Metal Nanogap Memory: Performances and Switching Mechanism.

The nanogap memory (NGM) device, emerging as a promising nonvolatile memory candidate, has attracted increasing attention for its simple structure, nano/atomic scale size, elevated operating speed, and robustness to high temperatures. In this study, nanogap memories based on Pd, Au, and Pt were fabricated by combining nanofabrication with electromigration technology. Subsequent evaluations of the electrical characteristics were conducted under ambient air or vacuum conditions at room temperature. The investigation unveiled persistent challenges associated with metal NGM devices, including (1) prolonged SET operation time in comparison to RESET, (2) the potential generation of error bits when enhancing switching speeds, and (3) susceptibility to degradation during program/erase cycles. While these issues have been encountered by predecessors in NGM device development, the underlying causes have remained elusive. Employing molecular dynamics (MD) simulation, we have, for the first time, unveiled the dynamic processes of NGM devices during both SET and RESET operations. The MD simulation highlights that the adjustment of the tunneling gap spacing in nanogap memory primarily occurs through atomic migration or field evaporation. This dynamic process enables the device to transition between the high-resistance state (HRS) and the low-resistance state (LRS). The identified mechanism provides insight into the origins of the aforementioned challenges. Furthermore, the study proposes an effective method to enhance the endurance of NGM devices based on the elucidated mechanism.

Physical mechanism of field modulation effects in ion implanted edge termination of vertical GaN Schottky barrier diodes.

In this study, the physical properties of F ion-implanted GaN were thoroughly studied, and the related electric-field modulation mechanisms in ion-implanted edge termination were revealed. Transmission electron microscopy results indicate that the ion-implanted region maintains a single-crystal structure even with the implantation of high-energy F ions, indicating that the high resistivity of the edge termination region is not induced by amorphization. Alternately, ion implantation-induced deep levels could compensate the electrons and lead to a highly resistive layer. In addition to the bulk effect, the direct bombardment of high-energy F ions resulted in a rough and nitrogen-deficient surface, which was confirmed via atomic force microscopy (AFM) and X-ray photoelectron spectroscopy. The implanted surface with a large density of nitrogen vacancies can accommodate electrons, and it is more conductive than the bulk in the implanted region, which is validated via spreading resistance profiling and conductive AFM measurements. Under reverse bias, the implanted surface can spread the potential in the lateral direction, whereas the acceptor traps capture electrons acting as space charges, shifting the peak electric field into the bulk region in the vertical direction. As a result, the Schottky barrier diode terminated with high-energy F ion-implanted regions exhibits a breakdown voltage of over 1.2 kV.

Toward High Carrier Mobility and Low Contact Resistance: Laser Cleaning of PMMA Residues on Graphene Surfaces.

ABSTRACT: Poly(methyl methacrylate) (PMMA) is widely used for graphene transfer and device fabrication. However, it inevitably leaves a thin layer of polymer residues after acetone rinsing and leads to dramatic degradation of device performance. How to eliminate contamination and restore clean surfaces of graphene is still highly demanded. In this paper, we present a reliable and position-controllable method to remove the polymer residues on graphene films by laser exposure. Under proper laser conditions, PMMA residues can be substantially reduced without introducing defects to the underlying graphene. Furthermore, by applying this laser cleaning technique to the channel and contacts of graphene field-effect transistors (GFETs), higher carrier mobility as well as lower contact resistance can be realized. This work opens a way for probing intrinsic properties of contaminant-free graphene and fabricating high-performance GFETs with both clean channel and intimate graphene/metal contact.

Hysteresis-free carbon nanotube field effect transistors without any post-treatment.

Out of hundreds of as-fabricated back-gated carbon nanotube field effect transistors we made, five devices which exhibit indiscernible hysteresis are found. The cause of these hysteresis-free devices is investigated based on the energy band and interface trap theory, and it is attributed to the bundling effect of single-walled carbon nanotubes which will result in a reduced bandgap in the single-walled carbon nanotube bundle compared with an isolated semiconducting carbon nanotube. A specific requirement of the SWNT bundle to satisfy the presumed explanation is also proposed.

Synthesis, characterization and properties of C60 nanocrystals, nanowires and hierarchical nanostructures.

