Nanoscale metal-based tunneling junction (MTJ) devices were fabricated using the electromigration method, and their electrical properties were studied after exposure to γ- and β-radiation. Irradiation caused the set threshold voltage (Vset) of the MTJ devices to increase, leading to a transition from a low-resistance state (LRS) to a high-resistance state (HRS). This shift in Vset was due to atom displacement from high-energy electrons excited by γ- and β-radiation. Unlike semiconductor devices, MTJ devices showed resilience to permanent damage and could be restored in-situ through multiple I-V (I is the drain current; V is the drain voltage) sweeps with appropriate configurations. This ability to recover suggests that MTJ devices have promising potential under irradiation. The reparability of irradiated MTJ devices is closely related to nothing-on-insulator (NOI) their structure, providing insights for other NOI and metal-based micro-nanoscale devices.

Nanoscale Co/CoO/Co point contact devices are promising for spintronics, magnetic sensors, single-electron transistors, and memory devices. We fabricated Co/CoO/Co point contact devices and studied their random telegraph signal (RTS) characteristics at various temperatures. In the time domain, the current fluctuates asymmetrically with respect to zero bias, with RTS behavior emerging above a certain voltage threshold. In the frequency domain, the RTS power spectral density exhibits Lorentzian lines. As temperature increases from 50 to 300 K, two RTS behaviors are observed: at ≤100 K, the high current state dominates; at ≥200 K, the low current state prevails, with RTS absent at 150 K. Single exponential fitting shows that RTS state lifetimes decrease with temperature. The charge capture and release model explains the RTS origins and temperature-dependent behavior from an energy band perspective. This study provides insights into charge dynamics, single-electron transport, and the noise mechanisms affecting the reliability of nanoscale devices.

Graphene is a promising material in the realm of flexible electronics, offering exceptional properties for advanced device fabrication. In this study, we present an optimized fabrication process for flexible graphene field-effect transistors (GFETs), achieving an intrinsic voltage gain of 6 dB. Furthermore, we demonstrate the development of multistage graphene inverter chains, successfully realizing a three-stage graphene inverter cascade on a flexible substrate, integrating six flexible GFETs with a total gain of 15.6 dB. The potential of these graphene-based inverter chains for signal amplification is illustrated through the successful amplification of heart rate signals, showcasing their application in flexible bioelectronics.

Frequency multipliers are essential components in communication systems, and graphene’s exceptional electrical properties make it highly promising for flexible electronics. This paper addresses the technical challenges of multi-frequency multipliers based on graphene field-effect transistors (GFETs) and introduces a novel fabrication method using graphene as the channel material and metals with different work functions as the top gate. By employing Ti and Pd with distinct work functions, we develop a dual-gate GFET device that exhibits stable M-shaped resistance characteristics on a flexible polyethylene naphthalate (PEN) substrate. We demonstrate frequency doubler, tripler, and quadrupler on the flexible substrate. The results show that the GFET-based frequency multiplier offers advantages such as low operating voltage (<1 V), high voltage conversion efficiency (up to 8.4% for tripler and 6% for quadrupler), and high spectral purity (up to 88% for tripler and 76% for quadrupler). The intrinsic maximum operating frequency of the frequency quadrupler reaches 54 GHz. The use of a monolayer graphene channel, dual-metal gate control enabling an M-shaped transfer curve, and flexible characteristics all contribute to its superior performance compared to conventional devices.

The nanogap resistive switch holds potential as a candidate for nonvolatile memory, although its durability needs enhancement. This study delves into the operational mechanisms through detailed morphological examination during continuous operation of nanogap resistive switches. By developing a finite element model of nanogaps, we reveal the mechanisms behind the formation of electrode surface hillocks and filaments during continuous switching. Our findings suggest that “set” operations include processes such as field evaporation, electric field-induced diffusion, and field-assisted migration within the gap. Conversely, “reset” operations, driven by Joule heating and electromigration, lead to filament breakage and the creation of a fine gap. This research elucidates device degradation issues, such as periodic fluctuations in set threshold voltage (Vset) and the presence of non-steep set curves, providing both theoretical and experimental insights to improve future device performance.

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.

In this investigation, atomic force microscopy (AFM) was employed to meticulously examine the electromigration phenomena occurring in pure gold nanowires possessing a mere 10 nm thickness. The study elucidates the dynamic evolution of the micromorphology of gold nanowires during the electromigration process, providing insights into the intricate processes leading to the formation of voids, hillocks, and nano gaps. Significantly, the intricate relationship between these morphological changes and the consequential impact on electrical properties, particularly the resistance of the nanowires, is systematically unveiled. A qualitative analysis of the formation mechanisms behind voids and hillocks was conducted, contributing to a deeper understanding of the electromigration-induced phenomena. This study holds significance in controlling the width and position of metal nanogaps, offering promising avenues for the advancement of gap-based nanoelectronic and memory devices.

