INFRAREDELECTRO-THERMO-OPTICALDEVICESBASEDONTHEPHASE TRANSITIONOFVANADIUMDIOXIDE by NoraicaDávilaMeléndez ADISSERTATION Submittedto MichiganStateUniversity inpartialful˝llmentoftherequirements forthedegreeof ElectricalEngineering-DoctorofPhilosophy 2015ABSTRACT INFRAREDELECTRO-THERMO-OPTICALDEVICESBASEDONTHEPHASE TRANSITIONOFVANADIUMDIOXIDE by NoraicaDávilaMeléndez Inthiswork,electro-thermo-opticaldevicesweredevelopedfornear-infrared(NIR)wave- lengthsbyexploitingtheopticalpropertiesofvanadiumdioxide(VO 2).VO 2undergoesinto aninsulator-to-metaltransition(IMT)inwhichitsoptical,electrical,andstructuralproper- tieschangeabruptlyasafunctionofitstemperature.Thechangesinthesepropertiesshow hystereticbehavior. Sol-geldepositionprocesswasinvestigatedforgrowingVO 2thin˝lmsonSiO 2andSi/SiO2substrates.ItscompositionandcrystallizationwerecharacterizedbyX-raydi˙rac- tionandRamanspectroscopy.Atomicforcemicroscopyanda3-Dsurfacecontactpro˝lome- terwereusedtoanalyzethesurfacetopographyofthe˝lms.Thesethin˝lmswerecompared withVO 2thin˝lmsdepositedbypulsedlaserdeposited(PLD).VO 2thin˝lmsdeposited byPLDwereusedtodevelopthedevicespresentedinthiswork. Inthepresentwork,theIMTisinducedthermally,usingphoto-orelectro-thermaltech- niques.Twodevicesweredeveloped:1)aNIRimageprojectorand2)afullyelectronic variableopticalattenuator(VOA)tooperateintheNIRregion.The˝rstdeviceusesthe inherenthysteresisintheopticalpropertiesofVO 2acrossitsphasetransition,whichallowed fortheprogrammingofpatternsontotheVO 2thin˝lm.Theseconddeviceemergedfrom theopticaltransitioninVO 2,inwhichtheelectro-opticalchangesinthe˝lmwereinduced electro-thermally(i.e.apticalThestrongcorrelationbetween theelectricalandopticalpropertiesinVO 2enabledtheimplementationofaself-sensing technique,whichreducedtheopticalhystereticbehaviorandsimpli˝edthemodelingand controloftheattenuation. Tothosewhoinspiredorsupportedme, butwillnotreadit. iiiACKNOWLEDGMENTS IwanttothanktomythesisadvisorNelsonSepúlvedaforacceptingmeinhisgroup,his help,supportandguidanceduringtheseyears.Ienjoyedandlearnedalotworkingin hisgroup.Mycommitteemembers:TimHoganforhistechnicalsupport,assistanceand thoughtfuldiscussionsaboutexperimentsandmaterialcharacterizationtechniques;Andre Leeforallhishelpwiththesol-geldepositionprocessdevelopment,recommendationsand helpfulinsights;PremChahalforthougfuldiscussionsaboutmyprojectandexperiments. AlsoIwouldliketothankmycolleagueRafmagCabreraforhishelpandguidancewith technicalexperimentsandcareeradvise.Itwasapleasureworkingwithhimthroughthese years.TomycolleagueandhusbandEmmanuelleMercedforbeingtherealwayslifecoaching meintimesofstrugglesandsuccesses. Tomyfamilyandfriendswhoremindedmewhatreallymattersinlifeandprovidedme emotionalsupportduringmystruggles.They˝lledthisjourneywithgreatmemoriesthatI willcarry. ivTABLEOFCONTENTS LISTOFTABLES..................................viii LISTOFFIGURES.................................ix CHAPTER1INTRODUCTION.........................1 1.1ProblemDescriptionandMotivation......................3 1.2ThesisStatement.................................5 1.3ThesisContributions...............................5 1.4DissertationOutline...............................6 CHAPTER2BACKGROUND..........................7 2.1OptoelectronicDevices..............................7 2.2InfraredImageProjectors............................8 2.2.1CurrentTechnologies...........................9 2.2.1.1EmissiveIRprojectors.....................9 2.2.1.2Re˛ectiveIRProjectors....................10 2.2.1.3TransmissiveIRProjectors..................11 2.2.1.4OtherTechnologiesforIRProjectors.............12 2.3VariableOpticalAttenuators(VOAs)......................13 2.3.1CurrentTechnologies...........................14 2.3.1.1Micro-electro-mechanicalsystems(MEMS)VOAs......14 2.3.1.2Micro˛uidicsandFerro˛uidicsVOAs.............16 2.3.1.3Solid-stateVOAs........................18 2.3.2DeviceCon˝guration...........................19 2.3.3AttenuationControl...........................20 2.3.4Applications................................22 2.3.4.1OpticalCommunicationNetworks...............22 2.4VanadiumDioxide................................23 2.4.1StructuralTransition...........................25 2.4.2ElectricalTransition...........................26 2.4.3OpticalTransition............................27 2.4.4OpticalMemory..............................27 2.4.4.1OtherOpticalMemorySystemsandMaterials........28 2.4.5Self-sensingFeedbackTechnique.....................31 2.5VO 2DepositionProcesses............................31 2.5.1ChemicalVaporDepostion(CVD)...................33 2.5.2AtomicLayerDeposition(ALD).....................35 2.5.3Sputtering.................................36 2.5.4Evaporation................................36 2.5.5Sol-gelDeposition.............................36 2.5.6PulsedLaserDeposition(PLD).....................40 v2.6Summary.....................................43 CHAPTER3VO 2THINFILMSDEPOSITION................45 3.1Sol-gelDepositionProcess............................45 3.1.1Substrate.................................46 3.1.2Synthesis.................................46 3.1.3GelDeposition..............................48 3.1.4DryingTreatment.............................48 3.1.5Annealing.................................48 3.2CharacterizationofVO 2depositedbysol-gel..................53 3.2.1ElectricalTransition...........................53 3.2.2Composition................................54 3.2.3Topography................................56 3.2.4Thickness.................................59 3.2.5OpticalTransition............................59 3.3PulsedLaserDeposition(PLD)Process.....................61 3.3.1DepositionConditions..........................61 3.4CrystallizationofPLDgrowthVO 2thin˝lmsassessedbyXRD.......62 3.5Summary.....................................64 CHAPTER4NIRIMAGEPROJECTIONBASEDONVO 2OPTICALMEMORY..............................65 4.1SamplePreparation................................65 4.2VO 2OpticalMemory...............................66 4.3ProjectionSetup.................................66 4.4Non-pixelatedImageCalibration........................69 4.5Results.......................................70 4.6DeviceCharacterization.............................75 4.7Summary.....................................76 CHAPTER5VO 2-BASEDVARIABLEOPTICALATTENUATOR...78 5.1Electro-opticalVO 2PropertiesCorrelation...................78 5.2Self-SensingFeedback...............................78 5.3DeviceFabrication................................79 5.4Electro-thermo-opticalSetup...........................80 5.5Results.......................................81 5.6DeviceCharacterization.............................84 5.7Summary.....................................88 CHAPTER6SCALINGOFMICROVARIABLEOPTICALATTEN- UATORDEVICE..........................89 6.1DeviceDesignandStructure...........................89 6.2DeviceFabrication................................94 6.3ExperimentalSetup................................95 6.4ResultsandDiscussion..............................98 vi6.4.1Powerconsumption............................107 6.5Summary.....................................108 CHAPTER7SUMMARY.............................109 7.1SummaryofContributions............................109 7.2ListofProblemsSolvedinthisthesis......................109 APPENDICES....................................111 AppendixA:CompositionstudyforVO 2thin˝lmsbysol-gel...........112 AppendixB:AdditionaltestingforVO 2-basedVOA................116 BIBLIOGRAPHY..................................119 viiLISTOFTABLES Table3.1:Solvente˙ectongelprecipitatesformation..................47 Table3.2:PLDdepositionconditions..........................61 Table6.1:CorrelationErrorforalldevices.......................104 Table6.2:Closed loopError...............................105 Table6.3:PowerConsumption..............................107 viiiLISTOFFIGURES Figure1.1:OpticaltransitioninVO 2forawavelengthof 1550nmasafunctionof temperatureforaheatrateof ˘0:4C=s...................2 Figure1.2:ElectricalresistancechangeinVO 2asthephasetransitionisthermally inducedwithaheatrateof ˘0:4C=s....................3 Figure2.1:Classi˝cationofIRimageprojectorsandVOAsoptoelectronicdevices..8 Figure2.2:SuspendedmembraneresistorsarrayusedinemitterIRprojectors[24]..9 Figure2.3:IRlaserdiodearrayusedinemitterIRprojector[12]............10 Figure2.4:IRprojectorbasedonlaserscanningtechnology[12]............10 Figure2.5:Projectionsetupwithre˛ectiveIRmicromirrorsarray[25].........11 Figure2.6:TransmissiveIRprojectorusingaliquidcrystallightvalve(LCLV)[12]..12 Figure2.7:MEMS-basedVOAwithopticalshutter[45].................14 Figure2.8:MEMSvariableopticalattenuatorbyretro-re˛ectionofIRlight(a) deviceSEMimage,(b)workingprinciple:initialstate(left)attenuation state(rigth)[34]................................14 Figure2.9:Re˛ectivemirrorMEMSvariableopticalattenuator[38]..........15 Figure2.10:Ellipticalmirrorvariableopticalattenuator(a)topview,(b)sideview[32].15 Figure2.11:VOAdevicebasedonvoltagecontrolledliquidlens[46]...........16 Figure2.12:Opto˛uidicdevicewith˛uidratetunablecorewidthandattenuation[48].16 Figure2.13:Ferro˛uidbasedVOA[37]...........................17 Figure2.14:Ferro˛uiddopedPDMScantileverwaveguideactuatedbyelectromag- nets[49].....................................17 Figure2.15:Polymer-basedelectrochromicvariableopticalattenuator(a)devicedi- agram,(b)deviceimageinneutral(top)active(bottom)[52].......18 Figure2.16:In-lineVOAcon˝guration[55]........................19 ixFigure2.17:Fiber-gapVOAcon˝gurationswithcouplers(left),lens,dualcollimator, mirroranddi˙ractiongrating(right)[59]..................20 Figure2.18:Fiber-gapVOAcon˝gurationwithsplit˝berandnoadditionaloptical components[45]................................20 Figure2.19:Closed-loopcontrolsystemforaVOA[61]..................21 Figure2.20:Opticalnetworkfordensewavelengthdivisionmultiplexing(DWDM)[63].22 Figure2.21:Arrayedwaveguidegratingandvariableopticalattenuatorintegration (AWG-VOA)[67]................................23 Figure2.22:Insulator-to-metaltransitionindi˙erentvanadiumoxides[70].......24 Figure2.23:VO 2structuraltransitionrepresentationoftheunitcells[5]........25 Figure2.24:BanddiagramofVO 2insulatorstate(left),metalstate(right)[88]....26 Figure2.25:MultipleopticalstatesinVO 2programedbyphotothermalactuation[91].28 Figure2.26:OpticalreadoutsystemtoretrievestoreddatainCDs[94].........29 Figure2.27:3-dimensionalmemory[98]..........................30 Figure2.28:Self-sensingtechniqueusedinVO 2toestimatede˛ectionbysensingthe resistance[111].................................32 Figure2.29:Vanadium-oxidephasediagram[112].....................33 Figure2.30:X-raydifractionforVO 2grownbyCVDoverSisubstrate[115]......34 Figure2.31:RamanspectrumforVO 2grownbyAPCVD[118,120]...........34 Figure2.32:X-raydi˙ractionforVO 2grownbyALDonglasssubstrate.VO 2ob-tainedat 475C[123].............................35 Figure2.33:X-raydi˙ractionforVO 2grownbysol-gelonSi(100)substratesat di˙erentannealingtemperatures[129]....................38 Figure2.34:X-raydi˙ractionforVO 2grownbysol-gelonSi(100)substratesat di˙erentannealingtemperatures[137]....................38 Figure2.35:XRDresultsforaVO 2thin˝lmsonSiforanannealingtemperatureof 450Cfor15min[128]............................39 Figure2.36:XRDresultsforaVO 2thin˝lmsonSiO 2foranannealingtemperature of450Cfor40min[128]...........................39 xFigure2.37:AFMtopographyofVO 2thin˝lmsbysol-gelonSi(100)[137]......40 Figure2.38:ThicknessmeasurementofVO 2thin˝lmsusingSEMcross-sectional imaging[138]..................................40 Figure2.39:SchematicforaPLDsystem[118]......................41 Figure2.40:XRDspectraforVO 2thin˝lmsoverSi(100),SiO 2,andAl 2O3byPLD [141].......................................41 Figure2.41:RamanspectraforVO 2thin˝lmsonSi(100)substratefordi˙erent annealingtimes(2,5,10,30,and90min).VO 2strongpeaksshownfor the30mindata[113].............................42 Figure2.42:RamanspectraforvanadiumoxidesonSiO 2substratefordi˙erentan- nealingtimes:2,5,10,30and90min[113].................43 Figure2.43:OpticaltransitionofVO 2overSiO 2for1550nm[113]...........43 Figure2.44:HysteresiscontrastforPLDdepositedVO 2thin˝lmsonAl 2O3,Siand SiO2[113]....................................44 Figure3.1:Summarizedsol-gelprocessusedtodepositVO 2thin˝lms.........46 Figure3.2:VO 2/SiO2/SiwafercutintopieceA,B,CandDforfurthercharacterization.47 Figure3.3:Annealingat 500Cvaryingtheannealingtime...............49 Figure3.4:AnnealingtemperaturestudyonSiO 2from400Cto 558C........50 Figure3.5:AnnealingstudyonSiO 2foratemperaturerangeof 417Cto 496C...51 Figure3.6:AnnealingconditionsvariationsinSiO 2/Sisubstrates............52 Figure3.7:ElectricalcharacterizationofVO 2/SiO2/SithewaferpiecesA,B,CandD.53 Figure3.8:XRDcharacterizationofVO 2/SiO2sampleannealedat 428Cfor2hrs at37mTorrandVO 2/SiO2/Sisampleannealedat 461Cfor2hrsat 15mTorr....................................54 Figure3.9:Ramanspectroscopycharacterizationofa)VO 2/SiO2sampleandb) VO 2/SiO2/Sisample..............................55 Figure3.10:VO 2thin˝lmtopographymeasuredwiththe3-Dsurfacepro˝lometer..56 Figure3.11:Atomicforcedmicroscope(AFM)surfacecharacterizationforVO 2/SiO2/Si.57 xiFigure3.12:AFMsurfacetopographyforVO 2/SiO2/Sisubstratemeasuredina1 mx1 msquarearea............................58 Figure3.13:Thicknessmeasurement............................59 Figure3.14:FIBtrapezoidalcutforVO 2thicknessmeasurement............60 Figure3.15:SEMcross-sectionalthicknessmeasurementforVO 2/SiO2/Si.Note: Thevaluelabeledonthese˝gurescorrespondstothemeasurementbe- tweenthewhitebars..............................60 Figure3.16:TransmissionforVO 2depositedbysol-gelmethodasafunctionoftem- perature(heatrate: ˘0:4C=s)for =1550µm...............61 Figure3.17:XRDcharacterizationofaVO 2thin˝lmandSiO 2substratedeposited underAr/O 2atmosphere...........................62 Figure3.18:XRDcharacterizationofVO 2thin˝lmandSiO 2substratedeposited underO 2atmosphere.............................63 Figure3.19:XRDresultsforSiO 2/VO 2depositedbysol-gel...............63 Figure4.1:Opticaltransmittanceofa ˘300nmthickVO 2˝lmdepositedbyPLD acrossthephasetransition..........................67 Figure4.2:Experimentalopticalsystemwithdi˙userusedtoprogramanimage ontoaVO 2thin˝lm.............................67 Figure4.3:Experimentalopticalsetupeliminatingthedi˙userusedtoprograman imageontoaVO 2thin˝lm,a)schematicdiagramb)photographofthe setup......................................68 Figure4.4:Pixelscalibrationbycontrolofthemicromirrorsscanningspeed......69 Figure4.5:Redlaserbeampro˝lealongthex-direction.Insetshowtheintensity contour.....................................69 Figure4.6:Micro-mirrorsscanningspeedcalibration...................70 Figure4.7:IRopticaltransmittancethroughVO 2/SiO2as˝lm'stemperatureincreases.71 Figure4.8:NIR =1:55µmtransmittancethroughVO 2duringheatingandcooling temperaturecycles.Thedi˙erentopticalstatesareidenti˝edasan imageisprogrammedandprojected.Onlysomeminorloopsareshown forclarity....................................72 xiiFigure4.9:ProgrammedandprojectedNIRimage:a)at 55Cbeforelaserscan- ning,b)duringwritinglaserscanning,c)rightafterthewritinglaser scan˝nished,andd)programmedimage5minutesafterscanning. `Grainy'pro˝lewasfoundtobecharacteristicofthedi˙user........73 Figure4.10:SquarepatternprogrammedintotheVO 2andprojectedNIRimage: a)at 55Cbeforelaserscanning,b)duringwritinglaserscanning,c) rightafterthewritinglaserscan˝nished,andd)programmedimage5 minutesafterscanning.