Biorealistic Neuronal Temperature-Sensitive Dynamics within Threshold Switching Memristors: Toward Neuromorphic Thermosensation.

Neuromorphic nanoelectronic devices that can emulate the temperature-sensitive dynamics of biological neurons are of great interest for bioinspired robotics and advanced applications such as neuroscience. In this work, we demonstrate the biomimetic thermosensitive properties of two-terminal VO memristive devices and showcase their similarity to the firing characteristics of thermosensitive biological neurons. The temperature-dependent electrical characteristics of VO-based memristors are used to understand the spiking response of a simple relaxation oscillator.

Optically Tunable Electrical Oscillations in Oxide-Based Memristors for Neuromorphic Computing.

The application of hardware-based neural networks can be enhanced by integrating sensory neurons and synapses that enable direct input from external stimuli. This work reports direct optical control of an oscillatory neuron based on volatile threshold switching in VO. The devices exhibit electroforming-free operation with switching parameters that can be tuned by optical illumination.

Thermal transport in metal-NbO-metal cross-point devices and its effect on threshold switching characteristics.

Volatile threshold switching and current-controlled negative differential resistance (NDR) in metal-oxide-metal (MOM) devices result from thermally driven conductivity changes induced by local Joule heating and are therefore influenced by the thermal properties of the device-structure. In this study, we investigate the effect of the metal electrodes on the threshold switching response of NbO-based cross-point devices. The electroforming and switching characteristics are shown to be strongly influenced by the thickness and thermal conductivity of the top-electrode due to its effect on heat loss from the NbO film.

Thermal Conductivity of Amorphous NbO Thin Films and Its Effect on Volatile Memristive Switching.

Metal-oxide-metal (MOM) devices based on niobium oxide exhibit threshold switching (or current-controlled negative differential resistance) due to thermally induced conductivity changes produced by Joule heating. A detailed understanding of the device characteristics therefore relies on an understanding of the thermal properties of the niobium oxide film and the MOM device structure. In this study, we use time-domain thermoreflectance to determine the thermal conductivity of amorphous NbO films as a function of film composition and temperature.

Engineering the Threshold Switching Response of NbO-Based Memristors by Ti Doping.

Two terminal metal-oxide-metal devices based on niobium oxide thin films exhibit a wide range of non-linear electrical characteristics that have applications in hardware-based neuromorphic computing. In this study, we compare the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped NbO and Ti-doped NbO and show that doping offers an effective means of engineering the device response for particular applications. In particular, doping is shown to improve the device reliability and to provide a means of tuning the threshold and hold voltages, the hysteresis window, and the magnitude of the negative differential resistance.

Metal-oxide interface reactions and their effect on integrated resistive/threshold switching in NbO .

Reactive metal electrodes (Nb, Ti, Cr, Ta, and Hf) are shown to play an important role in controlling the volatile switching characteristics of metal/NbO/Pt devices. In particular, devices are shown to exhibit stable threshold switching under negative bias but to have a response under positive bias that depends on the choice of metal. Three distinct responses are highlighted: Devices with Nb and Ti top electrodes are shown to exhibit stable threshold switching with symmetric characteristics for both positive and negative polarities; devices with Cr top electrodes are shown to exhibit stable threshold switching but with asymmetric hysteresis windows under positive and negative polarities; and devices with Ta and Hf electrodes are shown to exhibit an integrated threshold-memory (1S1M) response.

Electric Field- and Current-Induced Electroforming Modes in NbO.

Electroforming is used to initiate the memristive response in metal/oxide/metal devices by creating a filamentary conduction path in the oxide film. Here, we use a simple photoresist-based detection technique to map the spatial distribution of conductive filaments formed in Nb/NbO/Pt devices, and correlate these with current-voltage characteristics and in situ thermoreflectance measurements to identify distinct modes of electroforming in low- and high-conductivity NbO films. In low-conductivity films, the filaments are randomly distributed within the oxide film, consistent with a field-induced weakest-link mechanism, while in high-conductivity films they are concentrated in the center of the film.

Nanoscale modification of one-dimensional single-crystalline cuprous oxide.

In this work we report for the first time a method to modify the surface of CuO nanowires in a controllable way and physically weld them into a network form, which contributes to higher electrical conductivity as well as a strong water-repelling nature. We have used state-of-the-art theoretical calculations to support our experimental observations. We demonstrate how varying the irradiation fluence can modulate the surface and decorate the nanowire with a uniform distribution of CuO nanocrystals due to preferential sputtering.

Two-dimensional photonic crystals increasing vertical light emission from Si nanocrystal-rich thin layers.

We have fabricated two-dimensional photonic crystals (PhCs) on the surface of Si nanocrystal-rich SiO layers with the goal to maximize the photoluminescence extraction efficiency in the normal direction. The fabricated periodic structures consist of columns ordered into square and hexagonal pattern with lattice constants computed such that the red photoluminescence of Si nanocrystals (SiNCs) could couple to leaky modes of the PhCs and could be efficiently extracted to surrounding air. Samples having different lattice constants and heights of columns were investigated in order to find the configuration with the best performance.

Room temperature synthesis of HfO/HfO heterostructures by ion-implantation.

