Ultra-low Voltage And Multilevel Resistive Switching Of NiO Films Modified With HfO₂ Interfacial Layers

Resistive switching random access memory?ReRAM?based on oxide thin films are widely used in new generations of non-volatile memory devices,artificial intelligence logic circuits and neuro computers due to its simple structure,low energy consumption,good scalability and compatibility with silicon-based semiconductor technology.NiO has been extently studied since 1960's,and is one of the most promising materials applied for commercial ReRAMs.Since 2009,researchers have constantly optimized technical processes and device structures to improve resistive switching performance parameters of NiO film-based ReRAM cell.However,the problems of large set/reset voltages and wide voltage dispersion have not been solved well.In this paper,NiO films with one specific crystal orientation are selected as the resistance inversion layer material.By inserting HfO₂interfacial layer with different thicknesses at different positions of NiO films,multi-layer stack structure devices are constructed.Comparatively study resistive switching performance parameters of those devices before and after inserting the HfO₂ interfacial layer,especially for the numerical value and dispersion of reset/reset voltagesFirstly,NiO resistive switching layers grown along the?111?and?001?crystal orientations and HfO₂ polycrystal interfacial layers were deposited by using magnetron sputtering.Then,current-voltage characteristics of NiO-based ReRAM cells with/without HfO₂ interfacial layer and different upper electrodes?Ag or Pt?were comparatively studied.Subsequently,effects of NiO film thickness,HfO₂ interfacial layer thickness and inserting position,and measurement parameters on resistive switching performance for NiO-based ReRAM cells were investigated.Finally,resistive switching mechanism of the optimized NiO-based ReRAM cell was discussed.Main research contents and conclusions are listed as following:?1?For?111?-highly textured NiO films with different thickness and deposited on different substrates?Pt-MgO,Pt-STO and Pt-Si?::100 nm-thick NiO?111?films deposited on Pt-MgO substrates have relatively good resistive switching characteristics,such as acceptable high/low resistance ratio?20⁷0?,low set/reset voltage?^?0.5 V?.At 80^°C-testing temperature,its resistive switching parameters still meet the minimum requirements for ReRAM application.?2?Thickness effects of HfO₂ interfacial layers between NiO?111?films and Ag upper electrodes:10 nm-thick HfO₂ interfacial layer greatly improves resistive switching performance of NiO?111?-based ReRAM cell:the leakage current of ReRAM cell on high resistance state greatly decreases which makes the high/low resistance ratio is more than 10⁴ at 0.1Vread voltage;the set/reset voltage further decreases and with a narrow distribution?^?0.15 V?;the endurance is more than 10³ cycles.?3?Under optimized device technology and deposition conditions,the?001?-highly textured NiO films with 80 nm thickness were deposited on Pt-Si substrate.Then,comparatively study resistive switching characteristics of NiO?001?-based ReRAM cells before and after inserting 10 nm-thick HfO₂interfacial layer on the upper,middle and bottom surface of the NiO?001?film,respectively.Under optimized measurement conditions?compliance current is 10 mA and the scanning voltage is lower than?0.5 V?,the C1 Cell?the HfO₂ interfacial layer is inserted between the NiO?001?film and Pt bottom electrodes?,it exhibits excellent resistive switching characteristics;set/reset voltage is symmetrical around?0.2 V,the high/low resistance ratio is more than 10⁵,endurance is more than10⁴ cycles Under optimized measurement conditions?compliance current is 1 mA and the scanning voltage is?0.4 V?,the C2 Cell?the HfO₂ interfacial layer is both inserted between the NiO?001?film and Pt bottom?Ag upper?electrodes?exhibits abnormal bipolar resistive switching behaviors with 1or 2 SET processes?defined as S?1?and S?2??respectively:S?1?state shows a high/low resistance ratio of>10⁴,a ultra-low set/reset voltage of<?0.2 V,a good retention of>9.6?10³s and a long endurance of 7200 cycles.S?2?state exhibits an endurance of 1000 cycles and the two-step SET process with a high/low resistance ratio of 10² and 10³,respectively.The C3 cell?the HfO₂ interfacial layer is simultaneously inserted in the upper,middle and bottom surface of the NiO?001?film?exhibits multi-level resistive switching characteristics:three unstable states?S1,S2 and S3?and one stable state?S4?.S4 state can maintain a high/low resistance ratio of>10³ after continuous 2000 cycles.In summary,the HfO₂ interfacial layer improves the formation of conductive filaments of oxygen vacancies in NiO films,thus decreases the set/reset voltage and improves its dispersion.C2 Cell based on NiO?001?films modified with two HfO₂ interfacial layer exhibits stable multilevel resistive switching behaviors,showing potential applications on new generations of ultra-low operating voltage and multi-state switching memory devices.