非晶矽薄膜電晶體低溫特性與光漏電流之研究 - 9lib TW

The main stream of Large-Area Displays is TFT-LCD(Thin Film Transistor-Liquid Crystal Display) and it's applied a-Si:H TFT (the hydrogenated Amorphous Silicon ... menu Upload menu Loading... Home &nbsp 其他 非晶矽薄膜電晶體低溫特性與光漏電流之研究 95  26  Download (0) 顯示更多(94頁) 顯示更多(頁) 立即下載(95頁) 全文 (1)國立中山大學光電工程研究所碩士論文.非晶矽薄膜電晶體低溫特性與光漏電流之研究InvestigationonElectricalCharacteristicsatLowTemperatureandPhotoLeakageCurrentofa-SiThinFilmTransistor.研究生：黃靜美撰指導教授：張鼎張博士中華民國九十七年一月.(2)(3)(4)誌謝.本論文得以順利完成，首先要感謝我的指導教授張鼎張博士，除了提供優質完整的研究環境與設備，並且在兩年間辛勤的指導，於研究期間不只傳授許多寶貴的知識，更教導如何分析、處理問題，在處事態度方面也惠益良多，師恩難忘，永誌於心。

時光飛逝，研究生生活即將結束，在實驗室裡所有的點點滴滴都是最美好的回憶。

在這兩年生活中，感謝所有伴我走過這段日子的實驗室成員。

感謝伯鈞學長包容我的笨拙，耐心教導帶領我完成整個實驗；還有書瑋、世清、雅喨等學長於研究過程中給予建議；感謝漢博、書慶、詠恩、阿敏、冠張、正杰等實驗室的同學和學弟們，實驗過程中給予我幫助與鼓勵，實驗拖到半夜時通宵陪伴著我，今日我才能有這些研究上的成果。

感謝口試委員聯華電子黃正同經理、光電所朱安國教授和張榮芳學長撥冗審閱，給予論文指正與教導，特此致謝。

.(5)非晶矽薄膜電晶體低溫特性與光漏電流之研究研究生：黃靜美指導教授：張鼎張博士國立中山大學光電工程研究所碩士班.摘要人類生活中傳統的陰極射線管CRT已幾乎被平面顯示器(LCD、OLED、PDP)所取代，顯示器產業更被評為繼半導體產業之後的全球重大產業之一。

而市場上大面積的顯示面板主流為TFT-LCD(薄膜電晶體液晶顯示器)，正是使用了非晶矽薄膜電晶體為液晶顯示器的畫素開關。

a-SiTFT的製程中，主動層材料(a-Si)的光導係數較高，在光源照射下有較大的漏電流，使得畫素開關動作不完全造成顏色顯示上的問題。

若在主動層製程中通入了SiF4，使主動層偏P型半導體及增加了主動層的缺陷密度，對光漏電流的有抑制效果。

又因為液晶的特性，TFT-LCD顯示器的畫素開關的驅動將會施加偏壓，形同對TFT做STRESS。

因此針對SPL作DCSTRESS的實驗,發現摻雜SiF4的元件比較不會劣化,可靠度較佳。

本論文主要研究a-Si:FTFT在低溫下的光漏電流變化並了解STRESS對TFT影響.I.(6)InvestigationonElectricalCharacteristicsatLowTemperatureandPhotoLeakageCurrentofa-SiThinFilmTransistor.Postgraduate:Ching-MeiHuang.Advisers:Dr.Ting-ChangChang.InstituteofDept.ofElectro-OpticalEngineeringNationalSun-YetSanUniversity.AbstractSincethetraditionalCRT(CathodeRayTube)replacedbyFPD(FlatPanelDisplay),e.g.LCD、OLED、PDP,FPDindustryisregardedastheimportantoneofglobalindustryfollowingSemi-conductorindustry.ThemainstreamofLarge-AreaDisplaysisTFT-LCD(ThinFilmTransistor-LiquidCrystalDisplay)andit’sapplieda-Si:HTFT(thehydrogenatedAmorphousSiliconThinFilmTransistor)aspixel-switchdeviceonLCD.Ina-Si:HTFTCellprocess,theactiveregionmaterial(a-Si:H)withhigherPhotoconductivityresultsintohigheroff-statecurrentunderlightilluminationandthatcausescolorperformancediscrepancyasincompleteOn/Offoperationofpixel-switchdevices.AslongastheintroductionofFintoa-Si:Hmodifythedensityofstatesinthegapofa-Si:H(:F),thatmayresulttheshiftoftheFermileveltowardthevalencebandedgeandThedensity-of-statesincreasing.It’seffectivetodecreasethephotoleakagecurrent.Duetoelectro-opticalpropertiesofliquidcrystal(LC),todrivePixel-switchdeviceinTFT-LCDshallforceOn/OffvoltagetochangeTwistAngleofLCiscorrespondingtohaveStressonTFTdevice.AccordingtoDCStressexperimentresults,it’sfoundTFTdevicewithSiF4dopantcanreachbetterreliability.II.(7)Thisissueisaimedtoresearchthephotoleakagecurrentvariationofa-Si:HTFTatlowtemperatureandON/OffstateeffectbystressonTFTdevice..III.(8)ContentsChineseAbstract……………………...………………………….IEnglishAbstract…………………………..…………………….IIContent…………………………...…………………………………….IVTableCaptions…………………………………..…………………….VIIFigureCaptions………………………………………………………VIII.ChapterOne-Introduction1.1Introduction………………………………………………………………….11.1.1Overview1.1.2HydrogenatedAmorphousSilicon1.1.3AtomicStructureandtheElectronDensityofStates.1.2Photoleakagecurrentmechanism…...……………………….………….....51.3Somesolutionsforreducingphotoleakagecurrent……....……….......…..9.ChapterTwo-Fabrication2.1DepositionofHydrogenatedAmorphousSiliconbyPECVD……….….112.2DepositionofSiNxbyPECVD……………….…………………………….142.3Depositionofn+HydrogenatedAmorphousSiliconbyPECVD………162.4ProcessFlow………………………………………………………………..17.IV.(9)ChapterThree-ApparatusandParameters3.1ApparatusList………………………………………………………….…..183.2SetupinstrumentsforCurrent-Voltage(I-V)Measurement…...………193.2.1TheRoomTemperatureandHighTemperature3.2.2TheLowTemperature.3.3MethodofDeviceParameterExtraction…………..………...……….…...213.3.1Determinationofthethresholdvoltage3.3.2Determinationofthesubthresholdswing3.3.3Determinationofthefield-effectmobility.3.4DensityofStates……………………………………………………………243.4.1Overview3.4.2Evaluationofthedensityofstates--Activationenergymethod.ChapterFour–ExperimentsResultsandDiscussion4.1Overviewoftheexperimentssample..........................................304.1.1Characteristics4.1.2Relativeparameters.4.2CharacteristicsatLowTemperature……………………….…………..…334.2.1MotivationandExperimentSteps4.2.2Theelectricalcharacteristicsatlowtemperature4.2.3Measuredwithlightilluminatedatlowtemperature4.2.4Summary.4.3LightIlluminatedExperiment………………………….………..………..394.3.1MotivationandExperimentSteps4.3.2ADiscussiononExperimentResults4.3.3Summary.4.4ReliabilityExperiment—DCStress……………………………………..444.4.1MotivationandExperimentSteps.V.(10)4.4.2ADiscussiononExperimentsResults4.4.3Summary.ChapterFive–Conclusion…………………………………...……..47.References..…………………………………….……………..……….48Tables…………………………………………………………….…..56Figures…………………………………….................……….………..57.VI.(11)TableCaptionsChapterTwoTable2-1Physicalpropertiesofadevice-qualitya-Si:H..ChapterThreeTable3-1Typicalvaluesoftheparametersfortheexponentialmodelof.DOSforintrinsica-Si:HCompiledfromRef.10..VII.the.(12)FigureCaptionsChapterOneFigure.1.1Theelectronenergylevelsofsiliconindifferentbondingstates.Figure.1.2Aschematicdiagramoftheelectrondensityofstatesforanamorphoussemiconductor,suchasa-SiH.Figure.1.3TypicalTFTtransfercharacteristicsimulatedunderillumination.Figure.1.4TFTintheOFF-state(regime(III))underillumination.Figure.1.5Figure1.5SimulatedvariationsoftheelectronandholeFermipotentialsalongthemainTFTcurrentpath(theTFTbeingilluminated)..ChapterTwoFigure.2.1Cross-sectionalviewofaBCETFTwheretheSiNxgateinsulatorinthisexperiment..ChapterThreeFigure.3.1Microscope、Hotchuck、ProbestationFigure.3.2AgilentB2201A(Switch)andKeithley4200-SCS.Figure.3.3Temperaturecontroller.Figure.3.4Agilent4156C、AgilentE5250A(Switch)andAgilent41501B(highpower)Figure.3.5IllustrationoftheCryogenicsSystem(TTP-6ProbeStation)Figure.3.6Illustrationofthedensityofstates(DOS)inintrinsica-Si:H.(AfterShawandHack,Ref.10.©1988AIP.)Figure.3.7Representativedependenceoftheactivationenergyvs.gatevoltagedependenceforthetransistorswithincorporatedn+contactlayers..VIII.(13)Figure.3.8Densityofstatesvs.activationenergy.ChapterFourFigure.4.1TherelationshipbetweentheFermilevelandtheincreaseof.acceptor-likedeepstateSketchFigure.4.2TheID-VDtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithtemperatureat300Kand100K,individually.(a)300K,(b)100KFigure.4.3TheID-VGtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithtemperatureat300Kto100K.Figure.4.4ShowstheONresistanceoftheSiF4-9sccmanda-Si:HTFTatlowtemperatureFigure.4.5The△%ofVT(△VT/VT300K)andcarriermobility(△μ/μ300K)Figure.4.6TheID-VDtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithlightilluminatedbytemperatureat(a)300Kand(b)100K,individually.Figure.4.7ThetransfercharacteristicsVG-IDinthelinearregimeof(a)theSTDa-SiTFTdevicesand(b)theSiF4-9sccma-SiTFTwithlightilluminatedbytemperatureat300Kand100K,individually.Figure.4.8AtVG=-7V,-10V,-15V,changeofPhotoLeakageCurrentoftheSiF4-9sccma-SiTFTandtheSTDa-SiTFT.Figure.4.9Arrheniusplotofcarrierandemissiontimeforanindividualinterfacetrap.Figure.4.10Ionizedchargeconcentrationversustemperatureforsilicon.Figure.4.11TheparasiticalresistancesofN+from100Kto300K.Figure.4.12Thedevicewasilluminatedundertwodirection：(a)underfront-side.illumination(Top-Light)and(b)gate-sideillumination(Bottom-Light).Figure.4.13ThecomparisonofVG-IDcharacteristicsbetweentoplightilluminated.IX.(14)andbottomlightilluminated.(a)STD(b)9sccmFigure.4.14ThechangeofphotoleakagecurrentunderhighandlowlightilluminatedFigure.4.15ThecomparisonofVG-IDcharacteristicsbetweentoplightilluminatedandbottomlightilluminatedFigure.4.16Illustrationofregionforlightilluminatedandelectron–holepairst(a)Toplightilluminated(b)BottomlightilluminatedFigure.4.17ID-VGtransfercharacteristicsofstresswith(a)theSTDa-SiTFTand(b)theSiF4-9sccma-SiTFT,individually.Figure.4.18ID-VGtransfercharacteristicsoftheSTDa-SiTFT.Afterandbeforestresswith(a)Toplightilluminatedand(b)Bottomlightilluminated,individually.Figure.4.19ID-VGtransfercharacteristicsoftheSiF4-9sccma-SiTFT.Afterandbeforestresswith(a)Toplightilluminatedand(b)Bottomlightilluminated,individually.Figure.4.20Afterandbeforestress,(a)Theactivationenergy–gatevoltage(Ea-VG)and(b)DensityofstatescharacteristicoftheSTDa-SiTFTFigure.4.21Afterandbeforestress,(a)Theactivationenergy–gatevoltage(Ea-VG)and(b)DensityofstatescharacteristicoftheSiF4-9sccma-SiTFTFigure.4.22Afterandbeforestress,ID-VGtransfercharacteristicsoftheSTDa-SiTFTandtheSiF4-9sccma-SiTFTwith(a)no-lightilluminatedand(b)bottomlightilluminated,individually..X.