336nmforthelmsdepositedat4W.Forthelmat3W,theabsorptionedgefurthershiftedto310nm.ThiscorrespondstotheBurstein-Mossshiftandthussupportsthein-43

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creaseinthecarrierdensitywiththedecreaseintheamountofzincconcentration.Whileconsideringtheseobservations,itshouldnotedthattheORIELmonochromatorwasnotcalibratedforwavelengthsbelow350nm.Thusthetransmissionresponseobtainedinthisregionisneglected.Fromtheseobservations,itcouldbesaidthattheexcesszincproducedat4Wcouldhaveincreasedtheamountofnitrogendissolvedinthelmsresultinginthereductionofoxygenvacancies.Thiscouldcontributetothedecreaseintheelectroncarrierconcentrationandthen-typeconductivitywiththeincreaseintheZnsputterpower.ThisisfurthersubstantiatedwiththeEDSresults.FromTable4.3,thenitrogenconcentrationinthelmdepositedat3Wwashighcomparedtotheoneat4W.TheinstabilityinthecarriertypefromHallmeasurementswasalreadydiscussedintheprevioussection.Duetothesereasons,ourresearchatthispointwasmoreinclinedtoimprovingthedepositionconditionstoobtainp-typeZnOlms.Forthesamereason,thestudyonthevariationinthicknessintheabovesampleswaspostponedtofuturework. 300 350 400 450 500 0 20 40 60 80 100 Wavelength (nm)Transmittance (in %) 3W4W Figure4.4.TransmissionResponseatShorterWavelengthsforSamplesDepositedatDif-ferentZnConcentration44

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4.4DopinginanOxygen-andZinc-RichEnvironmentSincetheamountofnitrogenwasinsufcientincompensatingfortheoxygenvacan-ciesintheprevioussetofexperiments,insteadofincreasingthenitrogenpartialpressure,oxygengaswasintroducedintotheambient.Theexcessoxygengasisusedtosuppresstheoxygenvacanciesactingasholekillers.Theotherreasonforintroducingoxygenandnotincreasingthenitrogenpartialpressurewastoavoidanykindofetchingthatcouldoc-curduetoexcessinthenitrogenaspronouncedintheAZO:Nsamples.Thisideaofp-typedopinginhostanion-richenvironmenttosuppresstheanionvacancyisalsosupportedintheliterature[47].Inthissetofexperiments,ZnOlmsweredepositedunderdifferentnitrogenandoxy-genpartialpressures.Argonusedasthesputteringgaswasvariedtomaintainthetotalsputteringpressureataconstantvalueof5mTforthissetofexperiments.Table4.5in-cludestheelectricalpropertiesobtainedfromtheHallmeasurementsandthefour-pointprobeequipment.Table4.5.ElectricalPropertiesofZnOFilmsDepositedUnderVariousPartialPressuresofOxygen Resistivity Carrier S.no. PP PP Thick Carrier Mobility from from ofO ofN -ness Conc (incm 4ptprobe Hall type (mT) (mT) (A) (incmCP V-s) -cm -cm 1 0.2 0.5 1700 7.219E17 4.857E0 2.1 1.787 n/p 2 0.3 0.5 1000 5.647E18 2.311E-1 2.16 3.168 n 3 0.5 0.5 700 7.249E18 4.56E-1 2.06 3.686 n 4 0.6 0.5 650 1.6E19 5.228E-1 0.948 0.789 n 5 0.3 0.8 700 4.016E16 1.181E-1 3.408E2 p Note:PPofO-PartialPressureofOxygen,PPofN-PartialPressureofNitrogenThelmsdepositedatanambienceof0.2mTofoxygenand0.5mTofnitrogen(S.no.1)showedann-typeresistivityof2.1-cmwithamobilityof4.9cm/V-sandacarrierconcentrationof7.2x10cm.TheHallmeasurementsshowedaninconsistentvalueforthecarriertype.Thiscouldbeduetothefactthattheamountofoxygenintroduced45

