Improving the High Resistance State Retention Degradation of Al-Doped HfOx Based on Ab Initio Simulations

The Al doping effects on retention of HfOx based resistive switching memory were studied in detail using density functional theory. Al doping has many merits such as reducing forming voltage, improving uniformity and enhancing the ON state retention, but it was found to deteriorate the OFF state retention. This study finds that the migration barriers of oxygen vacancies in and out of the filament in the presence of Al are severely altered as compared to those far from Al. When the filament is electron-depleted, the oxygen diffusion into the filament, which corresponds to the OFF state, becomes thermodynamically unstable near Al, indicating strong implications on the OFF state retention. On the other hand, oxygen vacancies far away from Al showed similar characteristics as in undoped HfOx, with gradually decreasing barriers by the electron depletion process under the electric field. Therefore, a potential solution to address both ON and OFF state retention improvement is to consider using Al partial doping only near the inert electrode device region. This will facilitate the filament growing process at lower forming voltages and help with the ON state retention. At the same time, it will prohibit the influence of Al on the active switching area and therefore eliminate the OFF state retention degradation issue.