(15)ChapterOne-Introduction1.1Introduction1.1.1OverviewThin-filmtransistors(TFTs)includinganactivelayerofamorphoussiliconorpolycrystallinesiliconhavebeenwidelyemployedasthepixel-drivingelementsofaliquidcrystaldisplay(LCD).a-Si:HTFTisparticularlyadvantageoustotheproductionoflargescreendisplaysandfacilitatesmassproduction.[1-1]Whenemployingana-Si:Hlayer,themainobjectivesaretoenhancethefieldeffectmobilityandtoreducetheoff-stateleakagecurrentunderlightillumination.Theincreaseoffieldeffectmobilityresultsinwideapplicationofa-Si:HTFTsinhighresolutionLCDs.Ontheotherhand,a-Si:Hhashighphotoconductivitywhichresultsinhighoff-stateleakagecurrentsofa-Si:HTFTunderlightillumination.[1-2]Theoff-stateleakagecurrentunderlightilluminationis,inparticular,aseriousproblemintheprojectionand/ormultimediadisplaysthatrequirehighintensitybacklightillumination.Recently,thefullyself-aligneda-Si:HTFTwasdevelopedtoreducetheparasiticcapacitancebetweensource/drainandgateelectrodes.However,ithashighoff-stateleakagecurrentunderlightilluminationcomparedtoconventionalTFT.[1-3]Therefore,toreducetheoff-stateleakagecurrentina-Si:HTFTunderilluminationisveryimportanttoobtainahighqualityTFT-LCD..1.1.2HydrogenatedAmorphousSiliconHydrogenatedAmorphousSilicon(a-Si:H)firstcametoprominenceasa.1.(16)potentiallyusefulelectronicmaterialthroughtheworkofSpearandcoworkersinthe1970s.[1-4]Sincethen,therehasbeensignificantresearchanddevelopmentworldwidetobringa-Si:HtothecurrentstateinwhichitisroutinelyusedinTFTswithalargeswitchingratio(＜106)andalowoffstatecurrentbetweenthesourceanddrainwhichissuitableforcontrollingAMLCDs.[1-5]Alargebodyofresearchintothismaterialhasbeenrequiredduetoitscomplexnature.Althoughmanyofthepropertiesofa-SI:Harereminiscentofcrystallinesilicon,theamorphousnatureofthematerialhasaneffectupontheelectronenergybandstructureandthecarriertransportproperties.Thepresenceofhydrogenanddanglingbondsinsidethedisorderedstructurefurthercomplicatesthesystem.[1-6].1.1.3AtomicStructureandtheElectronDensityofStatesCrystallinesilicon(c-Si)hasawell-definedtetrahedrallatticestructurewithabondlength(a)of0.35nmbetweenadjacentatomsandacorrespondingbondangle(θ)of109°.Inthecaseofa-Si:H,althoughthematerialhaslittleornolongrangeatomicorder,onshortlengthscales(upto~1nm)thetetrahedralstructureofthesiliconnetworkispreserved.Thetetrahedralstructureisadirectresultoftheelectronicstructureofthesiliconatom,whichhasfourvalenceelectrons.Whenunbonded,twoelectronsexistsinthe3sstateandtwointhe3pstate.Fig.1.1showsthissituationintermsoftheenergyoftheelectronsineachstate.[1-7]Ineachstate,oneelectronexistsin“spinup”configurationandoneina”spindown”configurationsoastosatisfythePauliExclusionPrinciple.However,suchaconfigurationonlyallowstwocovalentbondstobeformedwithneighboringsiliconatoms.Inordertomaximizethe.2.(17)numberofbondspossible,thesandpelectronstatescombinetoformfoursp3statesatanenergybetweenthesandtheplevels,andthispermitsbondingwithuptofoursiliconatomsatthetetrahedralangles,aseachbondisequivalent.ThisisshownasthebondingconfigurationinFig.1.1.Whenansp3orbitalformsduringbonding,asecondelectroniscontributedtothestatebytheotheratom,andinteractionbetweenthetwoelectronslowerstheenergyofthestate.Therefore,thesp3energylevelsplitsintoabondinglevel,andananti-bondinglevel(Fig.1.1).Inarealsolid,thereisaCoulombinteractionbetweentheatomcoresandelectrons,andsothebondingandanti-bondingenergylevelssplitformingacontinuumofextendedstates,withthebandofbondingstatesandtheanti-bondingstatesbeingtheconductionband.Intheapproach,thea-Si:HnetworkisconsideredtoconsistofidenticalSi－Sibonds,asinc-Si..Inreality,theamorphousstructureintroducesadegreeofdisorderintothesystemsothatthereisarangeofbondlengths(△a)andbondangles(△θ)aroundthecrystallinecase.Fordevicequalitya-Si:H,itisfoundthat△aisabout2%ofthecrystallinebondlength(a)while△θisabout10%ofthecrystallinebondangle(θ).Thisleadstowhatisbestdescribedasa“continuousrandomnetwork”ofsiliconatoms.[1-7]Thebondingdeviationina-Si:Hperturbstheenergyofthebondingandanti-bondingstatesinaparticularbond.Therefore,whereastheedgesoftheconductionandvalencebandsarewelldefinedinc-Si,inthea-Si:Hthebandedgesaresmearedout,leadingtotheformationoflocalized“bandtail”electronstateswhichextendintothebandgap,decreasingexponentiallyasshowninFig.1.2.[1-8]Asthedeviationinlengthandangleofaparticularbondincrease,thebondwillbecomeweakerandtheperturbationoftheelectronenergylevelswillincrease.Therefore,thebandtailsmaybeassociatedwiththeweakestbonds,thewidthof.3.(18)thebandtailsbecomesameasureofthedisorderinthenetwork.ThewidthofthetailsisknownastheUrbachenergy(orUrbachslope)[1-8],andmaybedeterminedexperimentallyusingeithertheconstantphotocurrentmethod(CPM)orphotothermaldeflectionspectroscopy(PDS).Itisnotsurprisingthatifaparticularbondishighlydeformed,thenitbecomessoweakthatthebreakingofthebondtoformtwo“danglingbonds”(otherwiseknownasco-ordinationdefects)representsalowerenergy.Theenergyofthisstatewillbeatthe“bondingconfiguration”sp3levelshowninFig.1.1.Abondbetweentwosiliconatomsnormallycontainstwoelectrons,andso,afterbreaking,eachdanglingbondwillcontainoneelectron.Eachdanglingbondhasthepotentialtocontaintwoelectrons(onespin-upandonespin-down)andsois50%occupied.TheFermilevelis,bydefinition,theenergyatwhichthereisa50%probabilitythatanelectronstateisoccupied.Therefore,theFermilevelmustbeatthedanglingbondsp3energy.Thebondingandanti-bondingstatesdonotsplitevenlyaroundthesp3energy(Fermilevel),andinfacttheconductionbandisfoundtobeslightlyclosertotheFermilevelthanthevalenceband.Thisleadstothephenomenon,whichisobservedexperimentally,thatundopeda-Si:Hisslightlyn-type.[1-9]Forcomparison,undopedcrystallinesilicon,bydefinition,hasnodanglingbonds,andsotheFermilevelliesexactlyinthemiddleofthemobilitygap,andsoisgenuinelyintrinsicinnature.[1-6].4.(19)1.2Photoleakagecurrentmechanism[1-10]Thenumericalsimulationprogramsolvessimultaneouslythethreeequationsdescribingthesteadystateconductioninsemiconductors(Poissonequation,electronandholecontinuityequations),outofthermalequilibrium,usingtheShockleyReadmodelforthecarrierrecombinationrate.Theamorphoussilicondensity-of-statesiscomposedoftwoexponentialbandtailsandtwogaussiandistributionsofmonovalentstates,wheretherecombinationprocesstakesplace.Theprogramtakesintoaccountboththeinterface(a-Si/a-SiNandbackchannelinterfaces)andthebulkstatedensities.Thegateinsulatorisassumedtobeidea,whichmeansthatthereisnochargetrappingorfixedchargepresentintheinsulatorlayer.TheTFTisilluminatedfromthesource-drainside,andtheoriginalassumethatthewholea-Si:Hlayerisexposedtothelight.Fig.1.3isatransfercharacteristicsimulatedunderauniformillumination.TheanalysisofthesimulatedresultshasshownthattheoriginalcandistinguishboththeTFTelectronandholethresholdvoltages,thatdelimitthreemainTFToperatingregimes:(I):electronaccumulationlayercreatedatthea-Si:H/a-SiNinterface,(II):noaccumulationlayer,(III):holeaccumulationlayercreatedatthea-Si:H/a-SiNinterface.Intheregime(I),theTFTdraincurrentisassociatedwiththedrift-diffusionofelectronsintheaccumulationlayer(conductionchannel).Intheregime(II),theTFTdraincurrentresultsfromthesamemechanismthaninregime(I),buttheconductionoccursuniformlyinthewholeamorphous.5.