`Grainy'pro˝lewaseliminatedbyremovingthe di˙user.....................................74 Figure4.11:Finiteelementmethod(FEM)simulationat(a)operatingtemperature, (b)after 25msofredlaserradiation.....................76 Figure5.1:Schematicrepresentationoftheclosed-loopcontrolsystemfortheVOA usingself-sensing................................79 Figure5.2:Setupusedforperformingopticaltransmissionandelectricalresistance characterizationoftheVOA.Thesetupisalsousedforcontrollingthe VOAinclosed-loopusingtheself-sensingtechnique.............81 Figure5.3:a)ElectricalcharacterizationforVO 2thin˝lmasfunctionoftempera- ture;b)OpticalcharacterizationoftheVO 2-basedVOAfor 1:55µm...82 Figure5.4:a)Transmissionasafunctionofresistanceandpolynomial˝tmodel;b) Errorbetweenthepolynomialmodelandactualtransmissionpercent- age.Thetableshowsthecoe˚cientsforthemodel.............83 Figure5.5:VO 2-basedVOAattenuationresponse....................85 Figure5.6:a)Stepresponseforvariableopticalattenuatorwithself-sensingfeed- backcontrol.b)Errorcalculationamongthesetpoint,measuredand self-sensingsignals.Theinsetshowsasmallerscalefortheerror(y-axis).86 Figure5.7:a)Sinusoidalresponseforavariableopticalattenuatorwithself-sensing feedbackcontrolduringtheVO 2transition.b)Errorbetweentheset- point,measuredandself-sensingsignals...................87 Figure5.8:a)Opticaltransitionasafunctionoftemperature.b)Opticaltransition asafunctionofresistance...........................87 Figure6.1:Schematicstructureofthemonolithicintegrated VOAdesign......89 Figure6.2:TemperaturedistributionsforVO 2squarewindowsofa)100x100 m2,b)200x200 m2,c)300x300 m2,andd)400x400 m2assimulated usingJouleheatingwithaheaterinputcurrentof 35mA..........91 xiiiFigure6.3:SimulatedtemperatureatthecenteroftheVO 2windowforallfour devices.Theelectrodeswereincludedinthesimulation..........92 Figure6.4:MaskdesignforthemonolithicVOAmicrodevice.............93 Figure6.5:FabricationprocessfortheVO 2-basedVOAa)SiO 2substrate,b) heaterandelectrodesmetallization,c)SiO 2insulatinglayer,d)open- ingtotheelectrodes,e)SiO 2depositionandwindowpatterning,andf) openingtocontactpadsforelectricalconnections..............94 Figure6.6:SEMimagesofthefabricatedVO 2devices.................96 Figure6.7:OpticalmicroscopeimagesofthefabricatedVO 2devices.........97 Figure6.8:Electro-opticalsetupusedfortestingthe VOAsdevices..........98 Figure6.9:Timeconstantmeasurements:a) 100µmdeviceresponse(V R)toastep input(I H)andb)timeconstantforthescalingdownofthe mdevices.99 Figure6.10:Temperatureasafunctionofcurrentforelectro-thermalactuationin theVO 2deviceofa)400 m,b)300 m,C)200 m,andd)100 m.InsetshowsIRimageduringactuationat ˘30mA.............100 Figure6.11:Simultaneousmeasurementoftheelectricalandopticaltransitionin VO 2windowsfora)400 m,b)300 m,c)200 m,andd)100 mdevice.101 Figure6.12:Electro-opticalVO 2correlationandbidoseself-sensingmodel.......102 Figure6.13:Correlationmodelerrorbetweencalculatedtransmission%(zeroline) andmeasureddata...............................103 Figure6.14:Transmissionclosed-loopcontrolblockdiagram...............104 Figure6.15:Closed-loopresultsfor:a)400 m,b)300 m,c)200 mandd)100 mdevice....................................105 Figure6.16:Self-sensedandmeasuredsignalerrorswithrespecttothesetpointdur- ingclosed-loopexperimentsfora)400 m,b)300 m,c)200 mand d)100 mdevices...............................106 FigureA.1:RamanspectrumforVO x/SiO2depositedsamplesbysol-gelannealed at417Cfor2hrsunderapressureof37mTorr..............112 FigureA.2:RamanspectrumforVO x/SiO2depositedsamplesbysol-gelannealed at428Cfor2hrsunderapressureof37mTorr..............113 FigureA.3:RamanspectrumforVO x/SiO2depositedsamplesbysol-gelannealed at450Cfor2hrsunderapressureof37mTorr..............113 xivFigureA.4:RamanspectrumforVO x/SiO2/Sidepositedsamplesbysol-gelan- nealedat 406Cfor2hrsunderapressureof15mTorr..........114 FigureA.5:RamanspectrumforVO x/SiO2/Sidepositedsamplesbysol-gelan- nealedat 439Cfor2hrsunderapressureof15mTorr..........114 FigureA.6:RamanspectrumforVO x/SiO2/Sidepositedsamplesbysol-gelan- nealedat 461Cfor2hrsunderapressureof15mTorr..........115 FigureB.1:Minimumspotsizediameter( 70µm)achievedbytheelectro-opticalsetup.116 FigureB.2:VO 2resistanceasafunctionofheaterdrivencurrentfordi˙erentirra- diatedopticalpoweronthe200 mdevice.................117 xvCHAPTER1 INTRODUCTION VanadiumdioxideVO 2exhibitsasolid-to-solidphasetransitioninwhichthecrystallographic structureofthematerialchangesfrommonoclinic(M)torutile(R).ThistransitioninVO 2,alsoreferredtoasaninsulator-to-metaltransition(IMT),canbeinducedthermally,inwhich caseoccursatatemperatureofaround 68C[1].Theoptical,electrical,andmechanical propertiesofVO 2changesimultaneouslyduringtheIMT.ThemultifunctionalityofVO 2acrossitsIMThasenabledtheuseofthismaterialinmanyspeci˝cdevices,including temperatureandopticalsensors[2,3],micro-electro-mechanicalsystems(MEMS)actuators [4,5]andopticalactivecomponents,suchasmodulators[6]andshutters[7]. TheopticaltransmittanceofVO 2decreasesfromneartomidinfrared(IR)wavelengths (900nmto 2500nm)acrosstheIMT[8,9].Themagnitudeofthischangeincreaseswith wavelengthuntilterahertzfrequenciesaround1THz[10].ThetransmittanceofVO 2changes abruptlyshowingthehystereticbehavioracrossitsphasetransitionforwavelengthsaround 1550nmasshownin Figure1.1.Inthis Figure1.1thetransmissionacrossa ˘300nm VO 2thin˝lmonSiO 2substratewasmeasuredbyalaserbeampro˝ler.The˝lmwasheated ataconstantrateof ˘0:4C=swaiting ˘3sbetweensamplingfortemperaturestabilization. TheopticalsignalwasnormalizedwithrespecttotheVO 2transmissionat ˘25Ctoseta 100%transmissionfortheVO 2'sMphase.TheRphaseofthematerialcorrespondstoits transmissionat 100CaftertheIMT.Thissignalwasattenuatedto ˘1%fromtheinitial transmission,whichmakesVO 2desirableforopticalapplications. 1Figure1.1:OpticaltransitioninVO 2forawavelengthof 1550nmasafunctionoftemper- atureforaheatrateof ˘0:4C=s.TheelectricalresistanceinVO 2dropsfromtwotothreeordersofmagnitudewhenthe solid-to-solidphasetransitionisinduced[11].Themagnitudeandshapeofthisdropdepends onmanymicrostructuralproperties,butmainlyonthe˝lm'sstoichiometry,grainsizeand crystalorientation.Theelectricaltransitionforthesame ˘300nmVO 2thin˝lmonSiO 2substrateisshownin Figure1.2.Duringthismeasurementtheheatratewas ˘0:4C=s.It revealsitshystereticbehavior,similartothechangeintransmissionshownin Figure1.1.2 !& "<%æ ˇ keepsgrowing,thereisaneedforIRprojectorstomatchthecurrentIRcamerastechnology demands,includingstableimages.Threedi˙erenttechnologieshavebeenusedtodevelop IRprojectors:thermalresistivearrays,modulation-baseddeformablemicro-mirrors,and quantumphotonics.Theseprevioustechnologiesexhibitedissuessuchaslargepowercon- sumption,highpeakcurrents,complexandexpensivefabricationmethods,slowprocessing speedsand˛ickeringimages[12].StableIRimagescanbeachievedintegratingVO 2thin˝lmswithasimpleopticalsystemandprocess. Near-IRvariableopticalattenuators(VOAs)areusedinopticstocontroltheintensityofa transmittedopticalsignal.VOAscanprovidediscreteorcontinuousattenuation.Mechanical systemswithmotorizedstagesfacilitatetheswitchingamongattenuationlevels,butnoise isintroducedwiththemechanicalmovement.Suchdeviceshavelimitedattenuationlevels despitebeingcontinuousandarenoto˙eredwithfully-electroniccontrol.Electroniccontrol oftheopticalIRtransmittancethroughVO 2canbeachievedbythestrongcorrelation betweenthematerial'sopticalandelectricalproperties. Theproblemsaddressedinthisthesisare: ‹Studyofasol-gelprocesstodepositVO 2thin˝lmsonlargeareassuchas4inches wafers. ‹CharacterizeVO 2thin˝lmsgrownbysol-gelprocessbyX-raydi˙raction,Raman spectroscopy,surfacetopography,electricalandopticalbehavior. ‹ProgramingopticalstatesinVO 2thin˝lmsbylocalizedheating,usingtheoptical memoryofVO 2toachievemultipleopticalstates. ‹UsethedevelopedVO 2-basedIRprojectortostoreandprojectstableIRimages. ‹Characterizethespeed,contrastandresolutionofthedevelopedIRimageprojector. ‹MeasuredtheopticalandelectricalVO 2propertiessimultaneouslytodeterminea modeltoaccommodatetransmission-to-resistanceexperimentaldata. 4‹Useself-sensingfeedbacktechniquestoelectronicallycontroltheopticaltransmission ofadesignedandfabricatedmonolithicmicrovariableopticalattenuator. ‹Characterizethetimeresponseandattenuationparametersofthedevelopedmicro VOA. 1.2ThesisStatement ThisworkpresentsthedevelopmentoftwoopticaldevicessuitableforIRwavelengths:1. anIRimageprojector,and2.afullyelectronicallycontrolledvariableopticalattenuator (VOA).Twodi˙erentVOAdesignswerefabricatedandactuatedbyconductiveheatingin twodi˙erentways:onebyusinganexternalPeltierheaterandtheotherbyusingintegrated resistiveheaters. ThesisStatement: ThedevelopmentofanIRimageprojectorisaccomplishedbyex- ploitingtheVO 2opticalmemorycapability,whichimprovescurrentimageprojectorstechnol- ogyissueso˙eringasimpleopticalsystemandprocess.Afullyelectronicallyvariableoptical attenuator,whichusesthestrongcorrelationbetweentheelectricalandopticalproperties inVO 2canprovideadvantagesintermsofsimplefabricationandattenuationself-sensing control. 1.3ThesisContributions Thisthesisaddressedtheproblemsdescribedin Section1.1 .Thecontributionsofthis thesisare:1.thecomparisonofsol-geldepositionprocessandpulsedlaserdepositionfor growingofVO 2thin˝lms,2.thedevelopmentofanear-infrared(NIR)imageprojector and3.anelectronicallycontrolledVOA.Localizedphoto-thermalexcitationwasusedto triggertheVO 2phasetransition,creatingapatternonthethin˝lmwhichallowedforthe 5projectionofanNIRimage;self-sensingtechniquewasimplementedtoreducethehysteresis oftheopticaltransmissionacrosstheVO 2thin˝lmduringitsphasetransition. 1.4DissertationOutline Theremainingworkisorganizedasfollowed: Chapter2 presentsthebackgroundofVO 2thin˝lmspropertiesanddepositionprocesses,electro-thermo-opticaldevicesincludingIR sceneprojectorsandvariableopticalattenuators.Theoptical,electricalandstructural propertiesofVO 2arediscussed.VO 2depositionisdiscussedin Chapter3 .In Chapter4aNIRimageprojectionbasedontheopticalmemoryofVO 2isdevelopedanddiscussed. AvariableopticalattenuatorforNIRispresentedin Chapter5 .Ascalingstudyofmicro sizevariableopticalattenuatordevicesispresentedin Chapter6 .Last Chapter7 shows thecontributionsofthisworkandtheproposedworkforfurtherinvestigation. 6CHAPTER2 BACKGROUND 2.1OptoelectronicDevices Optoelectronicdevicescombineelectricalandopticalsignalstodetectorconvertonesignal intoanothersignal.Nowadays,solarcellsarethemostcommonlyknownoptoelectronic devices.Theyusephotonstocreateelectron-holepairsinasemiconductor,whichgener- ateanelectriccurrent[13].Otheroptoelectronicdevicesuseelectricalsignalstoproduce, detect,manipulateorcontrolopticalsignalssuchasopticalsourcesanddetectors.Optical sourcesuseelectricalsignalstoproduceopticalradiation(e.g.photodiodes,LEDs,LASERS) [14,15],whilephotodetectorsdetectopticalradiationandconvertitintoelectricalsignals (e.g.cameras,charged-coupleddevices(CDDs))[16].IRimageprojectorsandelectronically controlledvariableopticalattenuators(VOAs)areoptoelectronicdevices,whichuseelectri- calsignalstomanipulateopticalsignals. Figure2.1showstheclassi˝cationofthesetwo optoelectronicdevices.Manyothertypesofoptoelectronicdevicesexist;buttheemphasisis giventoIRimageprojectorsandVOA,sincethosearethedevicesdevelopedinthisthesis. Opticalattenuatorsreducetheintensityofopticalsignals.Thesecomponentscanbe passiveoractivedependingontheusedtechnologyandapplications.Activeoptoelectronic devicesrequireanexternalpowersourceinordertobeoperated,whilepassivecomponents donot. Imageprojectortechnologiesarebasedinopticalsourcescombinedwithdi˙erentphysics andmaterialstodrivetheseprojectionprocessesforvisibleandIRwavelengths.Projectors technologiesuseplasmadisplays,liquidcrystaldisplays(LCD),anddigitallightprojectors (DLPs)[17,18,19]. 7Figure2.1:Classi˝cationofIRimageprojectorsandVOAsoptoelectronicdevices. 2.2InfraredImageProjectors Infraredimageprojectors,alsoknownasinfraredsceneprojectors,havebeendevelopedsince 1970'stotestIRsensors[20].Athermalsourcecoveredwithashadowmaskwasusedasan emittingIRradiationtoproduceastaticsyntheticimage.Soonafter,demandsfordynamic imageprojectorstechnologiesemerged.In1982theBlycellwasreportedasthe˝rstdynamic IRprojector[21].AvisiblelightwasconvertedtoIRbyusingaBlycell,whichconsisted ofasuspendedmembrane(gold/cellulosenitrate)invacuum.Avisibleimagewasprojected intothecell,absorbedandemittedintheIRregionbythemembrane.Extensionstothis technologyemergedduringthesamedecadewiththermalresistorarraysandliquidcrystal valvetechnologies[20].ThesedevelopedtechnologiesforIRprojectorscanbedividedin threecategories:emissive,re˛ectiveandtransmissive. 82.2.1CurrentTechnologies 2.2.1.1EmissiveIRprojectors EmissiveIRprojectorsarebasedondevicesemittingIRradiationbyJouleheating.These optoelectronicdevicescanbeformedbycurrent-driventhin˝lmresistors,suspendedmem- braneresistors,siliconbridgeresistors,Blycellsarrays,orlaserdiodearrays.Metalsand SiGearesomeofthematerialsusedtofabricateemittingresistors[22].Thermoelectricele- mentshasbeeninvestigatedforsinglepixels,whichshowedanemissivityof0.95at 130C[23].TheycomparedthisperformancewithSiemittershaving ˘0.1emissivityat 140C.PreviouslydescribedresistorsarrayshavebeenmonolithicallyfabricatedwithCMOSinte- gratedcircuitstoindependentlyactivateeachresistorasapixeltoprojectanimage[24,22]. Figure2.2showsamicro-electro-mechanical(MEMS)suspendedmembraneresistorsarray. Inthisdevicetheactiveemitterarea(imagepixel)istheisolatedheatingelement. Figure2.2:SuspendedmembraneresistorsarrayusedinemitterIRprojectors[24]. IRlaserdiodesarepartofthisemissiontechnologycategory.IRradiationfromacurrent- drivenlaserdiodearrayswereprogramedtoprojectanimageasshownin Figure2.3[12]. 