Preparation And Characterization Of Resistive Random Access Memory Based On Solution-processed Tin Oxide

The requirements for the information storage technology are getting higher and higher with the rapid development and explosive growth of the amount of information. The research for the memory is now mainly focused on the huge capacity, high density storage, and rapid and easy reading capability. A variety of new non-volatile random access memories (NVRAM), such as magnetic RAM, phase change memory, ferroelectric RAM, and resistive RAM, have been developing as the conventional silicon-based memory is approaching the limit of physical size and processing technique. Among the new NVRAMs, RRAM based on the resistance change of active material is becoming one of the most promising candidates due to the advantages of huge storage capacity, high storage density, low power consumption, fast reading and writing, good repeatability, long retention time, and compatible with CMOS processing.The cell of RRAM has a simple metal-insulator-metal (MIM) configuration with the insulating film sandwiched between two conductive electrodes. The insulating material plays a crucial role on the performance of the RRAM device. As a transition metal oxide with wide band gap, tin oxide (SnO2) film has been studied extensively due to the chemical and thermal stability, and corrosion resistance. Therefore, the SnO2 film is expected to exhibit good bitable electrical behavior. In this thesis, we will focus on the preparation and characterization of the RRAM based on solution-processed SnO2 film. The improvements for the performance of the RRAM devices are also investigated.Firstly, the stable sol solution is prepared with SnO2 and ethanol. Secondly, the SnO22 layer is deposited by spin coating on the ITO-coated glass substrate and annealed in the atmosphere of oxygen. The crystal structure of SnO2 film is determined by X-ray diffraction and the film thickness is characterized through cross-sectional scan electron microscopy (SEM). Then, the Al/SnO2/ITO device is fabricated after the top Al electrode is deposited by electron beam evaporation. Finally, electrical measurements are conducted on the Al/SnO2/ITO device and the switching mechanism is analyzed according to the I-V results. The effects of annealing temperature and dopant in SnO2 film on the electrical switching behavior are investigated. The main results of this thesis are as following:1. Preparation and characterization of SnO2 thin filmsThe stable sol solution is prepared through sol-gel method by dissolving the precursor SnCl2·2H2O in anhydrous ethanol with the concentration of 0.1 mol/mL. SnO2 films are obtained by spin-coated deposition of the sol solution on the ITO-coated glass. The thickness and crystallization of SnO2 films changed with the spin coating speed, and the annealing atmosphere and temperature. SEM measurements showed that the film surface was smooth and the thickness was 140 nm. XRD analysis indicated that the strong (111) and (021) diffraction peaks exist in tin oxide films which proved the successful preparation of SnO2 films.2. Resistive switching characteristics and mechanism analysisThe top electrode Al was deposited on the SnO2 film by electron beam evaporation. I-V results indicated that the Al/SnO2/ITO device with SnO2 film annealed at 500℃ exhibited the bistable bipolar resistive switching characteristics with the set voltage (VSET) of 1.1 V, reset voltage (PRESET) of-3.8 V, and the current on-off ratio (ION/OFF) of 50 at the read voltage of-1.0 V. The retention test indicated that switching characteristics could remain for more than 10 s. The endurance performance showed that the device is stable within 30 cycles.To better understand the conduction and resistive switching mechanism of the memory device, the I-V curves were re-plotted in a log-log scale for the positive voltage sweep. An ohmic conduction behavior with linear relationship was observed in the LRS, which is evidence of a confined filamentary conduction. The switching characteristic in the HRS was in good agreement with the space charge limited current (SCLC) model. This resistance mechanism can be attributed to the formation and annihilation of the conducting filaments.3. The dependence of the SnO2-based device performance on the annealing temperatureSnO2 films were annealed at 450℃,550℃, and 650℃ respectively. I-V measurements of RRAM devices based on the annealed SnO2 films indicated that the device resistive switching characteristics gradually decrease while the stability of the device increases with the annealing temperature increasing. The RRAM device with SnO2 film annealed at 550℃ showed the best resistive switching characteristics with the VSET 1.3 V, the VRESET-4.1 V, and the ION/OFF ratio more than 102. The retention time is the same as that for the device with SnO2 film annealed at 500℃. However, the endurance measurement displayed that the ION/OFF ratio of device did not show remarkably degradation within 100 cycles.In order to improve the properties, Ti was doped into SnO2 films. I-V measurements of the new device indicated that Ti doping could increase the ION/OFF ratio and it still need to be further researched.