Effect of back-gate on contact resistance and on channel conductance in graphene-based field-effect transistors

We study the contact resistance and the transfer characteristics of back-gated field effect transistors of mono- and bi-layer graphene. We measure specific contact resistivity of ~ 7 k Ω μm² and ~ 30k Ω μm² for Ni and Ti, respectively. We show that the contact resistance is a significant contributor to the total source-to-drain resistance and it is modulated by the back-gate voltage. We measure transfer characteristics showing a double dip feature that we explain as the effect of doping due to charge transfer from the contacts causing minimum density of states for graphene under the contacts and in the channel at different gate voltage.     

Pd ohmic contacts to p-SiC 4H, 6H and 15R polytypes

A new metallization process at sample temperatures up to 600°C during deposition of the metals is employed for producing low resistivity narrow Pd ohmic contacts to p-SiC epitaxial layers of 4H, 6H and 15R polytypes. It is found that the values of the specific contact resistances were equal for the same Na–Nd concentration in all investigated polytypes. On epitaxial layers with Na–Nd=4×10¹⁸ cm⁻³ a specific contact resistance of 4×10⁻⁴ Ω cm² has been measured. Electrical and structural features of these contacts were investigated.     

Microwave absorption and radiation from large-area multilayer CVD graphene

Here we experimentally study the microwave absorption and near-field radiation behavior of monolayer and few-layer, large-area CVD graphene in the C and X bands. Artificial stacking of CVD graphene reduces the sheet resistance, as verified by non-contact microwave cavity measurements and four-probe DC resistivity. The multilayer stacked graphene exhibits increased absorption determined by the total sheet resistance. The underlying mechanism could enable us to apply nanoscale graphene sheets as optically transparent radar absorbers. Near-field radiation measurements show that our present few-layer graphene patches with sheet resistance more than 600 Ω/sq exhibit no distinctive microwave resonance and radiate less electromagnetic power with increasing layers; however, our theoretical prediction suggests that for samples to be practical as microwave antennas, doped multilayer graphene with sheet resistance less than 10 Ω/sq is required.     

Carbon nanotubes horizontal interconnects with end-bonded contacts,diameters down to 50 nm and lengths up to 20 μm

This work describes a novel approach to fabricate horizontal nanotube interconnects with dimensions comparable to state-of-the-art copper interconnects. The interconnects consist of carbon nanotubes bundles with wall density ≈10¹³ cm⁻², wire lengths of tens of micrometers and wire diameters scalable to 50 nm. The nanotubes are first grown vertically, inside vias with diameters ranging from 300 to 200 nm, and then flipped on the horizontal direction. Symmetrical contacts are made at the tips of the nanotubes in the so-called end-bonded geometry via a metallization process with a key dry-etch step. The quality of the contacts and the nanotubes is evaluated from the electrical measurements by extracting the specific contact resistivity and the carbon nanotube resistivity, respectively. The measured contact resistivity is 3.9 × 10⁻⁸ Ω cm² with Pd/Au contacts. This is the lowest value ever reported so far for nanotubes contacted in an end-bonded geometry. The nanotube resistivity is as low as 1.1 mΩ cm, a value among the best reported to date and only two decades higher than that of copper.     

Hydrogen-assisted pulsed KrF-laser irradiation for the in situ photoreduction of graphene oxide films