9Figure2.3:IRlaserdiodearrayusedinemitterIRprojector[12]. 2.2.1.2Re˛ectiveIRProjectors Re˛ectiveIRprojectorsarebasedonmaterialsorstructuresthatre˛ectIRwavelengths andmechanismstocontrolthere˛ectiondirection.Thesemodulation-basedelectro-optical devices,suchasscanninglasersandmicromirrorsarrays.TheintensityofanIRlaserbeam ismodulatedandsteeredvertically.Are˛ectivescanningmirrorisusedtodeterminethe horizontaldirectionprojectinganimageintoanimageplane(See Figure2.4).Figure2.4:IRprojectorbasedonlaserscanningtechnology[12]. 10Digitalmicromirrordevices(DMD)modulateIRradiationbycontrollingindividualIR re˛ectingmirrorselectronically[25]. Figure2.5showstheopticalsetupforprojectingan imagewithamicromirrorarray.AnIRlampirradiatesintothemicromirrorarray.These mirrorsareelectronicallycontrolledtotilt 20intoorpositions.Oneplane isprojectedasanIRimageandtheotherisabsorbed. Figure2.5:Projectionsetupwithre˛ectiveIRmicromirrorsarray[25]. 2.2.1.3TransmissiveIRProjectors TransmissiveIRprojectorsarebasedonmaterialsandsystemsthatcanmodulateorcontrol IRradiationpassingthroughthem.ThiscategoryincludesIRliquidcrystallightvalves (LCLVs)projectorsandgalvaniccells,whichcanmodulateIRradiation.IRLCLVprojectors convertsvisiblewavelengthsintoIRwavelengths[26].Avisibleimageisprojectedtoone sideoftheLCLV.Thepatternistransferredtotheliquidcrystal(LC)causingapolarization 11changeduetomolecularrearrangement.PolarizedIRradiationisfocusedontheLCsideand rotatedbythepolarizedpattern[12]. Figure2.6showstheuseofLCLVsasIRprojectors. Figure2.6:TransmissiveIRprojectorusingaliquidcrystallightvalve(LCLV)[12]. 2.2.1.4OtherTechnologiesforIRProjectors OthertechnologiesforIRprojectorshavebeenexplored,suchasbulksiliconphotonic(trans- missive),plasmadisplay(emissive)andelectron-beam-addressedmembranelightmodulation (re˛ective)[27,28,29].ThebulkSiphotonicprojectorisbasedonopticaldown-conversion technique,whichusestheindirectbandgapofSitoabsorbshortwavelengths( 1m)andemitlongerwavelengthsintheIRthermalregion[30].Plasmadisplaysarebasedon cavitiesstructures˝lledwithgas,whichareelectronicallydriventoproduceplasma.No- blegases,COandCO 2canbeusedforIRwavelengths[28].Membranelightmodulation, alsoknownasdeformablemirrorsspatiallightmodulation,wasaddressedbyascanninge beamgun[29].AnIRsourceirradiatedthemembranewhichwasusedtomodulatetheIR re˛ectionontoanimageprojectionplane. 122.3VariableOpticalAttenuators(VOAs) Variableopticalattenuators(VOAs)areanotherkeycomponentsinopticalcommunication systems.Opticalsignalswith˝xedintensityaredesiredinthesesystems.However,since theytravellongdistancesoftenareampli˝edthroughoutthetransmissionlines.Whenthese signalsreachtheroutingormodulationsystemstheyhavedi˙erentintensitiesfromwhich theyweregenerated.VOAsareusedtoadjusttheseintensitieswiththeequalizationand synchronizationofmultipleopticalsignalsandchannels. VOAsinvisibleandNIRwavelengthsarewidelyusedinoptics,photonicsandoptical networks.Themostcommonlyknownattenuatorsareusedinfreespacecon˝guration, whilemorecomplexdesignsarecombinedwithoptical˝bersinphotonicsandcommunication networks.Theyarecommerciallyavailableforasingleopticaldensity(OD),meaningasingle attenuationlevel.Formultipleattenuationvalues,astepvariableorcontinuousvariable attenuatorwithmanualcontrolisused. VOAsoptoelectroniccomponentsareelectronicallydrivenandinteractwithopticalsig- nalswithoutdetectingorproducingthem,similartooptical˝lters,couplers,splitters,mod- ulators,andwavelength-tunableoptical˝lters. Inthelastdecade,electronicallycontrolledVOAshavebeendevelopedtoassistcom- municationnetworksdemands.Commerciallyavailabledevicesarebasedonoptoceramic materials(PMN-PTandPLZT)[31].MEMS-basedVOAsthatarecompatiblewithoptical ˝bersandelectronicallycontrolledhavealsobeenreported.Someofthemusemicromirrors andre˛ectivematerials[32,33,34],othersarebasedonmicro˛uidics,ferro˛uidics[35,36,37], andothersareassistedwithpiezoelectricmaterials[38].Plasmonic-basedVOAshavealso beeninvestigatedusingwaveguides[39,40,41].Liquidcrystalcellsfornear-IRwavelengths havebeenexploredinVOAs[42].Morerecently,anelectronicallycontrolledVOAbasedon multilayeredgrapheneoperatingat 785nmwasreported[43]. 132.3.1CurrentTechnologies 2.3.1.1Micro-electro-mechanicalsystems(MEMS)VOAs MEMS-basedVOAscontroltheattenuationbyusingplanarre˛ectionwithdeformablemir- rorsorbyblockingtheradiationwithshutter-baseddevicesusingaknifeedgetoblockpart oftheradiationpassingthroughtheopticalpath[44]. Figure2.7showsadiagramofa VOAhavinganelectronicallycontrolledmechanicalshutter. Figure2.7:MEMS-basedVOAwithopticalshutter[45]. (a)(b)Figure2.8:MEMSvariableopticalattenuatorbyretro-re˛ectionofIRlight(a)deviceSEM image,(b)workingprinciple:initialstate(left)attenuationstate(rigth)[34]. Severaldevicesemergedfromthetechnologythatusesre˛ectiontocontroltheattenua- 14tionofanopticalsignal. Figure2.8showstwore˛ectiveMEMSmirrorsplacedwithinan opticalpathtocontrolitsattenuation.Inthiscase,electronicallycontrolledfolded-beam springsmovethemirrors[34].Piezoelectricmaterialswereusedtomovere˛ectivemirror inMEMSVOAs[38], Figure2.9and Figure2.10showare˛ectiveplateandanelliptical mirrorMEMS-basedVOA,respectively.Thisdeviceusesre˛ectiontoblocktheopticalpath [32].Figure2.9:Re˛ectivemirrorMEMSvariableopticalattenuator[38]. (a)(b)Figure2.10:Ellipticalmirrorvariableopticalattenuator(a)topview,(b)sideview[32]. 152.3.1.2Micro˛uidicsandFerro˛uidicsVOAs Micro˛uidic-basedVOAscombineopticsand˛uidsinmicrodeviceswithouthavingmoving parts.ThefabricationofthesedevicesissimilartothoseMEMS-basedVOAs.Micro˛uidic VOAsstartedbyplacingavoltagecontrolledliquidlensinbetweenanopticalbeampath [46].Theoutputofanoptical˝berwasalignedwiththislensandcoupledintoa˝ber.The liquidlenswasformedattheinterfaceoftwodi˙erent˛uids.Itscurvaturewaselectronically controlledbyelectrowettingwithanexternalappliedvoltage[47].Thebeamismisaligned whenthecurvatureofthelenschanges,thusitisde˛ectedfromthe˝berinput.The schematicforthisVOAapplicationisshownin Figure2.11Figure2.11:VOAdevicebasedonvoltagecontrolledliquidlens[46]. Anopto˛uidictunableattenuatorusesaliquid-corewaveguidetochangethetransmission ofacoupled˝ber.Thetransmittedpowerchangesbycontrollingthe˛uidrateofthe waveguide,thuschangingitscorewidth[48].Thetransmissiondecreasesasthecorewidth reduces.Figure2.12showsthisworkingmechanismforthisdevice. Figure2.12:Opto˛uidicdevicewith˛uidratetunablecorewidthandattenuation[48]. 16Aferro˛uid-basedVOAshownin Figure2.13usesa˛uidicshutterwhichiselectronically controlledbyanelectro-magnet[37]. Figure2.13:Ferro˛uidbasedVOA[37]. AmagneticVOAwasdevelopedbyusingferro˛uiddispersedonamicrochanneland withferro˛uiddopedPDMS[49].Anoptical˝berwasalignedintoacantileverwaveguide suspendedinsidethismicrochannel.Thiscantileverwasactuatedbyapplyinganexternal magnetic˝eld,thusde˛ectingthewaveguideoutput.The˝berinputgetsattenuatedby thisde˛ection.UsingasimilarfabricationprocessaPDMScantileverwasdopedwiththe sameferro˛uidandactuatedbyanexternalmagnet. Figure2.14:Ferro˛uiddopedPDMScantileverwaveguideactuatedbyelectromagnets[49]. 17Figure2.14showstheattenuationofthelaserbeamwiththecantileverde˛ection. 2.3.1.3Solid-stateVOAs Solid-stateVOAs,alsoknownaselectro-chromicVOAs(ECVOA)arebasedonelectro-optic (EO)orelectro-chromic(EC)materials.TheopticalpropertiesoftheseEOorECmaterials changewhenanelectric˝eldisapplied.Someofthesematerialsarepolymercomplexes, optoceramics,organicmaterialsandmaterialsoxides[50]. RedoxelectroactivepolymershavebeenexploredinVOAsdevices,suchasPoly(3,4- ethylenedioxythiophene)(PEDOT),Poly(3,4-alkylenedioxythiophene)(PADOT),anddual polymerlayers:PDDA/Naph-SO 3Na[51,52,53].Thesepolymer-basedVOAsoperatein re˛ectivemodeasshownin Figure2.15withaswitchingspeedbetween1to5s. (a)(b)Figure2.15:Polymer-basedelectrochromicvariableopticalattenuator(a)devicediagram, (b)deviceimageinneutral(top)active(bottom)[52]. OptoceramicssuchasPb(Mg 1/3Nb2/3)O3-PbTiO3(PMN-PT),Pb 1-xLax(ZryTi1-y)1-x/4O318(PLZT)andLiNbO 3havebeenusedincommerciallyavailableVOAsoperatingintransmis- sionmode[31].Dinuclearmolybdenumcomplex(oxo-Mo(V))wasusedinanIRVOAwith appliedvoltageof1.5V[54]. 2.3.2DeviceCon˝guration VOAscanbeimplementedinfree-space,in-linewithoptical˝bersorin˝ber-gapcon˝g- uration[55,45].Thiscon˝gurationisbasedonhowthepropagatinglightinteractswith theVOA.Infree-spacecon˝gurationthelightpropagatesthroughairbeforeandafterthe VOAdevice.In-linecon˝gurationaccommodatesbare˝bersintothedeviceasshownin Figure2.16.Fiber-gapcon˝gurationusesasplitbare˝berasshownin Figure2.17.These split˝berscanbecollimatedandconditionedbeforereachingtheVOAbyusingexternal passivecomponents.Thistypeofcon˝gurationisinawayacompactformoffree-space con˝gurationsincethelightpropagatesthroughairafterthe˝berissplit.Other˝ber-gap con˝gurationshaveasplit˝berwithnoextraopticsasshownin Figures2.7and2.18.In generalthefree-spacecon˝gurationcanbeadaptedwithbare˝ber[56],dual˝bercollimator [57],or˝beroutput/inputcollimator[58]. Figure2.16:In-lineVOAcon˝guration[55]. 19Figure2.17:Fiber-gapVOAcon˝gurationswithcouplers(left),lens,dualcollimator,mirror anddi˙ractiongrating(right)[59]. Figure2.18:Fiber-gapVOAcon˝gurationwithsplit˝berandnoadditionalopticalcompo- nents[45]. 2.3.3AttenuationControl Severalopticalsystemswithattenuationcontrolusingvariableopticalattenuatorshavebeen developedforMEMSandEObasedVOAs[60,61,62].Theattenuationleveliscontrolled byclosed-loopfeedbackasshownin Figure2.19.TheoutputopticalsignalfromtheVOA issplitintoaphotodetectorandtheoutputofthesystem.Thephotodetectorsignalisused forthefeedbackcontrol. 20Figure2.19:Closed-loopcontrolsystemforaVOA[61]. 212.3.4Applications 2.3.4.1OpticalCommunicationNetworks Opticalcommunicationsnetworksuseadvancedmodulationsystemstoroutesignalsthrough- outlongdistances.Thesemodulationsystemsneedtominimizesignalscrosstalkandregen- eration.Densewavelengthdivisionmultiplexing(DWDM)systemsaremodulationsystems widelyusetopackandrouteopticalsignals.DWDMisacost-e˙ectivesolutionforoptical networkingbecauseitsharesopticalcomponentsforallthechannelsconnectedtoit[63]. Figure2.20showsthebasicopticalnetworkforDWDM.Opticalsignalswithalimited wavelengthwindow( 1˘n)arepackedbyamultiplexer(MUX)intoatransmissionopti- cal˝ber.Signalsareampli˝edasneededaccordingtothetransmissiondistance.Anoptical add-dropmultiplexer(OADM)routesthesignalsbyaddingorremovingthem.Thesesig- nalsarepackedagainintoanopticaltransmission˝berandrecoveredbyademultiplexer (DEMUX).Figure2.20:Opticalnetworkfordensewavelengthdivisionmultiplexing(DWDM)[63]. OADMs,alsoknownasrecon˝gurableopticaladd-dropmultiplexers(ROADMs)are extensivelyusedinDWDM[64].All-opticalROADMsmodulateopticalsignalswithout convertingthemintoelectricalsignalsbytransmittancemodulation[63]. ROADMtechnologiesarebasedonpolymerthermo-opticarray,waveguideandBraggs grating,opticalswitchesandvariableopticalattenuators(VOAs)[65,66,64,67].Amono- lithicROADMcon˝gurationbasedonarrayedwaveguidegrating(AWG)andVOAsintegra- 22tionisshownin Figure2.21.This9-channelsROADMusestwoAWGsand4VOAs.The packedinputsignal( 19)isrecoveredintoninesignalsby AWG1 .1goestotheoutput channel.Fourchannels( 25)aresettoaddordropsignalsattheinputof AWG1 andoutputof AWG2 .FourVOAscontroltheintensityfor 69andconnectbothAWGs. Theoutputsignalsaremultiplexedby AWG2 intofourdropchannelsandonepackedsignal containing 1and69.ElectroniccontrolofVOAarraysarerequiredtocontrolopticalsignalsandtoadvance thisROADMtechnology. Figure2.21:Arrayedwaveguidegratingandvariableopticalattenuatorintegration(AWG- VOA)[67]. 2.4VanadiumDioxide Vanadiumdioxide(VO 2)isatransitionmetaloxideexhibitinganinsulator-to-metaltran- sition(IMT).Theelectrical,optical,mechanicalandstructuralpropertiesofVO 2change duringtheIMT.Thethermally-inducedIMTtransitionoccursaround ˘68C[1,8,68,69]. VO 2isoneofthestablephasesofvanadiumoxides,whichexhibitsatransitionnearroom temperature[70]. Figure2.22showsthetransitiontemperatureforothervanadiumoxides. Vanadiumpentoxide(V 2O5)alsoexhibitsanIMTforatransitiontemperatureof ˘280C23[71].Manyresearche˙ortshavebeenputtoinvestigatewaystomanipulatethistransi- tiontemperatureinVO 2.ThetransitiontemperatureofVO 2coulddecreaseorincreaseby substitutionaldopingwithCr,Ti,W,Fe,amongotherelements[72,73,74,75]. Figure2.22:Insulator-to-metaltransitionindi˙erentvanadiumoxides[70]. ThedrivenmechanismsforthistransitioninVO 2stillunderdebateinthescienti˝ccom- munity.Recently,thebehaviordescribedbyMott-Peierlstransitionwasreported[76].The Peierlsmechanism(relatedtotheatomicstructure)andtheMotttransitiondescribedasthe 24closureoftheMottbandgap(around0.7eV)havebeenextensivelystudiedasindependent mechanisms,butintheparticularcaseofVO 2,bothmechanismsarerelated[77,78,79,80]. Duringthistransition,VO 2undergoestheinsulator-to-metaltransition(IMT)anda structuralphasetransition(SPT).TheIMTisrelatedtothe˝lm'schangeinelectrical resistivityandtheSPTisassociatedtothecrystallinestructuretransition[1,81,82].These transitionsarestronglycorrelatedsincethechangeintheatomicstructurecausesareduction intheenergybandgapofthematerial.ThisVO 2'stransitionisfullyreversibleanditoccurs relativelyfast( ˘2ns)whentriggeredbyvoltagepulses[83]. 2.4.1StructuralTransition ThestructuralphasetransitioninVO 2issolid-to-solidphasetransition,inwhichthematerial crystalstructurechangefrommonoclinic(M 1)atroomtemperaturetorutile(R)athigher temperatures.Duringthestructuraltransitiontheinterplanardistancesa,bandcchangeas thecrystalstructureisreordered.Thedistancesare am=5:75Å,bm=4:53Å,cm=5:38Åand ar=br=4:55Å,cr=2:85Åforthemonoclinicandrutilephase,respectively.Thec rdirectioncontractsaftertheSPTtransition[84].Thestructurechangeisshownin Figure2.23.Figure2.23:VO 2structuraltransitionrepresentationoftheunitcells[5]. PreviouslycharacterizedVO 2thin˝lmsgrownPLDshowedhighorientationinthe(011) monoclinicplanesatroomtemperaturewiththec rdirectionorientedparalleltothesubstrate 25(SiO2andSi).Duringthetransition,asthec rcontracts,stressisintroducedonthe˝lm, changingthematerial'smechanicalproperties. 