Li Qiang Zhu

Artificial synapse network on inorganic proton conductor for neuromorphic systems

The basic units in our brain are neurons, and each neuron has more than 1,000 synapse connections. Synapse is the basic structure for information transfer in an ever-changing manner, and short-term plasticity allows synapses to perform critical computational functions in neural circuits. Therefore, the major challenge for the hardware implementation of neuromorphic computation is to develop artificial synapse network. Here in-plane lateral-coupled oxide-based artificial synapse network coupled by proton neurotransmitters are self-assembled on glass substrates at room-temperature. A strong lateral modulation is observed due to the proton-related electrical-double-layer effect. Short-term plasticity behaviours, including paired-pulse facilitation, dynamic filtering and spatiotemporally correlated signal processing are mimicked. Such laterally coupled oxide-based protonic/electronic hybrid artificial synapse network proposed here is interesting for building future neuromorphic systems.

Freestanding Artificial Synapses Based on Laterally Proton-Coupled Transistors on Chitosan Membranes.

Freestanding synaptic transistors are fabricated on solution-processed chitosan membranes. A short-term memory to long-term memory transition is observed due to proton-related electrochemical doping under repeated pulse stimulus. Moreover, freestanding artificial synaptic devices with multiple presynaptic inputs are investigated, and spiking logic operation and logic modulation are realized.

Characterization of ZnO:N films prepared by annealing sputtered zinc oxynitride films at different temperatures

N-doped ZnO films were prepared by annealing zinc oxynitride films deposited by rf reactive sputtering. Two Raman peaks were observed at 274 and 580 cm−1. According to the variation of the integral intensity of these two peaks, the nitrogen activation at 500 °C [the activation temperature (AT)] has been obtained. Below the AT, the integral intensities of them show a different variation trend. X-ray photoelectron spectroscopy (XPS) indicates the N chemical state variation for them and finds the activated Zn-N bond. Further analyses by photoluminescence (PL) spectra and spectroscopic ellipsometry (SE) have been carried out. The activated sample exhibits a symmetric emission peak at 3.22 eV assigned to be the AX0 emission at room temperature. SE investigation takes account of samples within the different temperature span divided by the AT. Different factors, such as nitrogen dopant (N)O and the nanocrystal growth, which affect the redshift of the absorption edges, have been discussed.

Microstructure and optical properties of ultra-thin zirconia films prepared by nitrogen-assisted reactive magnetron sputtering

High-k ZrO2 films were prepared by nitrogen-assisted direct current reactive magnetron sputtering on n-type silicon (100). The microstructure and optical properties in relation to thermal budgets were investigated. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical states. Atomic force microscopy (AFM) analysis indicated that the annealing temperature had significant effects on surface roughness. By using Fourier transform infrared spectroscopy (FTIR), the resistance to the interface growth after the additional thermal budgets was observed. The thickness and pseudodielectric constants of ZrO2 thin films correlating to annealing temperature were determined by Tauc–Lorentz spectroscopic ellipsometry (SE) dispersion model fitting. Optical band gaps (Eg) were also obtained based on the extracted absorption edge.

Proton-Conducting Graphene Oxide-Coupled Neuron Transistors for Brain-Inspired Cognitive Systems.

Proton-conducting graphene oxide electrolyte films with very high electric-double-layer capacitance are used as the gate dielectrics for oxide-based neuron transistor fabrication. Paired-pulse facilitation, dendritic integration, and orientation tuning are successfully emulated. Additionally, neuronal gain controls (arithmetic) are also experimentally demonstrated. The results provide a new-concept approach for building brain-inspired cognitive systems.

Effect of post deposition annealing on the optical properties of HfOxNy films

We report in this letter that Hafnium oxynitride (HfOxNy) gate dielectrics were prepared by radio frequency (RF) reactive magnetron sputtering and optical properties of the HfOxNy thin films were investigated by spectroscopic ellipsometry with photon energy of 0.75–6.5eV at room temperature. The investigations showed that the annealing temperatures have a strong effect on the optical properties of HfOxNy thin films. With increased annealing temperature, the refractive index n is observed to increase, while the extinction coefficient k decreases, respectively. The changes of the complex dielectric functions and the optical band gap Eg with the annealing temperature are also discussed.

Optimization of microstructure and optical properties of VO2 thin film prepared by reactive sputtering

VO2 (M) thin films with good optical switching properties have been grown by reactive sputtering method. The influence of sputtering parameters on the structural and optical properties of the as-grown VO2 thin films was investigated, and the correlation between the microstructure and optical switching properties were studied. It was found that the phase transition temperature, hysteresis width, and the amplitude of the transition depend on the sputtering gas pressure, and the amplitude of the transition can reach as high as 70% with an approximately zero infrared transmission in metal state at a wavelength of 2.5 μm. The anomalous optical properties of the VO2 thin films were analyzed and discussed together with the studies of the refractive index and optical band gap.

Structural, interfacial and optical characterization of ultrathin zirconia film grown by in situ thermal oxidation of sputtered metallic Zr films

High dielectri cc onstant ZrO2 gate dielectric thin films hav eb een prepared by means of in situ thermal oxidation of sputtered metallic Zr films. XRD reveal st hat the as-oxidized samples are amorphous, but can be made polycrystalline with a highly (¯ 111)-preferential orientation by increasing the annealing temperature. AFM measurements confirm that high temperature annealing results in increase of the roughness root mean square value of the films. The growth and properties of the interfacial SiO2 layer formed at the ZrO2/Si interface are observed by using Fourier transform infrared spectroscopy. It has been found that the formation of the interfacial layer depends on the post-deposition annealing temperature. On the basis of a parametrized Tauc–Lorentz dispersion model, the optical properties of the as-oxidized and annealed films related to the annealing temperature are systematically investigated by spectroscopic ellipsometry. The increase in the refractive index and decrease in extinction coefficient with increase of the annealing temperature are discussed in detail. (Some figures in this article are in colour only in the electronic version)

Memory and learning behaviors mimicked in nanogranular SiO2-based proton conductor gated oxide-based synaptic transistors

In neuroscience, signal processing, memory and learning function are established in the brain by modifying ionic fluxes in neurons and synapses. Emulation of memory and learning behaviors of biological systems by nanoscale ionic/electronic devices is highly desirable for building neuromorphic systems or even artificial neural networks. Here, novel artificial synapses based on junctionless oxide-based protonic/electronic hybrid transistors gated by nanogranular phosphorus-doped SiO2-based proton-conducting films are fabricated on glass substrates by a room-temperature process. Short-term memory (STM) and long-term memory (LTM) are mimicked by tuning the pulse gate voltage amplitude. The LTM process in such an artificial synapse is due to the proton-related interfacial electrochemical reaction. Our results are highly desirable for building future neuromorphic systems or even artificial networks via electronic elements.

Synaptic Behaviors Mimicked in Flexible Oxide-Based Transistors on Plastic Substrates

In the human brain, synapses are the crucial connective parts between neurons, which endow neurons with significant computational abilities. Here, indium-zinc-oxide (IZO) based flexible synaptic transistors are fabricated on a plastic substrate by a simple self-assembly method. Proton conducting phosphorus-doped nanogranular SiO2 electrolyte is used as the gate dielectric. Excitatory postsynaptic current, paired-pulse facilitation, and long-term memory are mimicked in the flexible artificial synapses. Such IZO-based flexible electronic synapses are promising for building neuromorphic systems.