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

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 La1-xSrxMnO3 (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) AlOx 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 AlOx 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.