(20)siliconlayer(channelandbulk),asthereisnocarrieraccumulationattheamorphoussilicon/siliconnitrideinterface.Becausethecarrierdensitiesinthewholeamorphoussiliconlayerdependmostlyonthelight-inducedgenerationandcarriersrecombinationrates,inthisregime,theTFTdraincurrentissensitivetotheamorphoussiliconthicknessandtothebulkdensity-of-statesofamorphoussilicon.Intheregime(III),theexistenceoftheholeaccumulationlayeratthea-Si:H/a-SiNinterfaceincombinationwiththen+a-Si:Hsource/draincontactlayers,resultsinthecreationoftwoPNjunctionlocatedatthesourceanddrainaccessareas.Forapositivedrainvoltage,astheoriginalhaveshowninFig.1.4,thesourceanddrainjunctionsarerespectivelyintheON-andOFF-state.Withintheregime(III),thereareactuallytwodifferentconductionregimesthatdependontherelativeimportanceoftwodifferentphysicalmechanisms.Inregime(III.a),thedrift-diffusionofholeintheaccumulationlayer(conductionchannel)ispredominant,whereasinregime(III.b)therecombinationofelectronsandholesinthedrain(OFF-state)PNjunctiondepletionregionisthemostimportant.TheoriginalhaveplottedFig.1.5thesimulatedvariationsoftheelectronandholeFermipotentialsalongtheusualTFTcurrentpathbetweenthesourceanddraincontacts.Wecanseethat,intheregime(III.a)(VG=-7.5V),themainvariationoftheFermipotentialhappensintheaccumulationlayer,wherethedrift-diffusionofholesoccurs.TheconductioninthisregimeissimilartotheconductionwhentheTFTisinaccumulationregime(gatevoltagelargerthantheelectronthresholdvoltage);however,inthisregime,themajoritycarriersaretheholesinsteadoftheelectrons.Onthecontrary,intheregime(III.b)(VG=-20V),themainvariationoftheFermipotentialsoccursinthedrainPNjunctiondepletionregion,wheretherecombinationofelectronsandholesistakingplace.Inthiscase,theTFTcurrent6.(21)dependsonlyonthelight-inducedgenerationandrecombinationofelectronsandholesinthedrainPNjunctiondepletionregion.Actually,forhighnegativevoltages,therecombinationratecanbeneglectedandtheTFTcurrentisthereforeonlysetbythelight-inducedgenerationofelectronsandholes(asinaphotodiodecase).Inparticular,inthisregime,thereisnolongeranyinfluenceofthegatevoltage(saturationphenomenon)orchannellengthontheTFTdraincurrent.TheparametersthatinfluencetheTFTdraincurrentdependontheTFToperationregimetobeconsidered.Theanalysisoftheoriginalsimulatedresultshasshownthat:Theelectronandholethresholdvoltagesdependmostlyontheamorphoussilicondensity-of-statesassociatedwithboththea-Si:H/a-SiNinterfaceandwiththebulkthedifferencebetweenthetwothresholdvoltagesishigherforalargerdensity-of-states.Intheregime(II),theTFTdraincurrentdependsstronglyontheamorphoussiliconthickness,butalsoonthea-Si:Hbulkdensity-of-states:itincreaseswiththea-Si:Hthicknessanddecreaseswiththea-Si:Hdensity-of-states.Thedraincurrentintheregime(III.b)dependsonlyonthelight-inducedgenerationofelectron-holepairsinthedrainPNjunctiondepletionregion(betweenthedraincontactandthea-Si:H/a-SiNinterface):itincreaseswiththeilluminationintensityandwiththePNjunctionarea.TherelativeimportanceofthetwomainmechanismsinvolvedintheTFTselectricalinstabilities(modificationofthea-Si:Hdensity-of-statesandchargetrappingintheinsulatorlayer)isnotyetperfectlyaccepted.Ingeneral,thechargetrappingintheinsulatorlayerresultsinvariationsofboththeelectronandtheholethresholdvoltagesinthesamedirection,whilemodificationsoftheamorphoussilicondensity-of-statesresultsinvariationsofthethresholdvoltagesinoppositedirection.7.(22)Moreprecisely,anincreaseinthebulkdensity-of-stateswillresultmainlyinashiftoftheelectronandholethresholdvoltagestowardmorepositiveandnegativegatevoltagesrespectively,andinareductionoftheTFTdraincurrentintheregime(II).Theinterfacedensity-of-stateswillalsoaffectthethresholdvoltages.Butintheregime(II),thewholea-Si:Hlayerisinvolvedintheconduction;thereforetheinfluenceofinterfacedensity-of-statesonthedraincurrentisnotsignificantincomparisontotheinfluenceofbulkdensity-of-states..8.(23)1.3Somesolutionsforreducingphotoleakagecurrenta-Si:Hhashighphotoconductivitywhichresultsinahighoff-stateleakagecurrentforana-Si:HTFTunderbacklightillumination[1-2].Theoff-stateleakagecurrentcanbeloweredbyreducingthethicknessofundopeda-Si:H,however,thisalsodecreasesthefieldeffectmobilityoftheTFT.[1-11]Theoff-stateleakagecurrentofa-Si:HTFTismainlyduetoholesinducedatthea-Si:Hinterfacetoagateinsulator.However,underlightillumination,electronsarethemajoritycarrierswhenanegativegatevoltageisappliedtotheTFTbecauseelectronmobilityismuchhigherthanthatofhole.[1-12]Theoff-stateleakagecurrentofa-Si:HTFTunderlightilluminationisrelatedwithitsphotoconductivity.Thephotoconductivityofa-Si:H(:Cl)isatleasttwoordersofmagnitudelowerthanthatofundopeda-Si:H.[1-13]Recently,Clincorporatedhydrogenatedamorphoussilicon[a-Si:H(:Cl)]hasbeenpreparedbyvariousdepositionmethodsusingSiH2Cl2mixturestoimprovefilmquality,[1-14]stability[1-15]ortoincreasedepositionrate.[1-16]However,theperformanceofthea-Si:H(:Cl)TFTswasfoundtodegradewithincreasing[SiH2Cl2]/[SiH4]ratiowhichwasusedtodepositthea-Si:H(:Cl).[1-17]Thea-Si:H(:Cl)filmsshowp-typeconduction,leadingtolowerphotoconductivity.Theoff-statedarkleakagecurrentofa-Si:H(:Cl)TFTsislowerthanthatofa-Si:HTFTs,whichisduetothepositionoftheFermilevelofa-Si:H(:Cl).TheFermilevelofa-Si:H(:Cl)thatexistsislowerthanthatofa-Si:H.But,withincreasing[SiH2Cl2]/[SiH4],thefieldeffectmobilitydecreasesslightlyandthethresholdvoltageincreases.Theincreaseinthethresholdvoltagemaybeduetotheincreaseinthedefect9.(24)densitybyClincorporationand/orduetotheshiftoftheFermilevelinthebandgaptowardthevalencebandedge.Thedensityofstatesinthegapofa-Si:H(:Cl)willbemodifiedbytheintroductionofClintoa-Si:H,resultingintheshiftoftheFermileveltowardthevalencebandedge.TheFermilevelofa-Si:Hisdeterminedfromthechargeneutralitycondition.Theacceptor-likestatesareextendedfromtheconductionbandedgeandthedonor-likestatesfromthevalencebandedge;danglingbondsexistaroundmidgap.Ofthese,somestatesarechargedpositivelyandotherstatesarenegativelychargedbythesameamount.ThepositionoftheFermilevelcanbeloweredbyareductionofdonor-likestatesorbyanincreaseofacceptor-likestates.ThemodificationinthedensityofthestatesinthegapbytheincorporationofClatomsina-Si:HshouldbestudiedinthefutureinordertounderstandtheoriginoftheshiftoftheFermileveltowardthevalencebandedgebyClincorporation.TheshiftintheFermilevelleadstoareductionofthephotoconductivityofa-Si:H(:Cl).Thephotoconductivityofa-Si:HisstronglyrelatedtothepositionoftheFermilevel.[1-18]Theoff-stateleakagecurrentofana-Si:HTFTunderlightilluminationisrelatedtoitsphotoconductivity,sothephotoconductivityforthea-Si:H(:Cl)filmswasinvestigated.Thephotoconductivityofa-Si:H(:Cl)isatleasttwoordersofmagnitudelowerthanthatofintrinsica-Si:H.ThepositionoftheFermilevelfora-Si:H(:Cl)filmliesbelowthemidgap,sothatthea-Si:H(:Cl)showsp-typebehavior.Itshouldbenotedthatthephotoconductivityofp-typea-Si:Hismuchlowerthanthatofn-typea-Si:Hbecausethemobilityofelectronsismuchhigherthanthatofholes..10.(25)ChapterTwo-Fabrication2.1DepositionofHydrogenatedAmorphousSiliconbyPECVDThea-Si:HandSiNxutilizedinTFTarrayfabricationarepreparedbyPECVD.TheTFTperformancedependsonthepreparationconditions,suchassubstratetemperature,RFpower,andgasdilution.Oxygenincorporationduringa-Si:HdepositioninparticularhasbeenfoundtogreatlydegradeTFTperformance,becauseitincreasesthedefectdensityintheundopeda-Si:H.Hydrogenatedamorphoussiliconhasashort-rangeorder,whichmeansthatthecoordinationnumber,bondangle,andbondlengthareclosetothoseforasinglecrystallineSiwithin2or3atomicdistances,butthereisnoperiodicityinthelongrange.Becauseofthisthereareaconsiderablenumberoflocalizedstatesinthegap.Thehydrogeninthea-Si:Hreducesthedanglingbondsbypassivation.Therefore,thedanglingbonddensityof~1020cm-3invacuum-evaporateda-Siorsputtereda-Siisreducedto1015~1016cm-3inPECVDa-Si:H,wherehydrogencontentis10-30at.%.Thehydrogenreducesthetail-statedensityinadditiontothereductionofdanglingbonds,becausethedisorderisdecreasedbyhydrogenincorporation.Typically,undpoeda-Si:HforTFTapplicationsispreparedatasubstratetemperatureof220-350℃.Thehydrogenina-Si:HmaybeincorporatedasSiHorSiH2;however,onlyfilmswithhydrogenbondedasSi-HaresuitableforTFTapplication.