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So far, it is still difficult to construct composites with a gradient distribution of graphene for decreasing the reflection and increasing the absorption of electromagnetic energy. Here, we introduce an electrochemical method to efficiently prepare a graphene/polyurethane composite with a gradient graphene distribution. And the composite shows obvious anisotropic reflection of electromagnetic waves, with low reflection loss (-30 dB) and high absorption (99.5%) in the whole X-band when electromagnetic waves are incident to the surface that has low graphene content. More importantly, the electrochemical method could be extended to the preparation of functional materials with similar structures based on the electrophoresis of charged nanoparticles.

Contact resistance (R C ) is of great importance for radio frequency (RF) applications of graphene, especially graphene field effect transistors (FETs) with short channel. FETs and transmission line model test structures based on chemical vapor deposition grown graphene are fabricated. The effects of employing traditional lithography solvent (Acetone) and strong solvents for photo resist, such as N, N-Dimethylacetamide (ZDMAC) and N-Methyl pyrrolidone (NMP), are systematically investigated. It was found that ZDMAC and NMP have more proficiency than acetone to remove the photo-resist residues and contaminations attached on graphene surface, enabling clean surface of graphene. However, strong solvents are found to destroy the lattice structure of graphene channel and induce defects in graphene lattice. Clean surface contributes to a significant reduction in the R C between graphene channel and metal electrode, and the defects introduced on graphene surface underneath metal electrodes also contribute the reduction of R C . But defects and deformation of lattice will increase the resistance in graphene channel and lead to the compromise of device performance. To address this problem, a mix wet-chemical approach employing both acetone and ZDMAC was developed in our study to realize a 19.07% reduction of R C , without an unacceptable mass production of defects.

(Al)GaN recess-free normally OFF technology is developed for fabrication of high-yield lateral GaN-based power devices. The recess-free process is achieved by an ultrathin-barrier (UTB) AlGaN/GaN heterostructure that features a natural pinched-off 2-D electron gas channel. The top–down manufacturing technique overcomes the challenges in etching of AlGaN barrier with well-controlled depth and uniformity, which is especially attractive for fabrication of normally OFF GaN-based high-electron-mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS-HEMTs) on large-size Si substrate. With SiNx passivation grown by low-pressure chemical-vapor deposition, on-resistance of the UTB-AlGaN/GaN-based power devices can be significantly reduced. High-uniformity low-hysteresis normally OFF HEMTs and Al2O3/AlGaN/GaN MIS-HEMTs are successfully demonstrated on the UTB AlGaN/GaN-on-Si platform. It is also a compelling technology platform for manufacturing high-performance GaN-based lateral power diodes, and normally OFF p-(Al)GaN heterojunction field-effect transistors.

A pre-ohmic micro-patterned recess process, is utilized to fabricate Ti/Al/Ti/TiN ohmic contact to an ultrathin-barrier (UTB) AlGaN/GaN heterostructure, featuring a significantly reduced ohmic contact resistivity of 0.56 Ω·mm at an alloy temperature of 550 °C. The sheet resistances increase with the temperature following a power law with the index of +2.58, while the specific contact resistivity decreases with the temperature. The contact mechanism can be well described by thermionic field emission (TFE). The extracted Schottky barrier height and electron concentration are 0.31 eV and 5.52 × 1018 cm−3, which suggests an intimate contact between ohmic metal and the UTB-AlGaN as well as GaN buffer. A good correlation between ohmic transfer length and the micro-pattern size is revealed, though in-depth investigation is needed. A preliminary CMOS-process-compatible metal–insulator–semiconductor high-mobility transistor (MIS-HEMT) was fabricated with the proposed Au-free ohmic contact technique.

The metal-graphene contact resistance has been identified to be a key bottleneck for achieving high performance of graphene transistors. It is crucial to understand the electrical properties of graphene and the carrier transport mechanism under the contact metal. Here, we have developed a new method of characterizing the electrical properties of graphene under the metal contact. It was found that the electrical properties of graphene under the metal can be tuned via the back-gate voltage and display ambipolar behavior. A quantum tunneling model for graphene-metal physical contact has been proposed. The probability of electric field-tunable tunneling has been derived from the results of measurements for the first time. The model predicts that even for physical contact the contact resistance can be much lower than 100 Ω μm when graphene is more heavily doped and the interfacial layer is eliminated. This study paves the way to achieving ultralow graphene-metal contact resistance in graphene devices for terahertz applications.