2.4.2ElectricalTransition Theinsulator-to-metal(IMT)transitioninVO 2wasinitiallydescribedbycrystal˝eldtheory [85].Itwasdescribedthattheunusualpropertiesintransitionmetaloxidesweredueto electronicreorderingintheouterdorbitals[86]. Thevanadiumatomsoccupytheoctahedralinterspacesbetweenoxygenatomsinthe tetragonalphaseasshownin Figure2.23.Hybridmolecularbands(d llandd ˇ)areformed bytheoxygen2pandvanadium3douterorbitals[87].Thesetwobandsareneartothe FermienergylevelandparticipateintheIMT[88].Thed llbandsplitintovalence(bonding, lowenergy)andconduction(anti-bonding)bandsresultinginabandgapof ˘0:7eVforthe insulatorstateasshownin Figure2.24.Duringthemetallicphase,thed ˇshifttolower energiesandthed llbandsmergeclosingtheenergybandgap. Figure2.24:BanddiagramofVO 2insulatorstate(left),metalstate(right)[88]. 262.4.3OpticalTransition DuringtheVO 2'sopticaltransitionthetransmission,re˛ectionandabsorptionallchange. Recentstudiessuggestthisthechangeintheopticalandelectricalpropertiescomesasthe resultofthe˝rst-orderinsulator-to-metal-transition(IMT)inVO 2,whichisnotdrivenby thestructuralphasetransition(SPT)[68,89]. TheVO 2transmissionchangesabruptlyfrom 900nmto 2500nmwavelengthsduringthe IMT[8].Themagnitudeofthischangeincreaseswithwavelengthuntilterahertzfrequencies (=300µm)[10].ThisopticalswitchingcapabilityofVO 2hasbeenexploredformid-IRto NIRimageconversionandopticalmemory[90,91]. Figure1.1showstheopticaltransition inVO 2for=1550nm.2.4.4OpticalMemory TheopticaltransitioninVO 2exhibitsahystereticbehavior.ThehysteresisinVO 2thin˝lmsmakeitpossibletouseitasanopticalmemory.Thismemorycapabilityhasbeen investigatedformultipleopticalstatesprogrammedintoaVO 2thin˝lmasasfunctionof photo-thermalexcitation(see Figure2.25)[91]. SeveraltechnologiesemergedfromthisopticalmemoryinVO 2.Amongthe˝rstdevices, singlecrystalVO 2wasusedtodevelopanIRopticalmodulator,whichexhibitedaveryslow switchingresponseduetothesingle-crystalthicknessandaresistiveheaterusedtoinduce theswitching[6].Later,thematerialâ•ŽsswitchingresponsewasimprovedbyusingaVO 2˝lmandJouleheatingthroughdoped-Siheatingresistors,andanIRshutterwasdeveloped [7].Morerecently,amillimeter-sizeswitchableIR˝lterarraywasdevelopedusingVO 2asthethermochromiclayer,diamondasaheatsink,chalcogenideglassforthermalinsulation withthesubstrate,andPtelectrodesasresistiveheaters[92]. 27Figure2.25:MultipleopticalstatesinVO 2programedbyphotothermalactuation[91]. 2.4.4.1OtherOpticalMemorySystemsandMaterials Opticalmemorysystemsemergedinthe1970swithholographicdatastorage[93].After decadesofresearchcompactdisks(CDs)werereleasedin1983[94].Thesesystemsevolved todigitalvideodiscs(DVDs)andblue-raydiscs.Inthesesystemsthedataisstoredas mechanicalindentsorpits(darkspots)ontoadisk.Thesedisksareopticallyreadbasedon areadoutlaserre˛ection[95]. Figure2.26showsthediagramfortheCDsopticalreadout system.Alaserbeamiscollimatedandpolarizedbeforepassingthroughaquarterwave( 4)plate,whichintroducesa90 phasedi˙erence.Thecircularpolarizedbeamisre˛ectedinto amirrorandfocusedintothedisk.There˛ectivecoatingontheCDre˛ectsthebeamback intothe 4platechangingitspolarizationagainwithaphaseshiftof90 .Thebeamis re˛ectedatthebeamsplitterinterfacetowardsthephotodiode.Thereadinglaserincident onthepitsontheCDwillnotreturntothereadoutsystemduetopoorre˛ectionand destructiveinterference[95].ThedatastoragecapacityofCDswasincreasedbyreducing 28thepitsizeandthereadinglaserspotsizeintheopticalreadoutsystem. Figure2.26:OpticalreadoutsystemtoretrievestoreddatainCDs[94]. Othertechnologieshavebeenexploredforopticaldatastorage,suchasliquidcrystals (LCs),photo-polymerization,photo-refractivecrystals,photo-bleachingand3-dimensional bits[96,97].CrystallinephasemoleculesaredispersedinapolymertoformLCs.These moleculesarepartiallyorientedatroomtemperature,knownasitsanisotropicphase.This orientationcanbechangetoanisotropicphasebyincreasingthetemperatureaboveitsglass temperatureT gorapplyinganexternalelectrical˝eld.DataisstoredinLCsbylocalized laserheating.Similarly,datacanbestoredbyphoto-polymerizationinagelcontaininga monomerandaphotosensitivematerial.Photo-inducedlocalizedpolymerizationchanges thedensityofthematerialandconsequentlychangesthematerial'srefractiveindex.The storedopticalpatternisbasedinthisrefractionindexchange. Opticalstorageinphoto-refractivecrystals(e.g.Fe:LiNbO 3)isalsobasedinthechange ofthematerial'srefractionindex.Thisindexchangeswhenanelectric˝eldisgenerateddue toaspace-chargeinthematerialwhenaphotonisabsorbedandfreeelectronsaregenerated. 29Photo-bleachingisusedforpermanentdatastorage.Apolymerblockmaterialcontaininga ˛uorophoreiswrittenbyafocusedhighintensitylaser( 1mW),whichdecomposeorbleach the˛ourophorepreventingittohavefurtheropticalemission.Alaserwithlessintensity( 1mW)incidentonthematerialwillinduce˛uorescenceintheareasnotwritten. 3-dimensionalbitsor3-Dopticalmemoriesarebasedonlayersofdi˙erentmaterials. Thedataisstoredindepthlayer Figure2.27showstheschematicdiagramfora3Doptical memory.Glasslayerswereusedforpermanentdatastoragesimilartoengraving[98].Also photo-polymers,photo-refractivematerialsandtransparentmaterials(glass)areusedfor these3-Dmemories.Aquantumopticaldatastoragewasreportedbyusingalocking techniqueforquantummemory[99]. Figure2.27:3-dimensionalmemory[98]. Photochromicmaterials(fulgides)andelectrontrappingmaterials(Sm 3+-dopedsul˝des Ca(Sr)SandY 2O3)areusedforopticalmemory[100,101].Moreover,chalcogenidesalloys suchasGe 2Sb2Te 5,Sb xSe1x,In x(Sb70Te 30)1x,Gex(Sb70Te 30)1x,Ag xIny(Sb70Te 30)1xyareusedasrewritablematerialsinphasechangememories[102].ThesedopedSb 70Te 30al-loysarecommerciallyusedmaterialsinrewritableDVDs.Ge 10Sb90amorphousnanodots wereinvestigatedforanewgenerationofrewritableopticaldisk[103].TheseGe 10Sb90nanodotscomparedwithGe 10Sb90thin˝lmsreducethethermaldi˙usionandshowrapid 30crystallization(nsrange)increasingtheopticalmemoryspeed.Thematerialwaswritten bylaser-inducecrystallizationusing300pspulses.Ahighdensitymemorywasachievedby growingGe 10Sb90nanodotsstructure.Theopticalmemoryofthisphase-changematerial wasstoredintheamorphousphasetocrystallinephasetransition. 2.4.5Self-sensingFeedbackTechnique Self-sensingfeedbacktechniqueisbasedonmeasurementandestimationoftwodi˙erent systemparameters.Oneparameterismeasuredwhileanotherisestimated.Thistechnique hasbeeninvestigatedformicroandnanomanipulationusingmainlyinactuatorsfordis- placementandappliedforcecontrol[104].Inconventionalmanipulationsystemsexternal sensorsorembeddedcomponentsarenecessarytoprovidefeedbackforthecontrolofthe displacement.Manipulatorsandactuatorsdisplacementcontrolcanbesimpli˝edbyusing thisself-sensingfeedbacktechniqueeliminatingtheneedforexternalcomponents. Severalsmartmaterials-basedactuatorshavebeenusedtoimplementtheself-sensing technique.Actuatorsbasedonbi-layercantilevers,piezoresistors,leadzirconatetitanate (PZT),ionconductingpolymermetalcomposite(IPMC),smartmemoryalloys(SMAs) andVO 2werereported[105,106,107,108,109,110].Thistechniquealsonamedasself- sensingactuation(SSA)enablesactivecontrolintheseactuatorsbytheestimationofthe displacementbasedonameasuredparameter.InthePZTbasedactuatorsthecapacitance ismeasured,whileintheotheractuatortheresistanceismeasuredorsensed. Figure2.28showstheself-sensingtechniqueusedtoestimatedisplacementinVO 2-basedmicroactuators. 2.5VO 2DepositionProcesses VO 2thin˝lmscanbedepositedbychemicalvapordeposition(CVP),sol-gelsynthesis, atomiclayerdeposition(ALD)andphysicalvapordeposition(PVD)processessuchaspulsed laserdeposition(PLD),electronbeamevaporation,andsputtering.Thedepositioncondi- 31Figure2.28:Self-sensingtechniqueusedinVO 2toestimatede˛ectionbysensingtheresis- tance[111]. tionsarecrucialtoobtainstoichiometryVO 2.Itcanbeobtainedformostdepositiontem- peraturesasshowninthevanadiumoxidesphasediagramin Figure2.29.Amixingofthe otherstablevanadiumoxidesphasesrepresentachallengetogrowstoichiometricVO 2.TheVO 2thin˝lmqualityandIMTisin˛uencedbythesedepositionprocessandthese- lectedsubstrate[113].VO 2hasbeenprimarilydepositedonsapphire(Al 2O3),glass(Quartz, fusedsilica),andsilicon(Si)substrates.Othersubstratessuchassiliconnitride(Si 3N4)[114], Germanium(Ge)[115],aluminum(Al)[116]andindiumtinoxide(ITO)[117]havebeen investigated. Thecharacterizationofthesethin˝lmsinmainlypresentedbyX-raydi˙raction(XRD) forthe 2angleandbyRamanspectroscopy.VO 2orientedinthe011planesshowsastrong peakaround 28°.VO 2Ramanspectroscopycharacteristicpeaksarearound192,224,309, 389,612and824 cm1.32Figure2.29:Vanadium-oxidephasediagram[112]. 2.5.1ChemicalVaporDepostion(CVD) VO 2thin˝lmsdepositedbyCVDandsol-gelsynthesisinvolveachemicalreactionorde- compositionprocessusingavanadium-basedprecursor.Similarprecursorshavebeenused forCVDandsol-gelsynthesisinvapororliquidstate,respectively.Someoftheusedpre- cursorsarevanadiumoxychloride(VOCL 3),vanadiumacetylacetonate(V (C5H7O2)4)and vanadiumtri-isopropoxideoxide(VO (OC 3H7)3)[118].Thedepositioncanbeachievedat roomtemperaturebyhydrolysisfollowedbypostannealing.CVDdepositionsathighertem- peratures( ˘400C)ofteninvolvepyrolysisthermochemicaldecompositiontoproduceVO 2thin˝lms[119].Low-pressureCVD(LPCVD),atmosphericpressureCVD(APCVD),and aerosol-assistedCVD(AACVD)havebeeninvestigatedtoproducethesethin˝lms[113]. Figure2.30showsXRDforVO 2grownbyCVDoverSi(100)substrate. Figure2.31shows 33theRamanspectraforVO 2onglassgrownbyCVD. Figure2.30:X-raydifractionforVO 2grownbyCVDoverSisubstrate[115]. Figure2.31:RamanspectrumforVO 2grownbyAPCVD[118,120]. 342.5.2AtomicLayerDeposition(ALD) ALDisasurfacereactionprocesswithaccuratethicknesscontrol.Thisprocessusereactant gasesandvanadium-basedprecursorsingasorliquidstate,similartoCVD.Insomecases apostannealingisnecessarytoproduceVO 2thin˝lms.ALDo˙ersaccuratethin˝lm thicknesscontrol.However,thisprocessisverysensitivetotheusedprecursorandthe depositionconditions.ThismakesitchallengingtoachievestoichiometricVO 2[121].A mixingofvanadiumoxidephasesoftenoccursduringtheseALDdepositions[122].Alsothese systemsareexpensivecomparetootherdepositionsystemsandrequirehighmaintenance. XRDresultsforVO 2depositedonglassatdi˙erenttemperaturesareshownin Figure2.32[123].Twodi˙erentphasesforVO 2monoclinic(MandM')wereobservedat 400C.ThisVO 2M'phasedoesnotexhibittheIMTanddisappearsforVO 2˝lmsdepositedat highertemperature.StoichiometricVO 2wasobtainedat 475Cusingvanadiumacetylace- tonateasaprecusorandoxygenasthereactantgasforthedeposition. Figure2.32:X-raydi˙ractionforVO 2grownbyALDonglasssubstrate.VO 2obtainedat 475C[123]. 352.5.3Sputtering SputteringdepositionprocessesincludeDCsputtering,radiofrequency(RF)sputteringand magnetronsputtering.Duringthisprocess,ionizedgashitatargetremovingmaterialatoms fromit.Theseatomsgetdepositedonasubstrate.Magnetronsputteringcanbeimple- mentedwithmagnetically-assistedDCorRFsputteringtoimprovetheuniformityofthe thin-˝lm[118,124]. Vanadiummetalorvanadiumoxide(VO 2,V 2O5)targetmaterialsareused[125]with inertgasoragasmixture(Ar-O 2)[124].StoichimetricVO 2grownbythistechniqueshows similarresultsasthosepresentedforXRD,Ramanspectroscopyandresistancedrop. 2.5.4Evaporation Electron-beamandthermalevaporationprocessesarealsousedtogrownVO 2thin˝lms. ThesePVDprocessesareverysimilartosputtering.Anelectronbeamisusedtoremove materialfromaVO 2targetoravanadiumtargetwithAr/O 2gas[113,126],whilethe substrateismaintainedatroomtemperature. ThermalevaporationdepositionconsistsofaVO 2thin˝lmsgrownbythermalevap- orationore-beamevaporation.Thesubstrateismaintainedatroomtemperatureduring thisdepositionprocess,whichisnotwidelyusedforVO 2thin˝lms[118].VO 2thin˝lms depositedbythismethodshowaresistancedropacrosstheIMToflessthantwoordersof magnitudeforSiandglasssubstrates[127]. 2.5.5Sol-gelDeposition Sol-geldepositionisawetchemicaldepositionprocess.SimilarlytoCVDandALDprocesses itusesavanadium-basedprecursors(sol)toproduceVO 2thin˝lms. Di˙erentV-precursorsusedtoproduceVO 2thin˝lmsarecommerciallyavailableinliquid phasesuchastriethoxyvanadyl,vanadiumoxyacetylacetone,andvanadiumisopropoxide 36[128,129,130].Otherscanbepreparedwithvanadiumpentoxide(V 2O5)orVpowders dissolvedinhydrogenperoxide(H 2O2)toformthesol[131,132].Otherswerebasedon ammoniumvanadate( NH 4VO 3)andvanadiumtetrachloride(VCL 4)[133,134,135].VO 2thin˝lmsdepositedbythisprocesscanbeeasilydopedbyaddingmetalpowdersintothe precursoratitsliquidstate. TheV-precursorisdilutedinasolventandthensynthesizetoformagel.Hydrolysis orpolycondensationreactionsoccurduringthesynthesiswhentheV-precursorreactswith wateroranothermetalhydroxide[136].Thesynthesizedgelisdepositedintoasubstrate bydipcoating,spraycoatingorspincoating.Thesubstrateisheatedafterthisdeposition toremoveremainingsolventsfromthesubstrate.Furtherthermalannealingisrequiredto crystallizethe˝lm.Thisdepositionmethodo˙erssomeadvantagescomparewithother depositionprocesses:1)low-cost,2)relativelylargeareadeposition3)lowdepositiontem- perature,andfeasiblemetaldopingforVO 2thin˝lms[118]. Thissol-gelprocessissensitivetotheannealingconditions.Thetemperatureandtime playsacrucialroleinthecrystallizationprocessofVO 2.Severalstudiesfocusedinthe optimizationoftheseannealingconditionstoimprovetheVO 2quality. TheXRDspectraforVO 2thin˝lmsgrownonSi(100)atdi˙erentannealingtemper- aturesareshowninFigures2.33and2.34.In2004Pan etal. ([129])reportedtheVO 2crystallizationstartingat 550ConSi(100)substrates.The˝lmswereannealedfor30min inN 2atmosphere.Additionalannealingat 600Cfor5minimprovestheVO 2crystallizationasshownin Figure2.33.Lateron,Shi etal. ([137])reportedVO 2crystallizationat 500CalsooverSi(100)fora 150nmthin˝lms(See Figure2.34).Theannealingtimewasthesameforalltemperatures (1.5hrs)inN 2atmospherewithaheatingrate 8C/min.Thesameyear,Vinichenko etal. reportedastudyforSiandSiO 2substratesusingalayerwisespincoatingmethod[128].VO 2thin˝lmsof190nmweregrownforanannealingtemperature 450CinAr/H 2atmospherefor15min.XRDresultsforthis˝lmareshownin Figure2.35.VO 2thin˝lmswiththe 37Figure2.33:X-raydi˙ractionforVO 2grownbysol-gelonSi(100)substratesatdi˙erent annealingtemperatures[129]. Figure2.34:X-raydi˙ractionforVO 2grownbysol-gelonSi(100)substratesatdi˙erent annealingtemperatures[137]. samethicknessweredepositedonSiO 2byusingthesameprocesssol-gelprocess,annealing temperatureandatmosphere.Theannealingtimewasincreasedto40mintoobtainthe VO 2stoichiometry.TheresultsforSiO 2˝lmsareshownin Figure2.36.Manyoptimizedconditionsarepresentedintheliterature.However,theseconditions 38Figure2.35:XRDresultsforaVO 2thin˝lmsonSiforanannealingtemperatureof 450Cfor15min[128]. Figure2.36:XRDresultsforaVO 2thin˝lmsonSiO 