[2-1]PrecursorgasesforPECVDdepositionofa-Si:HmaybeSiH4orSi2H6,withH2,He,and/orArbeingusedascarriers/diluentstodecreasethedensityoflocalizedstatesormodifythematerialintoμc-Si.Theμc-Siisdepositedby11.(26)PECVDundersimilardepositionconditionsusedfora-Si:H,exceptwithhighdilutionofsilanewithhydrogen.Thegrainsizeisusuallylessthan50nm,andfilmshowsacolumnargrowth.Becausethegrainsizeisquitesmallandtherearealotofdefectsinsidethegrainsthefield-effectmobilityofμc-Siiscomparabletothatofhigh-qualitya-Si:HTFTs.Moreover,theoff-statedraincurrentismuchhigherthanthatofa-Si:HTFTbecauseofitshighconductivity(＞10-6S/cm)andhighdensityofdefects.Thehydrogen/silanedilutionratioistypicallyhigherthan30forthegrowthofμc-Si,andtheRFpowerisalsoanimportantparameter.TheroleofhydrogenduringthedepositioninthePECVDchambercanbesummarizedasfollows:1.HydrogenatomscoverthegrowingsurfaceandincreasethediffusionlengthoftheSiprecursors.Therefore,theprecursorscanmigratetoamorestableposition.[2-2]2.AtomichydrogendiffusesintothesiliconnetworkdowntoafewnanometersandthusenhancestherelaxationofSiatoms,leadingtothemorestablestructure.[2-3]3.AtomichydrogenetchestheweakSi-SibondsandthusmorestableSi-Sibondsareformed.[2-4]Theprecursorfora-Si:HdepositionisSiH3.[2-5]Filmsofa-Si:HdepositedbyPECVDatlowerRFpowerhavebetterstepcoverageandlowdefectdensity,whereasa-SifilmsdepositedathigherRFpowergenerallyhavemoredefects.Particlescanbeformedbyplasmapolymerizationofradicalsand/orionsinthePECVDchamber.Inaddition,theplasmapotentialduringdepositioncancauseiondamageofthegrowingfilm,whichisespeciallyaproblemathighRFpower.[2-6]Amongthevariousdepositioncontrolvariables,suchasgasflowrate,gaspressure,RFpower,andsubstratetemperature,thehydrogencontentofthefinal.12.(27)filmisgreatlyaffectedbythesubstratetemperature.Thehydrogencontentdecreaseswithincreasingsubstratetemperaturebecauseoftheenhancedout-diffusionofhydrogenfromthefilm.Therefore,theopticalbandgapdecreaseswhenincreasingthesubstratetemperature,sincetheopticalbandgapincreaseswithhydrogencontentinthea-Si:H.Thedepositionrateincreasewiththegasflowrate,butathighflowratesthereismoregas-phase,plasmapolymerization.Attoolowaflowrate,moreiondamageisexpectedbecauseoftheincreasedplasmapotential.Therefore,anoptimumintermediateflowrateisimportantforthedepositionofdevice-qualitya-Si:H.Duringa-Si:Hdeposition,theprecursorarrivingatthegrowingsurfaceismainlySiH3,anditbondswiththesurfaceatoms,resultinginthedepositionofa-Si:Handout-diffusionofhydrogen.TheproductscanbeH2and/orSiH4.Depositionoccursbyheterogeneousreactionatthegrowingsurface[2-7],andionbombardmentofthesurfacecanaffectthefilmproperty.IncreasingtheRFpowerdecreasesthesurfacediffusionlengthofprecursorradicalsandincreasesthestickingcoefficienttothesubstrate.Table2.1showsthephysicalpropertiesofdevice-qualitya-Si:HusedforTFTfabrication..13.(28)2.2DepositionofSiNxbyPECVDPlasma-depositedsiliconnitride,SiNx,isusedforthepassivationofelectronicdevicesandasthegateinsulatorofchoicefora-Si:HTFTs.Asapassivationfilm,siliconnitrideprotectsagainstthediffusionofwatervapor,sodium,andoxygenintotheactivedevice.Thehydrogencontentofplasma-depositedsiliconnitrideis10-40at.%,andmostofthehydrogenatomsarebondedasSi:Hand/orN-H,dependingonthepreparationconditions,suchasRFpower,feedinggas,andsubstratetemperature.TotalhydrogencontentdecreaseswithincreasingsubstratetemperatureorincreasingRFpower.[2-8]Siliconnitridehasanamorphousstructure,andpropertiesdependontherelativeatomicconcentrationsofsilicon,nitrogen,andhydrogen.Forgood-electrical-qualitya-Si:HTFTs,PECVD-depositedSiNxismuchmoresuitablethanstoichiometricSi3N4.Siliconnitridedepositedat300-350℃(abbreviatedasSiNxorSiN:H)isquiteadifferentmaterialfromSi3N4producedbyCVDat700-900℃.[2-9]Hydrogeninsaturatingthetraps,sothedefectdensityismuchlessthanthatofaCVDSi3N4.Theinterfacechargedensitybetweena-Si:HandSiNxistypicallyintherangeof2×1011to7×1012eV-1cm-2,anditstronglydependsonthedepositionconditions:itincreaseswithdecreasingsubstratetemperatureordecreasingRFpower.NotethatthetrapdensityinSiNxincreasesonheatingabove400℃,becausethehydrogenisout-diffusedasaresultofbreakageofhydrogenbondsinSi-Hand/orN-Hmodes.[2-10]Theabilitytoformagoodinsulatingfilmatlowtemperature(lessthan350℃)havingalowinterfacestatedensitywitha-Si:H(~1011eV-1cm-2)makesSiNxa14.(29)goodgateinsulator.Inaddition,theabilitytoemploythesamePECVDequipmentusedtodepositthea-Si:Hisanimportantadvantage.SiNxistypicallydepositedfromamixtureofSiH4,NH3,N2,andHeat300-350℃.TheRFpoweristypicallyhigherthanusedfora-Si:Hdeposition,andthebestmaterialfortheTFTsisaN-richSiNx.[2-11]Thefield-effectmobilityinthelinearregionisofprimeimportanceforpixelcharginginTFT-LCDapplication.TheinterfacebetweenSiNxanda-Si:Haffectsthefield-effectmobility;inparticular,thesurfaceroughnessoftheSiNxlayerisimportantbecauseitaffectstheinitialgrowthofa-Si:H,whichformstheactivechannellayerfortheTFT[2-12]..15.(30)2.3Depositionofn+HydrogenatedAmorphousSiliconbyPECVDDepositionofann+-a-Si:Hlayerbetweenundopeda-Si:Handametalallowstheformationofanohmiccontactbetweenthem.Theohmiccontactalsoactsahole-blockinglayerbecauseitactstodepressthepositionofthecontactingsemiconductorvalencebandwithrespecttotheFermilevelofthemetal.Theresistivityofn+-a-Si:Hisabout100Ω-cm,whichismuchhigherthanthatofn+crystallinesilicon.Eventhoughtheresistivityisrelativelyhigh,thecurrentisnotgenerallylimitedbythen+contactitself,becausethedraincurrentisontheorderofmicroamps.FortypicalAMLCDTFTchannellengths(~10µm)andforthicknessofa-Si:Hbelow~100nm,theuseofn+poly-Siorn+µc-Sicontactlayersdosenotappreciablyincreasethedraincurrentoverthatofstandardn+-a-Si:Hcontact.Inmostcases,n+-a-Si:HisdepositedbyPECVDusingasilanemixturecontaining~1%PH3.PECVDdepositionusingthisdopinggasmixturedilutedataratioof~1:50inhydrogenallowstheformationofn+µc-Si.Addingmorethan~1%PH3tosilanedoesnotfurtherdecreasetheresistivityofn+-a-Si:H,becausethedefectdensityofthematerialbeginstogrowinproportiontotheaddeddopant,negatinganyfurtherenhancementofthefreecarrierpopulation.[2-13].16.(31)2.4ProcessFlowInverted-staggereda-Si:HTFTswithback-channel-etched(BCE)process,werefabricatedonglasssubstrateforthestudyofelectricalcharacteristics,whereAlisusedassource/drainmetalasshowninFig.2.1.Thedevicefabricationprocesswasdescribedasfollowed.Aftera3000-Å-thickCrgateelectrodewaspatternedontheglasssubstrate,a3000-Å-thicksilicon-nitride(SiNx)layer,a2000-Å-thicka-Si:H(:F)activelayeranda500-Å-thickn+-a-Si:Hwerecontinuouslydepositedbyplasmaenhancedchemicalvapordeposition(PECVD)method.Theundopeda-Si:H(:F)wasdepositedwithagasmixtureof9sccmSiF4and50sccmSiH4at200°C.Then+-a-Si:HlayerintheTFTchannelregionwouldtobeetchedoffusingthesource/drainpatternasamask,aftertheelectrodesareformedforTFTs.Inordertodecreasethedevicedegradationofa-Si:H(:F)layer,thea-Si:H/a-Si:H(:F)doublechannellayerstructurewasalsofabricatedforthisstudy.Thethicknessofa-Si:Handa-Si:H(:F)indoublelayerstructurewere40nmand160nmrespectively.ThechannellengthofTFTdevicesvariedfrom5to16µmandthechannelwidthwaskeptconstant24µm.Similarly,theconventionala-Si:HTFTwithoutgasmixtureofSiF4werealsofabricatedasareferencesample.ThecharacteristicsoftheamorphousSiTFTsdemonstratedthefiled-effectmobility~0.1cm2/V-s,theminimumsubthresholdswing~1.08V/dec,thethresholdvoltage~1.95Vdefinedatconstantcurrent10nA,andtheION/IOFFratio~106atVDS=10V.Theleakagecurrentthroughgateinsulatorislessthan10-13AThephotoleakagecurrentmeasurementwascarriedbylightilluminationtocomparethedifferenceintheoff-statephotoleakagecurrentsbetweenthea-Si:H(:F)TFTandtheconventionalone..17.(32)ChapterThree-ApparatusandParameters3.1ApparatusList3.1.1Theroomtemperatureandhightemperature(1)Microscope、Hotchuck、Probestation,asshowninFig.3.1.(2)AgilentB2201A(Switch)andKeithley4200-SCS,asshowninFig.3.2.(3)TemperaturecontrollerasshowninFig.3.3..3.1.2Thelowtemperature(1)Agilent4284A、Agilent4156C、AgilentE5250A(Switch)Agilent41501B(highpower),asshowninFig.3.4.(2)CryogenicsSystem--TTP-6ProbeStation,Fig3.5..3.1.3.Softwareofmeasurement.ICS(InteractiveCharacterizationSoftware)UsetheSoftwaretoget:1.VD-ID2.VG-ID(LinearRegionVD=1V,3V)3.VG-ID(SaturationRegionVD=10V)4.VG-GmNote：.VD:DrainVoltage.ID:Draincurrent.VG:GateVoltage.IG:Gatecurrent.Gm:VG-IDmaxslope.18.