A Mehonic

Resistive switching in silicon suboxide films

We report a study of resistive switching in a silicon-based memristor/resistive RAM (RRAM) device in which the active layer is silicon-rich silica. The resistive switching phenomenon is an intrinsic property of the silicon-rich oxide layer and does not depend on the diffusion of metallic ions to form conductive paths. In contrast to other work in the literature, switching occurs in ambient conditions, and is not limited to the surface of the active material. We propose a switching mechanism driven by competing field-driven formation and current-driven destruction of filamentary conductive pathways. We demonstrate that conduction is dominated by trap assisted tunneling through noncontinuous conduction paths consisting of silicon nanoinclusions in a highly nonstoichiometric suboxide phase. We hypothesize that such nanoinclusions nucleate preferentially at internal grain boundaries in nanostructured films. Switching exhibits the pinched hysteresis I/V loop characteristic of memristive systems, and on/off resistance ratios of 104:1 or higher can be easily achieved. Scanning tunneling microscopy suggests that switchable conductive pathways are 10 nm in diameter or smaller. Programming currents can be as low as 2 μA, and transition times are on the nanosecond scale.

Quantum Conductance in Silicon Oxide Resistive Memory Devices

Resistive switching offers a promising route to universal electronic memory, potentially replacing current technologies that are approaching their fundamental limits. In many cases switching originates from the reversible formation and dissolution of nanometre-scale conductive filaments, which constrain the motion of electrons, leading to the quantisation of device conductance into multiples of the fundamental unit of conductance, G0. Such quantum effects appear when the constriction diameter approaches the Fermi wavelength of the electron in the medium – typically several nanometres. Here we find that the conductance of silicon-rich silica (SiOx) resistive switches is quantised in half-integer multiples of G0. In contrast to other resistive switching systems this quantisation is intrinsic to SiOx, and is not due to drift of metallic ions. Half-integer quantisation is explained in terms of the filament structure and formation mechanism, which allows us to distinguish between systems that exhibit integer and half-integer quantisation.

Structural changes and conductance thresholds in metal-free intrinsic SiOx resistive random access memory

We present an investigation of structural changes in silicon-rich silicon oxide metal-insulator-metal resistive RAM devices. The observed unipolar switching, which is intrinsic to the bulk oxide material and does not involve movement of metal ions, correlates with changes in the structure of the oxide. We use atomic force microscopy, conductive atomic force microscopy, x-ray photoelectron spectroscopy, and secondary ion mass spectroscopy to examine the structural changes occurring as a result of switching. We confirm that protrusions formed at the surface of samples during switching are bubbles, which are likely to be related to the outdiffusion of oxygen. This supports existing models for valence-change based resistive switching in oxides. In addition, we describe parallel linear and nonlinear conduction pathways and suggest that the conductance quantum, G0, is a natural boundary between the high and low resistance states of our devices.

Emulating the Electrical Activity of the Neuron Using a Silicon Oxide RRAM Cell.

In recent years, formidable effort has been devoted to exploring the potential of Resistive RAM (RRAM) devices to model key features of biological synapses. This is done to strengthen the link between neuro-computing architectures and neuroscience, bearing in mind the extremely low power consumption and immense parallelism of biological systems. Here we demonstrate the feasibility of using the RRAM cell to go further and to model aspects of the electrical activity of the neuron. We focus on the specific operational procedures required for the generation of controlled voltage transients, which resemble spike-like responses. Further, we demonstrate that RRAM devices are capable of integrating input current pulses over time to produce thresholded voltage transients. We show that the frequency of the output transients can be controlled by the input signal, and we relate recent models of the redox-based nanoionic resistive memory cell to two common neuronal models, the Hodgkin-Huxley (HH) conductance model and the leaky integrate-and-fire model. We employ a simplified circuit model to phenomenologically describe voltage transient generation.