We report on the use of pulsed KrF-laser irradiation for the in situ reduction of graphene oxide (GO) films under both vacuum and partial hydrogen pressure. By exposing GO films to 500 pulses of a KrF-laser, at a fluence of 10 mJ/cm², their sheet resistance (R_s) is dramatically reduced from highly insulating (∼10¹⁰ Ω/sq) to conductive values of ∼3 kΩ/sq. By increasing the laser fluence, from 10 to 75 mJ/cm², we were able to identify an optimal fluence around 35 mJ/cm² that leads to highly conductive films with R_s values as low as 250 Ω/sq and 190 Ω/sq, under vacuum (10⁻⁵ Torr) and 50 mTorr of H₂, respectively. Raman spectroscopy analyses confirmed the effective reduction of the KrF-laser irradiated GO films through the progressive recovery of the characteristic 2D band of graphene. Furthermore, systematic Fourier-transform infrared spectroscopy analysis has revealed that KrF-laser induced reduction of GO preferentially occurs through photodissociation and removal of carboxyl (COOH) and alcohol (OH) groups. A direct correlation is established between the electrical resistance of photoreduced GO films and their COOH and OH bond densities. The KrF-laser induced reduction of GO films is found to be more efficient under H₂ background than under vacuum. It is concluded that our KrF-laser reduced GO films mainly consist of turbostratic graphite built from randomly organized few-layers-graphene building blocks, which contains some residual oxygen atoms and defects. Finally, by monitoring the KrF-laser fluence, it is shown that reduced GO films combining optical transmission as high as ∼80% along with sheet resistance as low as ∼500 Ω/sq can be achieved with this room-temperature and on-substrate process. This makes the laser-based reduction process developed here particularly attractive for photovoltaic hybrid devices using silicon substrates.     

One-step transfer and doping of large area graphene by ultraviolet curing adhesive

The application of graphene as transparent conductive electrodes remains to be a challenge mainly due to the difficulties in transfer and doping of large-area graphene. Here we report a new one-step transfer and doping method using chemical modified ultraviolet curing adhesive. This method enables faster transfer of monolayer graphene onto polyethylene terephthalate (PET) substrates up to 17 in., with a sheet resistance of 205 Ω/□ without any additional surface doping. The sheet resistances stay stable both at 20 and 80 °C in air for 50 days. Moreover, the transmittance of the graphene/PET is 90.8%, which is only 0.9% less than that of PET substrate. Finally, a well behaved capacitive-type touch panel based on this transferred method is demonstrated.     

Micro-scale aerosol-jet printing of graphene interconnects

This article addresses the deployment and characterization of a micro-scale aerosol-jet additive manufacturing technology to print graphene interconnects. A highly concentrated graphene ink with viscosity of 21 cP and 3.1 mg/ml graphene flakes with the lateral size below 200 nm was developed and adopted for this process to make a reliable and repeatable graphene deposition on the treated Si/SiO₂ wafers. To this end, the influence of the most significant process parameters, including the atomizer power, the atomizer flow rate, and the number of the printed layers, on the size and properties of graphene interconnects was studied. Results show that the aerosol-jet printing process is capable of printing micro-scale graphene interconnects with variable widths in the range of 10–90 μm. These patterns, as the finest printed graphene patterns, with resistivity as low as 0.018 Ω cm and sheet resistance of 1.64 kΩ/□ may ease the development of miniaturized printed electronic applications of graphene.     

SDS-stabilized graphene nanosheets for highly electrically conductive adhesives

We report sodium dodecyl sulfate (SDS) stabilization of graphene nanosheets, with two different sizes as auxiliary fillers inside the conventional electrically conductive adhesive (ECA) composite. Using this non-covalent modification approach we were able to preserve the single-layer structure of graphene layers and prevent their re-stacking inside the composite, which resulted in a significant electrical conductivity improvement of ECAs at noticeably low filler content. Addition of 1.5 wt% small and large SDS-modified graphene into the conventional ECAs with 10 wt% silver flakes led to low electrical resistivity values of 5.5 × 10³ Ω cm and 35 Ω cm, respectively, while at least 40 wt% of silver flakes was required for the conventional ECA to be electrically conductive. A highly conductive ECA with very low bulk resistivity of 1.6 × 10⁻⁵ Ω cm was prepared by adding 1.5 wt% of SDS-modified large graphene into the conventional ECA with 80 wt% silver flakes which is less than that of eutectic lead-based solders.     