(33)3.2SetupinstrumentsforCurrent-Voltage(I-V)Measurement.3.2.1TheroomtemperatureandhightemperatureThecurrent–voltagecharacteristicmeasurementofthinfilmtransistordeviceswasperformedby4200-SCSsemiconductorparameteranalyzerwithsourcegroundedandbodyfloating.Theelectricaltestsetupof4200-SCSsemiconductorparameteranalyzer,illustratedatFig.3.2,aprobestationsituatedinsideadarkbox.Thegroundprobestationisfurnishedwithanelectricallyisolated,water-cooledthermalchuck.ThechuckiscontrolledbyTEMPTRONICTPO315Athermalcontroller,whichcanoperatetemperaturefrom25℃to300oC.AnKeithley4200-SCSprecisionsemiconductorparameteranalyzerprovidesI-Vmeasurement,biasforBTS.The4200-SCSareconnectedtoE5250Alowleakageswitchmainframe,andthenlinktodarkbox.Thecurrent-voltage(I-V)characteristicsmeasurementsweregottenbyusingp-TFTstructurewithKeithley4200-SCSprecisionparameteranalyzer.Keithley4200-SCScanmeasuretheminimumleakagecurrent:1f(A)..3.2.2ThelowtemperatureThesystemisillustratedinFig.3.5.ThephotoshowstheTTPwiththe.19.(34)microscopeandcamerainstalled.TheremainingcomponentsnecessarytorunthesystemareconnectedtotheTTP.Tousethesystem,samplesareplacedinsidetheTTPchamber,thesystemisevacuatedwiththeturbovacuumsystem,andthesampleiscooledwithacryogen.Probesaremovedintoplaceonthesamplewhileobservingwiththemicroscope.Measurementsofsamplepropertiesarethenmadeviatheprobes[3-1].Whenthesystemisnotused,thechamberneedstobekeptthevacuum.Wemustventthechamberwithnitrogengastoloadasamplebeforemeasurement.Afterthat,sealthechamberandevacuateitusingturbopumpuntiltheIG(iongauge)readoutonthevacuumsystemis7.6E-5.InsertthebothendsofthetransferlineintoaDewarandthebayonetontheTTP,respectively,andsetthedesirabletemperatureonthetemperaturecontroller.Starttocoolthesampledowntothedesirablelowesttemperatureandperformtheexperiment.Weutilizemicroscopetoviewandmakemeasurementsofsampleviatheprobes.Then,enteringasetpointonthetemperaturecontrollertowarmthesystem,getbacktotheroomtemperatureorevenhightemperature.Afteraccomplishingtheexperiment,wemustmakethechambertemperatureatroomtemperaturebeforeventingit.Finally,unloadthesampleandkeepthechamberatvacuumstate.TheelectricaltestsetupofHP4156Csemiconductorparameteranalyzerisutilizedinthisexperiment,illustratedatFig.3.4.Thesameas4200-SCS,theAgilent4156CprecisionsemiconductorparameteranalyzercanprovideI-Vmeasurement,biasforBTS.WeemploytheICS(InteractiveCharacterizationSoftware)toobtaintheoutputandtransfercharacteristics,likeVD-ID,VG-ID(Linear),VG-ID(saturation),andextractthetypicalsemiconductorparameters..20.(35)3.3MethodofDeviceParameterExtractionInthissection,wewillintroducethemethodsoftypicalparameterextractionsuchasthresholdvoltage,subthresholdswing,field-effectmobilityµFE,channelresistanceandtheparasiticresistancefromdevicecharacteristics..3.3.1DeterminationofthethresholdvoltagePlentymethodsareusedtodeterminatethethresholdvoltagewhichisthemostimportantparameterofsemiconductordevices.ThemethodtodeterminatethethresholdvoltageinthisthesisistheconstantdraincurrentmethodthatthevoltageataspecificdraincurrentINistakenasthethresholdvoltage.ThistechniqueisadoptedinmoststudiesofTFTs.Itcangiveathresholdvoltageclosetothatobtainedbythecomplexlinearextrapolationmethod.Typically,thethresholdcurrentIN=ID/(Weff/Leff)isspecifiedat10nAforVD=-0.1Vand100nAforVD=-15Vinmostpaperstoextractthethresholdvoltageof.TFTs..3.3.2Determinationofthesub-thresholdswingSubthresholdswingS.S(V/dec)isatypicalparametertodescribethecontrolabilityofgatetowardchannel.Itisdefinedastheamountofgatevoltagerequiredtoincrease/decreasedraincurrentbyoneorderofmagnitude.Thesubthresholdswingshouldbeindependentofdrainvoltageandgatevoltage.However,inreality,.21.(36)thesubthresholdswingmightincreasewithdrainvoltageduetoshort-channeleffectssuchaschargesharing,avalanchemultiplication,andpunchthrough-likeeffect.Thesubthresholdswingisalsorelatedtogatevoltageduetoundesirablefactorssuchasserialresistanceandinterfacestate.Inthisexperiment,thesubthresholdswingisdefinedasone-secondofthegatevoltagerequiredtodecreasethethresholdcurrentbytwoordersofmagnitude.Thethresholdcurrentisspecifiedtobethedraincurrentwhenthegatevoltageisequaltothethresholdvoltage.[3-2].3.3.3Determinationofthefield-effectmobilityThefield-effectmobility(µFE)isdeterminedfromthetransconductancegmatlowdrainvoltage.Thetransfercharacteristicsofa-SiTFTsaresimilartothoseofconventionalMOSFETs,sothefirstorderI-VrelationinthebulkSiMOSFETscanbeappliedtothea-SiTFTs,whichcanbeexpressedasID=µFECox.Where.W12[(VG−VFB)VD−VD]L2.(3-1).Coxisthegateoxidecapacitanceperunitarea,Wischannelwidth,.Lischannellength,.VTHisthethresholdvoltage.IfVDismuchsmallerthanVG-VTH(i.e.VD<VTH,thedraincurrentcanbeapproximatedas:ID=µFECox.W(VG−VFB)VDL.(3-2).Thetransconductanceisdefinedas.22.(37)gm=.∂ID∂VG.VD=const..=.WCoxµFEVDL.(3-3).Therefore,thefield-effectmobilitycanbeobtainedbyµFE=.LgmCoxWVD.(3-4).Themobilityvaluewastakenfromeq.3-4withmaximumµFE..23.(38)3.3DensityofStates3.4.1OverviewThedensityofstates(DOS)inthemobilitygapofa-Si:Hhasbeenextensivelystudiedusingdifferentexperimentaltechniquessuchasfield-effectmeasurement[3-3]-[3-4],measurements[3-5],(DLTS)[3-6]..In.transient.andaddition,.and.deep-levelother.steady-state.photoconductivity.transient-capacitance.methods.such.as.spectroscopy.capacitance-voltage.characteristics(C-V)[3-7],anddependenceofcapacitanceontemperatureandfrequency.in.Schottky.diodes.and.metal-oxide-semiconductor.(MOS).structures[3-8]arealsousedforstudyoftheDOSina-Si:H.Basedontheseexperimentalstudies,itisdemonstrated[3-9]-[3-11]thatthedistributionofthelocalizedstatesina-Si:Hmobilitygapmaybemodeledbyexponentialdistributionsofdeepandtailstatesforbothacceptor-likeanddonor-likestates(seeFig.3.6).Thelocalizedstatesintheupperhalfofthemobilitygapclosertotheconductionbandedgebehaveasacceptor-likestates,whilethestatesinthelowerpartofthegapclosertothevalencebandedgebehaveasdonor-likestates.Acceptor-likestatesareneutralwhenemptyandnegativelychargedwhenfilledwithanelectron,whereasdonor-likestatesarepositivelychargedwhenemptyandneutralwhenfilledwithanelectron.BasedonthisexponentialDOSmodel,thedensityoftheacceptor-likestatesgA(E)asafunctionofenergyEmaybewrittenasfollow:.24.(39)−⎛g(E)=gexp⎜⎜EE⎝EA.tc.tc.C.⎞⎟+gexpdc⎟⎠.⎛E−EC⎞⎜⎟⎜⎟⎝Edc⎠.(3-5).whereECistheconductionbandedge,gtcandgdcthedensitiesofstatesattheconductionbandedgeforthetailanddeepacceptor-likestates,respectively,andEtcandEdctheassociatedslopeoftheexponentialdistributionofthetailanddeepacceptor-likestates,respectively.Similarly,thedensityofdonor-likestatesgD(E)maybewrittenasfollows:⎛E−E⎞⎛E−E⎞⎜⎟⎜⎟()=+gEgexp⎜⎟gexp⎜⎟V.D.tv.⎝.E.V.tv.⎠.dv.⎝.E.dv.⎠.(3-6).whereEVisthevalencebandedge,andparametersgtv,gdv,Etv,andEdvaresimilarlydefinedfortheexponentialdistributionofthetailanddeepdonor-likestates.Typicalvaluesoftheseparametersforintrinsica-Si:HarepresentedinTable3.1.AscanbeseenfromFig.3.6,theDOSisasymetricalina-Si:H,i.e.,thenumberofdonor-likestatesinthemobilitygapishigherthanthenumberofacceptor-likestates.Asaresult,followingtheneutralitycondition,thepositionoftheFermienergyinanintrinsica-Si:Hsampleinthedark(Ei)isclosertotheconductionbandedge.ThepositionoftheintrinsicFermienergyis~600mVbeneaththeconductionbandedgeandisdependentonthetemperatureduetotheasymmetricalDOSdistribution[3-2][3-12]..3.3.2.Evaluationofthedensityofstates--Activationenergymethod[3-13].Asitwasalreadymentioned,theFermilevelshiftwiththegatevoltageisstronglydependentonthedensityofstates(DOS).AthighdensityofstatesmorecarriersmustbeinducedinordertofillthestatesfromEFupwardanditisnecessarytoapplyhighergatevoltageinordertoinducemorecarriersinthe25.(40)channel.Onthecontrary,whenthedensityofstatesislow,thestatesfromEFupwardareeasilyfilledatlowconcentrationoftheinducedchargeandtheFermileveliseasilyshiftedatlowgatevoltages.ThiscorrelationbetweentheDOSandthegatevoltageallowstoobtaintheshapeofthedensityofstatesbystudyingthedependenceofEactvs.VGS.TheinformationonDOSshapeisimportantforunderstandingthephysicalmechanismsresponsibleforthedevicebehaviour.TheDOSshapeisrelatedtothethresholdvoltagevalue,subthresholdslope,fieldeffectmobilityandthestabilityoftheTFTs.Globusetal.