2foranannealingtemperatureof 450Cfor40min[128]. variedforeachsystemandtheselectedsubstrate.Theseresultsdemonstratetheimportance tooptimizetheannealingconditionsforwhenusingsol-gelprocess. VO 2thin˝lmsgrownbysol-gelareporouscomparewithotherdepositionmethods. Figure2.37showsthetopographyfortheVO 2˝lmof2x2 mareafromShi etal. results[137].SEMisusedtoevaluatethethicknessofthese˝lms[138]. Figure2.38showsacross- sectionmeasurementofaVO 2thin˝lm.Pro˝lometerisnottheprefermethodtomeasure sol-gelgrownthin˝lmsduetotheroughnessandtopographycharacteristicofthese˝lms. 39Figure2.37:AFMtopographyofVO 2thin˝lmsbysol-gelonSi(100)[137]. Figure2.38:ThicknessmeasurementofVO 2thin˝lmsusingSEMcross-sectionalimaging [138].2.5.6PulsedLaserDeposition(PLD) Pulsedlaserdeposition(PLD)isanotherPVDprocess,whichissuitableforoxidesdeposition[118]. Thisprocessisbasedonlaserablation,whereanexcimerlaser(ArF,KrF,XeCl)isfocused intoarotatingtarget(V,V 2O3)[139,140,141].Materialisremovefromthistargetand depositedintoasubstratelocatedoppositetothistargetasshownin Figure2.39.Ar/O 2gasmixtureorO 2gasareusedforthisprocess.Thesubstrateismaintainedatacertaintem- peratureduringthedeposition,usuallyatelevatedtemperatures[118].Oftenthisdeposition isfollowedbyapostannealingsteptoobtainstoichiometricVO 2.40Figure2.39:SchematicforaPLDsystem[118]. ThesubstratetemperatureforVO 2thin˝lmsrangedfromroomtemperatureto ˘630C[142,143,139].VO 2thin˝lmsweredepositedatasubstratetemperatureof 520Cwith90 mTorrofAr/O 2gaspressure.TheseconditionswereusedtodepositVO 2thin˝lmsonSi, SiO2andAl 2O3substrates.Figure2.40showstheXRDspectraforthesesubstrates. Figure2.40:XRDspectraforVO 2thin˝lmsoverSi(100),SiO 2,andAl 2O3byPLD[141]. DuringthissubstratestudySoltani etal. foundthatthethermochromicpropertiesof VO 2werenota˙ectedbytheselectedsubstratewhenusingthesamedepositionconditions 41[141].Moreover,Kim etal. showedaresistancedropof 4+ordersofmagnitudeforVO 2thin˝lmsgrowthonAl 2O3.Thesethin˝lmsweredepositedat 630Cwith30mTorrofO 2pressurewithnoannealing[139]. Marvel etal. studiedthein˛uenceofthreedepositionprocesses(includingPLD)on di˙erentsubstrates:Si(100),SiO 2andAl 2O3(0001)[113].AmorphousVO 2thin˝lms weredepositedonthesesubstratesatroomtemperaturewithapressureof11mTorrofO 2.These˝lmswereannealedat 450Cwith25mTorrofO 2fortimeintervalsof2,5,10, 30,and90min.HighlyorientedVO 2thin˝lmsresultedafterthiscrystallization/annealing step.RamanspectraforVO 2˝lmsoverSi(100)andSiO 2areshownin Figures2.41and 2.42,respectively.VO 2˝lmsstarttocrystallizeonSisubstratesafter5min.Itshowsthe strongestpeakaround640 cm1fora30minannealing.Amixingofvanadiumoxides phasesoccursforSiO 2substratewhendepositedafter5min. Figure2.43showstheoptical transitionofaVO 2thin˝lmoverSiO 2substrateannealedfor5min[113]. Figure2.41:RamanspectraforVO 2thin˝lmsonSi(100)substratefordi˙erentannealing times(2,5,10,30,and90min).VO 2strongpeaksshownforthe30mindata[113]. Thesesubstrateswerecomparedforitsopticalhysteresiscontrastasa˝gureofmeritfor thestudy. Figure2.44showshowAl 2O3outperformsamongSiandSiO 2substrates.42Figure2.42:RamanspectraforvanadiumoxidesonSiO 2substratefordi˙erentannealing times:2,5,10,30and90min[113]. Figure2.43:OpticaltransitionofVO 2overSiO 2for1550nm[113]. 2.6Summary Inthischapter,abackgroundoninfraredimageprojectors,electronicallycontrolledvariable opticalattenuatorsandVO 2waspresented.Electro-opticalmaterialssuitableforinfrared 43Figure2.44:HysteresiscontrastforPLDdepositedVO 2thin˝lmsonAl 2O3,SiandSiO 2[113].applicationswerediscussedwiththedevices.Threeoperationtechnologieswerediscussed forimagesprojectors(transmissive,re˛ectiveandemissive).Solid-state,MEMSandliquid crystalbaseddeviceswerediscussedaselectronicallyvariableopticalattenuators.Attenua- tioncontrolwasdiscussedasakeyparameterforopticalsystemintegration.Thetransition behaviorfortheelectrical,structuralandopticalpropertiesofVO 2wasdiscussed.Aback- groundonopticalmemorydevicesandsomeopticalstoragematerialswasprovided.VO 2thin˝lmdepositionprocesseswerepresentedwithemphasisinsol-gelsynthesisandpulsed laserdeposition(PLD).ThefollowingchapterspresenttheVO 2depositionprocesses;the opticalmemoryofVO 2anditsapplicationforinfraredimageprojectors;andvariableoptical attenuatorwithself-sensingcontrolbasedontheVO 2electro-opticalcorrelation. 44CHAPTER3 VO 2THINFILMSDEPOSITION VO 2thin˝lmsweregrownbypulsedlaserdeposition(PLD)andsol-gelsynthesis.PLD depositionsystemallowsfordepositionsonsubstratesaslargeas2inchesindiameter.Sol- gelprocesswasinvestigatedtodepositVO 2insubstratesaslargeas4inchesindiameter. Theinvestigatedmethodminimizedtheprecipitatesformedduringthesol-gelsynthesis, thusreducingthe˝lm'scracksformationasthesolventevaporates.Di˙erentsolvents,con- centrations,substrates,dyingmethods,andannealingconditionswereinvestigated.VO 2wassuccessfullygrownonfusedsilicaquartz 1:6cm2squaresubstrates(1mmthick)and SiO2/Si(100)(1/500 mthick)4incheswafers.TheVO 2thin˝lmswerecharacterized byx-raydifraction(XRD),Ramanspectroscopy,atomicforcemicroscope(AFM),scanning electronmicroscopeSEM,andasurfacepro˝lometer.Theelectricalandopticalproperties wereinvestigated. 3.1Sol-gelDepositionProcess Vanadium(V)triisopropoxideoxideprecursor(VTOP,Alfaaesar 96%,VO(OC 3H7)3)was usedasthesolforthissynthesis.AnhydrousmethanolwasaddedtotheVTOPsoltohelp withthesolubilityofthesolutionandtocontroltheviscosityofthegel.Distilled(DI)water wasaddedas˝nalcatalystcomponentintothesolutiontoinducethegelationprocess.The formedaqueousgelwasspincoatedintoSiO 2andSiO 2/Sisubstrates.Theexcessofsolvent onthe˝lmwasremovedbyadryingsteponahotplate.Thesampleswereannealedatone stepusingatubefurnacetocrystallizetheV 2O5thin˝lmandpromoteitsreductioninto VO 2.Theoverallreactionofthissynthesis, 2VO(OC 3H7)3+3H 2OV2O5+6OHC 3H745isbasedonhydrolysisreactionbyaddingH 2O.AmorphousV 2O5andsolventsaretheby productsofthisreaction.AfterremovingtheexcessofsolventOHC 3H7withthedryingstep theremainingV 2O5iscrystallizedandreducedtoVO 2.V2O52VO 2+12O2Adiagramoftheusedprocessisshownin Figure3.1.Figure3.1:Summarizedsol-gelprocessusedtodepositVO 2thin˝lms. 3.1.1Substrate VO 2thin˝lmswiththicknessofapproximately 100nmweregrownon 5cm2andsubstrates ofSiO 2.Squarefusedsilicaquartzsubstrateswithsurfaceareaof0.5x0.5inchesand thicknessof 1mmwereusedtodeterminetheprocessforgrowingVO 2.Inordertomake theprocesscompatiblewithstandardmicrofabricationprocess,substrateswereused.A 1µmlayerofSiO 2wasdepositedbylowthermaloxidation(LTO)processovera4inches diameterSiwafer(SiO 2/Si).Thesesubstrateswerecleanedinacetoneandmethanolwith ultrasonicbath.The4"waferwascutintofourpiecestoanalyzetheuniformityoftheVO 2thin˝lm. Figure3.2showstheresulted˝lms.Hereinafter,pieceA,B,CandDofthe SiO2/Sisubstrate. 3.1.2Synthesis Di˙erentsolventswereexaminedatthesameconcentrationtoconsidertheprecipitates formed.Thesolventwasdeterminedempiricallybasedontheobservedbehaviorofthe solution.Thesolutionswerepreparedwith624mgofVTOP,havingaconcentrationof 46Figure3.2:VO 2/SiO2/SiwafercutintopieceA,B,CandDforfurthercharacterization. 0.38M.Thesolventswereaddedatapproximatelythesameratetoavoidstirringupthe solution.Table3.1describesdi˙erentsolventbehavior.Anhydrousmethanolwasselected basedontheseexperiments. Table3.1:Solvente˙ectongelprecipitatesformation. Solvent Observedbehavior IsopropanolPrecipitatesformedinstantly. Solidlayerformed. Acetone Highlyviscousgelformedquickly, gelnotsuitableforspinning. EthanolPrecipitateswereformed, nonhomogenousgel. MethanolAqueoushomogenoussolution. Lastfor 1hbeforeaviscousgelformed. Anhydrous Samehomogeneityasmethanol, methanolrepeatableresultsaftertheannealingprocess. Thegelviscosityvariedwiththesolventconcentration.Highsolventconcentrations producedaqueousgel,aviscousgelwasachievedbydecreasingtheconcentration.Di˙erent anhydrousmethanolconcentrationswereexaminedinordertohaveanaqueousgelthat spreaduniformlyduringthespincoating.Theconcentrationofmethanolwasvariedfrom 60,65,70,75,and 80%oftheinitialvolume(4.3mL).Precipitate-freesolutions(tothe nakedeye)wereachievedwith 70%.473.1.3GelDeposition Thegelwas˝lteredwitha 0:45µmand 5µmporesizeMillipore˝lters.The 0:45µm˝lters weresaturatedduetoasmalloutputopeningwhilecoveringalargearea. 5µm˝lters demonstratedtoimprovetheuniformityofthe˝lmcomparedwithno˝ltering,asseen underthemicroscope. Thegelwasspin-coatedonthesubstrates.Thespinningprocessconsistedoftwosteps: spreadingthesolutionatlowspeedtocoveruniformlytheentirearea,andspinningat higherspeedtohaveauniformthickness.Thegelwasspreadfor 5sat600rpm,followed byaspinningat2,000rpmfor 30s.3.1.4DryingTreatment Adryingtreatmentfollowedthedepositiontoevaporatethesolventsby-productsofthe synthesisreaction.Twomethodswereevaluated:IRdryingandhotplatebaking.Inthis stepwasimportanttodetermineifthereisane˙ectonthe˝lmwhetherthe˝lmisheated fromtoporbottom.Thetemperatureonthesubstratewascalibratedusingthesame thermocoupleforbothmethods.Twosamplesweredepositedandtestedunderthesame heattreatmentfor4minat 190C.Acustom-builtIRdryerwasused.Thissampleshowed degradationanddiscontinuousspots.Therestoftheexperimentswerecarriedusingahot platetodryallthesamples. 3.1.5Annealing TheannealingconditionswerestudiedinitiallyforSiO 2substrates.xsampleswere annealedinatubefurnaceunderreducingatmospherewithAr-H 2(95-5%)gas˛ux.The temperatureheatingratewas ˘10C/minforallthecasestoavoidcrackformation.The coolingratewasnotcontrolled,andtheheaterswereturnedo˙attheendoftheannealing. ThetemperaturewascontrolledusingLabviewsoftwaretoensurearepeatableprocesswith 48accurateannealingtime.Thesubstratetemperaturewascalibratedwithrespecttothe controllertemperature.Thetemperaturepresentedisthesubstratetemperature. Abackgroundpressurebetween 1x104Torrand 7x105Torrwasreachedbeforein- creasingthetemperature.Theannealingtemperature,pressure,andtimewerestudied. Sampleswereannealedat 500Cand37mTorrfor1,1.5,2,3,and4hours. Figure3.3showstheresistanceasafunctionoftemperaturecurvesusingaheatrateof ˘0:4C=sfor themeasurablesamples.The4hourannealingprocessdegradedthesample.Anannealing timeof2hourswaschosenastheoptimumtimeforfurtherprocessing.Thissampleshowed hystereticcurvewithanoticeableresistancedropfrom383.6k to13.8k ,having ˘1.4ordersofmagnitudedrop. Figure3.3:Annealingat 500Cvaryingtheannealingtime. 49 %æ %æ %æ %æ %æ %æ %æ %æ %æ %æ %æ !& %æ %æ %æ %æ %æ %æ %æ !& %æ Figure3.7:ElectricalcharacterizationofVO 2/SiO2/SithewaferpiecesA,B,CandD. 3.2CharacterizationofVO 2depositedbysol-gel TheVO 2thin˝lmsgrownbysol-gelmethodwerecharacterizedtoinvestigateitscomposi- tion,topography,uniformity,electricaltransition,andopticaltransmission. 3.2.1ElectricalTransition Thephasetransitionwas˝rstcharacterizedbythedropinelectricalresistance.Theresults forthischaracterizationatdi˙erentannealingtimeswaspresentedintheprevioussection. 53ThecharacteristichystereticbehaviorofVO 2wasobservedasthetemperatureofthe˝lm wasvariedat ˘0:4C=sacrossthephasetransitiontemperature.Theresistancedroparound 68CandthetypicalhystereticbehaviorareanindicationofVO 2˝lm.3.2.2Composition ThecompositionandorientationoftheVO 2thin˝lmswereinvestigatedbyXRDandRaman spectroscopy.XRDmeasurementswereperformedusingaBruker-AXSHighResolutionX- raydi˙ractometer. Figure3.8showsthespectraforthe˝lmsdepositedonSiandSiO 2/Sisubstrates.Thesubstratebackgroundwasremovedfromthespectrum.TheVO 2character- isticpeakaround28 isobservedforbothsamples.Theseresultsdemonstratesomelevelof crystallizationinthesol-geldepositedVO 2thin˝lms. Figure3.8:XRDcharacterizationofVO 2/SiO2sampleannealedat 428Cfor2hrsat37 mTorrandVO 2/SiO2/Sisampleannealedat 461Cfor2hrsat15mTorr. 54Ramanspectroscopywasmeasuredusingwithawavelengthof532nm.Thin˝lms depositedonSiO 2andannealedat 417C,428Cand 450Cwereevaluated.Also˝lms depositedonSiO 2/Sisubstratesfor 406C,439Cand 461C.VO 2characteristicspectrum wasobtainedat 417CforSiO 2substrates.Amixingofvanadiumoxidesphasesappears asthetemperatureincreasessimilartothosepresentedin Chapter2 .See AppendixA: CompositionstudyforVO 2thin˝lmsbysol-gel fortheseresults. ForSiO 2/Sisubstrates,theVO 2characteristicspectrumisshowedatanannealingtem- peratureof 461Cfor2hoursand15mTorr.Attemperatureshigherthan 461CVO 2was observedtocrackanddegrade.Samplesevaluatedbelowthistemperatureshowedareduc- tionfromV 2O5phaseastheannealingtemperatureincreases.However,forSiO 2substratesamixingofV 2O3andV 2O5phaseswasobservedastheannealingtemperatureincreases. Theseresultsdemonstratetheimportanceofsurfaceconditioningandsubstrateselection duringsol-gelprocessing.Duringtheannealingprocessthethin˝lmisreducedbythecon- ditions(temperature,pressure,andpurginggas)butitcouldrecoveroxygenattheinterface withthesubstrate. Figure3.9showstheVO 2thin˝lmsRamanspectrumforSiO 2(left)andSi/SiO 2(right) substrates.(a)(b)Figure3.9:Ramanspectroscopycharacterizationofa)VO 2/SiO2sampleandb) VO 2/SiO2/Sisample. 553.2.3Topography The˝lmstopographywasinvestigatedbyusingacontactsurfacepro˝lometer(NanoMap- 500LS)fora5mmx5mmarea.Astylusforceof25.68mgwasappliedduringthesurface scanning.Figure3.10showsthetopographyforpieceoftheSiO 2/Siwafer.Similar resultswereobtainedforotherpieces.Theverticalresolutionoftheusedcontactsurface pro˝lometeris0.1nmforanidealenvironment.AFMwasconsideredtocharacterizethese ˝lmsatsmallerscale. Figure3.10:VO 2thin˝lmtopographymeasuredwiththe3-Dsurfacepro˝lometer. AnAFMsystemwasusedtoinvestigatethetopographyandroughnessofVO 2thin˝lm forpiecesA,B,CandD.Asquareareaof5 mx5 mwasmeasuredatapproximately thesameradialdistance.Theaverageroughnessforeachpiecewere27.2,42.6,33.57,and 32:8nm,respectively. Figure3.11showsthesemeasurements.Asmallerareaof1 mx1 msquareregionwasinvestigated,see Figure3.12.56(a)(b)(c)(d)Figure3.11:Atomicforcedmicroscope(AFM)surfacecharacterizationforVO 2/SiO2/Si.57Figure3.12:AFMsurfacetopographyforVO 2/SiO2/Sisubstratemeasuredina1 mx1 msquarearea. 58 "< ˇ Figure3.14:FIBtrapezoidalcutforVO 2thicknessmeasurement. (a)(b)(c)(d)Figure3.15:SEMcross-sectionalthicknessmeasurementforVO 2/SiO2/Si.Note:Thevalue labeledonthese˝gurescorrespondstothemeasurementbetweenthewhitebars. 