Design,Characterization And Optimization Of Resistive Switching Devices Based On Metal Oxide Films

Emerging electronic devices like memristor have drawn great attention in the last decades due to their advanced electrical performances and scalable integrating capability,that aims at plenty of applications.Among them,twoterminal resistive switching(RS)devices based on transition metal oxides(TMOs)are intensively studied.They have been used as alternative electronics to overcome the disadvantages of conventional complementary metal-oxide-semiconductor(CMOS)technology for next generation of information storage and computing.The widely accepted mechanism of a large class of RS devices relies on the electrical formation and dissolution of conductive filaments(CFs)inside the dielectric materials(e.g.,TMOs),resulting in the resistance changes of the entire cell between the high resistance state(HRS)and low resistance state(LRS).State-of-the-art,the mainstream of RS device prototype is based on TMOs materials like HfO2,Al2O3,Ta2O5,TiO2,in which the binary or multilevel/analog types of switching have been proposed for non-volatile memories,in-memory computing and as synaptic devices in neural networks.However,existing obstacles that impede the further development of TMOs based RS devices mainly come from the reliability concern,which leads to non-ideal RS performances,for example,LRS current overshoot during forming and set,stochastic set and reset voltages and transition time during the cycling,or non-linear synaptic prosperity in analog performance.This PhD thesis focuses on the study of TMOs based RS devices,typically,TiO2 and HfO2,to understand the properties from the dielectric breakdown and electroforming of the pristine device stacks,switching reliability and failure mechanisms with the combination of film characterization and a vast of electrical measurements.This thesis contains five chapters as following:Chapter 1 will start with the introduction of the emerging technologies and devices for information storage,among which resistive switching device shows to be one of the most promising candidates.Then,we discussed in details on the material-controlled RS mechanisms and crossbar architectures for device implementation.We also pointed out the potential applications of resistive switching devices in the fields of logic computing,analog computing and stochastic computing.In the end of this capture,starting from the topic selection of this paper,we discuss the main problems hindering the development of resistive switching devices,and disclose the research basis and content of this thesis.In Chapter 2,we study the dielectric breakdown and electroforming in TiO2 based memristors,where the TiO2 is deposited by two methods.After electrical analysis and comparison,we report the effects of different fabrication methods on the device switching performances.The experimental results show that magnetron sputtering is a desirable method and more favorable to grow defect-rich dielectric for resistive switching operation,with the evidence of lower electroforming current overshoot and reservable dielectric breakdown process in the sputtered devices.We also use different electrodes to optimize the resistive switching of both films.The programming condition(voltage polarity and mode)dependent electroforming of HfO2 based devices is also investigated with the with electrical measurement and multiscale simulations in Chapter 3.We observed that,comparing with inert electrode materials like Au,the oxygen active electrode Ni can effectively reduce the electroforming voltage and time.At the same time,under the same polarity but different kind of applied voltage modes,the devices always show relatively uniform and parallel electroforming dynamic.In Chapter 4,for the first time,we develop a new recipe to oxidize nonstoichiometric TaOx from the sputtered Ta metal substrate by using atomic layer deposition.This method allows us to prepare bilayer stacks with nonstoichiometric oxide/stoichiometric oxide in atomic layer deposition system directly.The well-layered structure and thickness of bilayer stack is confirmed by X-ray reflection(XRR)and Time-of-Flight secondary ion mass spectrometry(TOF-SIMS).Based on TaOx/HfO2,we designed and fabricated bilayer based resistive switching devices.The use of TaOx/HfO2 bilayer stack helps to optimize the electrical performance of HfO2 based device.Comparing to the single-layer HfO2 based device,TaOx/HfO2 based devices show reliable binary switching prosperities.The TaOx insertion efficiently alleviates the current overshoot during dielectric breakdown process,thus improves the electroforming yield and self-compliant operation.Comparing to Ti,the bilayer device with Ta as oxygen exchange layer promises much smaller cycle-to-cycle variability.We proved that the bilayer device holds improved switching endurances under wider and strong pulse programming conditions,and superior synaptic characteristics with enhanced conductance linearity than the single layer device under soft pulse programming conditions.Chapter 5 summarizes the work of this thesis.Two kinds of metal oxide materials and a variety of electrode materials are mainly studied.The purpose of this thesis is to improve and optimize the switching reliability of the RS devices by using different material synthesis methods(atomic layer deposition,magnetron sputtering or in-situ oxidation)and stacking structures.We evaluated the switching reliability of the device from the point of electroforming dynamic,and developed an in-situ oxidation recipe to grow non-stoichiometric oxide films in atomic layer deposition system,which is further used to optimize the resistive switching of HfO2 devices.The prospect of future research works is also proposed in the end.