Study On The Oxygen Vancancy Control And Performance Improvement In TaO_x-based Resistive Swiching Memory Devices

In the 21st century,the demand for high-density information storage is increasingly urgent in the current era of big data.Traditional flash memory will suffer from the physical size limit of 21nm technology nodes,which is difficult to meet the growing demand for high-density integrated applications.Therefore,four emerging nonvolatile memories have been proposed,such as ferroelectric memory,magnetic memory,phase-change memory,and resistive switching memory.Resistive random access memory?RRAM?is attracting growing interest due to its promising storage characteristics,which has been demonstrated in various materials.Among them,TaO_x-based RRAM device has been intensively studied because of its excellent performance such as high cycling endurance(>10¹²),ultra-fast programming speed?105ps/120ps?and high scalability?<10nm scale?.However,there are still several key bottlenecks remaining to be solved for practical applications,such as high operating current,large fluctuation of switching parameters and sneak path problem.As known,TaO_x-based RRAM device,as the non-volatile memory,is based on the migration of oxygen ions.The concentration,distribution and content of oxygen vacancy are three key factors that influence the device performance and mechanism of resistive switching.Aiming at performance optimization and in-depth mechanism analysis,our work focus on the above three aspects and the details are as follows:?i?Study on the control of the oxygen vacancy concentration and develop an effective means for demonstrating ultra-low power devices:First,the oxygen partial pressure was adjusted during film deposition in order to study the dependence of resistive switching characteristics on the defect concentration.Then,the introduction of Gd into the Ta₂O_5-x film was used to suppress the generation of oxygen vacancy,which was further confirmed by X-ray photoelectron spectroscopy?XPS?analysis.The resulting increase in the ion migration barrier can break the dilemma between ultra-low power operation and data retention.Herein,the reduction of oxygen vacancy concentration in the Ta?Gd?–O system is ascribed to the enlarged electronegativity difference between the cation and the anion.As a result,the optimized Gd-doped Ta₂O_5-x-x device can operate for more than 2000 cycles in DC mode at an ultralow operating current of 1?A,and almost no obvious degradation was observed.The extrapolated retention time can reach 10 years at 85^oC.An energy consumption as low as 60fJ/bit for RESET switching was achieved,which is comparable to that of the human brain with high energy efficiency?⁵0 fJ/bit?.?ii?Study on the control of the oxygen vacancy distribution and design reliable devices with highly localized switching:Ta₂O_5-x/Ta₂O_5-x:Si bilayer structure devices were designed to construct“tapered cone”conductive filaments?CFs?by means of Si dopant tuned the oxygen vacancy distribution.The structure design was aiming to controll the formation and also the rupture process of CFs,thus achieving the reliability of switching parameters.Herein,based on first-principles calculations and experimental verification?XPS?EDS?C-AFM?,we verified that Si-doping in Ta2O5-x based memory can facilitate VO formation,aggregation and migration.The Si doped Ta2O5-x was used as an ordered oxygen vacancy control layer?VCL?,which benefits for the growth of localized thinner CFs in the undoped switching layer?SL?.As expected,a localized“tapered cone”shaped filament in optimized bilayer stacks has been experimentally observed using C-AFM tomography.The rupture region would occur close to the hot spot at CF weakest region within the SL,which was calculated by the Fourier equation theory.Importantly,we report herein the observation and identification of switching sites?i.e.,SET/RESET processes?by in-situ TEM,thereby further clarifying the“tapered cone”CF geometry effects related to the bilayer stacks.?iii?Study on the control of the oxygen vacancy content and clarify the mechanism of CRS devices:Aiming at sneak path problem,the issues of preparation and optimization relative to CRS devices were clarified though studying the influence of oxygen vacancy content.Herein,though tuning the supply-demand relationship of oxygen defects,we discovered that the CRS behavior would occur under the condition of Q_S<Q_R,and further demonstrated the CRS behavior in TaOx based RRAMs.The generation of oxygen vacancy is driven by the electric field strength,which is estimated by E_F=V_F/t_oxide.So we carefully studied the dependence of CRS characteristics on the GO switching layer thickness(i.e.,t_oxide)and the forming process?i.e.,V_F?,respectively.The electric field dependence of CRS behavior was clarified to optimize the the power consumption and memory window of the devices,which provides the insight for future CRS crossbar arrays application.