Abstract With the rapid development of electronic engineering and nanotechnology, the electrical field-induced surface migration plays an increasingly important role in the fields of materials science. By comparing the changes in the surface migration behavior of gold nanogap electrodes under different electric field parameters, we reveal the kinetic processes and influencing factors of electric field-induced surface migration. Under the condition of only 1 pA current limit, under the continuous electric field, the hillcok only appears in the anode and gradually evolve into multiple peaks when rises to some extent. By establishing a finite element model of the nanogap, we find the hillock grows under the field strength gradient. Then hillock growth is biregulated by applying a positive or negative voltage to the drain end of the gap device. Located in the normal temperature atmosphere, the morphology will degenerate, and this feature is opposite to the hillock generated by electromigration.

We demonstrate an atomic-sized Pd tunneling junction memory that can operate in a vacuum, N 2 , or ambient environment. The device was fabricated by a simple 2-step electron-beam lithography/lift-off process and an electromigration process. Calculation based on quantum point contact model indicated the size of the tunneling gap is between 0.1 and 2nm, and the switching between high-resistance state and low-resistance state is achieved by a migration of a few atoms near the junction. The device's RESET speed is 25ns, and the ON/OFF ratio is from 2 to 10 3 , with an endurance of 1000 cycles. We proposed a phenomenological model to explain the reason for the imperfection of device performance, which could be useful for circuit design. It may be derived from two different migration processes, dominated by rapid changing electric field and current-induced Joule heat accumulation to affect the device performance, respectively. Compared to other reported works, our devices have the same fast RESET speed, better endurance, and potential for low power applications.

In this paper, we propose and implement a single-electron transistor (SET) based on cobalt oxide for the first time. The SETs were fabricated through nanofabrication process compatible with the CMOS technology. A cobalt oxide (Co1-yO) on the surface of the source (S) and drain (D) Co electrodes is utilized to act as a quantum dot (QD). The diameter of QD is calculated to be 2.3 nm. The interface between Co1-yO and Co thin films (S/D electrodes) is used as tunneling junctions. The SET based on CoO exhibits typical single electron transport characteristics. Owing to the high Coulomb blockade energy (~180 meV), the Coulomb blockade regions still remain at room temperature (300 K), revealing the nanometer scale CoO-based SETs have great potential to operate at room temperature.

Centimeter scale cobalt films with various thicknesses (8 nm ∼ 100 nm) were deposited by electron beam evaporation (EBE) and then annealed in a gas mixture of Ar and H 2 at temperatures ranging from 200 °C to 500 °C. Advanced characterization techniques (e.g., XRD, SEM, AFM and MFM) were employed to investigate the influence of annealing on the morphology, crystal structures and magnetic domain structures of Co thin films. The results of SEM and AFM suggest that there is no obvious change in the morphology of Co film before and after annealing especially for thicker films and the root-mean-square roughness of Co film surface is slightly reduced after annealing. The influence of thermal annealing on the magnetic domain structure of EBE Co thin films was investigated by magnetic force microscopy (MFM) for the first time. It is found that even if there is no clear domain structure in the as-deposited films, it is possible to obtain periodic stripe domains with perpendicular magnetic anisotropy (PMA) by thermal annealing owing to the development of HCP Co phase, which was confirmed by XRD analysis. The correlations between the film morphology, thickness and magnetic domain structure are discussed qualitatively. Based on the periodic stripe magnetic domains, the domain-wall energy density of annealed cobalt films is calculated and this study found that the annealing has almost no effect on the energy density of the domain-wall. This work provides an effective way to obtain the perpendicular magnetic anisotropy (PMA) for practical applications.

A prototype of a kind of two-bit data recording head based on the Ni 0.85 Co 0.15 thin film devices is proposed. Thin film devices with a two-terminal structure were fabricated by a two-step e-beam lithography (EBL) and e-beam evaporation (EBE) process, ensuring that this kind of devices possesses M-shaped magnetoresistance (MR) curves. The reading and writing of data can be realized by adjusting the magnetic field to change the magnetoresistance value. The consistency of the devices has been also experimentally confirmed.

An approach to fabricate graphene-based quantum dots (QDs) by reconnecting the electroburned nanogap is demonstrated. The structure and electrical properties of graphene QDs were studied in detail. We also investigated the characteristics of Coulomb oscillation and Coulomb diamond, and estimated the value of charging energy. This work reveals that graphene nanogap is robust structure and may have a possible application in quantum dot systems.

