ANABSTRACTOFTHEDISSERTATIONOF DavidHong forthedegreeofDoctorofPhilosophy in ElectricalandComputerEngineering presentedonNovember5,2008. Title: FabricationandCharacterizationofThin-filmTransistorMaterialsandDevices. Abstractapproved: JohnF.Wager A class of inorganic thin-film transistor (TFT) semiconductor materials has emerged in- volving oxides composed of post-transitional cations with (n-1)d10ns0 (n≥4) electronic configurations. This thesis is devoted to the pursuit of topics involving the development ofthesematerialsforTFTapplications: Depositionofzincoxideandzinctinoxidesemi- conductor layers via reactive sputtering from a metal target, and the characterization of indiumgalliumzincoxide(IGZO)-basedTFTsutilizingvariousinsulatormaterialsasthe gatedielectric. The first topic involves the deposition of oxide semiconductor layers via reactive sputtering from a metal target. Two oxide semiconductors are utilized for fabricating TFTs via reactive sputtering from a metal target: zinc oxide and zinc tin oxide. With optimizedprocessingparameters,zincoxideandzinctinoxideviathisdepositionmethod exhibitsimilarcharacteristicstoTFTsfabricatedviasputteringfromaceramictarget. Additionally the effects of gate capacitance density and gate dielectric material are explored utilizing TFTs with IGZO as the semiconductor layers. IGZO-based TFTs ex- hibit ideal behavior with improved TFT performance such as higher current drive at a given overvoltage, a decrease in the subthreshold swing, and a decrease in the magni- tude of the turn-on voltage. Additionally it is shown that silicon dioxide is the preferred dielectricmaterial,withsiliconnitrideapoorchoiceforoxide-basedTFTs. Finallyasimplemethodtocharacterizethebandtailstatedistributionnearthecon- duction band minimum of a semiconductor by analyzing two-terminal current-voltage characteristics of a TFT with a floating gate is presented. The characteristics trap en- ergy (E ) as a function of post-deposition annealing temperature is shown to correlate T very well with IGZO TFT performance, with a lower value of E , corresponding to a T moreabruptdistributionofbandtailstates,correlatingwithimprovedTFTmobility. Itis shown that as the post-deposition anneal temperature increases, the total number of band tail states does not change significantly, however the energy distribution of these states approachesthatofacrystallinematerial. (cid:176)cCopyrightbyDavidHong November5,2008 AllRightsReserved FabricationandCharacterizationofThin-filmTransistorMaterialsandDevices by DavidHong ADISSERTATION submittedto OregonStateUniversity inpartialfulfillmentof therequirementsforthe degreeof DoctorofPhilosophy PresentedNovember5,2008 CommencementJune2009 DoctorofPhilosophydissertationofDavidHong presentedonNovember5,2008 APPROVED: MajorProfessor,representingElectricalandComputerEngineering DirectoroftheSchoolofElectricalEngineeringandComputerScience DeanoftheGraduateSchool IunderstandthatmydissertationwillbecomepartofthepermanentcollectionofOregon State University libraries. My signature below authorizes release of my dissertation to anyreaderuponrequest. DavidHong,Author ACKNOWLEDGMENTS I would like to thank my family for their support and encouragement throughout my education and life. They have instilled in me a work ethic and ambition that has led methisfar. I would also like to thank all of my friends and coworkers who have contributed greatly during my graduate program. Several key people are Dr. Hai Chiang and Dr. Jeff Bender, who have been great resources for encouragement and cromulent insight into many topics as well as a great sounding board for my bizarre ideas. Professor John ”The Bossman” Wager has provided great direction and discussion as well as an endless supply of enthusiasm and energy. Chris Tasker provided a work ethic to follow, as well as great direction and support in and outside of the cleanroom. Additionally, Manfred Dittrich has provided exceptional fabrication work and mechanical knowledge as well as great discussions ranging from politics and religion