Reprint of "Palladium Ohmic contact on hydrogen-terminated single crystal diamond film"

Contact properties of Palladium (Pd) on the surface of hydrogen-terminated single crystal diamond were investigated with several treatment conditions. 150 nm Pd pad was deposited on diamond surface by thermal evaporation technique, which shows good Ohmic properties with the specific contact resistivity (ρ_c) of 1.8 × 10^− 6 Ω cm² evaluated by Transmission Line Model. To identify the thermal stability, the sample was annealed in Ar ambient from 300 to 700 °C for 3 min at each temperature. As the temperature increased, ρ_c firstly decreased to 4.93 × 10⁻⁷ Ω cm² at 400 °C and then increased. The barrier height was evaluated to be − 0.15 eV and − 0.03 eV for as-deposited and 700 °C annealed sample by X-ray photoelectron spectroscopy analysis. Several surface treatments were also carried out to determine their effect on ρ_c, among which HNO₃ vapor treated sample indicates a lower value of 5.32 × 10⁻⁶ Ω cm².     

Photovoltaic property dependence of dye-sensitized solar cells on sheet resistance of FTO substrate deposited via spray pyrolysis

F-doped SnO₂ (FTO) glass substrate was successfully fabricated via spray-pyrolysis deposition for use as a transparent conducting substrate in dye-sensitized solar cells (DSSCs). To investigate the performance dependence of DSSCs on the sheet resistance of the FTO films, three types of FTO films with sheet resistance values of 2 Ω/□, 4 Ω/□, and 10 Ω/□ were fabricated. Commercial FTO films having a sheet resistance of 15 Ω/□ were prepared for comparison. The structural, electrical, and optical properties of FTO films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), the four-point probe method, and UV–vis spectrometry. The photocurrent–voltage data show that DSSCs fabricated with a sheet resistance of 2 Ω/□ exhibit the best photoconversion effciency (～5.5%) among the four samples. The performance improvement of DSSCs is due to improved short-circuit current density (～13.7 mA/cm²) and fill factor (～62.3%).     

Exfoliated graphite with relative dielectric constant reaching 360, obtained by exfoliation of acid-intercalated graphite flakes without subsequent removal of the residual acidity

Exfoliated graphite (obtained by rapid heating of sulfuric-acid intercalated and subsequently deacidified graphite flakes) is optionally subjected to residual acidity removal, which involves repeated washing with water, such that the pH of the wash water increases from 2 to 7. Compared to washed exfoliated graphite, the unwashed material exhibits lower specific surface area (24 vs. 45 m²/g), a higher value (360 vs. 38 at 50 Hz) of the relative dielectric constant (real part), a similar value of the conductivity (50 S/m), a higher value of the specific carbon–contact interfacial capacitance (1.17 vs. 0.04 μF/m²), and a lower value of the carbon–contact interfacial resistivity (0.08 vs. 0.27 Ω cm²). The greater concentration of residual intercalate (containing sulfur and oxygen) present without washing contributes to the polarizability without interfering the conduction. The carbon–contact interface is superior when the exfoliated graphite has not been washed. At 2.0 MHz, the relative dielectric constant (real part) remains high (280) and the carbon–contact interfacial specific capacitance remains high (1.13 μF/m²). The imaginary part of the relative dielectric constant and the dielectric loss angle are not affected by the washing. The relative dielectric constant of 360 is even higher than the value of 121 for potassium-hydroxide-activated graphite nanoplatelet.     

Single-walled carbon nanotube/silicone rubber composites for compliant electrodes

Single-walled carbon nanotube (SWCNT)/silicone rubber composites that can be used in fabricating compliant electrodes are prepared by spraying a mixed solution of ionic-liquid-based SWCNT gel and silicone rubber onto an elastic substrate. Subsequently, the composites are exposed to nitric acid vapor. Scanning electron microscopy and atomic force microscopy images of the composites show that the SWCNTs are finely dispersed in the polymer matrix due to the addition of the ionic liquid. Doping of the SWCNTs by nitric acid can significantly lower the sheet resistance (R_s) of the composites; samples with 4 wt% of SWCNT content exhibit the lowest R_s value (50 Ω sq^?1). This sheet resistance corresponds to a conductivity value of 63 S cm^?1. In addition, the composites retain a high conductivity after several tensile strains are applied. Stretching the composite sample to 300% of the original length increased the R_s value to 320 Ω sq^?1 (19 S cm^?1). Even after 20th stretch/release/stretch cycle, the conductivity remains constant at a value of 18 S cm^?1. These results provide a scalable route for preparing highly stretchable and conductive SWCNT composites with relatively low SWCNT concentrations.     

