Property Optimization And Mechanism Analysis Of Resistive Switching Memory Based On Poly (o-Methoxyaniline)

The demand for high-speed,light,and reliable electronic products is increasing,therefore resistive random access memories(RRAMs)are becoming one of the most potential candidates in the next generation of non-volatile RAM(NvRAM)due to their unique advantages,including high storage density,low power consumption,good size scalability,simple structure,quick switching,etc.Till date,the physical insight into the electroforming(EF)and the resistive switching(RS)mechanism is still a controversy and not well understood.Various materials have been studied as the active layers in RRAM memories during the past several decades,however,most of them are inorganic compounds.It's worth mentioning that organic/polymer RRAMs are considered as one of the promising alternatives of next-generation data-storage devices due to their excellent properties of low cost,high-mechanical flexibility,good storage performance,and easy fabrication.Based on our previous work,the fabrication methods and RS mechanism of organic RRAM devices based on poly(o-methoxyaniline)(POMA)are further investigated and discussed in this thesis.Firstly,the fabrication of Al/POMA/ITO devices is optimized and the yield of devices is improved.On this basis,the measurements of electrical properties were also modified to lead and protect the forming process of the devices to obtain the stable RS behavior.Secondly,the effects of the deposition of top electrodes,the material of top and bottom electrodes,and the film thickness of POMA active layers on the RS properties were investigated,which could provide evidence to modulate the performance of POMA-based RRAM devices from multiple perspectives and analyze the RS mechanism.Finally,the RS mechanism based on conductive filaments was inferred from the linear fitting results of ?-? curves of different devices.The formation and rupture of conductive filaments were observed by C-AFM measurement,which confirmed the mechanism of POMA+cationic filaments.At the end of this thesis,the RS mechanism was further verified by the device degradation analysis in endurance test.Our work and results are as follows.(1)Fabrication optimization of POMA-based RRAM devicesITO-coated glass and metal-coated SiO2/Si substrates were used to fabricate POMA-based RRAM devices.Metal-coated SiO2/Si substrate was prepared by depositing a layer of metal on SiO2/Si by thermal or E-beam evaporation.The POMA films were spin-coated on ITO-coated glass or metal-coated SiO2/Si substrates,followed by thermal evaporation of patterned metal top electrodes on POMA films through shadow mask.The use of DECON 90 cleaning agent,which is widely recognized in Europe and the United States,ensures the effective cleaning of the substrates.The devices were fabricated and measured in the clean room to make sure the RRAM cells clean enough.Thermal evaporation is more suitable to deposit top electrodes(Al,Au,Ag,etc.)on organic/polymer layers,which ensures the quality of metal electrodes and prevents POMA films damaged.(2)The investigation of influencing factors on the resistive switchingIn this thesis,the same Al/POMA/ITO structures are obtained by depositing Al top electrodes through E-beam evaporation and thermal evaporation,respectively,and the results show that devices with thermal-evaporated A1 electrodes(shorten as thermal-evaporated devices)work well than those with E-beam-evaporated AI electrodes.Few of thermal-evaporated Al electrodes is darkened during the electrical measurements.Thermal-evaporated devices not only show higher quality,but also display a great improvement in performance.In addition,we note that the temperature in the chamber of E-beam evaporation is higher than that in thermal evaporation.Therefore,to some extent,it will affect the property of the polymer which is sensitive to the temperature.The POMA film thicknesses were altered by changing the organic solvent,spin speed,and the number of layers.The results revealed that the POMA film thickness has no significant effect on the performance of the devices.Devices with different POMA film thickness show the same EF process and RS behavior.A variety of devices with different electrodes pairs in a metal-insulator-metal(MIM)structure were fabricated,including POMA/ITO,Al/POMA/ITO,Au/POMA/ITO,Ag/POMA/ITO,Al/POMA/Al/SiO2/Si,Al/POMA/Au/Ti/SiO2/Si,and Al/POMA/PEDOT:PSS/ITO devices.Although different devices have their own unique performance,but there is a common characteristic among all devices that the types of RS are all the bipolar.We believe that this must be related to the POMA film.Among them,negative SET voltage and positive RESET voltage with positive forming processes are found for POMA/ITO and Al/POMA/ITO devices,exhibiting so-called clockwise characteristics.While forming-free and counterclockwise characteristics are observed for Au/POMA/ITO devices.The devices with Ag/POMA/ITO and Al/POMA/PEDOT:PSS/ITO structure exhibit forming-free and symmetrical bipolar resistive switching,which can be either clockwise or counterclockwise,depending on the direction of the first voltage sweep.Whereas the RS of Al/POMA/Al/SiO2/Si and Al/POMA/Au/Ti/SiO2/Si devices are not very stable,this may be caused by the deposition of bottom electrodes.It is difficult to guarantee the quality of the spin-coated POMA film on metal bottom electrodes.In this case,a small compliant current is necessary to prevent the breakdown of the device.Symmetrical bipolar RS are observed only in one or two cycles in devices with Al symmetric electrodes and then become extremely unstable in their high resistance state.And devices with Au bottom electrodes show counterclockwise bipolar RS behavior with 1 mA compliant current.The diffusion of Au into POMA films makes the device easy to be breakdown subsequently.(3)Study of switching mechanism in POMA-based RRAM devicesFirstly,the linear fitting of the ?-? curves of five devices with excellent performance were employed to analyze the RS mechanism of the device.The conducting behavior of POMA/ITO and Al/POMA/ITO devices are in good agreement with the Schottky emission model.The conduction of Au/POMA/ITO devices is in accordance with the P-F emission model in the low voltage region of HRS,and coincides with Schottky emission model in the higher voltage region of HRS in SET process,respectively.While the fitting of Ag/POMA/ITO and Al/POMA/PEDOT:PSS/ITO devices fully meet the SCLC model in high resistance state.The fitting results of these five kinds of devices show that the transition between the high and low resistance states of the devices should be attributed to the formation and rupture of conductive filaments in the POMA films.The conducting AFM(C-AFM)measurement was performed on the POMA/ITO devices without top A1 electrode and the results shown that the transition between the high and low resistance states corresponds to the formation and disappearance of the observed current peak.The appearance and disappearance of localized current peaks is a manifestation of the formation and annihilation of conductive filaments.So it is strongly confirmed that the RS mechanism of the POMA-based RRAM devices is ascribed to the formation and rupture of POMA+ cationic filaments,which corresponded to the redox reaction in POMA film.Finally,the mechanism of degradation was investigated based on Al/POMA/ITO devices.With the increase of switching cycles in the endurance test,the devices tend to degenerate into the intermediate state(IRS),which should be caused by residual filaments after multiple switching in the POMA film.The degradation of performance in RRAM devices supports and verifies the RS mechanism of conductive POMA+filaments.