to football and the proper usage of threadingtaps. A number of industry people deserve acknowledgements. Two specific people I wouldliketomentionareRandyHoffman,inventoroftheinorganictransparentthin-film transistor, and Greg Herman, both of whom have been great resources. In addition to providing many of the substrates used for this dissertation, they have provided a high levelofdiscussionandIamgratefulfortheirtimeandenergy. ThisworkwassupportedbytheHewlett-PackardCompany,theDefenseAdvanced Research Projects Agency (MEMS/NEMS: Science and Technology Fundamentals), the UnitedStatesDisplayConsortium,andtheNSF(IGERTgrantno. 0549503). Totheoptimisttheglassishalffull. Tothepessimisttheglassishalfempty. Totheengineer,theglassistwiceasbigasitneedstobe. -unknown TABLEOFCONTENTS Page 1. INTRODUCTION ..................................................... 1 2. OXIDESEMICONDUCTORSANDTHIN-FILMTRANSISTORS ......... 4 2.1 Oxidesemiconductors............................................. 4 2.1.1 Transparentconductingoxidesoverview ....................... 4 2.1.2 Zincoxide.................................................... 8 2.1.3 Amorphousoxidesemiconductors ............................. 9 2.1.4 Zinctinoxide................................................. 10 2.1.5 Indiumgalliumzincoxide..................................... 11 2.2 Oxidesemiconductordevices: Thin-filmtransistors................... 14 2.2.1 Thin-filmtransistors........................................... 14 2.2.2 Oxidesemiconductor-basedthin-filmtransistors................ 16 2.2.2.1 TFTswithsimpleoxidelayers: ZnO................... 16 2.2.2.2 TFTswithsimpleoxidelayers: In O ,SnO ,Ga O ... 20 2 3 2 2 3 2.2.2.3 TFTswithmulticomponentoxidelayers: ZTO ......... 21 2.2.2.4 TFTswithmulticomponentoxidelayers: ZIO.......... 23 2.2.2.5 TFTswithmulticomponentoxidelayers: IGO ......... 25 2.2.2.6 TFTswithmulticomponentoxidelayers: IGZO ........ 26 2.3 Conclusions...................................................... 29 3. TFTFABRICATIONANDCHARACTERIZATION ...................... 30 3.1 Thin-filmdepositionandprocessing ................................ 30 3.1.1 Evaporation................................................... 30 3.1.2 Sputtering .................................................... 31 3.1.3 Plasma-enhancedchemicalvapordeposition(PECVD) ......... 34 3.1.4 Atomiclayerdeposition(ALD)................................ 35 3.1.5 Post-depositionthermalprocessing ............................ 36 3.2 Hallmeasurement................................................. 37 3.3 Thin-filmtransistorfabrication..................................... 39 TABLEOFCONTENTS(Continued) Page 3.3.1 Fully-transparentTTFT ....................................... 39 3.3.2 Non-transparentTFT.......................................... 41 3.4 Transistoroverview ............................................... 41 3.5 Thin-filmtransistordevicecharacteristics............................ 46 3.5.1 DCcurrent-voltagemeasurements ............................. 46 3.5.2 Thresholdvoltageandturn-onvoltage ......................... 48 3.5.3 Mobility...................................................... 50 3.5.4 Draincurrentswinganddraincurrenton-to-offratio............ 57 3.6 Conclusions...................................................... 58 4. REACTIVESPUTTERINGOFOXIDESEMICONDUCTORS ............ 60 4.1 Introduction...................................................... 60 4.2 ReactiveZincOxide............................................... 61 4.3 ReactiveZincTinOxide........................................... 67 4.4 Conclusion....................................................... 75 5. THIN-FILMTRANSISTORDIELECTRICPERFORMANCE ............. 76 5.1 Introduction...................................................... 76 5.2 Experimental..................................................... 77 5.3 ChemicalVaporDeposited(CVD)SiliconDioxide................... 79 5.4 SiliconDioxide,AluminumOxide,andSiliconNitride ............... 88 5.5 SilaneandTetraethylOrthosilicate(TEOS)-basedCVD............... 98