Nanoscale transformations in metastable, amorphous, silicon-rich silica

Electrically biasing thin films of amorphous, substoichiometric silicon oxide drives surprisingly large structural changes, apparent as density variations, oxygen movement, and ultimately, emission of superoxide ions. Results from this fundamental study are directly relevant to materials that are increasingly used in a range of technologies, and demonstrate a surprising level of field-driven local reordering of a random oxide network.

Nanosecond Analog Programming of Substoichiometric Silicon Oxide Resistive RAM

Slow access time, high power dissipation, and a rapidly approaching scaling limit constitute roadblocks for existing nonvolatile flash memory technologies. A new family of storage devices is needed. Filamentary resistive RAM (ReRAM) offers scalability, potentially sub-10 nm, nanosecond write times and a low power profile. Importantly, applications beyond binary memories are also possible. Here, we look at aspects of the electrical response to nanosecond stimuli of intrinsic resistance switching TiN/SiOx/TiN ReRAM devices. Simple sequences of identical pulses switch devices between two or more states, leading to the possibility of simplified programmers. Impedance mismatch between the device under test and the measurement system allows us to track the electroforming process and confirm it occurs on the nanosecond timescale. Furthermore, we report behavior reminiscent of neuronal synapses (potentiation, depression, and short-term memory). Our devices therefore show great potential for integration into novel hardware neural networks.

Intrinsic Resistance Switching in Amorphous Silicon Suboxides: The Role of Columnar Microstructure

We studied intrinsic resistance switching behaviour in sputter-deposited amorphous silicon suboxide (a-SiOx) films with varying degrees of roughness at the oxide-electrode interface. By combining electrical probing measurements, atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM), we observe that devices with rougher oxide-electrode interfaces exhibit lower electroforming voltages and more reliable switching behaviour. We show that rougher interfaces are consistent with enhanced columnar microstructure in the oxide layer. Our results suggest that columnar microstructure in the oxide will be a key factor to consider for the optimization of future SiOx-based resistance random access memory.

Probing electrochemistry at the nanoscale: in situ TEM and STM characterizations of conducting filaments in memristive devices

Memristors or memristive devices are two-terminal nanoionic systems whose resistance switching effects are induced by ion transport and redox reactions in confined spaces down to nanometer or even atomic scales. Understanding such localized and inhomogeneous electrochemical processes is a challenging but crucial task for continued applications of memristors in nonvolatile memory, reconfigurable logic, and brain inspired computing. Here we give a survey for two of the most powerful technologies that are capable of probing the resistance switching mechanisms at the nanoscale – transmission electron microscopy, especially in situ, and scanning tunneling microscopy, for memristive systems based on both electrochemical metallization and valence changes. These studies yield rich information about the size, morphology, composition, chemical state and growth/dissolution dynamics of conducting filaments and even individual metal nanoclusters, and have greatly facilitated the understanding of the underlying mechanisms of memristive switching. Further characterization of cyclic operations leads to additional insights into the degradation in performance, which is important for continued device optimization towards practical applications.

Resistive Switching in Oxides

Resistive switching in oxides, the phenomenon whereby the resistance of samples of the matrix can be cycled between states with contrasts of up to several orders of magnitude, has received growing attention over the past decade thanks to the possibility of exploiting this effect in novel memory technologies. Here we summarise the current state of the art in the field, paying particular attention to the underlying mechanisms of switching, which involves the creation of defects in the oxide. We also describe potential technological applications.

Multiple Diode-Like Conduction in Resistive Switching SiO x -Based MIM Devices

Filamentary conduction in resistive switching metal- insulator-metal devices is often modeled from the circuital viewpoint using diode-like structures with series resistances. We show in this letter which arrangement of diodes and resistances is compatible with experimental multilevel set and reset I-V characteristics in electroformed TiN/SiOx/TiN structures. The proposed model is based on the solution of the generalized diode equation corresponding to N diodes arranged in parallel with a single series resistance. The model is simple yet accurate and it is able to capture the essential features exhibited by the I-V curves in the low and high bias regimes, revealing that a single equation can deal with both the low and high resistance states. An exact expression for the differential conductance suitable for small-signal analysis and circuit simulators is also provided.