HfO2-Based RRAM: Electrode Effects, Ti/HfO2 Interface, Charge Injection, and Oxygen (O) Defects Diffusion Through Experiment and Ab Initio Calculations

We investigate in detail the effects of metal electrodes on the switching performance and conductive filament (CF) stability of HfO 2 -based RRAM. The current- voltage characteristics of the devices exhibit different electrodedependent RESET profiles which we attempt to clarify. With the insight from the experimental data, we employ first-principles calculations to have a better microscopic understanding of the devices. We study the charge injection, formation of Frenkel pairs, and diffusion of oxygen defects (oxygen vacancies Vo and oxygen interstitials Oi) that are important in the CF creation and stability during the device operation. Since the presence of Ti in RRAM has been associated with the creation of substoichiometric TiOy region at the Ti/HfO 2 interface, we also explore different Ti and Hf suboxides to understand the possible composition of that interface. Our calculations suggest that the composition of the interface would be Ti 2 O/Hf 2 O 3 from thermodynamic perspective. By combining the experimental and calculations results, we show that the concentration of oxygen interstitial (Oi) ions in the oxide after CF formation is larger for RRAM devices with inert electrodes (like Pt) compared with O reactive electrodes (like Ti) which results in degraded device performance. The lower Oi concentration in HfO 2 layer with Ti electrodes results in improved CF thermal stability and device variability.