Search

International audience; This article presents the small-signal and noise characterization of different technologies based on chemical vapor deposition (CVD) and silicon-carbide (SiC) graphene field-effect transistors (GFETs). The noise model, built on noise figure measurements under 50 Ω using the F 50 method, was verified by additional source-pull measurements, with special care for the GFET stability. The four noise parameters were then extracted by using the validated F 50 model up to 18 GHz, and the correlations between noise and small-signal parameters were shown for two different configurations: top-gated and back-gated GFETs.

Applied Sciences 10(6), 2183 (2020). doi:10.3390/app10062183, Published by MDPI, Basel

oral; International audience; In the framework of the European project Flagship Graphene, we have developed several process technologies, for fabricating graphene field-effect transistors, for radio frequency (RF) applications. Depending on the technique used to synthesize graphene, different transistors topologies were designed, given rise to different applications. Graphene materials under consideration include graphene growth on silicon carbide, graphene growth by chemical vapor deposition (CVD) on copper foil. After fabrication of transistors on rigid and on flexible substrates, high frequency characterization of devices is made. Based on the performance of transistors, RF circuits where designed and fabricated.

Current rectifying p–n junction devices are demonstrated from solution exfoliated two-dimensional (2D) molybdenum disulfide (MoS2), and excitonic effects are elucidated for solution dispersions of not only MoS2, but also another refractory 2D sulfide, tungsten disulfide (WS2). The excitonic enhancement effects are correlated to new solution chemistries using environmentally friendly terpineol. The role of sonication time and centrifugation are analyzed in the presence of terpineol with isopropyl alcohol and surfactant ethyl cellulose, where the red-shift in the excitonic peaks is correlated to the size distribution of the nanoparticles using optical interferometry. This was correlated to the data obtained using photoluminescence, Raman spectroscopy, scanning electron microscopy and particle size analysis which also yielded results that were consistent with this finding. The terpineol dispersion exhibits the least red-shift of 5 meV from the top to the bottom of the vial, in contrast to non-terpineol dispersions where the red-shift is calculated to be as high as 90 meV, indicating terpineol's effectiveness in exfoliating a larger population of mono-to few-layer nanomembranes. Analysis of the optical absorption spectra allows for the extraction of the energy band gap for MoS2 and WS2. These results clearly show evidence of quantum confinement effects in solution dispersions of chemically exfoliated 2D MoS2 and WS2 which can be harnessed for a wide variety of optoelectronic devices that are amenable to scalable and high-throughput synthesis routes, using environmentally friendly solution chemistries.

For optoelectronic application, two-dimensional materials such as molybdenum disulfide (MoS2) are very promising candidate with their interesting electronic and optical properties. The layered structure of these materials makes them amenable to mechanical exfoliation to form scalable 2D atomic crystals. For width range of applications, liquid phase exfoliation using sonication and centrifugation in appropriate solvent is needed. This simple and scalable technique gives high quality of exfoliation of 2D materials without chemical reactions. In this paper, we report an example of the optical and electronic characterizations on MoS2 synthesized by liquid exfoliation in specific solvent.

Transition metal dichalcogenides (TMDs) are emerging among the potential alternatives to graphene. The monolayer of TMDs can easily be exfoliated mechanically and their electronic properties can also be tuned by controlling the number of layers. TMDs possess an advantage over graphene by controlling band gap magnitude appropriate for the electronic and optoelectronic applications. Here we show, mechanically exfoliated TMDs such as NbSe2 and MoTe2 exhibit metallic and fluctuating conductance behavior respectively. Metallic conduction in NbSe2 was investigated under atmospheric conditions and compered with vacuum conditions. Furthermore, NbSe2 resistance was measured at low temperature up to 5.6 K. The above electronic investigations clearly demonstrate ohmic and fluctuating conduction in NbSe2 and MoTe2 respectively which could be applicable for electronic and optoelectronic devices.