Electrical properties of ultrananocrystalline diamond/amorphous carbon nanocomposite films

The electrical surface properties of ultrananocrystalline diamond/amorphous carbon composite films have been investigated by four-point probe I/V and Hall measurements, whereas impedance spectroscopy has been used to establish the electrical bulk properties of the films. It turned out that the surface is p-type conductive with a resistivity of 0.14 Ω cm and a sheet carrier concentration of 7.6 × 10¹³ cm^?2. The bulk resistivity is higher by almost seven orders of magnitude (1.3 × 10⁶ Ω cm). The bulk conduction is thermally activated with an apparent activation energy of 0.17 eV. From Cole–Cole plots of the impedance spectra it can be concluded that there are three different contributions to the bulk conductivity. In order to try to identify these three components contributing to the electrical bulk conduction, Raman spectra have been recorded at five different wavelengths from the IR to UV region. These measurements showed that the UNCD/a-C films consist of at least three components: diamond nanocrystallites, an amorphous carbon matrix, and trans-polyacetylene-like structures probably at the interface between these two.     

Formation of a heavily B doped diamond layer using an ion implantation technique

In this report, we present a study on lattice and electronic structures of B doped layers formed using B implantation into diamond. Boron layers were produced using the multiple-energy B ion implantation (total dose: 2.1 × 10¹⁵ to 1.7 × 10¹⁷ cm^− 2) into type IIa diamond at ~ 400 °C. Optical absorption and Hall effects were measured in the range of 80−1000 K for investigating the change of the lattice and electronic structures with the B concentration in diamond. The p-type carrier conduction was observed at 80−1000 K in all the samples. While a lightly B doped sample displays typical semiconductive, temperature-dependent valence-band conduction, heavily B doped samples have the very weak or almost zero temperature dependence of the carrier concentrations, resistivity and Hall mobility in this temperature region, suggesting characteristics of a p-type degenerate semiconductor. In such heavily doped samples, broad optical absorption bands, most likely corresponding to Drude absorption originating from free holes, were observed. The minimum resistivity and the sheet resistance at room temperature among the samples were 1.4 mΩcm and 56 Ω/□, respectively. These results indicate that very low-resistive p-type degenerate semiconducting layers were produced, preserving diamond lattice (preventing graphitization), despite high-dose ion irradiation.     

Improving the electrical properties of niobium-doped titania sputtering targets by sintering in oxygen-deficient atmospheres

Niobium-doped titania (TNO) film can be used as a transparent conductive oxide (TCO) film due to its excellent conductivity and visible transparency. The performances of TNO sputtering targets are thus critical issues in optimizing sputtered films. This study clarifies the influences of inert and reducing atmospheres on the microstructure, densification, crystal structure, and electrical properties of TNO sputtering targets. The results indicate that a sintering atmosphere of 90% Ar–10% H₂ can result in a lower sintered density, larger grain size, and lower resistivity than can an atmosphere of Ar, followed by one of air. Sintering in 90% Ar–10% H₂ or Ar obviously decreases the resistivity of TiO₂, from >10⁸ Ω cm to <10⁻¹ Ω cm, and the TNO target, from >10¹ Ω cm to <10⁻¹ Ω cm. The resistivity of TNO target sintered at 1200 °C in 90% Ar–10% H₂ is as low as 1.8×10⁻² Ω cm.     

