Structural and Electrical Properties of Nickel Oxide for Resistance Memory Devices

다국어 초록 (Multilingual Abstract)

Nickel oxide (NiO) has recently been receiving a great deal of attentions as a material for the Resistance Random Access Memory (RRAM) as a substitutional device of the conventional Si memory devices. Therefore the characterization of NiO material properties is crucial for the progress of NiO-based RRAM. The DC bias analysis and ellipsometry measurements were applied to the NiO thin films to understand electrical, topological, and optical phenomena and the origin of bistable resistive switching. The crystallinity of a deposited NiO thin film was polycrystalline according to TEM, XRD patterns and AFM images. After the electric field was applied to NiO thin film, conducting filaments formed under and around the top Pt electrode. The three electronic states, such as forming, “on”, and “off” were monitored using current-voltage measurement. These NiO thin films deposited at 500~600℃, oxygen partial pressure of 5~7%, and deposition power of 200W have better switching characteristics. The endurance of NiO RRAM device is 10^(6) times of writing cycles and 10^(12) times of reading cycles with sensing margin (R_(reset)/R_(set)) of 1 order of magnitude. The switching speed is also measured by programmed pulse cycle, thus this NiO RRAM has less than 10 ns “set” speed and less than 5 μs “reset” speed. We also measured temperature dependence for NiO thin film. NiO thin film was deposited on silicon substrates at room temperature and at 500℃ using a nickel and a nickel oxide target by DC and RF magnetron sputtering. We annealed the NiO thin films which were deposited at room temperature. By using transmission electron microscopy, we found that the thin film grew in nano-columns. Furthermore, ellipsometric analysis showed that oxygen rate during the deposition process would affect considerably to the electrical switching phenomena. We conclude that oxygen vacancy could be detrimental to the resistance switching phenomenon. Although stoichiometric NiO is an insulator, its resistivity can be lowered by an increase of Ni^(3+) ions resulting from the addition of monovalent atoms or by the introduction of nickel vacancies and/or interstitial oxygen in NiO crystals. However these mechanisms may be valid just for the bipolar resistance change memory. Positive charges (i.e. such as Ni^(2+) or Ni^(3+) in the NiO thin film) do not show the same behavior in the negative bias. From the XPS result we suggest that, the conduction through the nonstoichiometric polycrystalline NiO thin film is result of metallic Ni content plays an important role in bistable resistance switching.

목차 (Table of Contents)

• CHAPTER1 INTRODUCTION = 1
• 1.1.Background = 1
• 1.2.Non-volatileResistanceRandomAccessMemory = 3
• 1.3.VariousmaterialcandidatesforRRAM = 5
• 1.4.ArrangementoftheDissertation = 8
• CHAPTER2 THEORETICALBACKGROUNDS = 10
• 2.1.TransitionMetalOxide(TMO) = 10
• 2.1.1.SurfaceDefects = 12
• 2.1.2.NickelOxide,NiO(100)singlecrystal = 14
• 2.2.MemoryswitchingeffectsofRRAM = 16
• 2.2.1.NegativeDifferentialResistance = 16
• 2.2.2.Thechangeofresistanceduetothebiasvoltageofdifferencepolarity = 18
• 2.2.3.Thechangeofresistanceduetothresholdvoltage = 20
• 2.4.ThinFilmTechnology = 22
• 2.4.1.PrincipleofSputtering = 22
• 2.4.2.Atomiclayerdeposition(ALD) = 27
• 2.4.2.1.DepositionmechanismofALDprocesses = 27
• 2.4.2.2.TheGrowthratevariationofALDprocess = 28
• 2.4.2.3.AdvantagesanddisadvantagesofALDprocess = 30
• 2.4.2.4.ConsiderationforALDprocess = 30
• 2.5.OpticalConstantsofThinFilm = 32
• CHAPTER3 EXPERIMENTALDETAILS = 33
• 3.1.Fabricationofnickeloxidethinfilms = 33
• 3.1.1.ALDsystemNiOthinfilm = 33
• 3.1.2.SputteringsystemNiOthinfilm = 36
• 3.2.CharacterizationTechniques = 39
• 3.2.1.ScanningElectronMicroscopy(SEM) = 39
• 3.2.2.EnergyDispersiveX-RaySpectroscopy(EDXS) = 41
• 3.2.3.X-rayDiffraction = 42
• 3.2.4.AugerElectronSpectroscopy(AES) = 44
• 3.2.5.AtomicForceMicroscopy(AFM) = 46
• 3.2.5.1.ConductanceImagingAFM = 50
• 3.2.6.Ellipsometry = 52
• 3.2.6.1.Ellipsometricangles = 52
• 3.2.6.2.VariableAngleSpectroscopicEllipsometry(VASE) = 54
• 3.2.6.3.VASETheory = 54
• 3.2.7.TransmissionElectronMicroscopy(TEM) = 56
• 3.3.MemorySwitchingMechanismofRRAM = 57
• 3.3.1.ConductingFilamentModel = 60
• 3.3.2.SpaceChargeLimitedCurrent = 62
• 3.3.3.Chargetrapmodel = 64
• 3.3.4.InterfaceModel = 66
• CHAPTER4 RESULTANDDISCCUSION = 68
• 4.1.PropertiesofdifferentannealingtemperatureofNiOthinfilms = 68
• 4.1.1.Samplefabrication = 68
• 4.1.2.Topologicalproperties = 69
• 4.1.3.Electricalproperties = 74
• 4.1.4.Opticalproperties = 77
• 4.2.PropertiesofdifferentthicknessofNiOthinfilms = 79
• 4.2.1.Samplefabrication = 79
• 4.2.2.Topologicalproperties = 80
• 4.2.3.Electricalproperties = 81
• 4.2.4.Opticalproperties = 84
• 4.3.PropertiesofdifferentoxygenpartialpressureofNiOthinfilms = 90
• 4.3.1.Samplefabrication = 90
• 4.3.2.Topologicalproperties = 91
• 4.3.3.Opticalproperties = 96
• 4.4.GeneralelectricalandopticalpropertiesofNiOthinfilms = 99
• 4.4.1.Electricalproperties = 99
• CHAPTER5 CONCLUSIONS = 106
• REFERENCES = 109