We studied nonlinear absorption characteristics of exfoliated Molybdenum disulphide (MoS2) dispersion in 1-Methyl-2-pyrrolidinone and demonstrate a dual absorption characteristic at 532nm nanosecond pulsed laser wavelength. A number of recent reports demonstrate a saturable absorption in MoS2 and other 2D materials at low fluences and a deviation from this saturable absorption at higher fluence using open aperture Z scan (OZ scan) technique. It has been suggested that this deviation at higher fluences is due to nonlinear optical scattering. We have recently developed a new technique which combines OZ scan and photoacoustic Z-scan (PAZ-scan). It can measure photoacoustic and optical transmission signals simultaneously. The data obtained from both signals are employed to find nonlinear absorption parameters in non-linear optical materials. Our results reveal that non-linear scattering is not the cause of deviation of 2D materials from saturable absorption at higher fluences. We propose that the optical limiting behavior at higher fluence in these 2D materials is dominated by free carrier absorption.

International audience; Graphene is a promising candidate as channel material for flexible wearable radio frequency devices. In this work we fabricated double gate flexible graphene field effect transistors and characterized their DC and RF performance. Moreover, we performed a fatigue test consisting on a 1000 times dynamical bending at 1 Hz. The banding radius was 40 mm, which correspond to a strain of 0.16%. The DC and RF characterization shows the device performance variation is around 10%. The finding demonstrates the robustness of our GFETs, further work will be needed to determine the physical mechanism that cause the performance change.

Two-dimensional graphene grown by chemical vapor deposition (CVD) without the sublimation of the silicon of SiC substrate was used to fabricate field effect transistors. Atomic force microscopy and Raman spectroscopy measurements show the high quality of the graphene. The study of DC, radio frequency and microwave noise characteristics demonstrate reasonable extrinsic value of transconductance $(g_{m})$ , current gain cut-off frequency $(f_{T})$ and minimum noise figure $(NF_{min})$ related to the transistor dimension. For devices with gate length $\pmb{L_{g}=150}$ nm, the transistors show extrinsic current gain cut-off frequency $\pmb{f_{T_{-}extr}=65\mathrm{GHz}}$ associated to the maximum frequency of oscillation $\pmb{f_{max}=28\mathrm{GHz}}$ . The measurement of noise performance shows $\pmb{NF_{min}=4\mathrm{dB}}$ @ 10 GHz.

npj 2D materials and applications 2(1), 5 (2018). doi:10.1038/s41699-018-0051-9, Published by Nature Publishing Group, London

International audience; In this paper we present high frequency field effect transistors based on graphene nanoribbons arrays (GNRFETs). The nanoribbons serve as a channel for the transistors and are fabricated with a process based on e-beam lithography and dry etching of high mobility hydrogen intercalated epitaxial graphene. The widths of the nanoribbons vary from 50 to 20 nm, less than half those measured in previous reports for GNRFETs. Hall measurements reveal that the devices are p-doped, with mobility on the order of 2300 cm(2)/Vs. From DC characteristics, we find that the maximum ratio I-MAX/I-MIN is 5 obtained at 50 nm ribbons width. The IV characteristics of the GNRFETs are slightly non-linear at high bias without a full saturation. Therefore, despite the aggressive scaling of the graphene nanoribbon width, a bandgap is still not observed in our measurements. The high frequency performances of our GNRFETs are already significant at low bias. At 300 mV drain source voltage, the highest intrinsic (extrinsic) cut-off frequency f(t) reaches 82 (18) GHz and the extrinsic maximum oscillation frequency f(max) is 20 GHz, which is promising for low power applications.

Graphene material exhibits a number of outstanding electronic and mechanical properties that make it very attractive for micro and nanoelectronic applications. Considering graphene field effect transistors (GFETs), considerable efforts were made during the recent years, and the devices at state of the art show impressive high frequency cut-off frequencies. The interest of GFET is particularly strong for some high frequency (HF) applications such as high linearity mixer in W-band, THz detection, …. In this paper we will give an overview on the GFETs and will present the devices developed in our Lab. Based on high frequency measurements of noise characteristics of our GFETs, we will point out the impact of technological aspects (such as contact resistance, device structure) on the HF noise performances. The implications of our findings for the development of graphene-based circuit such as low noise amplifiers will be also discussed.

