Fabrication,Storage Characteristics,and Memristive Functions Of Several Nanocomposite Oxide Resistance Switching Memory Devices By Atomic Layer Deposition

The resistance random access memories(RRAMs)based on oxides show exhilarating advantages for next generation of nonvolatile memories,such as simple structure,low power consumption,high density,fast access speed,three-dimensional integration,compatible with microelectronics technology,however,the dispersion of resistive switching(RS)parameters limits its applications in memory fields.The memristor is a two-terminal device which "remembers" the flow of charges through it with time-dependant memristance feature.It is the most proper electronic component to simulate the human brain processes such as learning/memory and realize the "brain" artificial cognition.Synaptic bionic research is in the booming development,but the lack of high quality memristor material systems and large-scale manufacturing technology compatible with the microelectronics technology,becomes a bottleneck,restricting its rapid progress.Atomic layer deposition(ALD)is a kind of novel thin film deposition technique based on precursor surface chemisorption reactions on substrates.Its unique self-limitation and self-saturation mechanism ensures large area uniformity,excellent three-dimensional conformality,and precise sub-monolayer thickness control.ALD exhibits great potential prospects in pursuit of high density,miniaturization,and integration of microelectronics,optoelectronics,and nanotechnology.This thesis mainly focuses on the fabrication and applications of ALD technology in RRAM and memristor fields.Several nanocomposite oxide RRAM devices have been prepared by ALD,including Al2O3/HfO2/Al2O3 trilayer stacking structure and three kinds of metal nanocrystals(Co,Pt and CoPtx)/oxides(Al2O3 and HfO2)on TiN/Si substrates.The relationship between processing parameters,microstructure,and storage properties has been investigated systematically.The optimal performance has been achieved and the related RS mechanism has been proposed.The impact of metal nanocrystal(NC)size and distribution on the RS characteristics has been evaluated based on experiment results and theoretical calculation.Simultaneously,two kinds of Hf02-based bilayer memristors have been fabricated by ALD.The bionic simulation functions of electronic synapses,including nonlinear transmission characteristics,synaptic plasticity,learning and memory behaviors,have been characterized in depth.The main achievements are summarized as follows:I.Fabrication and storage properties of Al2O3/Hf02/Al2O3 trilayer stacking RRAM devices by ALD1.The memory units of Pt/Al2O3/Hf02/Al2O3/TiN/Si fabricated by ALD exhibit a typical bipolar RS behavior.The optimized tri layer stacking structure of 3 nm Al2O3/13 nm HfO2/6 nm Al2O3 shows better monodispersity of RS parameters and excellent RS performances,such as stable RHRS/RLRS ratio(>10),smaller set/reset voltages(-0.96 V/1.31 V),better endurance and long data retention(>10 years at 85 ?).The trilayer stacking structure of Pt/Al2O3/HfO2/Al2O3/TiN manifests modulation effect on electric field.And the nonuniform distribution of the oxygen vacancies in the trilayer structure,especially in interfacial layers of Al2O3/HfO2,reduces the randomness of conductive filament formation,linkage and rupture in RRAM device,and improves the consistency of RS parameters and device stability.2.The effect of bottom electrodes of TiN and Pt on the RS properties of trilayer stacking structure of Al2O3/HfO2/Al2O3 has been studied carefully.Both memory cells obey the conductive filament mechanism.The dominant conduction mechanisms in LRS and HRS are Ohmic behavior and space-charge-limited current,respectively.It is found that the chemical inert Pt and chemical active TiN with oxygen have great influence on the electrofonning polarity preference,ratio of high and low resistance,and dispersion of the operating voltages of trilayer-structure memory cells.The RRAM unit with Pt/TiN asymmetric electrodes has uniform RS parameters and lower power consumption,which is ascribed to the oxygen reservoir role of TiN bottom electrode.The reasonable explanation on the different electroforming polarity of both memory cells has been proposed.3.The effect of thickness of the trilayer stacking structure and annealing atmosphere on the RS performance has been examined systematically.The results elucidate that the appropriate thickness of the bottom layer of Al2O3(?3 nm)can effectively decrease the dispersion of the set/reset voltages;the thickness of the middle layer of HfO2 ranging from 6 to 13 nm has less influence on the device performance;and the increased thickness of top layer of Al2O3 leads to the increased resistance ratio,and the 8?10 nm Al2O3 layer is proper.With reducing the oxygen partial