Silicon nitride (SiNx) grown by low-pressure chemical vapor deposition (LPCVD) at a reduced growth temperature (650 °C), is utilized for fabrication of GaN metal–insulator-semiconductor (MIS) power devices. An atomically sharp interface between the LPCVD-SiNx and GaN was achieved, featuring a disordered region with only a thickness of 2.5 ~ 5 A. A fabricated LPCVD-SiNx/GaN/AlGaN/GaN MIS diode exhibits a sharp two-step capacitance–voltage behavior with small frequency dispersion of 0.4 V in the right step. The improved interface was quantified by a well-elaborated constant-capacitance deep-level transient Fourier spectroscopy (CC-DLTFS), delivering quite low density of 1.5 × 10 13 cm - 2 eV - 1 at the level depth of 30 meV and about 4 × 10 11 ~ 1.2 × 10 12 cm - 2 eV - 1 at the level depth of 1 eV. Distributions of interface states can be described by a proposed physics-based decoupling function featuring an exponential law. A discrete level at the SiNx/GaN border or in the barrier layer with the level depth and the capture cross section being 0.8 eV and 5.5 × 10 - 14 cm 2 respectively, can thus be detached from the interface states.

A silicon nitride (SiNx) film deposited at 500 °C by plasma-enhanced atomic layer deposition (PEALD) is employed as the gate dielectric for GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs). An interface enhancement technology featuring in situ low-damage NH3/N2 remote plasma pretreatments (RPPs) is developed prior to the SiNx gate dielectric deposition, which contributes to an improved surface morphology while remarkably suppressed interface oxides. It is revealed by constant-capacitance deep-level transient spectroscopy that both shallow and deep states at the PEALD-SiNx/III-nitride interface are reduced by about one order of magnitude by the RPP. The in situ RPP and PEALD-SiNx gate dielectric process are implemented into fabrication of enhancement-mode MIS-HEMTs on an ultrathin-barrier AlGaN/GaN heterostructure technology platform. The fabricated MIS-HEMTs deliver an improved threshold stability and maximum output current as compared with devices without the RPP.

We investigated the influence of the flake thickness for molybdenum disulfide (MoS2) field effect transistors on the effect of a 150 keV high-energy proton beam applied on these devices. The electrical characteristics of the devices with channel thicknesses ranging from monolayer to bulk were measured before and after proton irradiation with a proton fluence of 5 × 1014 cm−2. The subthreshold swing (SS), threshold voltage shift and electron mobility were extracted with the Y-function method after proton irradiation and significant degradation were observed. It is found that, with the increase of layer thickness, mobility degradation and threshold voltage shift both eased, but the SS degradation was insensitive to the MoS2 flake thickness increase. We also demonstrate that the threshold voltage shift is dominated by oxide charges; however, the mobility and SS degradations are mainly affected by the interface traps. Our study will enhance the understanding of the influence of high-energy particles on MoS2-based nano-electronic devices. By increasing the MoS2 flake thickness to a certain extent, one can hopefully find a balance between effectively resisting V T H shift and achieving high mobility and small SS degradation.

The stability of the subthreshold swing (SS) is quite important for switch and memory applications in logic circuits. The SS in our MoS2 field effect transistor (FET) is enlarged when the gate voltage sweep range expands towards the negative direction. This is quite different from other reported MoS2 FETs whose SS is almost constant while varying gate voltage sweep range. This anomalous SS enlargement can be attributed to interface states at the MoS2–SiO2 interface. Moreover, a deviation of SS from its linear relationship with temperature is found. We relate this deviation to two main reasons, the energetic distribution of interface states and Fermi level shift originated from the thermal activation. Our study may be helpful for the future modification of the MoS2 FET that is applied in the low power consumption devices and circuits.

