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Low resistance state (LRS) retention after 104 and 106 pulse cycles is compared to the uncycled LRS retention, based on the (40 × 40 nm)- HfO2/Hf bipolar RRAM devices in a 1T1R configuration. The LRS retention after 104 pulse cycles does not show degradation, while a larger failure bit tail is seen after 106 pulse cycles. The larger failure bit tail is found strongly related to the degradation of the cycled LRS state. From the LRS current fitting with a quantum point contact model, it is found that the total number of oxygen vacancies (Vox) in the filament decreases after 106 cycles, leaving a narrower switching constriction. The narrower switching constriction therefore suffers more from the self-diffusion of the (Vox)'s from the filament into HfO2 bulk, and results in degradation of the LRS retention.
Date of Publication: May 2013