In this article, we propose a simple and reproductive approach to prepare low dimension C60 nanostructures, which bases on the crystallization of C60 from volatile solution on the surface of substrates of different materials. The C60 nanowires that we obtained have a uniform shape with a length from 2 to 10 microm and a diameter from about 100 to 250 nm. In some cases, we could prepare the Y- or T-shaped nanostructures of C60, which may be due to the crystal growth simultaneous in different directions. Besides the nanowires, we have also obtained C60 nanoparticles with a diameter about 100 nm to 200 nm. The formation mechanism and electronic properties of the low-dimension nanostructures have also been investigated.

Gate-controlled rectifying behavior in C70@SWNT networks.

We report the gate-controlled rectification behavior in C(70)@SWNT networks at room temperature in air. The electrical transport characteristics can be fitted well with the conventional Schottky diode model. The origin of the rectifying behavior in fullerene peapod networks device is qualitatively discussed. This paper demonstrates a strategy for diode fabrication based on peapod networks.

Thermally assisted tunnelling in ambipolar field-effect transistors based on fullerene peapod bundles.

We report the first detailed studies of the electrical transport behaviour of C(70) fullerene peapod bundles at various temperatures from 400 K down to 4 K. With electrical breakdown, we have prepared ambipolar (i.e. both p- and n-type) field-effect transistors (FETs) using fullerene peapod bundles with high levels of performance. This paper focuses on the role of the Schottky barrier and the thermal activation energy in the transport behaviour of fullerene bundles. The temperature dependence of our measurements reveals that transport is dominated by thermally assisted tunnelling in fullerene bundles at low temperature.

Bicrystalline hematite nanowires.

Bicrystalline nanowires of hematite (alpha-Fe(2)O(3)) have been successfully synthesized by the oxidation of pure iron. The product was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM in combination with focal series reconstruction, energy-dispersive X-ray spectroscopy, and electron energy-loss spectroscopy. The bicrystalline nanowires have diameters of 20-80 nm and lengths up to 20 microm. All of the investigated materials are found to be alpha-Fe(2)O(3) with a rhombohedral crystal structure. Investigations indicate that most of the bicrystalline nanowires are nanotwins with ellipsoidal heads. The orientation relationship between the nanotwins can be described as (110)(M)//(110)(T), [110](M)//[0](T). An energy-filtered TEM investigation indicates that the ellipsoidal head is iron-rich. The growth mechanism of such unique nanostructures is considered to be a solid-phase growth via surface and internal diffusions of molecules from base to tip.

Discrete polarization absorption property of carbon nanotubes studied by using scanning near-field optical microscope.

By using scanning near-field optical microscopy (SNOM), we have observed the polarization phenomena of carbon nanotubes (CNTs), which were synthesized by pyrolysis of a metal phthalocyanine method. Using SNOM, an area of about 100 nm in the thin part of the honeycomblike aligned CNT can be investigated, where the CNTs are nearly parallel or cross aligned with only a few layers. Transmission intensity of the light absorption with constant height scan identified the thickness of the nanotube layers. By changing the linear polarization of the incident light, the dependence of transmission versus polarization angle was recorded. The observed different polarization cannot be explained by using the model of optical absorption of continuum medium. We have proposed a model of discrete polarization absorption, which can be used to identify the number of layers of CNTs and their relative orientations. The combination of SNOM technique and our model can be used for optical polarization of carbon nanotubes of discrete medium.

[Raman spectroscopic study on the iron oxide film prepared by iron oxidation method].

Micro-Raman spectroscopy was used to investigate the chemical composition, microstructure and crystalline phase of an iron oxide sample with three-layer macro-structure prepared by iron oxidation. Two laser lines of 514 and 633 nm with a power of 0.5 mW on the sample were employed to excite the Raman spectra. Comparing the sample spectra to that of bulk alpha-Fe2O3, the sample Raman peaks were assigned. And it was found for the top-layer that the Raman frequencies were down shifted and the peak widths broadened. Therefore we verified that the top-layer of the verified sample is a nano-structure alpha-Fe2O3, the main component of the middle-layer is Fe3O4 and the bottom-layer is most like bulk alpha-Fe2O3.