The ambipolar graphene field-effect transistors (GFETs) usually exhibit Λ-shaped resistance versus gate voltage characteristics (R-Vg curve) with the n- and p-type regions switching at the neutrality points of graphene. However, M-shaped R-Vg curves were frequently observed in our back-gated GFETs without intentional doping. Here, we proposed an implementation of a frequency multiplier using the M-like shape of the R-Vg curve. We first investigated the effect of the channel length and the contact transfer length in a GFET on the shape of the R-Vg curve and then evaluated the influence of the various shapes of R-Vg curves on the performance of the frequency multiplier (including tripler and quadrupler). Finally, a frequency tripler based on a single GFET has been experimentally demonstrated. When applying a sinusoid input signal with a fundamental frequency at a suitable operation area, around 80% output signal power is concentrated at the third harmonic. The excellent output spectral purity makes GFETs with the M-shaped R-Vg curve promising candidates for the frequency multiplier.

In this work, we investigate the performance of radio-frequency (RF) power amplifier based on chemical vapor deposition (CVD) graphene field-effect transistors (GFETs). The GFETs with gate length of 300 nm were fabricated on SiO2/Si substrate, which show an extrinsic current gain cut-off frequency (f T ) of 18.6 GHz and an extrinsic maximum oscillation frequency (f max ) of 19.8 GHz. The parameters of compact large-signal model for the GFETs were extracted from the measured direct current (DC) and RF characteristics of GFETs, followed by implementation of the compact model using Verilog-A for circuit simulation. Using electronic design automation (EDA) tools, we designed a GFET-based power amplifier. The power amplifier working at 2.5 GHz shows a gain of ∼7 dB, an output power of ∼0 dBm (1 mW), a power added efficiency (PAE) of 2.8% and third order intermodulation distortion (IMD) of ∼20 dBc, at the 1 dB compression point.

While many studies on radio-frequency (RF) graphene field-effect transistor (GFET) aimed to study its high performances, explorations of the long-term stability of RF GFETs in atmosphere may be required as well. In this paper, we investigated the stability of the RF GFETs stored in ambient for four years. Both the RF and direct current characteristics are measured and compared with the initial properties. Changes of current-gain cut-off frequency and field-effect mobility, reductions of maximum oscillation frequency, peak transconductance and gate capacitance are observed butthe extracted contact resistance remains unchanged. The permeation of water vapor and oxygen into the thin Al2O3 dielectric, the diffusion of Pd atoms from contact onto the graphene channel and the rougher Al2O3 surface may challenge the stability of RF GFETs. This work may pave the way for the fabrication of more stable RF GFETs, which further serve steady graphene-based RF circuits.

Here an adsorption phenomenon on the surface of multilayer graphene in air atmosphere over time was studied. We found that after 2~3 months, some unknown adsorbates absorbed on the surface of graphene. The adsorbate films (~2nm) have no regular shape and most of them distribute on the flat graphene surface, some along the wrinkle or around a nanoparticle. The longitudinal conductivity of the multilayer graphene is substantially increased (at least 30 times) owing to the presence of the adsorbates. A reliable method to remove the adsorbates is proposed using a visible laser. This work may open up the potential application of adsorbates in graphene devices, and a new field of reliability of graphene devices.

研究通过使用氧等离子体刻蚀光刻胶的方法,在化学气相沉积(chemical vapor deposition,CVD)法生长出的石墨烯表面制备活性碳材料,从而形成基于石墨烯的碳纳米复合结构.对该材料进行表征后发现,该碳纳米复合材料表面存在许多碳纳米线结构,因此具有更高的比表面积,使石墨烯可以被用作锂离子电池的电极、填充材料及超级电容器等.

A carbon-based inductor may serve as an important passive component in a carbon-based radio-frequency (RF) integrated circuit (IC). In this work, chemical vapor deposition (CVD) synthesized monolayer graphene and graphite inductors are fabricated and their Q-factors are investigated. We find that the large series resistance of signal path (including coil resistance and contact resistance) in monolayer graphene inductors causes negative Q-factors at the whole frequency range in measurement. Comparatively, some of the graphite inductors have all of their Q-factors above zero, due to their small signal path resistance. We also note that some other graphite inductors have negative Q-factor values at low frequency regions, but positive Q-factor values at high frequency regions. With an equivalent circuit model, we confirm that the negative Q-factors of some graphite inductors at low frequency regions are related to their relatively large contact resistances, and we are able to eliminate these negative Q-factors by improving the graphite-metal contact. Furthermore, the peak Q-factor (Qp) can be enhanced by lowering down the resistance of graphite coil. For an optimized 3/4-turn graphite inductor, the measured maximum Q-factor (Qm) can reach 2.36 and the peak Q-factor is theoretically predicted by the equivalent circuit to be as high as 6.46 at a high resonant frequency, which is beyond the testing frequency range. This research indicates that CVD synthesized graphite thin film is more suitable than graphene for fabricating inductors in carbon-based RF IC in the future.