Junhua Gao

Mechanism for resistive switching in chalcogenide-based electrochemical metallization memory cells

It has been reported that in chalcogenide-based electrochemical metallization (ECM) memory cells (e.g., As2S3:Ag, GeS:Cu, and Ag2S), the metal filament grows from the cathode (e.g., Pt and W) towards the anode (e.g., Cu and Ag), whereas filament growth along the opposite direction has been observed in oxide-based ECM cells (e.g., ZnO, ZrO2, and SiO2). The growth direction difference has been ascribed to a high ion diffusion coefficient in chalcogenides in comparison with oxides. In this paper, upon analysis of OFF state I–V characteristics of ZnS-based ECM cells, we find that the metal filament grows from the anode towards the cathode and the filament rupture and rejuvenation occur at the cathodic interface, similar to the case of oxide-based ECM cells. It is inferred that in ECM cells based on the chalcogenides such as As2S3:Ag, GeS:Cu, and Ag2S, the filament growth from the cathode towards the anode is due to the existence of an abundance of ready-made mobile metal ions in the chalcogenides rather than ...

Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions

ZnSnN2 is regarded as a promising photovoltaic absorber candidate due to earth-abundance, non-toxicity, and high absorption coefficient. However, it is still a great challenge to synthesize ZnSnN2 films with a low electron concentration, in order to promote the applications of ZnSnN2 as the core active layer in optoelectronic devices. In this work, polycrystalline and high resistance ZnSnN2 films were fabricated by magnetron sputtering technique, then semiconducting films were achieved after post-annealing, and finally Si/ZnSnN2 p-n junctions were constructed. The electron concentration and Hall mobility were enhanced from 2.77 × 1017 to 6.78 × 1017 cm−3 and from 0.37 to 2.07 cm2 V−1 s−1, corresponding to the annealing temperature from 200 to 350 °C. After annealing at 300 °C, the p-n junction exhibited the optimum rectifying characteristics, with a forward-to-reverse ratio over 103. The achievement of this ZnSnN2-based p-n junction makes an opening step forward to realize the practical application of the...

Synaptic devices based on purely electronic memristors

Memristive devices have been widely employed to emulate biological synaptic behavior. In these cases, the memristive switching generally originates from electrical field induced ion migration or Joule heating induced phase change. In this letter, the Ti/ZnO/Pt structure was found to show memristive switching ascribed to a carrier trapping/detrapping of the trap sites (e.g., oxygen vacancies or zinc interstitials) in ZnO. The carrier trapping/detrapping level can be controllably adjusted by regulating the current compliance level or voltage amplitude. Multi-level conductance states can, therefore, be realized in such memristive device. The spike-timing-dependent plasticity, an important Hebbian learning rule, has been implemented in this type of synaptic device. Compared with filamentary-type memristive devices, purely electronic memristors have potential to reduce their energy consumption and work more stably and reliably, since no structural distortion occurs.

Determination of the basic optical parameters of ZnSnN 2

Polycrystalline ZnSnN(2) thin films were successfully prepared by DC magnetron sputtering at room temperature. Both the as-deposited and annealed films showed n-type conduction, with electron concentration varying between 1.6×10(18) and 2.3×10(17)  cm(-3) and the maximum mobility of 3.98  cm(2)  V(-1) s(-1). The basic optical parameters such as the refraction index, extinction coefficient, and absorption coefficient were precisely determined through the spectroscopic ellipsometry measurement and analysis. The optical bandgap of the ZnSnN(2)films was calculated to around 1.9 eV, with the absorption coefficient greater than 10(4)  cm(-1) at wavelengths less than 845 nm. The easy-fabricated ZnSnN(2) possesses a sound absorption coefficient ranging from the ultraviolet through visible light and into the near-infrared, comparable to some typical photovoltaic materials such as GaAs, CdTe, and InP.

Determination of some basic physical parameters of SnO based on SnO/Si pn heterojunctions

P-SnO/n-Si heterojunctions were constructed by using e-beam evaporation in combination with ultra-violet lithography technique. The current-voltage and capacitance-voltage characteristics of the pn heterojunctions were systematically investigated, through which the diode parameters, such as the turn-on voltage, forward-to-reverse current ratio, series resistance, ideality factor, and build-in voltage, were also determined. In particular, the pn heterojunctions presented a relatively good electrical rectifying behavior, with a forward-to-reverse current ratio up to 58 ± 5 at ±2.0 V. The relative permittivity and work function of the SnO films were measured to be 18.8 ± 1.7 and 4.3 eV, respectively. The energy band diagram of the heterojunctions was depicted in detail, which can interpret the rectifying behavior very well.

