The mechanism of electroforming of metal oxide memristive switches

Abstract

Metal and semiconductor oxides are ubiquitous electronic materials. Normally insulating,oxides can change behavior under high electric fields—through ‘electroforming’ or‘breakdown’—critically affecting CMOS (complementary metal–oxide–semiconductor) logic,DRAM (dynamic random access memory) and flash memory, and tunnel barrier oxides.An initial irreversible electroforming process has been invariably required forobtaining metal oxide resistance switches, which may open urgently needed newavenues for advanced computer memory and logic circuits including ultra-densenon-volatile random access memory (NVRAM) and adaptive neuromorphic logiccircuits. This electrical switching arises from the coupled motion of electronsand ions within the oxide material, as one of the first recognized examples of amemristor (memory–resistor) device, the fourth fundamental passive circuit elementoriginally predicted in 1971 by Chua. A lack of device repeatability has limitedtechnological implementation of oxide switches, however. Here we explain thenature of the oxide electroforming as an electro-reduction and vacancy creationprocess caused by high electric fields and enhanced by electrical Joule heating withdirect experimental evidence. Oxygen vacancies are created and drift towards thecathode, forming localized conducting channels in the oxide. Simultaneously,O2− ions drift towards the anode where they evolveO2 gas, causing physical deformation of the junction. The problematic gas eruption andphysical deformation are mitigated by shrinking to the nanoscale and controlling theelectroforming voltage polarity. Better yet, electroforming problems can be largelyeliminated by engineering the device structure to remove ‘bulk’ oxide effects in favor ofinterface-controlled electronic switching.