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 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, O 2− ions drift towards the anode where they evolve O 2 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.

Cross-Point Resistive Switching Memory and Urea Sensing by Using Annealed GdOxFilm in IrOx/GdOx/W Structure for Biomedical Applications

“…After first cycle, the switching materials have been changed to nonstoichiometry because some amount of oxygen will be moved out due to Gd-O bonds breaking under external bias, which is neither metal nor insulator. Yang et al 38 have also reported the O 2 gas eruption from the mechanically weakest part of the thin films in Pt/TiO 2 /Pt structure under external bias. Therefore, the resistivity values of the SMs are found to be 2.58 × 10 −4 , 2.74 × 10 −4 , and 1 × 10…”

Relevance of non-equilibrium defect generation processes to resistive switching in TiO2

“…The generation of such defects, assisted by a non-equilibrium electric field and charge injection, is often discussed in the literature and included explicitly in some device simulations. 11,[20][21][22][23][24][25][26] However, resistive switching models are also developed considering field driven diffusion of pre-existing defects only, without defect generation. [27][28][29] Experimentally, definitive evidence for one view over the other has proven extremely challenging to obtain, and the uncertainty that remains presents an obstacle to addressing pressing technological issues, such as controlling cycle-to-cycle and device-to-device variability, increasing endurance, and improving reliability.…”

Effect of ultraviolet illumination on metal oxide resistive memory

“…CFs are formed by the alignment of structural defects, such as oxygen vacancies, which could preexist or be induced under an electric field. 13 Since oxygen vacancy is positively charged in nature, it drifts toward the cathode after generation. Once the oxygen vacancies reach cathode, the nucleation of CF begins.…”