Hehe Zhang

Volatile resistance states in electrochemical metallization cells enabling non-destructive readout of complementary resistive switches

Redox-based resistive memory cells exhibit changes of OFF or intermediate resistance values over time and even ON states can be completely lost in certain cases. The stability of these resistance states and the time until resistance loss strongly depends on the materials system. On the basis of electrical measurements and chemical analysis we found a viable explanation for these volatile resistance states (VRSs) in Ag-GeSx-based electrochemical metallization memory cells and identified a technological application in the field of crossbar memories. Complementary resistive switches usually suffer from the necessity of a destructive read-out procedure increasing wear and reducing read-out speed. From our analysis we deduced a solution to use the VRS as an inherent selector mechanism without the need for additional selector devices.

Internal Cell Resistance as the Origin of Abrupt Reset Behavior in HfO2-Based Devices Determined from Current Compliance Series

The resistive switching behavior in different HfO2/TiO2 nano crossbar structures of 100 x 100 nm2 size is analyzed by means of DC voltage sweeps. The devices fabricated from 3 nm thin ALD layers of HfO2 and TiO2 sandwiched between Pt and Hf or Ti electrodes show VCM-type bipolar resistive switching after electroforming. For increased compliance current (cc) during set from 50 μA to 800 μA, the set current runs into self- limitation while the reset behavior changes from gradual to abrupt. A model is defined with an internal resistance being in series with the local resistive switch. A recursive algorithm is applied to the cc series for calculation of the series resistor and evaluation of the intrinsic switching characteristic of HfO2-based cells. The intrinsic LRS turns out to be current compliance controlled and to follow the universal switching rule. Supported by compact modelling, we show that an abrupt reset behavior might arise even for materials with a gradual intrinsic reset characteristic in consequence of an internal series resistor.

Understanding the Coexistence of Two Bipolar Resistive Switching Modes with Opposite Polarity in Pt / TiO2 / Ti / Pt Nano-sized ReRAM Devices

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 TiO2 cells, is still not completely understood. Here, we report on nanosized (<0.01 μm2) asymmetric memristive cells from 3 to 6 nm thick TiO2 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/TiO2/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 TiO2 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/TiO2 Schottky interface at the position of the c8w filament. Therefore, the coexistence of c8w and 8w BRS in the nanoscale asymmetric Pt/TiO2/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/TiO2 interface.

Resistive Switching: Resistive Switching of Individual, Chemically Synthesized TiO2 Nanoparticles (Small 48/2015)

Resistive switching of individual, spherical TiO2 nanoparticles is studied by U. Simon and co-workers. Bottom-up synthesized TiO2 particles are immobilized on a conducting substrate and individually electrically addressed by an electrode tip in an in-situ scanning electron microscope, as shown in this image by T. Pössinger. On page 6444, two switching behaviors are described, bipolar and complementary, which are associated with the inner structure of the individual particles. This opens up new opportunities for materials design.