Effect of Doping Different Cu Valence States in HfO₂ on Resistive Switching Properties of RRAM

Metal dopants are important for HfO₂-based resistive switching mechanisms in resistive random-access memory (RRAM) because they can improve the performance of RRAM devices. Although Cu ions have been widely explored as metal dopants, Cu dopants with different valence states have received little attention. Using the first principles method and the Vienna ab initio simulation package (VASP), the effect of electron gain or loss in different doped Cu states in hafnium oxide (HfO₂) was investigated. The electron affinity, defect formation energy, and charge density difference suggest that Cu doping results in a loss of electrons, thereby stabilizing the system. The population, the isosurface of partial charge density, and the migration barrier of the Cu-doped systems with different ionic valence states (+2 and 0) were calculated. Furthermore, the impact of doping ions on the formation of conductive filaments and the stability of the system were investigated in this study. The results indicate that the average population of the Cu-doped (+2) system is smaller than that of the Cu (0) system, and the Cu-O bond length increases in the Cu-doped (+2) system. At the same isosurface level, the electronic local clusters in the Cu (+2) system are stable; however, by increasing the isosurface level, the conductive filament of the Cu (0) system breaks first. At the same starting and ending positions, the migration barrier of the Cu (+2) system was much lower. In the transition state of the Cu (+2) system, the number of atoms whose atomic structure changes by more than 0.1 Å is lower than that in the Cu (0) system, which has a relatively small displacement deviation. This study, which indicates that the Cu (+2) system helps to form conductive channels upon applying current or voltage, can provide theoretical guidance for preparing RRAM and improving its performance.