[3-14]proposedamethodforevaluationofDOSina-Si:HTFTs,fromthedependenceofEactvs.VGS.IfitisassumedthattheDOSdoesnotsuffersharpchangesforenergyintervalaboutkBT,thechargeofacceptor-likestatesQt,filledbythegatebiasisgivenby(3-7)whereqistheelectroniccharge,Vsisthesurfacepotential,EFoistheequilibriumFermilevelinthesiliconlayerg(E)isthedensityofstates.ThechargeQtcanalsobeexpressedas(3-8)whereqntisthesurfacecharge,VFBistheflat-bandvoltage,εianddiarethegatedielectricpermitivityandgatedielectricthickness,respectively,anddtisthethicknessofthespace-chargelayer.Permeasuringthedrain-sourcevoltageatdifferenttemperaturesandkeepingthesamedrain-sourcecurrent,theactivationenergy(i.e.theFermilevelposition)canbedeterminedfromtheslopeoftheArrheniusplot(log(IDS)vs.1000/T)bythefollowingequation:[3-15].26.(41)(3-9)Fromeqs.(3.7)and(3.9),differentiatingwithrespecttoVGS,canbeobtained(3-10)whereEact=EC-EFo-qVsistheactivationenergy,EF=EFo-qVsisthequasiFermilevel.Hence,thedensityoflocalizedstatescanberelatedtothederivativeoftheactivationenergywithrespecttogatebias:.(3-11).Ifitisassumedthatthebandbendinginthea-Silayerissmallcomparedtothecharacteristicenergyofthedensityofstatesvariation,thendt≈twheretisthea-Silayerthickness,andeq.(3.10)reducesto(3-12).Thismethodfordeterminationofthedensityofstatesisexplainedindetailsin[3-14].Accordingtoref.[3-14]thistechniqueonlyaccountsfortheacceptor-likestatesinthebandgap.Advantageofthemethodisitssimplicity.Itisnecessarytoperformonlyfield-effectmeasurementsatdifferenttemperatures.Usingthismethod,thedensityofstatescanbeevaluatedinrelativelylargeenergyintervalfromthebandgap.Itissuitableforevaluationofchangesinthedensityofstatesduetobiasstress.Eq.(3.12)wasemployedtocalculatethedensityofstatesinthecomplete.27.(42)devices(withincorporatedn+layer)accordingtotheabove-mentionedassumptions.AsthemeasurementsofEactinthesimplifiedsampleareaffectedbytheabsenceofn+contactlayer,wehavenotperformedanalysisofDOSforthissample.Fromtheexperimentallymeasuredactivationenergy,depictedinFig.3.7,weestimatedthedensityoflocalisedacceptor-likestatesintheupperpartofthebandgap-intheinterval0.34to0.15eVbelowEC.Thecalculateddensityofstates,inenergyinterval0.15-0.33eV,ispresentedinFig.3.8.Thepeakofthedeepbandgapstatescanbeobservedat0.34eV.TheFermilevelispinnedinthesedeepstateswhentheTFTisinoff-state.From0.30to0.20eVtheshapeoftheacceptor-likestatesisnearlyconstant17.-3.-1.about5⋅10cmeV.TheFermilevelisshiftedthroughthesestatesinthesubthresholdregionofoperationoftheTFT.From0.20eVbeginstheexponentialincreasingoftheacceptor-liketailstates.TheFermilevelispinnedat0.15eVfromECwhentheTFTentersintheon-state.ThisshapeoftheDOSisverysimilartotheobtainedfortheamorphous17.siliconTFTsbyGlobusetal.[3-14].Inaddition,thecalculatedvalueof5⋅10-3.-1.cmeVfor0.30to0.20eVisslightlylowerthanthevalueofthemaximaldensity18.-3.-1.ofdeepbulkstates(Nbs)max=3.6⋅10cmeVestimatedfromthesubthresholdslope.Thisisbecause(Nbs)maxwascalculatedassuming(Nss)max=0Theactivationenergymethodisfastandsimpleandissuitabletoobservethechangesinthedensityofstatesduetobiasstress,illumination,etc.Themaindisadvantageofthismethodandalsoofthemethodofthesubthresholdslopeisthattheydonotpermittheseparationofthebulkstatesfromtheinterfacesatesatthechannel/gateinsulator.Bothmethodsarebasedonfield-effectmeasurements.28.(43)thatarestronglydependentonthequalityoftheinterfacebetweenthechannelmaterialandthegateinsulator.ThisdoesnotpermittheexactevaluationoftheintrinsicDOSofthechannelmaterial(inourcasenc-Si:H),independentlyfromthepropertiesofthisinterface..29.(44)ChapterFour.ExperimentsResultsandDiscussion.4.1Overviewoftheexperimentssample4.1.1CharacteristicsBecausea-Si:Hisaphoto-sensitivematerial,themainobjectivesforflatpaneldisplayapplicationaretoenhancethefieldeffectmobilityandtoreducetheoff-stateleakagecurrentunderlightillumination.[4-1]However,thehigheroff-stateleakagecurrentunderlightilluminationcomparedtoaconventionalTFThasbeenobserved.[4-2]Asaresult,thereductionofoff-statephotoleakagecurrentina-Si:HTFTisveryimportantforTFT-LCDstechnology.Thethin-filmtransistorusingafluorineincorporatedamorphoussilicon[a-Si:H(:F)]andamorphoussilicon(a-Si:H)stackedactivelayer,inwhichconductionchannelisformedina-Si:Handa-Si:H(:F).Thea-Si:H(:F)TFTwithbacklightilluminationthethresholdvoltagedecreaseandfield-effectmobilityincreaseslightly.Thethresholdvoltagedecreaseresultintheoncurrentincrease.Withincreasing[SiF4]/[SiH4]ratio,thefield-effectmobilitydecreasesandthethresholdvoltageincreases.TheincreaseinthethresholdvoltagemaybeduetotheincreaseinthedefectdensitybyFincorporationandtheshiftofFermilevelinthebandgaptowardthevalencebandedge.[4-3]Thephotoconductivityofa-Si:HisrelatedtothepositionoftheFermilevel,sotheshiftintheFermilevelleadstoareductionofthephotoconductivity[4-4].Thea-Si:H(:F)TFTwithFincorporationhasshownthelargerthresholdvoltageandlowerfieldmobilitythanconventionalcounterpart.Inthea-Si:H(:F)channelthedensityofthedonor-likedefectstatesisreducedthanthedensityoftheacceptor-likedefectstates[4-5]-[4-6].Also,thea-Si:H(:F)TFTbehavesasasimilarp-typepropertycomparedwiththea-Si:HTFT,andresultsinthelittleelectrical30.(45)degradationwithFincorporation.ThecorrelationbetweentheDOSandthegatevoltageallowsobtainingthedistributionofthedensityofstatesbystudyingthedependenceofEavs.VG.Globusetal.[4-7]TheFermilevelisshiftedthroughthesestatesduringtheoperationofTFTdevicesinthesubthresholdregion.Theadditionaldeepstateshavebeenfoundinthea-Si:H(:F)TFT.Theseadditionaldeepstatesina-Si:H(:F)TFTtherebyresultedinlowerphotoleakagecurrentcomparedtothea-Si:HTFT,andillustratedbytheinsetofFig.4.1.Theincreaseofacceptor-likedeepstateina-Si:H(:F)materialleadstoslightlyelectricaldegradationona-Si:H(:F)TFT,andresultsintheshiftdownofFermilevel.[4-1]Therefore,increasing[SiF4]/[SiH4]ratio,theacceptor-likestatesincreasetomakemorerecombinationcentersforelectronsandholes.Morerecombinationcentersforelectronsandholescanincreasethecarriers’recombinationrate,sothephotoleakagecurrentdecreasesThephotoleakagecurrentalsorelatedtoitsphotoconductivity.Theacceptor-likestatesincreasecausesthepositionoftheFermilevelfora-Si:H(:F)filmliesbelowthemidgap,sothatthea-Si:H(:F)showsp-typebehavior.Duetothesetwoimportantfactor,thephotoleakagecurrentdecreases..4.1.2.Relativeparameters.AfteracompleteTFTmanufactureprocess,thecharacteristicsoftheundopedamorphousSiTFTs(a-Si:H)demonstratedthefield-effectmobilityform0.43cm2/V-sto0.6cm2/V-s,theminimumsubthresholdswingfrom0.58V/decto0.59V/dec,thethresholdvoltagewasdistributedfrom1.9Vto2.6Vdeterminedfromturn-oncurrent(Ion)extrapolationinthelinearregionofID-VGcurveat.31.(46)VD=0.1V.,andtheION/IOFFratio~107atVDS=10V.Besides,The9sccm-SiF4a-Si:H(F)TFTdemonstratedthefiled-effectmobilityfrom0.3cm2/V-sto0.58cm2/V-s,theminimumsubthresholdswingfrom0.65V/decto0.67V/dec,thethresholdvoltagefrom3.8Vto5.5V.Withincreasing[SiF4]ratio,thefieldeffectmobilitydecreases,thresholdvoltageincreases,andoncurrentdecreases.Leakagecurrentofboththroughgateinsulatorislessthan10-13A..32.(47)4.2CharacteristicsatLowTemperature.4.2.1MotivationandexperimentstepsAfteracompleteTFTmanufactureprocess,InordertoverifyElectricalcharacteristicsandphotoleakagecurrentvariationatlowtemperature,it’stoperformthemeasurementoflightilluminatedandno-lightilluminatedatlowertemperaturefortheSTDa-SiTFTandtheSiF4-9sccma-SiTFT.Inthisexperiment,wehavetwokindsofsample：1.StandardTFTwithactivelayera-Si:H2.SiF4(9sccm)TFTwithactivelayera-Si:H(:F)Theexperimentalprocedureisdepictedbelow：Loadthesamplestothecryogenicssystemandcoolthesampledowntothedesirablemeasurementtemperature.MeasuretheI-VcharacteristicsusingtheHP4156Csemiconductorparameteranalyzer.WemeasureI-Vcharacteristicsat100K,120K,150K,180K,200K,220K,250K,280Kand300K.Andobservethedifferencebetweenlightilluminatedandno-lightilluminated.Then,wedeterminetheallsemiconductorparametersfromthetransfercharacteristics..4.2.2TheelectricalcharacteristicsatlowtemperatureFig.4.2showstheVD-IDtransfercharacteristicscurveat100Kand300K,it’sdetectedcurrentcrowdingeffectatlowertemperature.Normally,theeffectofparasiticresistancecanbeclearlyobservedintheoutputcharacteristicsofa-Si:H33.