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inthedepositionenvironmentwasinsufcienttocompensatefortheoxygenvacanciesinthelms.Ariseinthepartialpressureofoxygenwasexpectedtoincreasetheamountofcompensationinthelms.Thelmthicknessandcarrierconcentrationwerefoundtobeaffectedbythisvariationinthepartialpressureofoxygen,althoughthecarriertypestillremainedthesame.ItisapparentthatexcessOsupresseslmgrowthasdoesexcessN,thoughtheeffectofOismuchlessdramatic.Theelectricalpropertiesofthelmsobtainedatdifferentvaluesofoxygenandnitrogenpartialpressuresisdiscussedfurtherbelow.4.4.1InuenceofOxygenPressureonCarrierConcentrationFromthevaluesindicatedforthecarrierconcentrationunderdifferentdepositioncon-ditionsfromTable4.5,agraphisplottedtoshowthevariationofthecarrierconcentrationinthelmswiththeincreaseintheoxygenpressure.ThisisshowninFigure4.5.Thecar-rierconcentrationincreasedfrom7.2E17cmto1.6E19cmwhenthepartialpressureofoxygenwasincreasedfrom0.2mT(S.no.1)to0.6mT(S.no.4).Thepartialpressureofnitrogenwasmaintainedatavalueof0.5mTandtheArgonpartialpressurewasad-justedcorrespondingtotheoxygenpressure.Thus,thetotaldepositionpressureof5mTwasmaintainedinthedepositionenvironment.Althoughtheconductivityvaluesindicatedapossibilityfornitrogenhavinganeffectonthelms,thisdifferencewasrelativelysmall.4.4.2OpticalCharacteristicsAneffectoftheincreaseinoxygenpartialpressureonthecarrierconcentrationwasshownintheopticalcharacteristicsofthelmsalso.TheBurstein-Mossshift,theshiftintheabsorptionwavelengthtolowerwavelengthsathighcarrierdensitieswasobservedfromthetransmissionresponseobtainedfortheselms.ThetransmissionresponseforthesamplesinTable4.5isshowninFigure4.6.Forthesamereasonmentionedbefore,thetransmissionresponseofthesamplesbelow350nmisneglected.Forsampleno.2,this46

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0.2 0.3 0.4 0.5 0.6 0.7 0 2 4 6 8 10 12 14 16x 1018 Partial Pressure of Oxygen (mT)Carrier Concentration Figure4.5.InuenceofOxygenPartialPressureonCarrierConcentrationabsorptionedgewasfoundat320nmwhileforsampleno.4,thisvaluewasfoundtobeat289nm.Thisisconsistentwiththeincreaseinthecarrierconcentrationwiththeincreaseintheoxygenpartialpressure.Althoughtheaboveresultsindicateanimprovementinthedoping,thehallmeasure-mentsdidnotindicateanychangeinthecarriertype.Thiscouldbepossiblyduetothefactthattheexcessoxygeninthechambercouldhavepossiblyaccountedforalltheoxy-genvacanciesinthelmbutthenitrogencontentwasinsufcienttoproduceap-typeconductivityinthelm.4.4.3InuenceofOxygenPressureonFilmThicknessFromFigure4.7itisobservedthat,withtheincreaseinthepartialpressureofoxygen,thethicknessofthelmdecreased.AsimilareffectwasobservedbyK.B.Sundarametal.[59]intheirattempttondtheeffectofoxygenpressureonthedepositionratewhenZnOlmsweredepositedbyrfmagnetronsputteringusinga99.99%pureZnOtarget.Thiseffectofoxygenonthelmthicknessathighpressurescouldbeattributedtothe47

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300 350 400 450 500 0 20 40 60 80 100 Wavelength (nm)Transmittance (in %) S. no.1S. no.2S. no.3S. no.4S. no.5ZnO Figure4.6.StudyontheShortWavelengthAbsorptionatDifferentDepositionConditionschemisorptionofoxygenonthetargetsurface.Thisformsalayerofadsorbedoxygenonthetargetsurfacethatretardssputtering.Alsothefactthatthesputteringyieldinoxygenislessthanargonmaycontributetothedecreaseinthedepositionratewiththeincreaseinthepartialpressureofoxygen.4.4.4EffectofIncreaseinNitrogenSincesampleno.1inTable4.5indicatessomep-typetendenciesintheHallmea-surements,itwasdecidedtofurtherexplorethesedepositionconditions.AZnO:Nlmdepositedat0.3mTofoxygenand0.8mTofnitrogenproducedalmwithathickness700A.TheHallmeasurementsfoundthelmtobep-type.Thebestresistivityobtainedwiththeselmswas3.4E2-cm.Thecarrierconcentrationoftheselmswas4.016x10cmwithavalueof0.12cmV-sformobility.48

