The mechanism of electroforming of metal oxide memristive switches

TLDR

Metal and semiconductor oxides are ubiquitous electronic materials that, while normally insulating, can undergo irreversible electroforming under high electric fields, a process that creates coupled electron–ion motion and underlies memristive switching but suffers from device repeatability issues. The study aims to elucidate oxide electroforming as an electro‑reduction and vacancy‑creation process driven by high electric fields and Joule heating, supported by direct experimental evidence. Oxygen vacancies drift toward the cathode forming localized conducting channels, while O2− ions move to the anode, evolve O2 gas, and cause junction deformation, which is mitigated by nanoscale scaling and voltage polarity control. Device‑structure engineering that favors interface‑controlled switching over bulk oxide effects largely eliminates electroforming problems.

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 for obtaining metal oxide resistance switches, which may open urgently needed new avenues for advanced computer memory and logic circuits including ultra-dense non-volatile random access memory (NVRAM) and adaptive neuromorphic logic circuits. This electrical switching arises from the coupled motion of electrons and ions within the oxide material, as one of the first recognized examples of a memristor (memory–resistor) device, the fourth fundamental passive circuit element originally predicted in 1971 by Chua. A lack of device repeatability has limited technological implementation of oxide switches, however. Here we explain the nature of the oxide electroforming as an electro-reduction and vacancy creation process caused by high electric fields and enhanced by electrical Joule heating with direct experimental evidence. Oxygen vacancies are created and drift towards the cathode, forming localized conducting channels in the oxide. Simultaneously, O2− ions drift towards the anode where they evolve O2 gas, causing physical deformation of the junction. The problematic gas eruption and physical deformation are mitigated by shrinking to the nanoscale and controlling the electroforming voltage polarity. Better yet, electroforming problems can be largely eliminated by engineering the device structure to remove 'bulk' oxide effects in favor of interface-controlled electronic switching.