Study On The Filament Morphology Of Resistive Switching Memory Devices Based On Atomic Force Microscopy

The development of new intelligent fields,such as artificial intelligence,autonomous driving and automatic speech recognition,has greatly promoted the update of information storage technology,and also puts forward higher requirements on storage density and storage speed.Due to limited physical size,storage density of conventional flash memory can hardly be promoted further.It is urgent to find the next generation of non-volatile memory.Among various kinds of new non-volatile storage technologies,resistive random access memory?RRAM?has attracted great attention because of its simple structure,long endurance,fast switching speed and long-time retention.Among numerous resistive switching materials,transition metal oxides?such as hafnium oxide?are widely used in device manufacturing because they have good insulation and high stability,and show the excellent resistive switching performance.However,the mechanism of vacancy-type filaments formation and evolution in HfO₂ based RRAM is not well understood,so we could not fully explain various phenomena in electrical operation in terms of filaments.This paper aims to strengthen our understanding of vacancy-type filaments formation.Through atomic force microscopy,the observation and analysis of vacancy-type filaments under different electrical behaviors were carried out as follows:1.We fabricated Pt/HfO₂/W configuring resistive switching devices and obtained various resistance states under different compliance currents and negative largest voltages.Through retention tests,we verified that many resistance states had long retention time,which indicates that our devices have good non-volatility.In addition,our device showed a fast switching speed and long cycling endurance in pulse mode,indicating good switching characteristics and cycling tolerance,and laid a foundation for the observation and research of vacancy-type filaments.2.We designed Au/HfO₂/W configuring resistive switching devices which Au electrodes can be removed,and observed the conductive dendritic structure under the Au electrodes by using conductive atomic force microscope?CAFM?.Different from conductive channels reported by predecessors,the dendritic structures here have certain conductivity,but not the whole structures are involved in switching behaviors.Our analysis shows that local sites with high conductivity in dendritic structure leads to the resistive switching,which can be verified by filaments observation in low resistance states?LRS?and high resistance states?HRS?.3.Aiming at the phenomenon that resistive switching devices require large Forming voltage?V_F?during the forming process,we studied the causes of this phenomenon and its influence on devices.In the experiment of UV ozone treatment at the Au/HfO₂ interface,we excluded the possibility that the residual organic impurities at the interface would lead to the increase of V_F during the electrode growth.We observed and analyzed the membrane material after forming failure,and concluded that large V_F can generate a large amount of joule heat and promote oxygen ion reduction.When large amount of generated oxygen passes through the films of HfO₂ and Au electrode,the crater structure appears and the device cannot be activated.If generated oxygen breaks through the Au electrode after several cycles,it will lead to reset failure.Due to the absence of large number of oxygen ions,the vacancy-type filaments cannot be recombined.The device will be always in a low resistance state.