Postcycling LRS Retention Analysis in HfO2/Hf RRAM 1T1R Device

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 (V_ox) 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 (V_ox)´s from the filament into HfO₂ bulk, and results in degradation of the LRS retention.