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0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 600 800 1000 1200 1400 1600 1800 Partial Pressure of Oxygen (mT)Thickness (in Angstroms) Figure4.7.InuenceofOxygenPartialPressureonFilmThickness4.4.4.1OpticalCharacteristicsFigure4.8depictsthetransmissionresponseforthep-typelm(sampleno.5)dis-cussedabove.ThetransmissionresponsewhencomparedwiththatoftheundopedZnOlm(seeFigure4.1)showsanabsorptionintheshortwavelengthregion.Thevaluefortheabsorptionwavelengthwas290nmwhichwasverylowcomparedtothevalueof340nmobtainedfortheundopedZnOlm.ThisindicatesthepresenceofasubstantialcarrierdensityintheZnO:Nlmindicatingsuccessfuldopingwithnitrogen.Asimilarshiftinthelowwavelengthregionwasproducedbythep-typeZnOlmsobtainedbyZ.-Z.Yeetal.[55].Thebandgapshiftwasalsoobservedintheabovementionedresult.Thoughthedifferenceinthebandgapwasaverysmallvalueof0.1eV,itcanstillconrmtheeffectofnitrogenontheZnOlms.49

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300 400 500 600 700 800 900 1000 1100 0 10 20 30 40 50 60 70 80 90 100 Wavelength (nm)Transmittance (in %) Figure4.8.TransmissionResponseofZnO:NDepositedat0.3mTofOxygenand0.8mTofNitrogen50

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CHAPTER5CONCLUSIONInthisresearch,variousexperimentswereperformedtoachievep-typedopinginZnOlms.Thecodopingtechniqueusingaluminiumwasusedtosuppresstheself-compensationthatoccursduringthedopingmechanismandtoimprovethesolubilityofthedopant.SimilarworkwasdonebyM.Josephetal.[54].IntheirworkGalliumwasusedastheco-dopant.P-typeZnOlmswitharesistivityof0.5-cmwereobtained.Sincealuminumandgalliumbelongtothesamegroupoftheperiodictable,itwassug-gestedthataluminiumcouldbeusedforthispurpose.ZnO:Allmsweredepositedatdifferentpartialpressuresofnitrogen.TheHallmeasurementsindicatedadecreaseintheconductivityoftheZnO:Allmwhenthepartialpressureofnitrogenwasincreased.Thelmswerecompletelyinsulatingat1.7mTofnitrogen.Thissuggestedthatafurtherin-creaseinthenitrogenpressuremightchangethecarriertypeofthelms.Alongwiththiseffect,itwasobservedthatnitrogenhadaneffectonthegrowthofthelm.Thusafurtherincreaseinthenitrogenpressureinanefforttochangethecarriertypetop-typeresultedinasubstratewithnolmdepositedonit.Thepartialpressureofargonwas3mT,enoughtosustainastableplasma.Itwasconcludedthatthevariationinthethicknessofthelmwasduetotheincreaseinthepartialpressureofnitrogen.TheLiterature[47],[55]suggestedthataZnrichenvironmentduringnitrogendopingwouldincreasethesolubilityofnitrogenintheZnOlms.Z.-Z.Yeetal.[55]sputteredametalliczinctargetinanoxygenandammoniaenvironment.Themetalliczincprovidedtheexcesszincrequiredfortheincreaseinthesolubilityofnitrogeninthelms.Inourresearch,thisideawasimplementedbydcsputteringaZnmetaltargetusingaseparate51

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gun.Thenitrogenpartialpressurewasvaried,andtheelectricalandopticalpropertieswerestudied.Itwasfoundthatanincreaseinthesubstratetemperatureofthelmsresultedinanincreaseinthequalityofthelm.Thiscausedanimprovementintheconductivityofthelm.TheHallmeasurementsonthissampleshowedaninconsistencyforthecarriertype,althoughthesheetresistanceandtheresistivityofthelmswereinagreementwiththevaluesobtainedfromthefour-pointprobemeasurements.SimilarinconsistentbehaviorwasobservedintheworkbyJosephetal.[48]inwhichp-typedopinginZnOwasachievedusingNOgasusingaPulsedlaserdepositiontechnique.AnincreaseintheNOgasshowedaninconsistentbehaviorinthecarriertypewhichultimatelyresultedinap-typeZnOlm.Butinourwork,sinceZnisalsousedinthedepositionenvironment,theexcesszincisexpectedtoimprovethesolubilityofnitrogen.Duetothedifferenceinthedepositiontechniqueandconditions,therecouldbeapossibilityforthepresenceofoxygenvacanciesinourlm.Amethodtosuppresstheoxygenvacancieswasfurtherdeveloped.Theintroductionofoxygengasinsidethechamberwasexpectedtosuppresstheholekillers(oxygenvacancies).TheZnO:Nlmsweredepositedatvariouspartialpressuresofoxygenandnitrogen.TheZntargetwassputteredatadcpowerof3W.ThisexcessZnenvironmentwaschosenfortwopurposes.Primarily,toenhancethesolubilityofnitrogenandsecond,tocombinewiththeexcessoxygenthuscontrollingtheamountofoxygenpresentinthedepositionenvironment.Thismightotherwiseleadtothereactionofoxygenwiththenitrogenresultinginthedecreaseofnitrogencontentincorporatedinthelms.Fromthedataobtainedfortheseexperiments,itwasobservedthatanincreaseinthepartialpressureofoxygenwiththenitrogenpressurekeptconstantresultedinanincreaseinthecarrierconcentrationofthelm.Thisalsoproducedablue-shiftinthetransmissionresponseofthelms.Theincreaseintheoxygenpressurealsoresultedinreductioninthelmthickness.52

