Research On Interfacial Modification Of Niobium Oxide-based Resistive Random Access Memory

CMOS technology is reaching its physical limit.Device scaling down will be not able to provide the required performance gain.On the other hand,Von Neumann's architecture puts performance obstacles in the way of the rapid development of artificial intelligence.In the post-Moore era,non-von Neumann architectures,such as Quantum computing,Optical computing,In-memory computing,and Neuromorphic computing,have been of great interests.As the hardware foundation of general artificial intelligence,Neuro-inspired Computing Chips could greatly accelerate artificial neural network algorithms.Electronic synapses based on nonvolatile resistive switching and the oscillation neurons based on volatile threshold switching are the basis of Neuro-inspired Computing Chips.Moreover,random access resistive memory is also regarded as a strong candidate for next-generation memory.Threshold switching could effectively solve the sneak-path currents problem in cross-point arrays without reducing chip integration.Niobium oxide could be used in both resistive switching and threshold switching devices.It is a very interesting and significant dielectric for Neuro-inspired Computing Chips and next-generation nonvolatile memory.In this thesis,amorphous niobium oxide films with different oxygen content were prepared by using reactive sputtering method.Then,resistive memory with metal/Nb₂O₅/NbO_x/metal structure were designed and developed.We investigated the rule and mechanism of tuning the interface between the top electrode and the Nb₂O₅ resistive dielectric layer.The main contents are as follows:1.NbO-rich amorphous films and Nb₂O₅ amorphous films were prepared by reactive magnetron sputtering methods.Natural oxidation of the niobium oxide films in air has been investigated.A Nb₂O₅ laye with highest valence Nb forms on the surface of exposed niobium oxide films.Continuous observation indicates that NbO-rich films can be oxidized to Nb₂O₅-rich films gradually.2.Electrode-induced polarity conversion has been found in Pt/Nb₂O₅/NbO_x/Ru and W/Nb₂O₅/NbO_x/Ru devices.The W electrode has extracted some oxygen ions from the surface of Nb₂O₅ film to form the WO_x oxygen ion reservoir layer at the W/Nb₂O₅ interface during sputtering.The oxygen ion reservoir layer changes the oxygen-ion distribution at the interface and causes the polarity conversion.The W/Nb₂O₅/NbO_x/Ru devices can reset immediately after electroforming during daul negetive voltage sweeping.The devices can also be in the high resistance state after the electroforming.3.Co-existence of threshold and resistive switching has been found in Ta/Nb₂O₅/NbO_x/Ru devices.During sputtering process,a Ta O_x oxygen ion reservoir layer forms at the Ta/Nb₂O₅interface by extracting oxygen ions from the surface of the Nb₂O₅ film.The Nb₂O₅ is reduced into a Nb₂O_5-x layer,and the interface of Ta/Nb₂O₅ degenerates to the Ta O_x/Nb₂O_5-x interface.The device is in a high resistance state after the electroforming under a negative bias voltage.In a Set process under positive bias voltage,oxygen ions have been injected from the Nb₂O_5-x layer to the Ta O_x oxygen ion reservoir layer.The conductive filaments are connected to the top electrode,and the device is in a low resistance state.The stable Ta O_x is difficult to release oxygen ions so that only a few oxygen ions return to the Nb₂O_5-x layer during a Reset process.The Nb₂O_5-x at the conductive filament tip transforms into NbO₂,a phase transition material.Threshold switching phenomenon occurs in the NbO₂ when the current compliance is low.When the compliance increases,oxygen ions will migrate between the NbO₂ phase transition channel and the Ta O_x oxygen ion reservoir layer.NbO₂ becomes NbO_x in metal phase,which causes the resistive switching phenomenon.The co-existence of threshold/resistive switching controled by current compliance indicates that Ta/Nb₂O₅/NbO_x/Ru resistive memory could be applied in both oscillation neurons and electronic synapses.4.Five types of TE/Nb₂O₅/NbO_x/Ru devices have been prepared using Ta,W,Mo,Ru,and Pt as top electrodes,respectively.The relationship between initial rectifier behaviors of the devices and the work functions of the top metal electrodes was analyzed.The change of rectifier behaviors after electroforming was observed.This phenomenon is attributed to the formation of a conductive filament with a work function between 4.6 e V and 4.7 e V in the Nb₂O₅ matrix.A compliance interrupting method is used to electroform the five devices.It is found that the devices with Pt,Ru,and W as the top electrodes are in a low resistance state,and the devices with Ta and Mo as the top electrodes are in a high resistance state.An oxygen ion reservoir model successfully explains the phenomena.Multi-level storage of the devices with Mo,W,and Ta top electrodes was demonstrated by tuning the current compliance during the Set process and the sweep voltage during the Reset process.It is demonstrated that the devices with oxygen ion reservoir layer have a great potential as electronic synapses.In conclusion,a reasonable selection of electrode materials can alter the metal/oxide interface structure of resistive memory,which is helpful to improve the device performance and develop new devices.The oxygen ion reservoir model can explain well the resistive behavior of transition metal oxide memory,which is theoretically significant for the research of resistive memory and the development of electronic synapses.