Implantation of Hf films with oxygen ions is shown to be an effective means of fabricating high-quality HfO/HfO heterostructures at room temperature, with the layer composition and thicknesses determined by the ion energy and fluence. Implantation with 3 keV O ions to a fluence of 1 × 10 ions cm produces a polycrystalline (monoclinic-) HfO layer extending from the surface to a depth of ∼12 nm, and an underlying graded HfO layer extending an additional ∼7 nm, while implantation with 6 keV O to a similar fluence produces a near-stoichiometric surface layer of 7 nm thickness and a graded substoichiometric layer extending to depth of ∼30 nm. These structures are shown to be broadly consistent with oxygen range data but more detailed comparison with dynamic Monte Carlo simulations suggests that the near-surface region contains more oxygen than expected from collisional processes alone.

Anatomy of filamentary threshold switching in amorphous niobium oxide.

The threshold switching behavior of Pt/NbO /TiN devices is investigated as a function device area and NbO film thickness and shown to reveal important insight into the structure of the self-assembled switching region. The devices exhibit combined selector-memory (1S1R) behavior after an initial voltage-controlled forming process, but exhibit symmetric threshold switching when the RESET and SET currents are kept below a critical value. In this mode, the threshold and hold voltages are independent of the device area and film thickness but the threshold current (power), while independent of device area, decreases with increasing film thickness.

Silicon nanocrystal-based photonic crystal slabs with broadband and efficient directional light emission.

Light extraction from a thin planar layer can be increased by introducing a two-dimensional periodic pattern on its surface. This structure, the so-called photonic crystal (PhC) slab, then not only enhances the extraction efficiency of light but can direct the extracted emission into desired angles. Careful design of the structures is important in order to have a spectral overlap of the emission with extraction (leaky) modes.

Coupling dynamics of Nb/NbO relaxation oscillators.

The coupling dynamics of capacitively coupled Nb/NbO relaxation oscillators are shown to exhibit rich collective behaviour depending on the negative differential resistance response of the individual devices, the operating voltage and the coupling capacitance. These coupled oscillators are shown to exhibit stable frequency and phase locking states at source voltages as low as 2.2 V, with frequency control in the range from 0.

Quadrupole shift of nuclear magnetic resonance of donors in silicon at low magnetic field.

Shifts from the expected nuclear magnetic resonance frequencies of antimony and bismuth donors in silicon of greater than a megahertz are observed in electrically detected magnetic resonance spectra. Defects created by ion implantation of the donors are discussed as the source of effective electric field gradients generating these shifts via quadrupole interaction with the nuclear spins. The experimental results are modeled quantitatively by molecular orbital theory for a coupled pair consisting of a donor and a spin-dependent recombination readout center.

Oxygen self-diffusion in HfO2 studied by electron spectroscopy.

High-resolution measurement of the energy of electrons backscattered from oxygen atoms makes it possible to distinguish between (18)O and (16)O isotopes as the energy of elastically scattered electrons depends on the mass of the scattering atom. Here we show that this approach is suitable for measuring oxygen self-diffusion in HfO2 using a Hf(16)O2 (20 nm)/Hf(18)O2 bilayers (60 nm). The mean depth probed (for which the total path length equals the inelastic mean free path) is either 5 or 15 nm in our experiment, depending on the geometry used.

Self-reported quality of life ratings of people with dementia: the role of attitudes to aging.

Background: Attitudes to aging have not previously been assessed in people with dementia. The possession of positive life attitudes into older age has the potential to induce resilience to health changes and may explain the discrepancy between self-reported and proxy ratings of quality of life (QoL). The aim of this study was to explore the attitudes of people with dementia to determine the main factors that predict these attitudes and any relationship that exists with self-reported QoL.

No evidence of age-related increases in unconscious plagiarism during free recall.

In three experiments younger and older participants took part in a group generation task prior to a delayed recall task. In each, participants were required to recall the items that they had generated, avoiding plagiarism errors. All studies showed the same pattern: older adults did not plagiarise their partners any more than younger adults did.

Self-assembled growth and luminescence of crystalline Si/SiOx core-shell nanowires.

Crystalline Si/SiOx core/shell nanowires (NWs) are self-assembled by annealing Ni-coated hydrogenated Si-rich SiOx (SRO:H) films at 1100 degrees C in the presence of Si powder. Plasma-enhanced chemical vapor deposition is used to grow 100 nm SRO:H thin films with varying silicon concentration (n(Si)). The NWs vary from SiOx nanowires to Si/SiOx core/shell structures depending on the composition of the SRO:H substrate, with the fraction of core/shell structures increasing with increasing Si concentration.

Gold bead-strings in silica nanowires: a simple diffusion model.

Silica nanowires grown from gold droplets deposited on the surface of a silicon crystal sometimes develop within them a regular series of gold beads distributed along the wire axis in what is often called either a bead-string or a pea-pod structure. This is generally attributed to a 'Rayleigh instability' driven by the surface free energy of the included gold core. Here a new model is proposed in which quasi-conical gold inclusions are developed by the diffusion-limited growth process and are subsequently modified to spherical shape by another diffusion process that is driven by surface free energy.