Effect of RESET Voltage on Distribution of SET Switching Time of Bipolar Resistive Switching in a Tantalum Oxide Thin Film

The distribution of SET switching time of bipolar switching tantalum oxide thin films is studied using pulse measurement techniques. SET switching times are measured by repeating SET and RESET operations in a single cell. It is found that the distribution measured with a high RESET voltage can be well described by a steep Weibull distribution with a shape parameter >1, but lowering the RESET voltage results in a broad distribution at high cumulative frequencies. Statistical analysis shows that this broadening of the distribution can be attributed to the variation of initial conditions for SET, whereas a steep Weibull distribution points to an aging process leading to SET under a voltage stress. It is also shown that although the power of the leakage current before SET determines the fastest limit of the SET switching speed, those SET in the broadened distributions are delayed due to gradual current increase prior to the SET. The waiting time for the start of the gradual current increase has a correlation with the power of the leakage current, showing that Joule heating effect is still a significant factor in the SET mechanism even if the SET time distribution is broadened due to the variation of initial states programmed by low RESET voltages.