Study Of Tantalum Oxide And Hafnium Oxide Based Random Resistive Access Memory

The mainstream nonvolatile memory is flash at present,which is based on the charges in the“floating gate”changing to store information.The thickness of“floating gate”is approaching to the physics limit because the thickness is too small to effectively store charges.However,reducing the size of device is the development trend for nonvolatile memory,and there is no room for thinning;Furthermore,there are many disadvantages for Flash,such as high operating voltage,low write/erase speed,and so on.Thus,these cases limit the application and development of Flash,and some emerging nonvolatile memories,such as,magnetic random access memory?MRAM?,phase random access memory?PRAM?,ferroelectric random access memory?FRAM?,resistive random access memory?RRAM?,have been attracted much attention.Among the memories,RRAM is a promising candidate for next generation memory due to its simple structure,low power consumption,high endurance,longer retention time,fast speed,high density for storage and compatibility with CMOS.Accordingly,RRAM is a strong candidate for next generation memory.However,stability,uniformity of device performance and reproducibility are to be improved.Therefore,it is necessary that RRAM is further investigated.This work focuses on investigation of the aforementioned aspects for tantalum oxide?TaO_x?and hafnium oxide?HfO_x?RRAM,mainly aims to improve resistive switching characteristics,such as,reducing reset current(I_reset),enhancing stability and uniformity,improving R_off/R_on ratio of device.The main content is as follow:By reactive sputtering of tantalum in the mixing atmosphere of argon and oxygen and then oxidizing of as-deposited tantalum oxide?TaO_x?with oxygen plasma,a bi-layer of Ta₂O₅/TaO_x is formed and can significantly improve device performance,such as reducing I_reset.A significant reduction of I_reset from 20mA to³0?A was achieved.For investigating the composition of tantalum oxide films and chemical state of element in tantalum oxide,x-ray photoelectron spectroscopy?XPS?was employed.Resistive switching mechanism was investigated via combining XPS spectra with resistive switching phenomenon.The nature that I_reset was significantly reduced was explored.It is demonstrated that plasma oxidation is an effective and simple method for improving device performance.In order to improve uniformity,stability of device performance,and R_off/R_on ratio for tantalum oxide-based RRAM device,controlling formation and rupture of filament is needed.We fabricated tri-layer structure of tantalum oxide with different oxygen ion concentration gradient?Ta₂O₅/TaO_y/TaO_x,where TaO_y film was deposited under ratios of oxygen partial pressure of 30%and 35%,respectively,while TaO_x was deposited under20%?via changing oxygen partial pressure ratio during sputtering tantalum and the duration of plasma oxidation,and device performance was improved with an appropriate compliance current(I_cc).In the case of oxidation of TaO_y for 600s and an I_cc of 40?A,the tri-layer structure device?Pt/Ta₂O₅/TaO_y/TaO_x/Pt,where TaO_y and TaO_x were deposited under the ratio of oxygen partial pressure of 35%and 20%respectively?exhibited an I_reseteset of⁴0?A,an R_off/R_on ratio increasing to more than 20 and the most uniform distribution of parameter.Comparisons among bi-layer structure and tri-layer structure devices were performed.According to XPS spectra of tantalum oxide films and resistive switching characteristics,the reasons for improving device performance were found out.In bi-layer structure,controlling the TaO_x layer fabricated by reactive sputtering can effectively control the filament forming and rupturing,which thus can improve uniformity of device.Appropriately increasing sputtering-temperature is favorable for controlling oxygen ion distribution.For further improvement of device performance,tantalum oxide-based device?Pt/Ta₂O₅/TaO_x/Pt?was fabricated via combining high temperature?320??sputtering of tantalum with further plasma oxidation of tantalum oxide.The results have demonstrated that device performances are stable,highly uniform,and R_off/R_on ratio is more than 10 with the plasma oxidation of TaO_x for 720s,and 900s.Voltage pulse measurement was performed for the device containing TaO_x oxidized for720s,and reliable,stable resistive switching can be achieved with a R_off/R_on ratio of more than 10.In addition,the process of filament forming and rupturing was observed by conductive atomic force microscopy for the device where TaO_x was oxidized for 720s,which further verifies that resistive switching is attributed to the formation and rupture of filament.Compared with the device containing tantalum oxide sputtered at room temperature and then oxidized,better stability and uniformity are demonstrated for high-temperature sputtered and oxidized tantalum oxide-based device.The location where filament forms and ruptures directly affects performances of RRAM device.In order to observe the effect of filament on device performance when filament consisting of oxygen vacancies forms and ruptures at the interface between two electrodes Pt and HfO_x,HfO_x-based RRAM devices were fabricated.Filament ruptured at different interfaces by a voltage sweep and its opposite sweep.The significant effect of interface between Pt and HfO_x on resistive switching characteristics is demonstrated.The results provide a reference for the study found that RRAM devices.