60Figure3.16:TransmissionforVO 2depositedbysol-gelmethodasafunctionoftemperature (heatrate: ˘0:4C=s)for =1550µm.3.3PulsedLaserDeposition(PLD)Process 3.3.1DepositionConditions PLDwasusedtodeposittheVO 2thin˝lmsusedinthedevicesdevelopedanddescribed inthisthesis.Table3.2summarizetheseconditions.Moredetailsaboutthedeposition conditionscanbefoundinfollowingchapters. Table3.2:PLDdepositionconditions Device1 Devive2 Device3 Temperature oC600595595ArFlux(sccm) 10--O2Flux(sccm) 152015RepetitionRate(Hz) 101010DepositionTime(min) 303025AnnealingTime(min) -3030613.4CrystallizationofPLDgrowthVO 2thin˝lmsassessedbyXRD XRDwasusedtocharacterizedtheVO 2thin˝lmsgrownbyPLD. Figure3.17showsthe spectrumonaVO 2thin˝lmdepositedbyPLDunderAr/O 2gasmixtureonSiO 2.TheSiO 2amorphoussubstratebroadpeakcenteredaround21 isshowninthisplot.Materialswith nocrystalstructure(suchastheamorphousfusedsilicausedassubstrate)producethese scatteredre˛ections. Figure3.18showsthespectrumforVO 2depositedbyPLDonan oxygenatmosphereonaSiO 2substrate.Thin˝lmsgrownwithonlyO 2gasshowedbetter orientationasevidencedbythestrongerpeak ˘28.TheXRDresultsforsol-geldepositedVO 2isshownin Figure3.19forcomparison purposes.Amuchweakerpeakisobserved ˘28forthis˝lm.Sol-geldepositedVO 2thin˝lmsshowlowercrystalorientation. Figure3.17:XRDcharacterizationofaVO 2thin˝lmandSiO 2substratedepositedunder Ar/O2atmosphere.62Figure3.18:XRDcharacterizationofVO 2thin˝lmandSiO 2substratedepositedunderO 2atmosphere.Figure3.19:XRDresultsforSiO 2/VO 2depositedbysol-gel. 633.5Summary Inthischapter,twodepositionprocesseswereinvestigatedforVO 2thin˝lmdeposition.Sol- geldepositionprocesswasusedforSiO 2andSi(100)/SiO 2substrates.Thesol-gelsynthesis, geldeposition,anddryingtreatmentswerediscussed.Theannealingconditions(e.g.time pressure,andtime)forthisprocesswerestudied.Electricalcharacterization(resistanceand afunctionoftemperaturecurves)wereusedasaparametertodescribethequalityoftheVO 2thin˝lms.OptimumannealingconditionsfoundforSiO 2substrateswereappliedtoSi/SiO 2substrates.TheseVO 2thin˝lmswerecharacterizedbyXRD,Ramanspectroscopy,surface contactpro˝ler,AFM,andSEM.Alsotheopticaltransitionofthese˝lmswasinvestigated forawavelengthof1550nm.VO 2thin˝lmsdepositedbyPLDwerecharacterizedbyXRD andcomparedwithsol-gelresults. 64CHAPTER4 NIRIMAGEPROJECTIONBASEDONVO 2OPTICALMEMORY InthischapterthehystereticbehaviorofVO 2isusedfortheprojectionofnear-IR(NIR) imagesbyprogramingmultipleopticalstatesontheVO 2thin˝lm.Theopticalstateswere programmedbyphotothermalactuation,scanningafocusedredlaseronlocalizedregions oftheVO 2thin˝lm.TheabruptchangeintransmissionofNIRwavelengthsacrossVO 2'ssolid-to-solidphasetransitionwasusedtore˛ectadi˙usednear-IRbeamintheregions scannedbytheredlaser.ThetransmittedNIRbeamwasprojectedonalaserbeampro˝ler. Theresultsofasinglescanfromtheredlasershowimageswithahighcontrastbetweenthe twophasesofthematerial[144]. 4.1SamplePreparation TheVO 2˝lmwasdepositedonSiO 2substratebypulsed-laserdeposition(PLD).Thedepo- sitionwasdonebyusingaKrFlaserat10Hzrepetitionratewithapulseenergyof 350mJintoarotatingvanadiumtargetwhilemaintaininganO 2-Argasatmosphere(15and10 standardcubiccentimetersperminute,respectively)at 20mTorr.Thesubstratetemper- aturewas 550C.Thesubstratehadwidth,height,andthicknessof,respectively,15,11, and1mm.ThedepositedVO 2˝lmthicknesswas ˘300nmâ•fiasmeasuredusingaVeeco Dektak6MPro˝lerâ•fiandshowedaresistancechangeclosetotwoordersofmagnitude whenthetemperaturewascycledbetween30and 90C.Thechangeinresistanceshowed hystereticbehavior,typicalofVO 2.654.2VO 2OpticalMemory TheVO 2opticalmemoryforawavelengthof 1550nmwasinvestigatedbymeasuringthe transmittanceasafunctionoftemperatureasthephasetransitionofVO 2wasinducedby conductiveheatingusingPeltierheaters.Thetemperaturesetpointwerecycledfrom 20Cto70Casshownin Figure4.1.Themajorandminorstransmittancehysteresisloops demonstratetheVO 2materialcapabilitytostoremultipleopticalstates. TheVO 2thin˝lmhasdi˙erenttransmittancevaluesoropticalstatesatthesametemper- aturebasedonthepreviousthin˝lm'stemperature,whichisknownas`memory'.Initially, theVO 2thin˝lmtemperatureincreasesfrom 20CtoatemperatureaboveitsIMTfol- lowingtheheatingcurvein Figure4.1(e.g. 70C).Asthe˝lm'stemperaturedecreases, nowfollowingthecoolingcurvein Figure4.1thetransmittanceoftheVO 2's˝lmwillbe di˙erentfromthanithadbeforeatcertaintemperature(e.g. 50C).Multipleopticalstates areobtainedbycyclingthetemperatureoftheVO 2'sthin˝lmasshownin Figure4.1,at 55C7opticalstatesareshown. 4.3ProjectionSetup NIRimagewereprogrammedandprojectedusingtheopticalset-upsshownin Figure4.2andFigure4.3.AnIRprojectinglaserbeam( =1550nm,90mW,spotsizediameter ˘1:5mm,ThorlabsFPL1055T)wasdi˙usedintoasquareshape( 7:25mm7:25mm)of uniformintensityontheVO 2˝lm.Theopticaldi˙userwasusedtoprovideuniformity,in termsofopticalpowerorintensitytotheimageplane.Thisopticaldi˙userwithadi˙using angle= 6°(EDS-ARPCPhotonics)wasalignedbetweentheIRlaserandtheVO 2sample.TheareaoftheVO 2thin˝lmilluminatedbythedi˙usedIRbeamistheplaneinwhichthe NIRimagetobeprojectedwillbeprogrammed.Awritinglaser( =650nm(red),power =100mW,spotsizediameter ˘600µm)wasfocusedontheVO 2˝lm.Ascanningsystem (ThorlabsGVS002-2DGalvoSystem)wasplacedinthepathofthewritinglaser.This 66Figure4.1:Opticaltransmittanceofa ˘300nmthickVO 2˝lmdepositedbyPLDacross thephasetransition. systemhasapairofmirrors,whichwerecomputer-controlledusingLabVIEWprogramming towriteapatternontheVO 2˝lm.InordertousethememorycapabilityofVO 2,itisnecessarytooperatewithinthe Figure4.2:Experimentalopticalsystemwithdi˙userusedtoprogramanimageontoaVO 2thin˝lm 67hystereticregion[91].Therefore,apre-heatingofthesampletoatemperatureclosetothe phasetransition(hereinafterreferredastheoperatingtemperature(OT))wasdonebyusing twoPeltierheaters,whichalsoservedthepurposeofholdingthesample.Theheaterswere controlledinafeedbackcon˝gurationbyatemperaturecontroller(Newport350B). Thedi˙usedIRbeamtransmittedthroughtheVO 2/SiO2samplewasfocusedintoa detectingLaserBeamPro˝ler(LBP)imagesensor(LBP-4Newport,sensorarea: 6:47mm4:83mm)byusingabiconvexlenstransparenttoIRwavelengthswithafocallengthof 30mm.Acalibrationprocesswasperformedtolimitthewritinglaserscanningtoaregion insidetheIRirradiatedarea,compensatefortheimageinversionduetothebiconvexlens, andfocustheIRdi˙usedbeamontheLPBimagesensor. Asimilarsetupeliminatingthedi˙userwasinvestigated.Inthiscaseahigherpower laserwasusedandthelaserbeamwasexpandedtohavearelativelyuniformintensityatthe imageplane.Ifwasfoundthedi˙usercoatingintroducevisiblegrainsontheimage. Figure4.3showstheexperimentalsetupusedfortheexperimentswithnodi˙user. Figure4.3:Experimentalopticalsetupeliminatingthedi˙userusedtoprogramanimage ontoaVO 2thin˝lm,a)schematicdiagramb)photographofthesetup. 68Figure4.6:Micro-mirrorsscanningspeedcalibration. astheheatinglaserspotsizewas ˘590µmasmeasurewiththeLBPasshownin Figure4.5.Largerdistancesbetweenthescanningmovementsintroducedgapswithinthewriting path.Thescanningspeedwasselectedto 25msbasedon Figure4.6.Lowerscanspeeds (e.g.50ms,100ms)a˙ectstheresolutionoftheimageheatingalargearea.Fasterscanning timewasnotenoughtoheatthe˝lmandinducethematerialtransition.Thewritinglaser wasscannedarateofapproximately5mm/swithinacontinuouspath-movingeach 25msadistanceof 125µm.4.5Results First,aproof-of-conceptexperimentwasperformedtodeterminethecapabilityofnear- IRprojectionandre˛ectionthroughVO 2asthematerialundergoesaphasetransition. Figure4.7showsopticaltransitionoftheVO 2thin˝lmastemperatureincreases.For thisexperimenttheIRlaser =1550nmwasprojectedthroughVO 2/SiO2sampleintoa laserbeampro˝ler(LBP).TheIRlaserwasoperatedatlowerpower( 4mW)andfocused tospotsizeof ˘3mm,andthetemperaturewascontrolledbythePeltierheaters.Asthe 70temperatureincreasesthematerialtransitionisinducedandtheIRradiationtransmission decreases.Figure4.7:IRopticaltransmittancethroughVO 2/SiO2as˝lm'stemperatureincreases. Thetransmittancecurves(shownin Figure4.8)werenormalizedwithrespecttothe powermeasuredbytheLPBwhennosamplewasplacedbetweenthedi˙usedIRbeam andtheLBP(i.e.powerofthedi˙usedIRbeamwas 100%).Thistransmittance(stillat roomtemperature)decreasedto 97%whentheVO 2/SiO2substratewasadded( 1%due totheSiO 2substrate,and 2%duetotheVO 2).Thedi˙erentloopsshownin Figure4.8arethemajorandminorloopsresultingfromheating/coolingsequencesofdecreasing amplitude;from 20Cupto 70Ctemperaturerange.Thisdemonstratesthatthepresented IRimageprojectionsystemcouldbeprogrammedtohavemultipleopticalstates,which wouldtranslateintoprogrammablecontrastlevelsintheprojectedimage.Inthiswork,only onecontrastlevelwasused. InordertoincreasethecontrastoftheprojectedIRimage,itisnecessarytoincrease thedi˙erenceintransmittanceoftheVO 2thin˝lm.Thiswilloccurwhentheoperating 71Figure4.8:NIR =1:55µmtransmittancethroughVO 2duringheatingandcoolingtem- peraturecycles.Thedi˙erentopticalstatesareidenti˝edasanimageisprogrammedand projected.Onlysomeminorloopsareshownforclarity. temperaturecorrespondstothelargestseparationbetweentheheatingandcoolingcurves. Thisoperatingtemperaturewasdeterminedtobe 55CfromtheVO 2opticaltransmittance characterizationexperiment.Itshouldbementionedthatthistemperatureof 55Cisthe temperatureasmeasured(andcontrolled)atthesurfaceofthePeltierheaters,anditcor- respondstoatemperatureof ˘52Catthecenterofthesample.Thesampletemperature wassetto 55CandafteritstabilizedtheactualtemperatureoftheVO 2˝lmswasmeasured usingathermocoupleplacedatthecenterofthesample. TheNIRtransmittanceattheoperatingtemperaturewasfoundtobe 74%(see Figure4.8).WhilethetemperatureatthesurfaceofthePeltierheaterswasmaintainedconstant, thewritinglaserincreasedthetemperatureinthescannedregionsabovethetransition temperature,followingthemajorheatingcurve.Afterthewritingprocessisover,thescanned regionreturnstotheoperatingtemperature,butnowfollowingoneofthecoolingcurves, 72whichendsatadi˙erenttransmittancevaluedependingonthespeci˝cminorcurvefollowed. ConsideringthetemperatureincreaseinducedintheVO 2bythewritinglaser(whichwas measureddirectlyonthesample),itwascon˝rmedthatthetemperatureincreases 6Cabove theoperatingtemperaturereturningtoatransmittancestateof 13:2%aspointedin Figure4.8.Acontrastratioofapproximately6:1wasobtainedbetweenthetwoprogrammedand storedopticalstates. IRimageswereprogrammed,stored,andprojectedbyusingtheopticalsetupsdescribed inSection4.3 .Figure4.9showsdi˙erentstagesoftheprogrammingsequenceofanimage ontheVO 2˝lmbyusingtheprojectionsetupin Figure4.2.Inthiscase,wechosethe capitalletterâ•ŸSâ•Žfromoura˚liationlogo;butthepatternshapecanactuallybedrawn inagraphicuserinterfacethatwascreated anyshapecanbeprogrammedandprojected. Figure4.10showstheprogrammingsequenceofasquareshapebyusingtheprojectingsetup describedin Figure4.3,whicheliminatesthedi˙user.Theseimagesdemonstratedthatthe `grainy'pro˝leobservedin Figure4.9isduetotheopticaldi˙userusedin Figure4.2.Figure4.9:ProgrammedandprojectedNIRimage:a)at 55Cbeforelaserscanning,b)dur- ingwritinglaserscanning,c)rightafterthewritinglaserscan˝nished,andd)programmed image5minutesafterscanning.`Grainy'pro˝lewasfoundtobecharacteristicofthedi˙user. 73DuringearlyexperimentstodevelopthisIRprojectortechnology,alowerintensitylaserwas used.ExpandingthelaserbeamwasnotenoughtocovertheprojectingplaneontheLBP andtheuseofthedi˙userwasnecessary.Inthiscasethedi˙userwaseliminatedbecausea higherintensitylaserwasusedandexpandedtocovermostoftheprojectingplane.However, non-uniformcornersareobservedin Figure4.10a.Thepatternwasmostlywritteninside theuniformintensityarea. Figure4.10:SquarepatternprogrammedintotheVO 2andprojectedNIRimage:a)at 55Cbeforelaserscanning,b)duringwritinglaserscanning,c)rightafterthewritinglaserscan ˝nished,andd)programmedimage5minutesafterscanning.`Grainy'pro˝lewaseliminated byremovingthedi˙user. TheprocessofprogrammingtheVO 2˝lmstartedwiththedi˙usedNIRprojectinglaser illuminatingthe˝lm.Theimagewasprogrammedattheoperatingtemperatureof 55C(Figure4.9-aand Figure4.10-a).ThewritinglaserwasturnedONandasinglewriting laserscan,whichtookabout2secondstocomplete,wasenoughtoprogramtheimage.This timelimitationisduetotheusedscanningmethodandisindependentfromthematerial timeresponse.WhenthewritinglaseristurnedOFFthepatternisstoredintheVO 2thin˝lmandprojectedsimultaneouslybytheprojectinglaser. 744.6DeviceCharacterization Theminimumfeaturesizeofthepattern(i.e.pixelsize)measuredontheLBPwas ˘600µm,whichcorrespondedto ˘860µmintheVO 2thin˝lmplane.Thedi˙usedNIRbeamwas optimizedtocoverthelargestareaoftheVO 2thin˝lm.Thedevelopedopticalsystemwas characterizedassumingapixelsizeof ˘600µm(see Figure4.5).Itshowedaresolutionof ˘10pixelsinthex-directionand ˘8pixelsinthey-direction.Usingthedevelopedsystem theprogrammedimagewascompletelyerasedbydecreasingtheoperatingtemperature. Thecompleteâ•œresetâ•šoftheopticalmemorystatesinVO 2wasdonebydecreasingthe operatingtemperaturetoroomtemperature. TheobservedbehaviorwasanalyzedbythetemperaturechangeintheVO 2˝lmdueto thewritinglaserradiation.Thischangeintemperaturewascalculatedbyusingthefollowing equation:T=Pt ˆVC pd;(4.1)whereˆ,Cpandarethedensity,speci˝cheat,andthermalconductivityofVO 2,respec- tively;Visthe˝lmheatedvolume;AistheareaoftheVO 2˝lmilluminatedbythewriting laser;disthe˝lmthickness,tistheirradiationtime,andPispowerabsorbedbytheVO 2˝lm.ThepowerabsorbedbytheVO 2wascalculatedtobe 52:9mWbyusingtheLambert- Beerâ•ŽsLaw.Thematerialextinctioncoe˚cientfortheinsulatingphase( ˘0.3)[[145]] andtheangleofincidence,( ˘45°wereconsidered). Asimulationwasperformedusing˝niteelementmethod(FEM)analysis(COMSOL Multiphysics)todeterminethetemperaturechangeandvalidatethecalculation.Thesample dimensions,parametersandalltheboundariesconditionsusedinthesimulationresembled thevaluesusedintheexperiment.Foraradiationexposuretimeof 25ms,Equation(4.1)andthesimulationgavea˝naltemperatureof 58:8Cand 67C,respectively. Figure4.11showstheFEMsimulationbeforeandafter 25msofredlaserradiation. Thecalculatedvalueagreeswiththe˝lmdirectmeasurementof 59C,suggestingthat 75(a)(b)Figure4.11:Finiteelementmethod(FEM)simulationat(a)operatingtemperature,(b) after25msofredlaserradiation. theprogrammedimagefollowedaminorlooppath(asshownin Figure4.8).Thesimulation showed 67Casthehighesttemperatureatthecenterofthespot;whichcoveredasmaller diameterof ˘400µm.Atemperaturedi˙erenceof 12Cwasseenbetweenthespotcenter andtheboundaryofthecircleusedtosimulatethewritingbeamspotsize( 600µmdiameter). Thedi˙erencebetweentheFEMsimulationandanalyticalmethodsisbelievedtobedueto thenon-uniformtemperaturedistributionofthePeltierheaterscon˝guration.Theoperating temperatureusedforthecalculationwasthedirectmeasurementonthe˝lm,whilethe simulationcalculatesitsownoperatingtemperatureafterthesystemreachessteadystate (43C).4.7Summary Inthischapter,aprogrammableNIRimageprojectorwasdemonstratedbysuccessfully usingtheopticalhysteresisofVO 2thin˝lmsacrossitsphasetransitionandarelatively simpleopticalset-upanddevice.Anopticaldi˙userforIRwavelengthswasusedinone projectingset-upresultingina`grainy'imageprojection.Agrainfreeimagewasachieved usingasimilaropticalset-upbyremovingthisopticalcomponent.Theminimumfeature 76sizeoftheprojectedimagewasdeterminedbythewritinglaserspotsize.Thesystemcan eraseanimageandreachtheoperatingtemperaturein ˘2s.Theimagesareprogrammed atawritingrateof5mm/s.Inthenextchapter,theopticaltransitionofVO 2isexplored todevelopaVOAoptoelectronicdevice. 