The physical mechanism for recovery of 2D electron gas (2DEG) in down-scaled AlGaN/GaN heterostructures with SiNx layers grown by low-pressure chemical vapor deposition (LPCVD) was investigated by means of Hall-effect characterization, scanning Kelvin probe microscopy (SKPM), and self-consistent Poisson–Schrodinger calculations. Observations using SKPM show that the surface potential of the AlGaN/GaN heterostructure remained nearly unchanged (∼1.08 eV) as the thickness of the AlGaN barrier was reduced from 18.5 to 5.5 nm and likely originated from the surface pinning effect. This led to a significant depletion of 2DEG from 9.60 × 1012 to 1.53 × 1012 cm−2, as determined by Hall measurements, toward a normally OFF 2DEG channel. Based on a consistent solution of the Schrodinger–Poisson equations and analytical simulations, approximately 3.50 × 1013 cm−2 of positive fixed charges were confirmed to be induced by a 20-nm LPCVD-SiNx passivation over the AlGaN/GaN heterostructures. The interface charge exerted a strong modulation of band bending in the down-scaled AlGaN/GaN heterostructure, contributing to the efficient recovery of 2DEG charge density (∼1.63 × 1013 cm−2). E-mode ultrathin-barrier AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors with a low ON-resistance (RON), high ON/OFF current ratio, and steep subthreshold slope were implemented using LPCVD-SiNx passivation.

A physical insight into the capture and emission behavior of interface/oxide states in a GaN-based metal-oxide-semiconductor (MOS) structure is of great importance to understanding the threshold voltage (VTH) instability in GaN power transistors. A time-dependent VTH shift in Ni/Al2O3/AlGaN/GaN MOS-HFETs (heterojunction field-effect transistors) and a distribution of Al2O3/III-nitride interface states (Dit) were successfully characterized by constant-capacitance deep level transient spectroscopy. It is found that in situ remote plasma pretreatments in plasma-enhanced atomic-layer-deposition could suppress Dit (EC-ET > 0.4 eV) down to below 1.3 × 1012 cm−2 eV−1. Under high applied gate bias (e.g., VG > 8 V), tunnel filling of oxide states in the Al2O3 dielectric comes into play, contributing to remarkable VTH instability in the MOS-HFETs. The tunnel distance between the 2D Electron Gas (2DEG) channel and oxide states ET,ox in the Al2O3 dielectric decreases from 3.75 to 0.82 nm as VG increases from 2 to 8 V. A further increase of VG to 11 V makes the Fermi level approach ET,ox (EC − ET ∼ 1.62 eV), which may enable direct filling. High electric field induced tunnel filling of gate oxide states could be an assignable cause for VTH instability in normally-OFF III-nitride MOS-HFETs.

Graphene is one of the materials with the most potential for post-silicon electronics because of its outstanding electrical, optical, and mechanical properties. However, the lack of a uniform stable doping method extremely limits the various possible applications of graphene. Here, we developed a uniform and stable graphene efficient p-doping method. Through etching a thin gold film on graphene with a KI/I2 solution, iodine complexes are produced as the dopant absorbing on the graphene surface, and induce extra holes in graphene. Utilizing this method, the graphene film can be effectively doped to p-type without producing undesirable defects, and the roughness of the graphene surface can still be maintained at an ultra-low nanoscale (RMS roughness ∼0.739 nm). The doping effectiveness can be clearly verified by the changes in the Raman spectrum, and the Dirac point shift of the graphene-based transistor, and the reduction of sheet resistance (∼27.2%). Furthermore, the substantially coincident transfer curves after 45 days reveal the long-term stable doping effects. Therefore, this doping method can exploit a way for various graphene-based applications, such as phototransistors, sensors, and organic thin-film transistors.

The quality of metal–graphene contact has a great influence on the performance of graphene-based electronic devices. In this paper, a simple, fast and accurate method is introduced to evaluate the quality of metal–graphene contact, which is based on measuring the contact end resistance. Using this method, the effect of rapid thermal annealing on the metal–graphene contact is investigated. It is common observation that before the rapid thermal annealing, contact end resistance has a large variation showing that the metal–graphene contact is in a relatively unstable state when the device is just prepared. After the rapid thermal annealing treatment, contact end resistances show a good consistency for all the devices. The results of contact end resistance are consistent with the images of scanning electron microscope observed before and after the rapid thermal annealing, which indicates that using the contact end resistance to evaluate the quality of metal–graphene contact is an efficient and effective way to predict the contact quality.

