Properties of graphene deposited on GaN nanowires: influence of nanowire roughness, self-induced nanogating and defects

We present detailed Raman studies of graphene deposited on gallium nitride nanowires with different variations in height. Our results indicate that different density and height of nanowires impact graphene properties such as roughness, strain, and carrier concentration as well as density and type of induced defects. Tracing the manifestation of those interactions is important for the application of novel heterostructures. A detailed analysis of Raman spectra of graphene deposited on different nanowire substrates shows that bigger differences in nanowires height increase graphene strain, while a higher number of nanowires in contact with graphene locally reduces the strain. Moreover, the value of graphene carrier concentration is found to be correlated with the density of nano wires in contact with graphene. The lowest concentration of defects is observed for graphene deposited on nanowires with the lowest density. The contact between graphene and densely arranged nanowires leads to a large density of vacancies. On the other hand, grain boundaries are the main type of defects in graphene on rarely distributed nanowires. Our results also show modification of graphene carrier concentration and strain by different types of defects present in graphene. Therefore, the nanowire substrate is promising not only for strain and carrier concentration engineering but also for defect engineering.This work was partially supported by the Ministry of Science and Higher Education in 2015-2019 as a research grant "Diamond Grant" (n degrees. DI2014 015744). The GaN nanowires were grown within the Polish National Science Centre (grants n degrees. UMO-2016/21/N/ST3/03381 and 2016/23/B/ST7/03745). This work was supported by the Research Foundation Flanders (FWO) under grant n degrees. EOS 30467715

Electrostatically-induced strain of graphene on GaN nanorods

Few-layer graphene deposited on semiconductor nanorods separated by undoped spacers has been studied in perspective for the fabrication of stable nanoresonators. We show that an applied bias between the graphene layer and the nanorod substrate affects the graphene electrode in two ways: 1) by a change of the carrier concentration in graphene and 2) by inducing strain, as demonstrated by the Raman spectroscopy. The capacitance of the investigated structures scales with the area of graphene in contact with the nanorods. Due to the reduced contact surface, the efficiency of graphene gating is one order of magnitude lower than for a comparable structure without nanorods. The shift of graphene Raman modes observed under bias clearly shows the presence of electrostatically-induced strain and only a weak modification of carrier concentration, both independent of number of graphene layers. A higher impact of bias on strain was observed for samples with a larger contact area between the graphene and the nanorods which shows perspective for the construction of sensors and nanoresonator devices

Tunable Carbon-Based Nanomaterials for THz Applications

In this work single-walled carbon nanotube tuning properties are studied for phase shifting applications. The dielectric rod waveguide with loaded carbon nanomaterials was experimentally studied in ultra-wide frequency band of 0.1-0.5 THz

Sub‐THz Phase Shifters Enabled by Photoconductive Single‐Walled Carbon Nanotube Layers

Materials with tunable dielectric properties are highly relevant for terahertz (THz) applications. Herein, the tuning of the dielectric response of single-walled carbon nanotube layers by light illumination is studied for applications to THz phase shifters. The dependence of the length of individual nanotubes on the THz photoconductivity of the network is experimentally investigated in the frequency range of 0.2–1 THz by time-domain spectroscopy (TDS). The effective conductivity of the networks is described by a theoretical model that fits the measured dielectric function. Terahertz phase shifters are realized with the carbon nanotube layers as the optically tunable element deposited on the wall of rectangular dielectric waveguides. The phase of the electromagnetic wave propagating in the waveguide is shown to be tunable by illuminating the nanotubes. A linear phase shift with the frequency is measured between 75 and 500 GHz with a low change in amplitude due to the illumination.QC 20221110</p

Graphene/AlGaN/GaN RF Switch

RF switches, which use a combination of graphene and two-dimensional high-density electron gas (2DEG) in the AlGaN/GaN system, were proposed and studied in the frequency band from 10 MHz to 114.5 GHz. The switches were integrated into the coplanar waveguide, which allows them to be used in any system without the use of, e.g., bonding, flip-chip and other technologies and avoiding the matching problems. The on-state insertion losses for the designed switches were measured to range from 7.4 to 19.4 dB, depending on the frequency and switch design. Although, at frequencies above 70 GHz, the switches were less effective, the switching effect was still evident with an approximately 4 dB on–off ratio. The best switches exhibited rise and fall switching times of ~25 ns and ~17 ns, respectively. The use of such a switch can provide up to 20 MHz of bandwidth in time-modulated systems, which is an outstanding result for such systems. The proposed equivalent circuit describes well the switching characteristics and can be used to design switches with required parameters

Graphene as a Schottky Barrier Contact to AlGaN/GaN Heterostructures

Electrical and noise properties of graphene contacts to AlGaN/GaN heterostructures were studied experimentally. It was found that graphene on AlGaN forms a high-quality Schottky barrier with the barrier height dependent on the bias. The apparent barrier heights for this kind of Schottky diode were found to be relatively high, varying within the range of &phi;b = (1.0&ndash;1.26) eV. AlGaN/GaN fin-shaped field-effect transistors (finFETs) with a graphene gate were fabricated and studied. These devices demonstrated ~8 order of magnitude on/off ratio, subthreshold slope of ~1.3, and low subthreshold current in the sub-picoamperes range. The effective trap density responsible for the 1/f low-frequency noise was found within the range of (1&ndash;5) &middot; 1019 eV&minus;1 cm&minus;3. These values are of the same order of magnitude as reported earlier and in AlGaN/GaN transistors with Ni/Au Schottky gate studied as a reference in the current study. A good quality of graphene/AlGaN Schottky barrier diodes and AlGaN/GaN transistors opens the way for transparent GaN-based electronics and GaN-based devices exploring vertical electron transport in graphene

Characterization of silver nanowire layers in the terahertz frequency range

Funding Information: Funding: The work was supported by the European Union’s Horizon 2020 FET Open project TERAmeasure (grant agreement No 862788), by the “International Research Agendas” program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund (No. MAB/2018/9), by the statutory sources of the Department of Structural Materials, Military University of Technology (project no. UGB 22–846/2021/WAT) and by the Ministry of Science and Higher Education of the Russian Federation (project no. FSRR-2020-0004), (Igor S. Nefedov). A. Krajewska was supported by the Foundation for Polish Science (FNP). Publisher Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude–Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range—significant for future wireless communication devices.Peer reviewe