Single-crystalline metal filament-based resistive switching in a nitrogen-doped carbon film containing conical nanopores

In this letter, we report on the resistive switching originating from the rupture/rejuvenation of single-crystalline Cu filaments in a nitrogen-doped porous carbon-based memristive device Cu/CN0.15/Pt. Cu filaments are confined in conical nanopores in CN0.15 thin films. Dislocations exist in the Cu filaments, resulting in obvious crystal lattice distortions. The Cu/CN0.15/Pt device shows outstanding high temperature retention performance for both ON and OFF states, indicating that it is promising for resistance memory applications. Furthermore, continuous RESET (ON-to-OFF switching) and SET (OFF-to-ON switching) processes could be realized indicating the adaptive learning ability of Cu/CN0.15/Pt, which has potential applications in synaptic devices.

Ultrasensitive Memristive Synapses Based on Lightly Oxidized Sulfide Films

For biological synapses, high sensitivity is crucial for transmitting information quickly and accurately. Compared to biological synapses, memristive ones show a much lower sensitivity to electrical stimuli since much higher voltages are needed to induce synaptic plasticity. Yet, little attention has been paid to enhancing the sensitivity of synaptic devices. Here, electrochemical metallization memory cells based on lightly oxidized ZnS films are found to show highly controllable memristive switching with an ultralow SET voltage of several millivolts, which likely originates from a two-layer structure of ZnS films, i.e., the lightly oxidized and unoxidized layers, where the filament rupture/rejuvenation is confined to the two-layer interface region several nanometers in thickness due to different ion transport rates in these two layers. Based on such devices, an ultrasensitive memristive synapse is realized where the synaptic functions of both short-term plasticity and long-term potentiation are emulated by applying electrical stimuli several millivolts in amplitude, whose sensitivity greatly surpasses that of biological synapses. The dynamic processes of memorizing and forgetting are mimicked through a 5 × 5 memristive synapse array. In addition, the ultralow operating voltage provides another effective solution to the relatively high energy consumption of synaptic devices besides reducing the operating current and pulse width.

Template-Free Growth of Well-Ordered Silver Nano Forest/Ceramic Metamaterial Films with Tunable Optical Responses

Currently, the limitations of conventional methods for fabricating metamaterials composed of well-aligned nanoscale inclusions either lack the necessary freedom to tune the structural geometry or are difficult for large-area synthesis. In this Communication, the authors propose a fabrication route to create well-ordered silver nano forest/ceramic composite single-layer or multi-layer vertically stacked structures, as a distinctive approach to make large-area nanoscale metamaterials. To take advantage of direct growth, the authors fabricate single-layer nanocomposite films with a well-defined sub-5 nm interwire gap and an average nanowire diameter of ≈3 nm. Further, artificially constructed multilayer metamaterial films are easily fabricated by vertical integration of different single-layer metamaterial films. Based upon the thermodynamics as well as thin film growth dynamics theory, the growth mechanism is presented to elucidate the formation of such structure. Intriguing steady and transient optical properties in these assemblies are demonstrated, owing to their nanoscale structural anisotropy. The studies suggest that the self-organized nanocomposites provide an extensible material platform to manipulate optical response in the region of sub-5 nm scale.

Silver nanoparticles with an armor layer embedded in the alumina matrix to form nanocermet thin films with sound thermal stability.

In this article, we demonstrate that the Al-alloyed Ag nanoparticle-embedded alumina nanocermet films lead to excellent thermal stability, even at 500 °C for 130 h under an ambient nitrogen atmosphere. The outward diffusion of Al atoms from the AgAl bimetallic alloy nanoparticles and their easy oxidation create an armor layer to suppress the mobility of Ag atoms. Then, the AlAg particles or/and agglomerates with a uniform spherical shape favor higher dispersion concentration within the host matrix, which is beneficial both for high absorptance in the visible range and for the solid localized surface plasmon absorption features in the AgAl-Al2O3 nanocermet films. Based on the AgAl-Al2O3 absorbing layer with sound optical and microstructural stability, we successfully constructed a high-temperature-endurable solar selective absorber. The multilayer stacked absorber demonstrates a high solar absorptance of ∼94.2% and a low thermal emittance of ∼15% (@ 673 K) after annealing at 450 °C for 70 h in an ambient nitrogen atmosphere.

Observations of 2000 DP107 in NAOC: rotation period and reflectance spectrum

Abstract Photometric observations of a Near-Earth-Asteroid (NEA) 2000 DP107 were made on four successive nights during October 2000, 1.4–4.7 with the 0.6-m/0.9-m Schmidt telescope of National Astronomical Observatories, CAS (NAOC). The derived rotation period of 0.1156 day was consistent with that obtained by Pravec et al. (IAU Circular No. 7504, 2000). In addition, the relative reflectance spectrum of the asteroid covering 0.35– 0.9 μm was obtained with the NAOC 2.16-m telescope adopting a low-resolution grating ( 10 A /pix) on October 2, 2000, which revealed that 2000 DP107 is an M-type asteroid.