Adnan Mehonic

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.

Slow access time, high power dissipation and a rapidly approaching scaling limit constitute roadblocks for existing non-volatile flash memory technologies. A new family of storage devices is needed. Filamentary resistive RAM (ReRAM) offers scalability, potentially sub-10nm, 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… Show more

Slow access time, high power dissipation and a rapidly approaching scaling limit constitute roadblocks for existing non-volatile flash memory technologies. A new family of storage devices is needed. Filamentary resistive RAM (ReRAM) offers scalability, potentially sub-10nm, 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. Show less

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… Show more

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. Show less

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… Show more

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. Show less

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… Show more

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. Show less

Filamentary conduction in resistive switching metal- insulator-metal (MIM) devices is often modeled from the circuital viewpoint using diode-like structures with series resistances. We show in this Research 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… Show more

Filamentary conduction in resistive switching metal- insulator-metal (MIM) devices is often modeled from the circuital viewpoint using diode-like structures with series resistances. We show in this Research 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. Show less

Redox-based resistive random access memory (RRAM) has the scope to greatly improve upon current methods of data storage, despite incomplete understandings of material switching mechanisms. We make use of atomic force microscopy (AFM), conductive atomic force microscopy (cAFM) and X-ray photoelectron spectroscopy (XPS) to characterise the physical processes occurring in the changes in conductance state in silicon-rich silicon oxide RRAM. Surface analyses of the insulating oxide layer of our… Show more

Redox-based resistive random access memory (RRAM) has the scope to greatly improve upon current methods of data storage, despite incomplete understandings of material switching mechanisms. We make use of atomic force microscopy (AFM), conductive atomic force microscopy (cAFM) and X-ray photoelectron spectroscopy (XPS) to characterise the physical processes occurring in the changes in conductance state in silicon-rich silicon oxide RRAM. Surface analyses of the insulating oxide layer of our devices are employed to establish the chemical and structural properties of pristine and switched states. The removal of oxygen from the active layer is observed to be concomitant with the appearance of varying degrees of surface distortion and regions of high conductivity in an otherwise-insulating material. These results support the currently-recognised model of a resistive switching mechanism that is reliant upon the migration of oxygen ions under an electrical bias. Show less

A simple analytic model for the electron transport through filamentary-type structures in Si-rich silica (SiOx)-based resistive switches is proposed. The model is based on a mesoscopic description and is able to account for the linear and nonlinear components of conductance that arise from both fully and partially formed conductive channels spanning the dielectric film. Channels are represented by arrays of identical scatterers whose number and quantum transmission properties determine the… Show more

A simple analytic model for the electron transport through filamentary-type structures in Si-rich silica (SiOx)-based resistive switches is proposed. The model is based on a mesoscopic description and is able to account for the linear and nonlinear components of conductance that arise from both fully and partially formed conductive channels spanning the dielectric film. Channels are represented by arrays of identical scatterers whose number and quantum transmission properties determine the current magnitude in the low and high resistance states. We show that the proposed model not only reproduces the experimental current-voltage (I-V) characteristics but also the normalized differential conductance (dln(I)/dln(V)-V) curves of devices under test. Show less

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… Show more

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. Show less

This invention relates to memory resistors, arrays of memory resistors and a method of making memory resistors. In particular, this invention relates to memory resistors having an on state and an off state, comprising: (a) a first electrode; (b) a second electrode; (c) a dielectric layer disposed between the first and second electrodes; wherein the dielectric layer comprises nanoparticles of semiconductor material, and wherein in the on state nanoparticles form at least one conductive filament… Show more

This invention relates to memory resistors, arrays of memory resistors and a method of making memory resistors. In particular, this invention relates to memory resistors having an on state and an off state, comprising: (a) a first electrode; (b) a second electrode; (c) a dielectric layer disposed between the first and second electrodes; wherein the dielectric layer comprises nanoparticles of semiconductor material, and wherein in the on state nanoparticles form at least one conductive filament encapsulated by the dielectric layer, thereby providing a conductive pathway between the first electrode and the second electrode. Show less

Resistive switching in a metal-free silicon-based material offers a compelling alternative to existing metal oxide-based resistive RAM (ReRAM) devices, both in terms of ease of fabrication and of enhanced device performance. We report a study of resistive switching in devices consisting of non-stoichiometric silicon-rich silicon dioxide thin films. Our devices exhibit multi-level switching and analogue modulation of resistance as well as standard two-level switching.

We demonstrate… Show more

Resistive switching in a metal-free silicon-based material offers a compelling alternative to existing metal oxide-based resistive RAM (ReRAM) devices, both in terms of ease of fabrication and of enhanced device performance. We report a study of resistive switching in devices consisting of non-stoichiometric silicon-rich silicon dioxide thin films. Our devices exhibit multi-level switching and analogue modulation of resistance as well as standard two-level switching.

We demonstrate different operational modes that make it possible to dynamically adjust device properties, in particular two highly desirable properties: nonlinearity and self-rectification. This can potentially enable high levels of device integration in passive crossbar arrays without causing the problem of leakage currents in common line semi-selected devices.

Aspects of conduction and switching mechanisms are discussed, and scanning tunnelling microscopy (STM) measurements provide a more detailed insight into both the location and the dimensions of the conductive filaments.
Show less

Researchers have revealed details of a promising way to make a fundamentally different kind of computer memory chip.
The device is a "memristor", a long-hypothesised but only recently demonstrated electronic component. A memristor's electronic properties make it suitable both for computing and for far faster, denser memory.

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… Show more

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. Show less