First principles study on the switching mechanism in Resistance Random Access Memory devices

The role of Resistance Random Access Memory (RRAM) is recently becoming extremely important in the field of developing non-volatile memory devices. The foreseen relevance of RRAM in this field is attributed to the switching of the electronic properties from metal to insulator, and vice versa, of the transition metal oxides (TMOs) included in RRAM by a set and reset pulse voltage. However, conclusive clarifications on the switching mechanism have not yet been fully realized. In this study, by using first principles calculation based on density functional theory, we investigated RRAM's switching mechanism through analysis of the change in the electronic properties of the bulk TMOs resulting from oxygen vacancies and charge carrier trapping for two known TMOs materials used in RRAM, HfO₂ and CoO. We found that an oxygen vacancy row with charge carrier trapping creates a conduction path and therefore the transition from insulator to metal. In addition, we perform calculations for slab models of the TMOs in contact with Ta electrodes and hence investigate the effects of oxygen vacancies at the interface between the TMO layers and the electrode layer. From the obtained results, we confirmed that our investigations on activation energy barrier for oxygen vacancy migration are consistent with the experimental data of voltages required for switching.