Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

Abstract

The resistance random access memory (RRAMä) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO 2 and Ta/HfO 2 systems, thereby providing a complete explanation of the RRAMä switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.

Routes for increasing endurance and retention in HfO2 -based resistive switching memories

“…Here we should note that in Ref. [25] the conductive path in HfO 2 ReRAM was attributed to negatively charged oxygen vacancy chains, which are responsible for the formation of partially occupied defect states in the electronic density of states. It was shown that the electroforming process is initiated by a supply of neutral oxygen Frenkel defect pairs, and the accumulation of the oxygen vacancies results in the formation of local metallic suboxide phases, HfO x , which remain conductive even as ultranarrow filaments in the insulating matrix.…”

“…Here we note that the relative position of I bam-Hf 2 O 3 and P4m2-Hf 2 O 3 strongly depends on the choice of U . However, for all reasonable values [25] of U eff above ∼3 eV with the PBE functional (and ∼4 eV for PBEsol), the Ibam-Hf 2 O 3 phase has a lower energy than the P4m2-Hf 2 O 3 one. For U eff 3 eV (or U eff 4 eV for PBEsol) including the calculations performed with U eff = 0 eV, P4m2-Hf 2 O 3 has the lowest energy, as reported previously [30,31].…”

“…We used the Perdew-Burke-Ernzerhof exchange and correlation functional revised for solids (PBEsol) [44], with DFT+U correction on the Hf d shell choosing U = 7 eV and J = 1 eV, as in Ref. [25]. In order to check whether the calculated composition diagram depends on the particular choice of the Hubbard U , we recalculated the diagram with a large set of U parameters, 0 U eff 6 eV (where U eff = U − J ) [45].…”

“…Previously, it was theoretically shown [24][25][26] that a chain of vacancies in the insulating hafnia host results in a conductive channel. The conductivity can be destroyed by recombination with oxygen, which restores the insulating state.…”

“…It should be noted, however, that the mentioned theoretical structural studies [30,31,35] were performed with the generalized gradient approximation for exchange and correlation energies [36]. This approach neglects possibly strong correlation effects within the Hf d shell, which may affect the Hf-O interaction and bonding [25].…”