New Paradigm Technology

The rapid development of computing technology is reflected in the fact that industry has consistently doubled the number of transistors per unit area on a semiconductor wafer every two years. Essentially the basic business model of the semiconductor industry, processor and memory technology has so far continued to roughly fulfil this doubling convention, despite technological barriers and fluctuating economic conditions. Many other aspects of computing technology have followed similar exponential laws, including hard drive space and internet connection speeds. However, the expiry of such rapid development has been forecast on multiple occasions as technological hurdles become increasingly more challenging

Light-activated resistance switching in SiOx RRAM devices

We report a study of light-activated resistance switching in silicon oxide (SiOx) resistive random access memory (RRAM) devices. Our devices had an indium tin oxide/SiOx/p-Si Metal/Oxide/ Semiconductor structure, with resistance switching taking place in a 35 nm thick SiOx layer. The optical activity of the devices was investigated by characterising them in a range of voltage and light conditions. Devices respond to illumination at wavelengths in the range of 410–650 nm but are unresponsive at 1152 nm, suggesting that photons are absorbed by the bottom p-type silicon electrode and that generation of free carriers underpins optical activity. Applied light causes charging of devices in the high resistance state (HRS), photocurrent in the low resistance state (LRS), and lowering of the set voltage (required to go from the HRS to LRS) and can be used in conjunction with a voltage bias to trigger switching from the HRS to the LRS. We demonstrate negative correlation between set voltage and applied laser power using a 632.8 nm laser source. We propose that, under illumination, increased electron injection and hence a higher rate of creation of Frenkel pairs in the oxide—precursors for the formation of conductive oxygen vacancy filaments—reduce switching voltages. Our results open up the possibility of light-triggered RRAM devices

Advanced physical modeling of SiOx resistive random access memories

We apply a three-dimensional (3D) physical simulator, coupling self-consistently stochastic kinetic Monte Carlo descriptions of ion and electron transport, to investigate switching in silicon-rich silica (SiOx) redox-based resistive random-access memory (RRAM) devices. We explain the intrinsic nature of resistance switching of the SiOx layer, and demonstrate the impact of self-heating effects and the initial vacancy distributions on switching. We also highlight the necessity of using 3D physical modelling to predict correctly the switching behavior. The simulation framework is useful for exploring the little-known physics of SiOx RRAMs and RRAM devices in general. This proves useful in achieving efficient device and circuit designs, in terms of performance, variability and reliability

Multi-channel conduction in redox-based resistive switch modelled using quantum point contact theory

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

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 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

Conductance tomography of conductive filaments in intrinsic silicon-rich silica RRAM

We present results from an imaging study of filamentary conduction in silicon suboxide resistive RAM devices. We used a conductive atomic force microscope to etch through devices while measuring current, allowing us to produce tomograms of conductive filaments. To our knowledge this is the first report of such measurements in an intrinsic resistance switching material

High Performance Resistance Switching Memory Devices Using Spin-on Silicon Oxide

In this paper, we present high performance resistance switching memory devices (RRAM) with an SiO 2 -like active layer formed from spin-on hydrogen silsesquioxane (HSQ). Our metal-insulator-metal (MIM) devices exhibit switching voltages of less than 1 V, cycling endurances of more than 10 7 cycles without failure, electroforming below 2 V and retention time of resistance states of more than 10 5 seconds at room temperature. We also report arrays of nanoscale HSQ-based RRAM devices in the form of multilayer nanopillars with switching performance comparable to that of our thin film devices. We are able to address and program individual RRAM nanopillars using conductive atomic force microscopy. These promising results, coupled with a much easier fabrication method than traditional ultra-high vacuum based deposition techniques, make HSQ a strong candidate material for the next generation memory devices

Intrinsic resistance switching in amorphous silicon oxide for high performance SiOx ReRAM devices

In this paper, we present a study of intrinsic bipolar resistance switching in metal-oxide-metal silicon oxide ReRAM devices. Devices exhibit low electroforming voltages (typically − 2.6 V), low switching voltages (± 1 V for setting and resetting), excellent endurance of > 107 switching cycles, good state retention (at room temperature and after 1 h at 260 °C), and narrow distributions of switching voltages and resistance states. We analyse the microstructure of amorphous silicon oxide films and postulate that columnar growth, which results from sputter-deposition of the oxide on rough surfaces, enhances resistance switching behavior

Resistive Switching in Silicon-rich Silicon Oxide

Over the recent decade, many different concepts of new emerging memories have been proposed. Examples of such include ferroelectric random access memories (FeRAMs), phase-change RAMs (PRAMs), resistive RAMs (RRAMs), magnetic RAMs (MRAMs), nano-crystal floating-gate flash memories, among others. The ultimate goal for any of these memories is to overcome the limitations of dynamic random access memories (DRAM) and flash memories. Non-volatile memories exploiting resistive switching – resistive RAM (RRAM) devices – offer the possibility of low programming energy per bit, rapid switching, and very high levels of integration – potentially in 3D. Resistive switching in a silicon-based material offers a compelling alternative to existing metal oxide-based devices, both in terms of ease of fabrication, but also in enhanced device performance. In this thesis I demonstrate a redox-based resistive switch exploiting the formation of conductive filaments in a bulk silicon-rich silicon oxide. My devices exhibit multi-level switching and analogue modulation of resistance as well as standard two-level switching. I demonstrate different operational modes (bipolar and unipolar switching modes) that make it possible to dynamically adjust device properties, in particular two highly desirable properties: non-linearity and self-rectification. Scanning tunnelling microscopy (STM), atomic force microscopy (AFM), and conductive atomic force microscopy (C-AFM) measurements provide a more detailed insight into both the location and the dimensions of the conductive filaments. I discuss aspects of conduction and switching mechanisms and we propose a physical model of resistive switching. I demonstrate room temperature quantisation of conductance in silicon oxide resistive switches, implying ballistic transport of electrons through a quantum constriction, associated with an individual silicon filament in the SiOx bulk. I develop a stochastic method to simulate microscopic formation and rupture of conductive filaments inside an oxide matrix. I use the model to discuss switching properties – endurance and switching uniformity

In situ transmission electron microscopy of resistive switching in thin silicon oxide layers

Silicon oxide-based resistive switching devices show great potential for applications in nonvolatile random access memories. We expose a device to voltages above hard breakdown and show that hard oxide breakdown results in mixing of the SiOx layer and the TiN lower contact layers. We switch a similar device at sub-breakdown fields in situ in the transmission electron microscope (TEM) using a movable probe and study the diffusion mechanism that leads to resistance switching. By recording bright-field (BF) TEM movies while switching the device, we observe the creation of a filament that is correlated with a change in conductivity of the SiOx layer. We also examine a device prepared on a microfabricated chip and show that variations in electrostatic potential in the SiOx layer can be recorded using off-axis electron holography as the sample is switched in situ in the TEM. Taken together, the visualization of compositional changes in ex situ stressed samples and the simultaneous observation of BF TEM contrast variations, a conductivity increase, and a potential drop across the dielectric layer in in situ switched devices allow us to conclude that nucleation of the electroforming—switching process starts at the interface between the SiOx layer and the lower contact