Switching mechanism of Al/La1−xSrxMnO3 resistance random access memory. I. Oxygen vacancy formation in perovskites

Abstract

Resistance random access memory is a promising next-generation non-volatile memory device due to its simple capacitor-like structure, ultrafast switching, and extended retention. A composite thin film of perovskite oxide such as La_1−xSr_xMnO₃ (LSMO) and reactive metal such as aluminum (Al) is a key material for such device, but lack of clear understanding of its microscopic switching mechanism hampers further development along this direction. We therefore carry out a series of density functional theory calculations tracking down a molecular-level hypothesis of the switching process: (1) oxygen vacancy (V_O) formation in LSMO and migration through LSMO towards the interface with Al and (2) AlO_x oxide formation at the interface. As the first step of this series of effort, Al/LSMO/Al model junction devices are built to represent four different oxygen-deficiency levels of LSMO, and their structure, energy, electronic structure, and current–voltage characteristics are calculated and compared. We find that the V_O formation in LSMO itself plays an interesting role in the resistive switching of the junction by initially reducing the number of majority-spin states around the Fermi level (becoming more insulating as expected) and then by increasing the number of minority-spin states through Mn–V_O–Mn–V_O filament-like pathways developed in the film (surprisingly becoming more conducting than stoichiometric LSMO). Assessment of the importance of this effect would require a comparison with the ON/OFF ratio induced by AlO_x formation, which will be done separately in the second step of our effort, but the control of the oxygen deficiency appears to be a very important and challenging task required for reliable device fabrication and operation. The calculation also shows that, at sufficiently high doping level x, the V_O formation energy is reasonably low and the V_O migration energy barrier is even lower, explaining the fast switching of this type of devices. On the other hand, the calculated energy barrier is high enough to avoid thermal random-walk O migration which could refill V_O sites, explaining the extended retention of such devices.