Orientation and concentration of Ag conductive filament in HfO2-based resistive random access memory: first-principles study

Abstract

HfO2-based resistive random access memory takes advantage of metal dopants defects in its principle of operation. Then, it is significantly important to study the performance of metal dopants in the formation of conductive filament. Except for the effects of the applied voltage, the orientation and concentration mechanism of the Ag dopants are investigated based on the first principle. First, five possible models of Ag in HfO2 are established in [001], [010], [100], [-111] and [110] directions, in each of which adequate and equal dopants of Ag are ensured. The isosurface plots of partial charge density, formation energy, highest isosurface value and migration barrier of Ag dopants are calculated and compared to investigate the promising formation direction of Ag in the five established orientation systems. The formations of conductive filament are observed in [100], [010], [001] and [-111] directions in the unit cell structure from the isosurface plots of partial charge density. But no filament is formed in [110] direction. And the highest isosurface value of Ag dopant is largest in [-111] direction. This indicates that the most favorable conductive filament formation takes place in this direction. The formation energy of Ag in the different direction is different, and the values in [-111] and [100] direction are minimum and close to each other, which shows that it is easy to form conductive filaments in these two directions. In addition, the smallest migration barrier of Ag in [-111] direction reveals that the [-111] orientation is the optimal conductive path of Ag in HfO2, which will effectively influence the SET voltage, formation voltage and the ON/OFF ratio of the device. Next, based on the results of orientation dependence, four different concentration models (HfAgxO2, x=2, 3, 4, 5) are established along the [-111] crystal orientation. The isosurface plots of partial charge density about those concentration models are compared, showing that the resistive switching phenomenon cannot be observed for the samples deposited in a mixture with less than 4.00 at.% of Ag content (HfAg4O2). The RS behavior is improved with Ag content increasing from 4.00 at. % to 4.95 at.%. However, the formation energy and highest isosurface value are calculated and it is found that the conductive filaments cannot be switched into a stable state when Ag content becomes greater than 4.00 at.%. Then, the total electron density of states and the projected electron density of states are also calculated for the two models. It indirectly shows that the conductive filament is mainly comprised of Ag atoms, rather than Hf atoms or oxygen vacancy. Also, it is not helpful to improve the ON/OFF ratio of the device when the Ag dopant concentration is higher than 4.00 at.%. Therefore, the best doping concentration of Ag is 4.00 at.% and it is more advantageous to change the resistance memory storage features. This work may provide a theoretical guidance for improving the performances of HfO2-based resistive random access memory.