Background: In the recent years, social networking sites (SNSs, also called social media) have been adopted in organ donation campaigns, and recruiting opinion leaders for such campaigns has been found effective in promoting behavioral changes. Objective: The aim of this paper was to focus on the dissemination of organ donation tweets on Weibo, the Chinese equivalent of Twitter, and to examine the opinion leadership in the retweet network of popular organ donation messages using social network analysis. It also aimed to investigate how personal and social attributes contribute to a user’s opinion leadership on the topic of organ donation. Methods: All messages about organ donation posted on Weibo from January 1, 2015 to December 31, 2015 were extracted using Python Web crawler. A retweet network with 505,047 nodes and 545,312 edges of the popular messages (n=206) was constructed and analyzed. The local and global opinion leaderships were measured using network metrics, and the roles of personal attributes, professional knowledge, and social positions in obtaining the opinion leadership were examined using general linear model. Results: The findings revealed that personal attributes, professional knowledge, and social positions predicted individual’s local opinion leadership in the retweet network of popular organ donation messages. Alternatively, personal attributes and social positions, but not professional knowledge, were significantly associated with global opinion leadership. Conclusions: The findings of this study indicate that health campaign designers may recruit peer leaders in SNS organ donation promotions to facilitate information sharing among the target audience. Users who are unverified, active, well connected, and experienced with information and communications technology (ICT) will accelerate the sharing of organ donation messages in the global environment. Medical professionals such as organ transplant surgeons who can wield a great amount of influence on their direct connections could also effectively participate in promoting organ donation on social media. Published version

Through adjusting the focal plane of the optical microscope, we have verifi ed that these trenches on the copper foil have a con-cave cross section. It is similar with the surface topography of copper foil, whose optical image is shown in the inset panel of Figure 1 a. The surface of copper foil shows a directional texture consisting of many parallel trenches with spacing on the order of micrometers, these trenches are considered to be produced during the foil rolling process used to fabricate the Cu foil, with the trenches running parallel to the shear/drawing direction.

The contact resistance of metal/graphene is becoming a major limiting factor for graphene devices. Among various kinds of contact resistance test methods, the transmission line model is the most common approach to extract contact resistance in graphene devices. However, experiments show that in some cases there exists large inaccuracy and instability using this method. In this study, we added a cross-bridge structure at the terminal of the transmission line as a supporting test. This modified transmission line measurement structure can easily compare not only the transmission line and Kelvin contact resistance, getting a more reliable value, but also the other contact-related parameters, such as specific contact resistivity, transfer length and the graphene sheet resistance under and outside contact metal at the same time. The new measurement test is very helpful in enabling us to study the contact property accurately. The specific contact resistivity in our experiment is in the range of 2.0 × 10(-6) Ω · cm(2) and 3.0 × 10(-6) Ω · cm(2) at room temperature. With the temperature decreasing from 290 K to 60 K, the transfer length fluctuates around 1.7 μm, and the specific contact resistivity reduces to less than 2.0 × 10(-6) Ω · cm(2).

The effect of source-gate spacing on graphene filed effect transistors has been investigated. Reducing the source gate spacing allows for a significant improvement on both the direct current and radio frequency (RF) performances. Instead of the generally considered output conductance, our results suggest that the access resistances at the un-gated region contribute more to the maximum oscillation frequency (fmax). Further analysis reveals that the ratio of cut off frequency (fT) to fmax is also sensitive to the resistances at source-gate spacing. This work can be used to guide the further optimization of graphene-based RF devices.

Current saturation in graphene field-effect transistor (GFET) is of significant importance to improve the maximum oscillation frequency (fmax). We investigated the direct current (dc) and radio frequency (rf) characteristics of a heavily p-type doped GFET based on chemical vapor deposition grown material. The drain current saturation is found in our device. It cannot be explained by the “pinch-off” effect associated with ambipolar transport, but can be attributed to nonlinear channel conductance and velocity saturation in unipolar channel. This study promotes understanding the behaviors of heavily doped GFETs and their radio frequency applications.