(48)TFT.Thelargeparasiticresistancewouldleadtocurrentcrowdingeffect.Underoperatedatsmalldrainvoltagesandhighgatevoltages,TheTFTONresistance,Ron,consistsofthechannelresistanceRchandtheparasiticresistanceR.parasitic..The.contactresistanceisalsoestimatedwiththefollowingequations：Ron=Rch+Rparasitic.(4-1).Rch=L/WµCi(VG-VTH).(4-2).WhereCiisthegatenitridecapacitanceperunitareaandW,L,andVTHaretheintrinsicdevicechannelwidth,length,andthethresholdvoltage,respectively.[4-8]Fig.4.4showstheONresistanceoftheSiF4-9sccmanda-Si:HTFTatlowtemperature.Duringtemperaturedecreasing,electronsactivationofjunctionalsogetdownandincreaseRparasitic.Duetotheparasiticresistanceincreasing,theTFTshouldbedrivedbyhigherdrainvoltages.From300Kto100K,it’sdetectedcurrentcrowdingeffectofVD-IDtransfercharacteristicscurveatlowtemperature.Incaseofcurrentcrowdingeffect,it’ssupposedtosetVD=3VfortheproperelectricalcharacteristicsdataduringVD-IDtransfercharacteristicscurvemeasurementtoavoidcurrentcrowdingeffectarea.Fig.4.3showsVG-IDtransfercharacteristicscurvesoftheSTDa-Si:HTFTandtheSiF4-9sccma-Si:H(:F)TFT.TherearetwoelementstochangeIDcurrent：thresholdvoltage(VTH)andfield-effectmobility(µFE).Wecanderivethedecreasingcurrentatlowtemperaturefromtheconductingcurrentformula：ID=µFEC.W12[(VG−VTH)VD−VD]L2.(4-3).WhenthetemperaturecoolingtheseriesresistanceatRparasiticofthesourceanddrain,willenlargeandleadthedevicesneedthelargerthresholdvoltagetoturnontheTFT..34.(49)Fig.4.5showsVTshiftpercentage(△VT/VT300K)andcarriermobilityshiftpercentage(△µ/µ300K).ComparingtheSiF4-9sccma-SiTFTandtheSTDa-SiTFT,themobilityisdecreasingalongwiththetemperaturegettingdown,Assample’smobilitydecreasingratioofbothisverysimilar,itpresentscarriermobilitydecreasesalongwiththeparasiticresistanceincreasing.However,it’soppositeofthresholdvoltageshiftpercentage：ThresholdvoltageincreasingratiooftheSiF4-9sccma-SiTFTatlowtemperatureishigherthanthatofthea-Si:HTFT,becausethea-Si:H(:F)TFTbehavesasasimilarp-typepropertycomparedwiththea-Si:HTFT,mobilityandthresholdvoltageareworse.Byformula(4-2),it’sestimatedchannelresistance1isincreasingmorethantemperaturedecreasingandthresholdvoltageshiftratioishigher..4.2.3MeasuredwithlightilluminatedatlowtemperatureDuetotheincreaseofcontactresistance,currentcrowdingeffectisobviousatlowtemperature.However,underlightillumination,thecurrentcrowdingphenomenonissuppressedbytheadditionalphotocurrent.Fig.4.6ShowsTheID-VDtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithlightilluminatedbytemperatureat300Kand100K,individually.Fig.4.7showsthetransfercharacteristicsVG-IDinthelinearregimeoftheSiF4-9sccma-SiTFTsandtheSTDa-SiTFTdeviceswithbottomlightilluminatedbydifferenttemperature,respectively.OncurrentofTFTunderlightilluminatedisdecreasingalongwiththetemperaturedescendingsameasoncurrentof.TFT.underno-illuminated；Nevertheless,it’soppositeofoffcurrent.Photoleakagecurrentisincreasingalongwiththetemperaturedescendingat250K~150Kbut.35.(50)photoleakagecurrentisdecreasingat100K.Inordertounderstandtherelationofphotoleakagecurrentalongwithtemperaturevariation,tomeasuretheSiF4-9sccma-SiTFTsandtheSTDa-SiTFTsphotoleakagecurrentvariation(△ID/ID)atdifferenttemperatureinVG=-7V,-10V,-15V,asshowinFig.4.8.TotreatPhotoLeakageCurrentvariationinto2parts:.(I)photo-leakage-currentincreasingalongwiththetemperaturedescendingRecombinationmechanismismainlyindirectrecombinationtocausecaptureandemissionthroughrecombinationcenters.Capturetimeandemissiontimeincreasingalongwiththetemperaturedescending.Thus,recombinationratebecomesworsebutphotoleakagecurrentbecominghigher.Fig.4.9showscapturetimeandemissiontimetotemperature,per2equationsbelow:Capturetime(τC)isestimatedwiththefollowingequations：[4-9].I(T)T2τC=(1/σ0α)exp[(∆EB/kT)]Whereα=l(3k/m*).1/2.(4-4)./qµ0Vdωt；l,w,andtarethechannellength,width,and.thickness,respectively.σ0and∆EBarethecrosssectionprefactorandcaptureactivationenergy.Emissiontime(τE)isestimatedwiththefollowingequations：[4-9].T2τE=(1/gσ0β)exp[(∆EB+∆ECT)/kT]Where.β=A(3k/m*)1/2；.(4-5).and∆ECTisthetrapbindingenergy..(II)PhotoLeakageCurrentdecreasingalongwithtemperaturedropsto200KAccordingtotheSeto’smodel,theelectronsactivatedfromthedopedatoms.36.(51)werefilledatthetrapstatesatthegrainboundaries.Forsimplify,theamountoffreecarrierinN+layers,Nfree,canbeequatedto:+.Nfree=ND−NT.(4-6).whereND+isthenumberofionizedimpuritiesandNTistheeffectivetrapdensity.Thus,theconductivityisproportionaltotheamountoffreecarriers.[4-8]Withthedecreasingofthetemperature,theamountsofactivatedelectronsaredecreasingandtheratiooftrappedelectronsisrisingbyassumingthattheamountoftrapstatesisnon-varied.Ionizedchargeconcentrationversustemperatureforsilicon,showninFig.4.10.[4-10]Attemperaturelowerthan200K,theionizationischangingfromCompleteionizationtoPartialionization.TheN+resistanceincreasingtocausedrain-sourcevoltagedecreasinginchannelsothemeasuredphotoleakagecurrentislower.TheparasiticalresistancesofN+from100Kto300K,areshowinFig.4.11.[4-8]It’sobviouslyobservedresistanceincreaseslopebecominghigher.That’sdefinitelyPhotoLeakageCurrentdecreasingbecauseoftheN+sheetresistanceincreasing.theN+dopantcouldnotfullyactivatedresultingintheN+resistanceincreasing.ComparingwiththeSiF4-9sccma-Si:H(:F)TFTsandtheSTDa-SiTFTsphotoleakagecurrent,photoleakagecurrentincreaseanddecreasetrendsarethesameatlowtemperature.Butphotoleakagecurrentofa-Si:H(:F)TFTincreaseanddecreaserationsmallerisduetotherecombinationcentersofa-Si:H(:F)TFTaremorethanthatofa-Si:HTFT..4.2.4SummaryA-SiTFTsworksasalargerresistoratlowtemperaturethanthatatroomtemperature.Asthemobilitybecominglower,Turn-oncurrentofTFTdevices.37.(52)mustbehigherandthresholdvoltageshiftsrightwardsandoncurrentdecreasing.Besides,fortheSiF4-9sccma-SiTFTcomparedwiththea-Si:HTFTsimilartothep-typepropertyandwithlowermobilitycomparedwiththea-Si:HTFT,Thethresholdvoltageincreasingratioishigheratlowertemperature.Furthermore,moreelectron-holepairsproducedcausecurrentcrowdingeffectdescendingbycurrentgain.Butindirectrecombinationrateatlowtemperatureisworsethatthatatroomtemperature.Underlightilluminated,photoleakagecurrentisalsoincreasingalongwiththetemperaturedescending.Astemperaturedropsto200K,theN+dopantcouldnotfullyactivatedresultingintheN+resistanceincreasing.Thus,thephoto-leakage-currentdecreases.Theseresultscanprovidethedesignerstoconsiderthetemperatureeffectsforthea-SiTFTapplicationinasuitabletemperaturerange..38.(53)4.3LightIlluminatedExperiment.4.3.1MotivationandexperimentStepsBecauseofequipmentlimitation,onlytop-lightilluminatedexperimentatlowetemperatureisperformed.It’stodistinguishthedifferenceofphotoleakagecurrentbetweentop-lightandbottom-lightilluminated.Toinvestigateelectricalcharacteristicsundertop-lightandbottom-lightilluminatedatroomtemperature.WehavetwotypesTFTswithdifferent[SiF4]/[SiH4]ratio:(1)StandardTFTwithactivelayera-Si:H(2)SiF4(9sccm)TFTwithactivelayera-Si:H(:F)Fig.4.12showsthedevicewasilluminatedundertwodirection：(a).under.front-side.illumination(Bottom-Light)..illumination(Top-Light)The.fluorine.and.incorporated.(b)amorphous.gate-sidesilicon.[a-Si:H(:F)]andamorphoussilicon(a-Si:H)wereilluminatedwithtop-lightandbottom-light.Thebacklightmodule’slightintensitywas3500nits..4.3.2AdiscussiononexperimentresultsFig.4.13showsthecomparisonofVG-IDcharacteristicsbetweentoplightilluminatedandbottomlightilluminated,(a)STD(b)9sccm.Whenthelightilluminatedthethresholdvoltagedecreasesandfield-effectmobilityincreasesslightly.ThedecreaseinthethresholdvoltageisduetotheTFT.39.