77CHAPTER5 VO 2-BASEDVARIABLEOPTICALATTENUATOR Inthischapter,thecorrelationbetweentheopticalandelectricalpropertiesofVO 2arepre- sentedforthedevelopmentofavariableopticalattenuatorinthenear-IRregion( =1550nm).ThisworkintroducestheVO 2capabilityofelectronictunabilityforNIRapplications,specif- icallyinfreespace.Amodeltoestimateopticaltransmissionbysensingtheelectricalresis- tanceofVO 2ispresented.Thismodelisimplementedwithself-sensingandfeedbackcontrol toattenuateanopticalsignalthroughVO 2.5.1Electro-opticalVO 2PropertiesCorrelation Recentstudiessuggestthatthechangeinthetheelectricalandopticalpropertiescomesas theresultofthe˝rst-orderinsulator-to-metal-transition(IMT)[68,89].Whilethechange inbothpropertiesshowhystereticbehavior(asshownin Section1 ),thefactthatboth changesarestronglycorrelatedcanbeusedtoreducethehysteresiswhenonepropertyis plottedagainsttheother.Aseriesofexperimentsareperformedtocon˝rmthiscorrelation bymeasuringbothpropertiessimultaneously. 5.2Self-SensingFeedback Inthiswork,self-sensingreferstotheestimationoftheopticalparameter(transmittance) basedonthemeasurementsofanothercorrelatedparameter(resistance)ofthesamematerial. Self-sensingofVO 2thin˝lmisusedtocontrolthetransmittanceacrossthematerial.The electricalresistanceofVO 2ismeasuredwhiletheopticaltransmissionisestimatedbya determinedmodelandcontrolledbyaclose-loopfeedbackthatincludestheself-sensing model. 78Aproportional-integral(PI)controllercon˝gurationwasdesignedandimplemented.The controllergainvaluesweredeterminedfromasetofperformancespeci˝cationsandthe systemdynamics(whicharedominatedbythethermaldynamicsfromtheheater).Dueto therelativelyslowsystemresponse,whichischaracterizedbyanopen-looptimeconstantof 18:5s,thetargetedresponsetimefortheclosed-loopcontrolledtransmittanceis 10s.The measurementsetupdescribedin Section5.4 isusedtoperformtheVOAclosed-looptests withacon˝gurationrepresentedbytheblockdiagramshownin Figure5.1.Theresistance oftheVO 2(R)issensedandusedinthemodeltocalculatethecorrespondenttransmission (Tc).Thisvalue( Tc)isthencomparedwiththedesiredtransmission( Ts).Theerror( E)goestothePI,whichcontrolsthecurrent( I)thatissenttothePeltierinordertochange thetemperatureofthesample,thereforecontrollingthetransmissionpercentage( T).5.3DeviceFabrication AlthoughthephasetransitioninVO 2hasbeendemonstratedtooccuratultrafasttime scales,mostofthecurrentVO 2-baseddevicesareoperatedbyheatingthematerialabove thetransitiontemperature.TheVO 2-basedVOAreportedinthisletterwasoperatedby conductiveheatingfromaPeltierheater.ThedeviceconsistsofaVO 2thin˝lmwith coplanarelectrodes.TheVO 2thin˝lm( 200nmthick)wasgrownonsinglecrystalquartz Figure5.1:Schematicrepresentationoftheclosed-loopcontrolsystemfortheVOAusing self-sensing.79(SiO2)substrate( 12mmwide, 11mmlong,and 250µmthick)bypulsedlaserdeposition (PLD)at 600C.AKrFexcimerlaserwasusedwitha˛uenceof ˘2J/cm2at10Hzrepetitionrateonavanadiumtarget.DuringVO 2deposition,thebacksideofthesample wascoveredbyasinglecrystalsiliconwaferpiece.Thiswasdonetoavoidthedepositionof materialresidueonthebacksideofthesubstrate,whichwasobservedinpreviousdepositions andexperimentallyveri˝edtohaveanimpactontheopticaltransmittancebehaviorof thesample.Alsoduringdeposition,ashadowmaskwasusedonthesamplefrontside witha 5mmopening,whichallowedforthedepositionofaVO 2rectangularpatchthat extendedthesubstratewidthof 12mm.The˝lmwasdepositedfor 30minunder 20mTorr ofoxygenpressure.Thedepositionwasfollowedwithanannealingstepof 30minatthesame temperatureandpressureconditionsusedduringdeposition.AftertheVO 2deposition,the samplewastakentoacustom-builtevaporatorforthedepositionofaluminum,whereanother shadowmaskwasusedtoformthetwocoplanarelectrodesontheVO 2˝lm.Thedevicewas thenmountedonaPeltierheaterwithacenteredhole(Thorlabs,TEC1.4-6).Theinseton Figure5.2showsthe˝nalassemblyofthedevicewiththepatternedVO 2centeredonthe heater.5.4Electro-thermo-opticalSetup Thedevicewasmountedintheelectro-opticalsetupshownin Figure5.2.ANIRdiodelaser withwavelengthof 1:55µmandaspotsizeof ˘5mm(Thorlabs,FPL1055T),anoptical powersensor(Thorlabs,S144C),andapowermeter(Thorlabs,PM100D)wereusedforthe opticalmeasurements.Beforetheexperiments,itwasnecessarytoverifythatthephase transitionoftheVO 2˝lmwasnotinducedbytheabsorptionofIRradiationcomingfrom thelaser.Tothisend,thelaserpowerwasinitiallysetatitsminimum( 12mW).Then,the powerwasincreasedto 150mWandthespotsizereducedinordertoincreasetheradiation intensityonthesample,whilethe˝lmwaskeptatroomtemperatureanditsresistancewas beingmonitored.Nonoticeablechangeinresistancewasmeasuredintheprocess,which 80Figure5.2:Setupusedforperformingopticaltransmissionandelectricalresistancecharac- terizationoftheVOA.ThesetupisalsousedforcontrollingtheVOAinclosed-loopusing theself-sensingtechnique. indicatedthatthephasechangewasnotbeinginducedphotothermally. Fortheexperiments,alaserpowerof 12mWwasused.Theelectricalmeasurements wererecordedusingadataacquisitionsystem(NationalInstruments,cRIO),whichwas constantlymeasuringtheVO 2'sresistancethroughconnectionstotheAlcoplanarelec- trodes.Atemperaturecontroller(Thorlabs,TED4015)wasemployedtocontrolthePeltier heaterandtriggerthematerialtransitionintheVO 2.Acomputerinterface(usingavirtual instrumentinLabviewsoftware)wasusedtomonitortheoutputofthedataacquisitionsys- tem,thepowermeter,andtoinputthetemperature(foropen-loopexperiments)ordesired transmittance(forclosed-loopexperiments). 5.5Results TheVO 2˝lmwascharacterizedbymeasuringitselectricalresistanceandopticaltransmit- tanceasafunctionoftemperature.Bothmeasurementsweredonesimultaneously,asthe temperaturewasvaried.The˝lm'sresistanceacrossthephasetransitionshowedadropof almostthreeordersofmagnitude;whichevidencesthegoodqualityandorientationofthe 81Figure5.3:a)ElectricalcharacterizationforVO 2thin˝lmasfunctionoftemperature;b) OpticalcharacterizationoftheVO 2-basedVOAfor 1:55µm.˝lm(see Figure5.3-a).Thedropintheopticaltransmissionacrossthephasetransition wasnormalizedusingasreferencethetransmissivityatroomtemperature( 1:6mW).After thisnormalizationthetransmissiondropwasfrom 100%to 2%(see Figure5.3-b).Itcanbenoticedthatthewidthsoftheresistanceandtransmittancehystereticcurves areverysimilar.However,themidpointsoftheheatingandcoolingcurvesfortheresistance andtransmissioncurves(labeledin Figure5.3)aredi˙erent.Perhapsthemostintuitive explanationforthiswouldbethetemperaturegradientacrossthe˝lmandthedi˙erent probinglocationsforbothproperties(i.e.electricalresistanceandopticaltransmittance) opticaltransmittanceoftheVO 2wasmeasuredatthecenterofthe˝lm,whilethe resistancewasmeasuredacrossthe˝lm'swidththroughthecoplanarelectrodes.However,it wasveri˝edthatthedi˙erenceintemperatureacrossthesampleisonly ˘1C,asmeasured withathermocouple.Furthermore,thetemperatureisloweratthecenterofthesample (likeitwasmeasuredpreviouslyonasimilarset-up[144]),andthiswouldhaveresultedin TTsgreaterthanT Rs,whichisnotwhatwasobserved. Typically,theresponseofVO 2-baseddevices,showshystereticbehaviorbetweenthe 82Figure5.4:a)Transmissionasafunctionofresistanceandpolynomial˝tmodel;b)Error betweenthepolynomialmodelandactualtransmissionpercentage.Thetableshowsthe coe˚cientsforthemodel. input(usuallytemperature)andtheoutput.However,iftwostronglycorrelatedproperties areused,therelationshipbetweenthemshowssigni˝cantreductionofthehysteresis. Figure5.4-ashowstheplotofthetwodi˙erentresponsesoftheVO 2˝lm(opticaltransmissivity vs.electricalresistance).Thelargestseparationbetweentheheatingandcoolingcurves occursatthe 1kW/20%region,whichcorrespondstotemperaturesnearthetransition.This separationisgreatlyin˛uencedbythedi˙erenceinthetransitiontemperatureforboth properties.Thisnearlyone-to-onecorrespondencebetweenelectricalresistanceandoptical transmittanceallowsfortheimplementationofaself-sensingapproachthatwouldeliminate theneedforcomplicatedhysteresiscompensationorinversionalgorithmsforcontrollingthe device.83Aninthorderpolynomial˝twasusedtomodeltheexperimentaldataasshownin Figure5.4-abyusingtheleastsquaremethod.Thepolynomialcoe˚cientsareshownin Figure5.4.Theerrorbetweenthemodelandthemeasurementsoftheactualtransmissiondata wascalculatedintermsoftransmissionpercent. Figure5.4(b)showsamaximumerrorof 2%.Thecurve˝twasusedasfeedbackintheimplementationofasimple(yet,e˙ective) closed-loopcontrolcon˝guration.Thecompletesystemformsafullyelectricallytunable VO 2-basedVOAforNIRasdescribedin Section5.2 5.6DeviceCharacterization Intermsofdecibels,theattenuationincreasedfrom 0dBto 19:24dBasshownin Figure5.5.Thisresultsinadynamicrangeof 19:24dB,whichiscomparablewiththetypicalrange of25dBforcommerciallyavailableVOAs. Asequenceofinputsteps( Ts)wasusedtovalidatetheperformanceofthepresented VOAintermsoftransientandsteadystateerror. Figure5.6-ashowstheresponseofthe systemtosuchsequence.Thedurationofeachsetpointwas 60s.Anaverageerrorof 0:55%wascalculatedforthemeasuredtransmissionwithrespecttothesetpointinsteadystate. Also,acompleteerrorrangeof 2%isshownin Figure5.6-b,whichalsoshowsswitching noise.Theseresultsshowthee˚cientuseoftheself-sensingmechanismproposedintermsof transientandsteadystateperformance.Accuratetransmittancevalueswereobtainedusing theself-sensingmethodproposedconsideringtheaverageerrorvalue. Figure5.7showstheresponsetoasinusoidalinputsignalconsistingofthesumofthree sinusoidalwaveformswithfrequenciesof 0:5mHz,1mHz,and 5mHz;allwithanamplitude ofT=44%.Themodelandthemeasuredsignalfollowedthecontinuouslychangingsetpoint withanaverageerrorof 1:15%.Theerrorrangeforthesinusoidalresponseisshownin Figure5.7-b.Thisexperimentdemonstrateshowthesigni˝cantlyreducedhysteresisallows fortheaccurateattenuationcontroloftheVO 2-basedVOAdevice,evenduringthetransition region.Themeasuredopticaltransmissionfrom Figure5.7isalsoshownin Figure5.8(a-b)84Figure5.5:VO 2-basedVOAattenuationresponse. asafunctionoftemperatureandresistance,respectively.Itcanbenoticedthattheminor hystereticcurvesaresigni˝cantlyreducedin Figure5.8-bduetothestrongproperties correlationoftheVO 2˝lm.85Figure5.6:a)Stepresponseforvariableopticalattenuatorwithself-sensingfeedbackcontrol. b)Errorcalculationamongthesetpoint,measuredandself-sensingsignals.Theinsetshows asmallerscalefortheerror(y-axis). 86Figure5.7:a)Sinusoidalresponseforavariableopticalattenuatorwithself-sensingfeedback controlduringtheVO 2transition.b)Errorbetweenthesetpoint,measuredandself-sensing signals.Figure5.8:a)Opticaltransitionasafunctionoftemperature.b)Opticaltransitionasa functionofresistance. 875.7Summary Thischapterpresentedthedevelopmentofavariableopticalattenuator(VOA)fornear-IR wavelengthsthatiscompletelyelectricallytunableanddoesnotrequiresamplingormonitor- ingtheopticalbeamforcontrollingtheattenuationlevel.TheVO 2-basedVOAfundamental operationisbasedonthestrongcorrelationoftheelectricalandopticalpropertiesofVO 2acrossitsIMT.ThemaincontributionisthedemonstrationofanewVOAtechnology,which reliesontheuseofself-sensingtechniquestocontroltheattenuationofVOAsonafeedback con˝gurationthatalsoresultsintemperaturestability.Ahighlyaccurateclosed-loopcontrol oftheattenuationisachievedusingtheself-sensingtechniqueshown. 88thin˝lmmaterialbyusingaresistiveheaterloopandelectrodesasshownin Figure6.1.Thisdesignaimstomakethistechnologycompatiblewithopticalinterconnectsanddecrease theresponsetimebyreducingthethermalmass.ASiO 2(fusedsilica,doublesidedpolished) substratewasselectedbasedonitstransmissionforNIR( 1550nm)wavelengths.Platinum (Pt)metallizationmaterialwasselectedtosustaintheVO 2depositiontemperature.The heaterloopisusedtoinducetheopticaltransitionintheVO 2windowelectro-thermally(i.e. usingJouleheating).AtemperatureabovetheVO 2transitiontemperature(T c˘68C)isrequiredinsidetheVO 2windowtocompletetheIMT.Finiteelementmethod(FEM) simulationswereperformedinComsolMultiphysicstocon˝rmthetemperaturedistribution insidethewindow.VO 2squarewindowsof400,300,200and 100µm2wereconsideredfor thisdesign. Figure6.2showsthetemperaturedistributionusingtheJouleheatingmodule withacurrentof 35mA.Thenon-uniformityofthetemperaturedistributionincreasedwith thesizeofthewindow,butacurrentof 35mAresultedinatemperaturehigherthan 68CatthecenterofallthefourVO 2windows. ThetemperaturewasevaluatedatthecenteroftheVO 2windowsforcomparingthe di˙erentdevicesizes. Figure6.3showsthistemperatureasafunctionofthepowerinput tothedevice.Power( I2R)insteadofcurrentwascomparedbecausetheheaterresistanceis di˙erentforeachdevice.Theseresultsdemonstratelowerpowerconsumptionforthesmaller devices,asexpected. Adieof 4mm4:5mmcontainedallfourwindowsasshownin Figure6.4.Thefour maskslayersare:1)metal(yellow),2)SiO 2etch(blue),3)VO 2pattern(purple),and4) SiO2etchforcontacts(green).(See Figure6.4.)90(a)(b)(c)(d)Figure6.2:TemperaturedistributionsforVO 2squarewindowsofa)100x100 m2,b)200 x200 m2,c)300x300 m2,andd)400x400 m2assimulatedusingJouleheatingwith aheaterinputcurrentof 35mA.91Figure6.3:SimulatedtemperatureatthecenteroftheVO 2windowforallfourdevices.The electrodeswereincludedinthesimulation. 92Figure6.4:MaskdesignforthemonolithicVOAmicrodevice. 9315µmforthe400 mand300 mdevices. AninsulatinglayerofSiO 2(˘400nmthick)wasdepositedbyplasmaenhancedchemical vapordeposition(PECVD)inthreeconsecutivestepsof 133nmeachtopreventpossiblevoids propagationthroughtheSiO 2thickness(stepc).TheSiO 2layerwasetchedbyreactiveion etching(RIE)toopenelectricalcontactpathsfromthePtelectrodestotheVO 2thin˝lm (stepd).AVO 2layer, ˘170nmthick,wasdepositedbyPLDusingaKrFlaseroperated at10Hzwith ˘2J=cm 2˛uencefor25minutes(stepe).Thesubstratewasmaintained at595Cunder15mTorrO 2pressure.Thisdepositionstepwasfollowedbyanannealing stepundersamepressureandtemperatureconditionsfor30minutes.Thebacksideofthe waferwascoveredduringthedepositiontopreventundesiredresidualsaccumulationonthe substrate,whichcouldhavea˙ectedopticaltransmissionexperiments.VO 2squarewindows werepatternedbyphotolithographyprocessandRIE(stepe).AnotherSiO 2etchingstep (f)wasdonetoopencontactspadstotheheaterandelectrodes.Finally,thewaferwas dicedintodiesofapproximately4x 4mm2.Figure6.6and Figure6.7showscanningelectronmicroscope(SEM)imagesandoptical microscope(KeyenceVHX-S15)imagesofthefabricateddevices. 