Study Of The Oxide Stack Structure RRAM And Its Bioelectronic Synapse Function

As the down-scaling of charge storage based nonvolatile memory(NVM),such as flash memory,approaches its physical limitations,to develop new types of NVM with high performance,high density and low cost has become the research focus for the semiconductor memory industry.Resistance random access memory(RRAM),as a promising NVM device,has attracted great attention due to its simple structure,fast switching speed,low power consumption,high-density integration capability and excellent scalability.It is well known that the greatest advantage of RRAM as a new type NVM device is that it has an excellent scalability,thus it can obtain a high-density storage.However,the down-scaling of device size depends on the improvement of micro-fabrication technology,resulting in the higher and higher cost.The realization of multilevel cell(MLC)storage characteristics in single RRAM cell provides another effective way to improve the storage density.If the transverse dimension of the conductive filament in RRAM device is controlled at the atomic scale,it is possible to observe an unique phenomenon--quantized conductance.The controlled and stable quantized conductance has wide application in ultra-high density memory.In addition,in recent years,it has been found that the RS effect not only can be applied to NVM but also has potential application in neuromorphological computing(especially for mimicking the functions of biological synapses)or logic circuits.Therefore,in view of the above key issues about the research of RRAM device,this thesis mainly focuses on to building stack tructure RRAM devices to improve its RS performance,especially to improve the memory window(RHRS/RLRS)to obtain stable and reliable MLC storage characteristics.Besides,for the first time the intrinsic phenomena of quantized conductance and negative differential conductance with an oscillatory behavior are observed at the as-fabricated HfOx/Al2O3 multilayer structure RRAM devices.Finally,the potential applications of the RS effect in biosynaptic simulation are also stuided.The main research contents and results are listed below:In the first part,the effects of three types of bottom electrode(BE)materials,such as such as active Cu and Ag,inert Pt and Au,and oxygen-affinity W and Ti,on the RS characteristics of HfO2/TiOx bilayer RRAM devices were systematically investigated.The results show that the RRAM devices with oxygen-affinity BE(W or Ti)can significantly improve the uniformity of main switching parameters(VSet,VReset,RLRS and RHRS)compared to the other two types of BE materials.In addition,due to the presence of interface layer(IL),which can be served as an oxygen reservoir,the RRAM devices with oxygen-affinity BE(W or Ti)show gradual changes in resistances in the Reset process.The gradual Reset characteristics can be used to obtain MLC storage capability by controlling the Reset stop voltage(VReset-stop),resulting in the increase of storage density of the RRAM devices.By comprehensive consideration of the effect of BE materials on RS performance,for the HfO2/TiOx bilayer structure RRAM device,the optimum combination of BE and TE electrodes is Ti and Pt,respectively.These results provide insights into the proper selection of oxide-based switching layer and electrode materials to improve the performance of the related RRAM devices.This part has been accepted by IEEE ICSICT 2018.In the second part,it is demonstrated that inserting a thin Al2O3 layer at the interface of Ti BE/HfO2 and HfO2/Pt TE,respectively,can significantly improve the memory window(RHRS/RLRS),thus improving the stability and reliability of MLC operation.Four stable and distinct resistance states(LRS,HRS1,HRS2 and HRS3)were obtained by controlling the VReset-stop(1.5,2 and 2.5 V).In addition,the as-fabricated RRAM devices also exhibit other excellent RS performance,such as ultra-fast program/erase speed(?10 ns,the resolution limit of our measurement equipment)and good pulse endurance(?105 cycles),which are suitable for application in high density and high speed NVM.Then,the role of the inserted Al2O3 layers in the RS and conduction mechanism of the Ti/Al2O3/HfO2/Al2O3/Pt device is discussed in details.Finally,the initial switching dynamics of the RRAM devices are studied by measuring the dependence of Set and Reset switching characteristics on the pulse width and amplitude.The results show that under the same pulse amplitude,the initial Set progress is faster than the initial Reset progress.The study of switching dynamics can help us to design the proper combination of varying pulse amplitude and width to optimize the device operation parameters.This part has been published in J.Phys.D:Appl.Phys.,51(2),2017.In the third part,for the first time,we demonstrated an HfOx/Al2O3 multilayer structure RRAM device with 3-bit storage in each memroy cell.In addition,an obvious quantized conductance phenomenon was observed at both DC sweep and pulse-train operation mode.More importantly,as the external bias(applied to Pt TE)increases,a progressive decrease in conductance followed by a quantized conductance and finally a multilevel Reset phenomenon can be clearly observed in the Reset process.The Reset dynamics in Ti/HfOx-Al2O3 MLs/Pt device can be interpreted as the evolution process of conductive filaments(CFs)as follows:gradual thinner followed by atomic point contact and finally disconnect layer by layer in the Al2O3 and HfOx film.The multilevel Reset process can be seen as a negative differential conductance(NDC)phenomenon with oscillatory behavior with respect to the applied voltages.The intrinsic physical mechanism of the oscillatory behavior in NDC is not clear at present,which needs to be further studied.In the fourth part,under pulse-train operation mode,we have successfully mimic the synaptic functions,such as short-term plasticity/long-term plasticity(STP/LTP)and synaptic potentiation and depression(P/D)behaviors,by using the as-fabricated Ti/HfO2/TiOx/Pt memristor.The measured results of P&D characteristics suggested that the synaptic weight can be precisely tuned by the number or amplitude of the input pulse-train.In STP behavior,the conductance increased temporally at the moment of each pulse stimuli and spontaneously exponentially decayed with time until the application of the next pulse.While increasing the pulse amplitude or width or decreasing the interval of two adjacent pulses of the input pulse-train can result in the memristor behavior from STP into LTP transition,which plays an important role in learning and memory function in brain.This part has been published in Nanotechnology,29(41),2018.