First Principale Calculation of Resistive Switching in ZnO

The resistance of the metal oxides under external bias has been observed to show brain-like synapses. Switching of resistance forms the base criterion for realizing stable neuromorphic electronic devices. Resistive switching involves the alternative change of resistance with the applied external bias. With a suitable metal-metal oxide-metal configuration, it is possible to create Resistive-Random Access Memories (ReRAMs). Unlike conventional RAMs, the RERAMs are characterized by faster switching time, low power consumption and enhanced stability. Under the condition of ON state, i.e., low resistance state, a conductive filament formed in the oxide layer due to segregation of oxygen vacancy or metal ions. In the case of the OFF state, this filament gets ruptured, leading to a high resistive state. Several metal oxides, like TiO2, HfO2, SrTiO3, ZnO, etc., have shown this behaviour. The physical nature, electrical properties, and morphology of conductive filament formation have been studied through various in-situ and ex-situ techniques like transport measurement, transmission electron microscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. However, the nature of electron distribution and subsequent contribution from other orbital remains tricky. In this regard, we have explored the resistive switching phenomena of ZnO on the basis of charge injection and charge removal using density functional theory. In the case of ZnO, the oxygen vacancy migration and subsequent charge transfer occur through the conductive filament. Oxygen vacancies can exist in three different states , , and with two, one and zero electron, respectively. The band structure clearly shows that these oxygen vacancies act as a deep donor. The appeared near the VBM and was found to be stable at higher energy than and . When an electron from is removed, it forms state, a metastable paramagnetic and unstable state, but upon removing the other electron, the state is very stable. The formation energy of both and is less comparable to state thus, oxygen vacancy acts as a negative U-centre point defect. For the state, the four Zn nearest neighbours are displaced inward by 12 % of the equilibrium Zn–O bond length, whereas for the and states, the displacements are outward by 2 % and 23 %, respectively. In order to mimic the conductive filament, vacancies are created along [010] direction, and rupture is replicated by replacing a vacancy with oxygen. Electron localisation and band decomposed charge density calculations have confirmed the observation of conductive filament. A better understanding of the nature of resistive switching will help in the development of ReRAMs and neuromorphic electronic devices.