Resistive switching mechanisms in random access memory devices incorporating transition metal oxides:TiO2, NiOand Pr0.7Ca0.3MnO3

Abstract

Resistance change random access memory (RRAM) cells, typically built as MIM capacitorstructures, consist of insulating layers I sandwiched between metal layers M, wherethe insulator performs the resistance switching operation. These devices can beelectrically switched between two or more stable resistance states at a speed ofnanoseconds, with long retention times, high switching endurance, low read voltage,and large switching windows. They are attractive candidates for next-generationnon-volatile memory, particularly as a flash successor, as the material properties can bescaled to the nanometer regime. Several resistance switching models have beensuggested so far for transition metal oxide based devices, such as charge trapping,conductive filament formation, Schottky barrier modulation, and electrochemicalmigration of point defects. The underlying fundamental principles of the switchingmechanism still lack a detailed understanding, i.e. how to control and modulate theelectrical characteristics of devices incorporating defects and impurities, such asoxygen vacancies, metal interstitials, hydrogen, and other metallic atoms acting asdopants. In this paper, state of the art ab initio theoretical methods are employed tounderstand the effects that filamentary types of stable oxygen vacancy configurations inTiO2 and NiO have on the electronic conduction. It is shown that strong electronicinteractions between metal ions adjacent to oxygen vacancy sites results in theformation of a conductive path and thus can explain the ‘ON’ site conduction inthese materials. Implication of hydrogen doping on electroforming is discussed forPr0.7Ca0.3MnO3 devices based on electrical characterization and FTIR measurements.