pressure in annealing atmosphere,the oxygen vacancy concentration of the device also rises.After post annealing at 550 to 600 ? in high pure N2 atmosphere,the RRAM devices can be obtained with low power consumption.II.Fabrication and storage characteristics of oxide-based RRAM devices with embedded metal NCs by ALD1.Al2O3 or HfO2-based nanocomposite structures with embedded Co,Pt,or CoPt,NCs have been prepared on TiN-coated Si substrates by combination of thermal ALD(TALD)and plasma-enhanced ALD(PEALD).A comparative study has been performed on the effect of metal NCs types and ALD cycle numbers of 60 and 100 on the RS behaviors.It is found that Pt/Al2O3/100 cycle-CoPtx NCs/TiN/Si sample follows the conductive fiilament model with the best storage performances,including highly concentrated RS parameters,smaller set/reset voltages(-0.72 V/0.93 V),stable resistance ratio(?102)of OFF/ON states,better switching endurance up to 104 cycles,and longer data retention over 105 seconds.A series of analyses show that a monolayer CoPtx NCs(x = 0.4?0.6)deposited on TiN bottom electrode has been embedded into Al2O3 thin films successfully with mixed phases of ferromagnetic fct CoPt,disordered fcc CoPt,and Co3O4.This possibly implies the formation of a core-shell structure of CoPt NCs ?Co3O4 with weak ferromagnetism.The nanocomposite structures of CoPtx NCs/oxide might also bring new characteristics for RRAM due to the ferromagnetic property of CoPtx in multifunctional electronic devices.2.The dependence of ALD cycles of 50?130 during Pt or CoPtx NCs growth on the RS parameters of nanocomposite oxide-based RRAM devices has been characterized systematically.Both memory cells with embedded Pt or CoPtx NCs show similar trends:with increasing ALD cycles,the forming voltage,set/reset voltage,LRS/HRS,and resistance ratio firstly decrease and then increase.And in the middle region of about 90 and 100 cycles,the lower RS parameters are obtained with flat change and better RS properties.When ALD cycles exceed a critical value of about 110 to 120,the RS parameters suddenly become large with degraded RS performances.The impact of metal NCs size and distribution on electric field strength of RRAM devices has been calculated by using the finite element method.Although all metal NCs with various sizes enhance the electric field strength,only metal NCs with proper grain size and areal density(9 nm/6?10 × 1011/cm2 in this work)can effectively produce stronger localized electric field at the tip of metal NCs,compared to at the planar region.The conductive filament prefers to grow at the location of metal NC site under applied low voltage.Hence,they are easily formed in the same paths in reduplicate during set process,which is advantageous to minimize the distribution of the RS parameters and reduce the power consumption.?.Fabrication and bionic simulation functions of HfO2-based bilayer memristors by ALD1.Two ultrathin HfO2-based inorganic bilayer memristor devices with the structure of Pt/HfOx/ZnOx/TiN and Pt/AlOx/HfOx/TiN have been fabricated via combination of TALD and PEALD.Multilevel resistance states(no less than 6)are obtained in both HfO2-based memristor devices by varying the programming voltage amplitudes during the pulse cycling.Several essential synaptic functions are simultaneously achieved in such HfOx/ZnOx and AlOx/HfOx devices,including nonlinear transmission properties,such as long-term plasticity(LTP),short-term plasticity(STP),and spike-timing-dependent plasticity(STDP).The learning and memory behaviors are further confirmed in memristive device of Pt/HfOx/ZnOx/TiN,especially for the transformation from STP to LTP induced by repetitive pulse stimulation.The device conductance can also be continuously increased or decreased from cycle to cycle with better endurance characteristics up to about 3 × 103 cycles.2.The memristive mechanism of two HfO2-based inorganic bilayer structures follows the oxygen vacancy migration/diffusion model.For Pt/HfOx/ZnOx/TiN,ZnOx with deficient oxygen on TiN bottom electrode is conductive layer,HfOx with relatively rich oxygen near Pt top electrode is insulator layer.The oxygen vacancy migration/diffusion between oxygen-deficient layer and oxygen-rich layer under different bias voltages modulates the device conductance.There exist various oxygen vacancy concentrations in bilayer structures of HfOx/ZnOx and AlOx/HfOx with asymmetric Pt/TiN electrodes.This leads to significant asymmetry set/reset voltage in HfO2-based memristors during continuous set and reset processIn conclusion,ALD is explored to prepare new type of RRAMs and memristors.The related storage properties and memristive functions have been characterized in-depth.This work helps to promoting practical applications of new RRAMs and memristors with material and manufacturing foundations in the future.