Investigation On The Mechanism And Application Of Nanoscale NiO Memristors

Memristor is promising for many important applications,including nonvolatile memory,logic circuits,and neuromorphic networks,etc.Since the successful commercialization of any application demands a robust and predictive understanding of its underlying mechanisms,the mechanism investigation of memristor is essential.On the other hand,when memristor is applied to the crossbar structure,it will inevitably suffer from the so-called crosstalk issue,which should be well addressed to facilitate the device applications.Since NiO is among the most promising materials for memristor applications,in this thesis we have studied at the nanoscale both the mechanism of bipolar memristive behavior of NiO,and the intrinsically rectifying-memristive behavior(IR-MB)of NiO that can be utilized to solve the crosstalk problem.The main work can be divided into two parts as follows:Migration of oxygen vacancies has been proposed to play an important role in the bipolar memristive behaviors since oxygen vacancies can directly determine the local conductivity in many systems.However,a recent theoretical work demonstrated that both migration of oxygen vacancies and coexistence of cation and anion vacancies are crucial to the occurrence of bipolar memristive switching,normally observed in the small-sized Ni O.So far,experimental work addressing this issue is still lacking.In this work,with conductive atomic force microscope and combined scanning transmission electron microscope & electron energy loss spectroscopy,we reveal that concentration surplus of Ni vacancy over O vacancy determines the bipolar memristive switching of Ni O films.Our work gives an experimental support for the first time to the dual-defects-based model,which is of fundamental importance for understanding the memristor mechanisms beyond the well-established oxygen-vacancy-based model.Moreover,this work provides a methodology to investigate the effect of dual defects on memristive behaviors.Recently,IR-MB has been found as an effective way to address the crosstalk issue.In these systems,generally,interfacial Schottky diodes are formed in the low resistance states to suppress the sneak current at reverse bias.Besides this type of interface-dominated IR-MB,the IR-MB originating from the built-in homojunction is also possible in a metal/oxide/metal structure with ohmic contacts.On the other hand,IR-MBs in previous work were limited to macroscopic samples with micron-order pad-type electrodes.It is imperative to reduce the dimension to the real device size.In this work,we have fabricated Ni O nanodots of high storage density and high uniformity with ultrathin AAO templates.The Ni O nanodots show two types of IR-MBs,namely bipolar self-rectifying MB and switchable diode-like MB,with good device performance in terms of retention,endurance,switching ratio and rectifying ratio.Based on the metal/oxide contact theory,the IR-MB mechanism is exclusively attributed to the built-in isotype homojunction that is modulated by the oxygen migration.This is the first work reporting the IR-MB dominated by the built-in homojunction,which was studied at the nanoscale.The feasibility for selection device-free memory application has been demonstrated,by calculating the maximum crossbar size with 10% read margin under the worst-case scenario to be 3 Mbit.