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Zhang, Hehe,Yoo, Sijung,Menzel, Stephan,Funck, Carsten,Cü,ppers, Felix,Wouters, Dirk J.,Hwang, Cheol Seong,Waser, Rainer,Hoffmann-Eifert, Susanne American Chemical Society 2018 ACS APPLIED MATERIALS & INTERFACES Vol.10 No.35
<P>Redox-type resistive random access memories based on transition-metal oxides are studied as adjustable two-terminal devices for integrated network applications beyond von Neumann computing. The prevailing, so-called, counter-eight-wise (c8w) polarity of the switching hysteresis in filamentary-type valence change mechanism devices originates from a temperature- and field-controlled drift-diffusion process of mobile ions, predominantly oxygen vacancies in the switching oxide. Recently, a bipolar resistive switching (BRS) process with opposite polarity, so-called, eight-wise (8w) switching, has been reported that, especially for TiO<SUB>2</SUB> cells, is still not completely understood. Here, we report on nanosized (<0.01 μm<SUP>2</SUP>) asymmetric memristive cells from 3 to 6 nm thick TiO<SUB>2</SUB> films by atomic layer deposition, which reveal a coexistence of c8w and 8w switching in the same cell. As important characteristics for the studied Pt/TiO<SUB>2</SUB>/Ti/Pt devices, the resistance states of both modes are nonvolatile and share one common state; i.e., the high-resistance state of the c8w mode equals the low-resistance state of the 8w-mode. A transition between the opposite hysteresis loops is possible by voltage control. Specifically, 8w BRS in the TiO<SUB>2</SUB> cells is a self-limited low-energy nonvolatile switching process. Additionally, the 8w reset process enables the programming of multilevel high-resistance states. Combining the experimental results with data from simulation studies allows to propose a model, which explains 8w BRS by an oxygen transfer process across the Pt/TiO<SUB>2</SUB> Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO<SUB>2</SUB>/Ti/Pt cells is understood from a competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO<SUB>2</SUB> interface.</P> [FIG OMISSION]</BR>