Ion implantation and anneal to produce low resistance metal–diamond contacts

Contacts to boron-doped, (100)-oriented diamond implanted with Si or with Si and B were formed and the effects of dose, implantation energy and anneal treatment on the specific contact resistance were examined. Ti/Au contacts on heavily implanted diamond (10¹⁶ Si ions cm⁻², E_i=30 keV or 10¹⁷ Si and B ions cm⁻², E_i=15 keV (Si) and E_i=10 keV (B)) had a specific contact resistance lower than the best contacts produced on unimplanted diamond. A specific contact resistance of (1.4±6.4)×10⁻⁷ Ω cm⁻² was achieved following a 450°C anneal. The results were consistent with a reduction in barrier height brought about by silicide formation. Light silicon implantation (10¹³ ions cm⁻²) or relatively light dual implantation (B, Si<10¹⁶ ions cm⁻²) did not reduce the specific contact resistance. Increasing the diamond conductivity by 4×10⁴ decreased the specific contact resistance by over three orders of magnitude, in agreement with the trend observed by Prins (J.F. Prins, J. Phys. D 22 (1989) 1562).     

Application of nitrogen-doped graphene nanosheets in electrically conductive adhesives

Nitrogen-doped graphene nanosheets (N-GNSs) were used as a conductive filler for a polymer resin adhesive and as a performance improver for a silver-filled electrically conductive adhesive (ECA). The N-GNS samples were prepared by the chemical-intercalation/thermal-exfoliation of graphite followed by a thermal treatment in NH₃. Only 1 wt.% of N-GNSs was required for the adhesive to reach a percolation threshold, and the performance using N-GNSs was much better than that obtained using carbon black or multi-walled carbon nanotubes (MWCNTs). The effect of N-GNS or MWCNT additives on reducing the electrical resistivity of Ag-particle filled ECAs at low Ag loading ratios was also investigated. With 30 wt.% of Ag filler, the polymer resin was still non-conducting, while a resistivity of 4.4 × 10⁻² Ω-cm was obtained using an Ag/N-GNS hybrid filler fortified with only 1 wt.% of N-GNSs due to large specific surface area, high aspect ratio, and good electrical conductivity of the doped graphene.     

Bilayer graphene synthesis by plasma treatment of copper foils without using a carbon-containing gas

Bilayer graphene has been synthesized by using hydrogen plasma treatment of copper foils for 30 s at the temperature of 850 °C together with joule-heating treatment of the foils without using a carbon-containing gas such as methane in order to suppress the nucleation density of graphene. The effect of plasma provides active species of carbon atoms on copper substrate and a selective bilayer graphene formation of AB-stacking in a very short time. Carbon to be precipitated is delivered from the copper foil and/or the environment in the reaction chamber. The domain size of synthesized graphene, the controllability of a few layers and the electrical conductivity have been significantly improved compared with plasma chemical vapor deposition (CVD) using carbon-containing gas. The sheet resistance of bilayer graphene exhibits 951 Ω in average. The carrier mobility shows 1000 cm²/V s in maximum at room temperature. The sheet resistance of 130 ± 26 Ω has been attained after the doping by gold chloride solution.     

Activation of graphene aerogel with phosphoric acid for enhanced electrocapacitive performance

We present a facile and efficient route to introduce in-plane nanopores on the graphene sheets by activation of graphene aerogel (GA) with phosphoric acid (H₃PO₄). Results from N₂ adsorption and TEM images showed that H₃PO₄ activation created mesopores with pore size of 2–8 nm on the graphene sheets. With such nanopores on graphene sheets, the activated GA exhibits a specific capacitance of 204 F g⁻¹, enhanced rate capability (69% capacitance retention from 0.2 to 30 A g⁻¹), reduced equivalent series resistance (3.8 mΩ) and shortened time constant (0.73 s) when comparing with the hydrothermally-derived pristine GA and thermally annealed GA in the absent of H₃PO₄. The excellent capacitive properties demonstrate that introduction of nanopores on GA by H₃PO₄ activation not only provides large ion-accessible surface area for efficient charge storage, but also promotes the kinetics of electrolyte across the graphene two-dimensional planes.