Device physics of Correlated Electron Random Access Memory

Correlated Electron Random Access Memory (CeRAM) is a particular transition metal oxide (TMO) based Resistance Random Access Memory (RRAM) that does not involve electroforming and exhibits reliable unipolar resistance switching properties. A physical model of NiO based CeRAM/RRAM is proposed in the present work.;The two stable resistance states in NiO are attributed to intrinsic metallic and insulating states of the material, rather than the creation and rupture of filaments. Mott transition theory and the Hubbard model have been reviewed in details. By incorporating the long range Coulomb interactions (screening effects) we demonstrate that a high free electron concentration may lead to a metallic NiO. An effective Hubbard U has been proposed as an ansatz and the first order insulator-to-metal (SET) and metal-to-insulator (RESET) transitions in NiO are well understood within the new picture.;Transport in CeRAM involves quantum mechanical effects as well as electron correlation and phase transition. Analytical current-voltage formulae for CeRAM are given both on the metal and insulator sides by putting the appropriate solutions of the modified Hubbard model into the mesoscopic Meir-Wingreen transport equation. The RESET phenomenon is explained by a sufficient separation of Fermi levels in the electrodes and hence a Mott transition can be triggered in the anodic region of NiO due to a lack of electrons. The SET behavior originates from a tunneling current which removes the insulating region near the anode.;Several experimental evidences, either in the literature or from our own results, are presented to support this model. That the insulating region in NiO RRAM is near the anode has been confirmed separately by TEM inspection and sputtering damage experiments. The RESET mechanism has been bolstered by the fact that VRESET is very stable while IRESET is prone to dispersion. On the other hand, the strong dispersion of VSET and its film thickness independent nature once more validate our inference of a small insulating region in the OFF state. Last but not least, electroforming phenomena in other NiO RRAMs could also be well explained by this model.