Two-dimensional black phosphorous (BP) is a novel material with great potential for implementation in a new generation of flexible and optoelectronics devices. 2D BP, an intrinsically p-type semiconductor with high hole mobility, has a hexagonal honeycomb structure with strong anisotropic properties, including mechanical, thermal, electrical, and optical properties, along the zigzag against the armchair direction. In contrast with other semiconductor layered materials, such as Transition Metal Dichalcogenides (TMDs), the band gap in black phosphorous remains direct both in bulk as in monolayer. Also, as the number of layers is reduced the band gap is open up from ~ 0.3 eV in bulk to ~ 2 eV or more in monolayer, opposite effect in TMDs. BP exhibits a fast photoresponse, it can be operated in near infrared spectrum, and the photodetection can be tunable with an external electric field. In this work, we have mechanically exfoliated multilayer black phosphorus. We have conducted a stability and degradation study of the exfoliated membranes when exposed they are exposed to air and high temperatures up to 400 °C. In addition, a two-terminal broadband photodetector has been designed and fabricated based on multilayer black phosphorus. The optoelectrical measurements exhibit relatively high electrical transport levels (~ 100 uA at 1 V) compared to previous reports. The photoresponse of our device is analyzed here where the photocurrent was measured as a function of the source-drain bias voltages.

Thin film production with solution-based materials can complement or replace semiconductors in high performance and low cost thin film devices. The possibility of inkjet printing to eventually create devices, is explored in this work.

The two dimensional layered crystals represent promising candidate materials for enabling novel photonics and nanoelectronics devices. In this paper, we present our early work on deposition and characterization of functional 2D materials.

Correction for ‘On the chemically-assisted excitonic enhancement in environmentally-friendly solution dispersions of two-dimensional MoS2 and WS2’ by Dalal Fadil et al., J. Mater. Chem. C, 2017, DOI: 10.1039/c7tc01001j.

International audience; We have observed the in-plane magnetic domain arrangement during magnetization reversal in a 40 nm thick La0.7Sr0.3MnO3/SrTiO3 (001) thin film patterned into 500 lm long microbridges of width 50 or 100 lm. Magneto-optical Kerr effect microscopy was used at room temperature and magnetic hysteresis loops were deduced from local averaging of intensity over the microbridge areas. Magnetization reversal proceeds by nucleation and propagation of 180 domain walls. When the magnetic field was applied parallel to the bridge, we observed the nucleation of only one or two domain walls and the reversal occurred by the propagation of them. When the magnetic field was applied perpendicular to the bridge, the reversal occurred mostly by the nucleation of several domain walls. The low field magnetoresistance (MR) and the low frequency noise at zero magnetic field were measured at room temperature. In addition to the linear and reversible colossal MR effect, hysteretic MR versus magnetic field curves could be observed, showing two maxima (minima) when the magnetic field is parallel (perpendicular) to the bridge length. The observed hysteretic MR behaviour is attributed to anisotropic MR inside the 180 Ne'el domain walls.

The paper proposes a structure of a new low noise high output impedance DC current source which acts as a quasi‐ideal source for frequencies lower than 10 kHz. It uses a feed‐back structure with low noise operational amplifiers. The DC current, the small signal output impedance and the noise level are controlled by the value of one single resistance. The performances of our structure are compared with the ones of the simple current source that consists in a voltage source and a resistance in series. It is shown that our structure exhibits the same noise level whereas the output impedance can be one thousand time higher.

We report low frequency noise measurements performed in patterned La0.7Sr0.3MnO3 (LSMO) thin films of various thicknesses (20 nm up to 200 nm) deposited onto SrTiO3(STO) or buffered Silicon substrates. From an application point of view, the latter is very important because it demonstrates the possibility to integrate LSMO sensors or devices with standard industrial microelectronic circuits. The noise level in LSMO on buffered Si substrate is about 2 orders of magnitude higher than on STO substrates. Contact noise is also given.