6.3ExperimentalSetup Theelectro-opticalsetupshownin Figure6.8wasusedtotestthe VOAdevicesbyusing afree-spacecon˝guration,similartothatdescribedin[45].Thediecontainingthe VOAs wasmountedandwire-bondedintoacircularpackagewithacentered-hole,asshowninthe Figure6.8inset.AcustommadePCBwithacentered-holewasbuildto˝tthecircular package,routetheelectricalconnections,andactasasampleholder.ThePCBwasmounted onaX-Y-ZtranslationalstagetoaligntheNIRlaserbeamwithnormalincidenceonthe VO 2windows.TheNIRdiodelaser( =1550nm,FPL1055T)wasoperatedatitsminimum stablepowerof ˘25mWandfocusedbyalens( f=15mm)ontheVO 2window.Aminimum diameterof ˘70µmwasachievedasshownin AppendixB:AdditionaltestingforVO 2-95Figure6.6:SEMimagesofthefabricatedVO 2devicesbasedVOA .Thelenswasmountedonamicropositionertomanipulatethedistance betweenthe VOAandthefocusinglens,thus,allowingcontrolofthebeamdiameterson eachwindow.Aneutraldensity(ND)˝lterbetweenthelensandthelaserwasusedto furtherreducethelaserintensity.Thetransmissioncurvesweremeasuredfordi˙erentNDs (seeAppendixB:AdditionaltestingforVO 2-basedVOA ).The˝lterwasadjusted to2opticaldensity(OD)forthe100 mand200 mdevicesmeasurements,1ODfor300 mandto0.6ODfor400 mdevice.Alaserbeampro˝ler(LBP)wasusedtoassistin thealignmentofthefocusedlaserbeam.AfterthisalignmenttheLBPwasreplacedbyan opticalsensor(S144C,Thorlabs)connectedtoapowermeter(PM100D,Thorlabs),which 96Figure6.7:OpticalmicroscopeimagesofthefabricatedVO 2devicescommunicateswithaLabviewcomputerinterfaceduringthemeasurements. ElectricalcontactsonthePCBwereusedfordataacquisitionandcontrol(cRIO,National Instruments).Theheatercurrent( IH)drivesthedevicewhilethevoltageacrossthe VO 2(VR)wasmeasured. VRwasusedtocalculatetheVO 2resistance( RVO 2)byusingavoltage dividerasshownin Figure6.8.Aseriesresistorof Rs=22kWandsupplyvoltageof VC=2VwereselectedtomeasuretheVO 2resistanceandpreventself-heatingonthe VO 2thin˝lm.ThissetupallowedtosensetheVO 2resistancechangeanddrivethedevice heatersimultaneouslyinaclosed-loopcon˝guration.Thisallowstoelectronicallycontrolthe transmissionforNIRwavelengthsthroughaVO 2window.Thesamplingrateforelectrical measurementsandactuationwassetto 50µs(whichismuchfasterthanthesystemresponse time),whiletheopticalsensingwaslimitedto 50msduetothesensorbandwidth. 97Figure6.9:Timeconstantmeasurements:a) 100µmdeviceresponse(V R)toastepinput (IH)andb)timeconstantforthescalingdownofthe mdevices. thermalactuationinthesedevicesisdominatedbyheatdissipationduringcooling. Theelectro-thermalactuationandtemperaturedistributionwasinvestigatedbyIRther- malimaging(OptoTherm,InfraSightMI320).Thedeviceswereactuatedelectro-thermally totheirmaximumcurrentlimit(I H)asdeterminedbyIMTcharacterizationexperiments. Figure6.10showsthetemperatureofthefourdevicesasafunctionoftheheatercurrent (IH).ThistemperatureisanaverageofaregionoutsidetheVO 2window(labeledas#2 ontheinsetin Figure6.10).EvaluatingthetemperatureinsidetheVO 2windowwillgive anincorrectvaluesinceVO 2emitslessthermalradiationfortemperaturesabovetheIMT [146].Themaximumtemperaturewas 108:4C,101:6C,102:7Cand 104:8Cfor400 m,300m,200 mand100 mdevice,respectively. TheVO 2thin˝lmwascharacterizedbysimultaneouslymeasuringitsopticalandelec- tricaltransitionasshownin Figure6.11.TheVO 2resistancedropsmorethan3ordersof magnitudeforalldevices,resultinginaresistanceON/OFFratio( R25C=R100C)of2.22k, 2.16k,1.56kand1.76kforthe400 m,300 m,200 mand100 mdevice,respectively. Theaverageresistancedropisfrom 685kWto367WhavinganaverageresistanceON/OFF ratioof ˘1.9k.99(a)(b)(c)(d)Figure6.10:Temperatureasafunctionofcurrentforelectro-thermalactuationintheVO 2deviceofa)400 m,b)300 m,C)200 m,andd)100 m.InsetshowsIRimageduring actuationat ˘30mA. Duringthesemeasurementsthemaximumcurrentappliedtothedevicewaslimitedto 29.7,28.7,22and32.7 mAforthe400,300,200and100 mdevice,respectively. TheintensityoftheNIRlaseroneachdevicewasadjustedbychangingtheneutral density˝lterODandthelensposition.The˝lterODwasselectedbymeasuringtheVO 2resistancebeforeandafterturningontheNIRlaseruntilnochangewasdetected.The transmittedpowerat 25Cwere11.94 mW=mm 2,20.37 mW=mm 2,3.31 mW=mm 2and9.93mW=mm 2forthe400 m,300 m,200 mand100 m,respectively. 100Thetransmissionin Figure6.11wasnormalizedwithrespecttothetransmittedpower at25Ctodemonstratethetransmissiondropduetothedevice.Theinsertionlosses(before theIMT)of-3.6dB,-3.9dB,-3.4dBand-4.6dBweremeasuredforthe400,300,200and 100mdevices,respectively.Thehighestinsertionlossesweremeasuredforthe100 mdevice.Theirradiatedpoweronthe100 mdevicewasnotreducedanyfurtherbyaND ˝ltersincethepowerreadingsfromthesensorwouldhavebeennearitsresolutionlimit.It canbenoticedin Figure6.11thattheperformanceintermsofminimumtransmissionand totaltransmissiondropwassimilarbetweenthetwolargerandthetwosmallerdevices. ThestrongcorrelationbetweentheelectricalandopticaltransitioninVO 2isobserved Figure6.11:SimultaneousmeasurementoftheelectricalandopticaltransitioninVO 2win-dowsfora)400 m,b)300 m,c)200 m,andd)100 mdevice. 101byplottingthetransmissionpercentage(%T)asafunctionofitsresistance. Figure6.12showsthiscorrelationforthedevicesusingthesameresultspresentedon Figure6.11.This datawas˝ttedintoacorrelationmodel(sigmoidal,bidosemodel)withan R20.99for eachdevice.The%TiscalculatedusingthebidoseequationfromOriginLabSoftware, %T=A1+p(A2A1)1+10 [X1Log(R)]h1+(1p)(A2A1)1+10 [X2Log(R)]h2(6.1)whereRistheVO 2resistance,A1,A2,X1,X2,h1,h2andpare˝ttingparameters.Matlab curve˝ttingwasusedtodeterminethebest˝ttingparametersforeachdevice.These parametersareshownin Figure6.12foreachdevice.Itcanbeobservedfrom Figure6.12thattheshapeofthecorrelationcurveschangesasthedevicescalesdowninsize. Figure6.12:Electro-opticalVO 2correlationandbidoseself-sensingmodel. 102Thebidosemodelwaschosenbasedonthefunctionimplementationinthedataacquisition system.OthersigmoidfunctionssuchasthedoubleBoltzmannequationcanbeuseto˝t thisdata.However,thebidosemodelproducedthebest˝tforthemeasureddatainthese experiments. Thiscorrelationmodelallowsfortheestimationofthemeasuredtransmission%(% Tm)bysensingtheVO 2resistance( RVO 2)thusallowingforself-sensingmodel.Thedi˙erence betweenthemeasureddataandtheself-sensingmodelisshownin Figure6.13.Table6.1 showsasummaryforthesecorrelationerrors.The400 mdevicehasthehighererrorswhile the200 mdevicehasthelowest.The300 mand100 mdeviceshaveasimilarbehavior, Figure6.13:Correlationmodelerrorbetweencalculatedtransmission%(zeroline)and measureddata. 103bothwithaRMSerrorof ˘0.8%. Table6.1:CorrelationErrorforalldevices DeviceSizeMaximumMinimumRMSAverage (m)Error(%)Error(%)Error(%)Error(%) 4005.9-4.71.22 6:91063004.5-3.30.82 2:21052002.5-1.960.65 3:11081003-3.70.86 3:3103Thisself-sensingmodelwasusedtoestimatethetransmission% Tssbymeasuring RVO 2inthe VOAandcompleteaclosed-loop(CL)systemasshownin Figure6.14blockdiagram. Figure6.14:Transmissionclosed-loopcontrolblockdiagram. Theerrorbetweenthetransmissionsetpoint( %Ts)and %Tsswasminimizedbya proportional-integral(PI)controller.ThePIgainswereobtainedexperimentallyforeach devicesize.Theoutputvoltage( V)fromthePIcontrollerwasconvertedintoacurrent (IH)byanexternalcircuit,whichdrivestheintegratedheaterofthe VOA.Usingthis controltechnique,% Tmwaselectronicallycontrolledasitstransmission%andtheresistance changedasafunctionof IH.TheCLresultsforthe VOAsareshownin Figure6.15.% Tschangesin1sstepsfrom 100%to40%. Figure6.16showstheerrorforthemeasuredsignalandtheself-sensed 104signal.Theself-sensedsignalfollowsthesetpointforalldevicesminimizingthesteady-state error.Table6.2showstheclosed-looperrorsforalldevices. Figure6.15:Closed-loopresultsfor:a)400 m,b)300 m,c)200 mandd)100 mdevice. Table6.2:Closed loopError DeviceSizeMaximumSteady-stateAverage (m)Error(%T)Error(%T)Error(%T) 40071.631.32 30083.013.53 20040.03-0.23 10060.71-0.39 The300 mdeviceshowsamaximumCLerrorof ˘8%andamaximumaverageerror of3.53%.Howeverthe400 mdevicehasaslightlyhigherRMSerror.Thetransmission 105forthesedevices(400and300 m)wasstablewithin1s(di˚culttoseefromtheseresults). The400 mdeviceandthe300 mweretestedwith1.5sswitchingintervalstoallowmore timeforstabilizationduetotheirslowertimeconstants.Theseshowedresultssimilarto thosepresented.Theresultsforthe200 mdeviceshowsthelowestRMSandaverageerrors of3.15%and-0.23%,respectively.The400 mand300 mdevicesresultedinhighererror ˘7-8%duringCLcontrol.The100 mdeviceshowed ˘6%oferrorduringCL. Figure6.16:Self-sensedandmeasuredsignalerrorswithrespecttothesetpointduring closed-loopexperimentsfora)400 m,b)300 m,c)200 mandd)100 mdevices. 1066.4.1Powerconsumption Thepowerconsumptionwascomparedforthese VOAdevicesshowninTable6.3.The heaterresistancewasmeasuredacrosstheheatercontactpadsatroomtemperature.The currentvalueusedinthecalculationwasthatoftheintersectionpointoftheheatingand coolingcurvesaftertheIMT,whichwasobtainedfrom Figure6.11forthefourVOAs.The resultsshowalowerpowerconsumptionasthedevicegetssmaller,withtheexceptionofthe 200VOAdevice. Table6.3:PowerConsumption DeviceSizeHeaterresistanceCurrentConsumedPower (m)(W)(mA)(mW) 400165.521.6377.43 300132.122.2565.40 20097.818.433.11 10066.223.536.56 1076.5Summary FourmonolithicallyintegratedVO 2-basedVOAsof400,300,200and100 2windows wereinvestigated.ThestrongcorrelationbetweentheelectricalandopticaltransitioninVO 2wasusedtodetermineaself-sensingmodelwithspeci˝c˝ttingparametersforeachdevice size.Similar˝ttingparameterswereusedforthe400,300and200 mdevices.Howeverthe 100mdevicecorrelationcurveshowedasharppositivesloperequiringdi˙erentparameters. The200 mdeviceshowedthelowestself-sensingmodelerrorof2.5%andasteadystate errorof0.03%. Thisself-sensingmodelenabledtheelectronicallycontrolofthetransmission%fora NIRlaser(1550nm)throughaVO 2windowforalldevices.TheseVO 2-basedVOAs canbecalibratedfordi˙erentlaserintensitiesasdemonstratedinthiswork,withbetter transmission%controlatlowintensities( ˘3.31mW=mm 2).Thesecanbecalibratedfor arangeofnear-IRwavelengthsfrom 900nmto 2500nmincluding˝beropticstransmission bandsO-band,C-bandandL-band.Furthermore,thissmallscaledevicescouldbeextended intoalargescaleVOAarraysimilartoMEMS-basedVOAarraybutreducingthecomplexity ofthefabricationprocess[147]. Closed-loop(CL)controlofthetransmission%wasachievedforalldevices.However, the200 mdeviceshowedabetterperformanceduringCLcontrolwiththelowestaverage CLerrorof0.23%.Thetransmission%dropoftheVO 2thin˝lm(upto ˘30%)could bea˙ectedbytheopticalqualityoftheSiO 2substrate(fusedsilica),theVO 2'soptical qualityasgrownonSiO 2insulatinglayerdepositedbyPECVDorinterfacere˛ectionsof theSiO 2/VO 2/airdevicelayers.Especially,theVO 2thin˝lmthicknesshasadirectimpact inthesere˛ections[148]. 108CHAPTER7 SUMMARY 7.1SummaryofContributions VO 2sol-geldepositionprocesswasinvestigatedforgrowingVO 2thin˝lmson4inches wafers.Filmsgrowthbythisprocesswerecomparedwithpulsedlaserdeposition(PLD) process.TwodevicesweredevelopedusingPLDgrownVO 2thin˝lms. Thedevelopmentofanear-IR(NIR)imageprojectorandaNIRvariableopticalatten- uator(VOA)arepresentedinthiswork.VO 2thin˝lmmaterialwasusedtostoreanimage informoflocalizedinducedtransitionbyphoto-thermalactuation.Thereversiblenatureof theopticaltransitioninVO 2allowstoconsecutivelyprogramimagesintotheVO 2thin˝lm andprojectthemintheNIRregion( =1550nm).Theimageprogrammingspeed,resolution andopticalcontrastwerecharacterizedforthisprojector. ThesimultaneousmeasurementoftheopticalandelectricalpropertiesofVO 2demon-stratedthestrongcorrelationbetweenthesetwoproperties,whichenablethehysteresis reductionbysensingitselectricalresistance.Arelativelysimpleself-sensingmodelwasused controltheopticaltransmissionthroughtheVO 2thin˝lm.AmonolithicVOAcompatible withoptical˝bersystemswasdeveloped. 7.2ListofProblemsSolvedinthisthesis Thisdissertationaddressesthefollowing: 1.Characterizethecomposition,crystallization,topography,electrical,andopticaltran- sitionsofVO 2thin˝lmsdepositedbysol-gelprocessin4incheswafers. 2.Useofphoto-thermalactuationtoprogram,storeandprojectaNIRimage. 1093.Characterizethecontrast,programmingspeedandresolutionofthedevelopedVO 2-basedimageprojector. 4.DemonstratehysteresisreductionintheopticaltransmissionofVO 2bysensingits electricalresistance,enabledbythematerial'sstrongcorrelation. 5.Developedamonolithicvariableopticalattenuator(VOA). 6.Demonstrateself-sensingfeedbackcontrolinthedevelopedVO 2-basedVOA. 7.CharacterizetheVO 2-basedVOAbasedonitsattenuationperformance. 110APPENDICES111AppendixA:CompositionstudyforVO 2thin˝lmsbysol-gel ThecompositionoftheVO 2solgeldepositedthin˝lmswasstudiedbyRamanspectroscopy. SampleswereanalyzedforSiO 2andSiO 2/Sisubstratesnearthepresentedoptimumtem- perature.Thespectrumwasmeasuresfordi˙erentranges100-800( 1=cm)and100-1100 (1=cm).Thisdatawasobtainedusingawavelengthof532nm. FigureA.1showstheRamanspectrumofVO 2depositedonaSiO 2substrateat 417C.Thespectrumshownin FigureA.2correspondstoasampleannealedataslightlyhigher temperatureof 428C.PeaksfromtheV 2O5spectrum[149]wereobservedforthissample. FigureA.3showsthespectrumformixedphasesofVanadium-Oxides,havingpeaksfrom V2O3andV 2O5.Thissamplewasannealedat 450C.Theresultsforthin˝lmsonSiO 2/Sisubstratesshowthereductionprocessasthetem- peratureincreases. FigureA.4showsV 2O5mixedpeaksforasampleannealedat 406C.FigureA.5alsoshowsaV 2O5mixedphasepeaksforanannealingtemperatureof 439C.FigureA.6showstheVO 2spectrumoverSiO 2/Sisubstrateswhenannealedat 439C.At T439CVO 2thin˝lmwilldegrade. FigureA.1:RamanspectrumforVO x/SiO2depositedsamplesbysol-gelannealedat 417Cfor2hrsunderapressureof37mTorr. 112FigureA.2:RamanspectrumforVO x/SiO2depositedsamplesbysol-gelannealedat 428Cfor2hrsunderapressureof37mTorr. FigureA.3:RamanspectrumforVO x/SiO2depositedsamplesbysol-gelannealedat 450Cfor2hrsunderapressureof37mTorr. 113FigureA.4:RamanspectrumforVO x/SiO2/Sidepositedsamplesbysol-gelannealedat 406Cfor2hrsunderapressureof15mTorr. FigureA.5:RamanspectrumforVO x/SiO2/Sidepositedsamplesbysol-gelannealedat 439Cfor2hrsunderapressureof15mTorr. 114FigureA.6:RamanspectrumforVO x/SiO2/Sidepositedsamplesbysol-gelannealedat 461Cfor2hrsunderapressureof15mTorr. 115AppendixB:AdditionaltestingforVO 2-basedVOA Smallest( ˘70m)laserspotsizeachievedfortestingthe VOAdeviceinshownin FigureB.1.FigureB.1:Minimumspotsizediameter( 70µm)achievedbytheelectro-opticalsetup. 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