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TheaboveexperimentsconrmedthatonlybyndingtherightcombinationofNandOpartialpressures,p-typeconductivitycouldbeachievedinZnO.Theincreaseinthepartialpressureofnitrogenfrom0.5mtto0.8mTwith0.3mTofoxygenand3.9mTofargonresultedinap-typeZnOlm.Thehallmeasurementsshowedacarrierconcentrationof4.0x10cmandamobilityof0.12cm/V-s.Thebestresistivityobtainedattheseconditionswas3.4E2-cm.5.1FutureWorkThepartialpressuresofoxygenandnitrogencouldbevariedfurthertoimprovetheconductivityofthelms.Literaturesuggeststhatinanattempttoachievep-typedopingusingphosphorusdopedZnO,RapidThermalProcessannealingathightemperaturesinnitrogenat-mosphereshelpedinmakingthenitrogenelectronicallyactive.Similarconditionscouldbetriedonthelmstodecreasetheobtainedresistivityof3.4E2-cm.53

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ABSTRACT: Zinc Oxide falls under the classification of transparent conductive oxides. They typical optical transmittance of Zinc Oxide is 90% in the visible wavelength region. Though stoichiometric ZnO is an insulator, due to the presence of internal defects such as Zn interstitials and Oxygen vacancies, it exists as a n-type conductor. The other important property of ZnO which could be used by the optical field is its widebandgap. ZnO has a wide bandgap of 3.2eV -3.3eV. The additional advantage of being a direct bandgap semiconductor has increased the probability of using ZnO for short wavelength applications. These practical applications are directly related to the fabrication of homostructural p-n junctions. ZnO can be readily doped n-type. Doping ZnO P-type is very difficult due to its native defects and the self-compensation that occurs during doping. But when P-type doping is obtained in ZnO it could be used in various optical applications such as light emitting diodes and laser diodes. This provided the motivation for this research. Theoretical studies have proposed nitrogen as a suitable material to achieve p-type ZnO. Literature provides a set of conditions that could be used to improve the doping in ZnO films. In this research, a set of these conditions were used to implement p-type doping in ZnO films. A sputtering system with a setup to support two Torus 5M guns was used to deposit the ZnO films. A codoping technique using an aluminium doped zinc oxide target was the first method. Though an improvement in the nitrogen incorporation was found in this method in the beginning, a further increase in the nitrogen pressure did not show further improvement. A co-sputtering technique of a 99.999% pure ZnO target and a 99.99% pure Zn metal target was the second method. The ZnO target was rf sputtered while the Zn target was dc sputtered using the two guns provided in the deposition chamber. The extra Zinc obtained from sputtering the metallic Zn target was used to improve the incorporation of nitrogen. The films were later deposited in an oxygen ambient where the excess oxygen was used to suppress the oxygen vacancies that act as hole killers during the doping process. Four point probe measurement and Keithley 900 series Hall equipment were used for the electrical characterization of the films. An ORIEL monochromator was used to optically characterize the films. Hitachi S-800 T EDAX analysis system was used to measure the atomic weight % of nitrogen incorporated in the ZnO:N films. Deposition at an oxygen partial pressure of 0.3mT and 0.8mT of nitrogen produced p-type ZnO films. These films showed a carrier absorption in the short wavelength region. The carrier concentration and the mobility obtained for these films were 4.0x10 cm and 0.12 cm/V-s respectively.
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Wideband gap semiconductors.
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