(54)activelayerilluminatedbylight,causesalargenumberofelectron–holepairscreatedtoleadtheshiftoftheFermilevelinthebandgaptowardtheconductionbangedge.TheFermilevelinthebandgaptowardtheconductionbangedgeleadthethresholdvoltagedecreasesandthetransistorturnonmoreeasily.Forphotoleakagecurrentundertop-lightilluminated,ismuchhigherthanthatunderbottom-lightilluminatedofinverted-staggereda-Si:HTFT.Top-lightilluminatedirectlyintochannelbutbottom-lightilluminatedthePNjunctionofsource&drainduetolightcoveredbygatemetallayerunderbottom-lightilluminated.Fig.4.14showsVG-IDtransfercurvesofSTDandSiF4-9sccma-SiTFTsunderthedifferentlightilluminatedintensity；Oncurrentdoesnotincreasealongwithlightilluminatedintensityisraising,duetoturnonoperationofTFTdeviceslocatedatSaturationarea.Butoffcurrentisincreasingalongwithlightilluminatedintensityraising.AsS.SvariationasVg=-5V~0V,photoleakagecurrentbyVg=-20V~-5VisincreasingalongwithlightilluminatedintensityraisingandVG-IDtransfercurveisverticallylifted.Ontheotherhand,VG-IDtransfercurveisverticallyfallingdownbylightilluminatedintensitydescendingandit’shelpfultocomparewiththeVG-IDtransfercurvemeasuredofbottomlightilluminatedandtoplightilluminated.Toadjustlightilluminatedintensityandhavephotoleakagecurrentvalueoftoplightilluminatedisclosedtobottomlightilluminated.ThecomparisonofVG-IDcharacteristicsbetweentoplightilluminatedandbottomlightilluminatedshowatFig.4.15.AsVg-10Visoppositeofthat.Theoff-statedarkleakagecurrentofa-Si:HTFTmainlyoriginatesfromthephoto-inducedholecurrentattheinterfacebetweena-Si:HandgateSiNlayers.Incontrast,electronsarethemajoritycarriersofoff-statecurrentforthea-Si:HTFTunderlightillumination,sinceelectronmobilityismuchhigherthanthatofhole.[4-3]Duetohigherverticalelectricalfieldatsourceanddrainseparatedtheelectron-holepairsinchannelandholecurrentoccurring,photoleakagecurrentisincreasingalongwithgatebiasraising.[4-10]Also,duetotheSiF4-9sccma-SiTFTincreaseinthedefectdensitybyFincorporationandduetoshiftoftheFermilevelinthebandgaptowardthevalencebandedge[4-11].PhotoLeakageCurrentoftheSiF4-9sccma-SiTFTsishigherthanthatofSTDa-SiTFTsshowninFig.4.16(b).Underbottomillumination,photo-leakage-currentofthea-Si(:F)TFTishigherthanSTDTFTatlargernegativegatevotage(-15V~-20V)duetotheholeaccumulation.Toplightilluminated：BywayofID-VGtransfercharacteristicsmeasured,photoleakagecurrentnotincreasingasVgni^2).Asaresult,thetrapstatesplayedtheroleofrecombinationcenters.AsVG=-5V~-20V,off-statecurrenttransfercurveupanddownalongwithlightilluminatedintensityraisingordescendingbutthetrendshiftshorizontallyduetotheoff-statephotoleakagecurrentconductedbyelectron–holepairsproducedduringlightilluminated.Theoffcurrentjustraisedwithoutanyleakagebehaviorchange.Thephotoleakagecurrentofa-Si:H(:F)TFToperatedinthesmallnegativegatevoltage(VG＞-10V)and1Vdrainvoltageislessthanconventionala-Si:HTFTs.Becausetheelectricfieldisnotlargeenoughtoseparatethephotoinducedelectron-holepairs,theincreaseddensityofstatesservingasrecombinationcentersina-Si:H(:F)channelmaterialhasresultedinthelowerphotoleakagecurrent.However,withtheholeconductionregion(VG＜-10V),thelargerphotoleakagecurrentwasobserved.Accordingtopreviousstudy,[4-6]Thelargeroff-statephotoleakagecurrentisduetothefasterholechannelaccumulationinthelargernegativegatevoltage.Thesmallerholeaccumulatedvoltagealsoindicatedthattheundopeda-Si:H(:F)showsnearp-type-likebehavior.Thereisnohighelectricalfieldtoseparatetheelectron-holepairsandtheholeconductionregionisasVG＜-10V.Thus,photoleakagecurrentisnotincreasing.Inotherwords,photoleakagecurrentundertoplightilluminatedisnotcausedbytheholeconductedasgatebiasraising42.(57)Typicala-Si:Hisn-typematerialeventhoughtheFermilevelliesnearthemidgapofundopeda-Si:Hbecausetheelectronmobilityisatleast10timeshigherthanthatofhole.[4-4]Theundopeda-Si:H(:F)TFTshaveshownlargerthresholdvoltageandsmallerfieldeffectmobility..43.(58)4.4ReliabilityExperiment—DCStress4.4.1MotivationandexperimentstepsTFTdeviceisthepixelon/offswitchofflatdisplayandlong-timeoperatedbybias.ThelifetimeofTFTdevicesisveryimportant.InordertoknowtheSiF4-9sccma-SiTFTsunderthelong-timeoperation,thereliabilityexperimentisperformed.Mainly,DCstressinchanneltomeasuretheelectricalcharacteristicsoftheSiF4-9sccm.a-Si.TFTs.and.the.STD.a-Si.TFT.before.and.after.BTS(bias-temperaturestress)andtoobservetheeffectofphotoleakagecurrentunderstressStressvoltage,VG=15V,VD=8V,VStoGND.Stresstime,800sec.TomeasuretheelectricalcharacteristicsonceperStressforcedandsamplingEaandDOSat0S,800S,1600S,and2400S..4.4.2AdiscussiononexperimentresultsID-VGtransfercharacteristicsofstresswith(a)theSTDa-SiTFTand(b)theSiF4-9sccma-SiTFT,individuallyshowninFig.4.17.Thresholdvoltageshiftrightwardsisduetocarriersshocka-SiduringthelongtimeandstrainedbondbrokentoformDanglingBondcausethedefectdensity.Stresseffectat2400sec.similarwiththatat1600sec,duetostrongerbondremainedafterstrainedbondbroken.Fig.4.18~Fig.4.19showID-VGcurveoftheSTDa-SiTFTandthe.44. 閱讀更多 閱讀更多 數據 Table3-1 TypicalvaluesoftheparametersfortheexponentialmodeloftheDOSforintrinsica-Si:HcompiledfromRef p.70 Table2-1 Physicalpropertiesofadevice-qualitya-Si:H. p.70 Fig.1.1Theelectronenergylevelsofsiliconindifferentbondingstates. p.71 Fig.1.2Aschematicdiagramoftheelectrondensityofstatesforanamorphoussemiconductor,suchasa-Si:H p.71 Fig.1.5SimulatedvariationsoftheelectronandholeFermipotentialsalongthemainTFTcurrentpath(theTFTbeingilluminated). p.73 Fig.3.1Microscope、Hotchuck、Probestation p.74 Fig.3.3Temperaturecontroller. p.75 Fig.3.4Agilent4156C、AgilentE5250A(Switch)andAgilent41501B(highpower) p.75 Fig.3.5IllustrationoftheCryogenicsSystem(TTP-6ProbeStation) p.76 Fig.3.6Illustrationofthedensityofstates(DOS)inintrinsica-Si:H.(AfterShawandHack,Ref p.76 Fig.3.7Representativedependenceoftheactivationenergyvs.gatevoltagedependenceforthetransistorswithincorporatedn+contactlayers p.77 Fig.3.8Densityofstatesvs.activationenergy p.77 Fig.4.2TheID-VDtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithtemperatureat300Kand100K,individually.(a)300K,(b)100K p.79 Fig.4.3TheID-VGtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithtemperatureat300Kto100K p.80 Fig.4.4ShowstheONresistanceoftheSiF4-9sccmanda-Si:HTFTatlowtemperature p.81 Fig.4.6TheID-VDtransfercharacteristicsoftheSiF4-9sccmanda-SiTFTwithlightilluminatedbytemperatureat(a)300Kand(b)100K,individually p.82 Fig.4.7showsthetransfercharacteristicsVG-IDinthelinearregimeof(a)theSTDa-SiTFTdevicesand(b)theSiF4-9sccma-SiTFTswithlightilluminatedbytemperatureat300Kand100K,individually p.83 Fig.4.9Arrheniusplotofcarrierandemissiontimeforanindividualinterfacetrap p.84 Fig.4.8AtVG=-7V,-10V,-15V,changeofPhotoLeakageCurrentoftheSiF4-9sccma-SiTFTandtheSTDa-SiTFT p.84 Fig.4.10Ionizedchargeconcentrationversustemperatureforsilicon p.85 Fig.4.13ThecomparisonofVG-IDcharacteristicsbetweentoplightilluminatedandbottomlightilluminated p.87 Fig.4.15ThecomparisonofVG-IDcharacteristicsbetweenToplightilluminatedandBottomlightilluminated p.88 Fig.4.14Thechangeofphotoleakagecurrentunderhighandlowlightilluminated p.88 Fig.4.16Illustrationofregionforlightilluminatedandelectron–holepairst(a)Toplightilluminated(b)Bottomlightilluminated p.89 Fig.4.17ID-VGtransfercharacteristicsofstresswith(a)theSTDa-SiTFTand(b)theSiF4-9sccma-SiTFT,individually p.90 Fig.4.18ID-VGtransfercharacteristicsoftheSTDa-SiTFT.Afterandbeforeofstresswith(a)Toplightilluminatedand(b)Bottomlightilluminated,individually p.91 Fig.4.19ID-VGtransfercharacteristicsoftheSiF4-9sccma-SiTFT.Afterandbeforeofstresswith(a)Toplightilluminatedand(b)Bottomlightilluminated,individually p.92 Fig.4.20Afterandbeforestress,(a)Theactivationenergy–gatevoltage(Ea-VG)and(b)DensityofstatescharacteristicoftheSTDa-SiTFT p.93 Fig.4.21Afterandbeforestress,(a)Theactivationenergy–gatevoltage(Ea-VG)and(b)DensityofstatescharacteristicoftheSiF4-9sccma-SiTFT p.94 Fig.4.22Afterandbeforestress,ID-VGtransfercharacteristicsoftheSTDa-SiTFTandtheSiF4-9sccma-SiTFTwith(a)no-lightilluminatedand(b)bottomlightilluminated,individually p.95 閱讀更多 參考文獻 Updating... 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