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Graphene metrology needs to keep up with the fast pace of developments in graphene growth and transfer. Terahertz time-domain spectroscopy (THz-TDS) is a non-contact, fast, and non-destructive characterization technique for mapping the electrical properties of graphene. Here we show several case studies of graphene characterization on a range of different substrates that highlight the versatility of THz-TDS measurements and its relevance for process optimization in graphene production scenarios.

International audience; Atomically thin 2D materials have drawn considerable attention in the past years with potential ranging from transistors to optoelectronics. As such, they are now foreseen as strong candidates for epitaxy-free technologies and the tetrad of size-weight-power-and-cost (SWAP-C) reduction. Targeting radiofrequency (RF) applications, the 2D semiconducting transition metal dichalcogenides (TMDC) family can offer the opportunity of wide tunability of their electronic properties, providing a large variety of band gaps. However, evaluation and integration of those materials into discrete components requires a stabilization of their properties. This work focuses on the evaluation of a large-scale compatible fabrication/passivation process on large area (>1000 μm2) monolayers of the prototypical 2D semiconductor MoS2. The process is developed including pre- and post-patterning protection/passivation layers. It is shown to reduce the initial natural p-doping of the sample, leading to lower transistor threshold voltages, a 10^6 ION/IOFF ratio, and an effective averaged field-effect mobility under ambient conditions of 20 cm2 V−1 s−1 (up to 35 cm2 V−1 s−1 for some devices), which represents an increase by a 40-fold factor compared to a conventional process carried on the large scale platform. This work represents an important step toward the integration of 2D TMDCs in discrete RF circuits and components.

International audience; We report on the achievement of a large-scale tungsten disulfide (WS2) 2D semiconducting platform derived by pulsed-laser deposition (PLD) on both insulating substrates (SrTiO3), as required for in-plane semiconductor circuit definition, and ferromagnetic spin sources (Ni), as required for spintronics applications. We show thickness and phase control, with highly homogeneous wafer-scale monolayers observed under certain conditions, as demonstrated by X-ray photoelectron spectroscopy and Raman spectroscopy mappings. Interestingly, growth appears to be dependent on the substrate selection, with a dramatically increased growth rate on Ni substrates. We show that this 2D-semiconductor integration protocol preserves the interface integrity. Illustratively, the WS2/Ni electrode is shown to be resistant to oxidation (even after extended exposure to ambient conditions) and to present tunneling characteristics once integrated into a complete vertical device. Overall, these experiments show that the presented PLD approach used here for WS2 growth is versatile and has a strong potential to accelerate the integration and evaluation of large-scale 2D-semiconductor platforms in electronics and spintronics circuits.

We report on spin transport in WS

International audience

International audience; We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spec-troscopy. The WS2 layers are then integrated in complete Co/Al2O3 /WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi-and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.

© 2016 American Chemical Society. The remarkable properties of graphene, such as broadband optical absorption, high carrier mobility, and short photogenerated carrier lifetime, are particularly attractive for high-frequency optoelectronic devices operating at 1.55 μm telecom wavelength. Moreover, the possibility to transfer graphene on a silicon substrate using a complementary metal-oxide-semiconductor-compatible process opens the ability to integrate electronics and optics on a single cost-effective chip. Here, we report an optoelectronic mixer based on chemical vapor-deposited graphene transferred on an oxidized silicon substrate. Our device consists in a coplanar waveguide that integrates a graphene channel, passivated with an atomic layer-deposited Al2O3 film. With this new structure, 30 GHz optoelectronic mixing in commercially available graphene is demonstrated for the first time. In particular, using a 30 GHz intensity-modulated optical signal and a 29.9 GHz electrical signal, we show frequency downconversion to 100 MHz. These results open promising perspectives in the domain of optoelectronics for radar and radio-communication systems.

We report on the large-scale integration of graphene layers over a FePd perpendicular magnetic anisotropy (PMA) platform, targeting further downscaling of spin circuits. An L10 FePd ordered alloy showing both high magneto-crystalline anisotropy and a low magnetic damping constant, is deposited by magnetron sputtering. The graphene layer is then grown on top of it by large-scale chemical vapor deposition. A step-by-step study, including structural and magnetic analyses by x-ray diffraction and Kerr microscopy, shows that the measured FePd properties are preserved after the graphene deposition process. This scheme provides a graphene protected perpendicular spin electrode showing resistance to oxidation, atomic flatness, stable crystallinity, and perpendicular magnetic properties. This, in turn, opens the way to the generalization of hybrid 2D-materials on optimized PMA platforms, sustaining the development of spintronics circuits based on perpendicular spin-sources as required, for instance, for perpendicular-magnetic random-access memory schemes.

International audience; In this work we demonstrate thermal rectification at the nanoscale between doped Si and VO2 surfaces. Specifically, we show that the metal–insulator transition of VO2 makes it possible to achieve large differences in the heat flow between Si and VO2 when the direction of the temperature gradient is reversed. We further show that this rectification increases at nanoscale separations, with a maximum rectification coefficient exceeding 50% at ∼140 nm gaps and a temperature difference of 70 K. Our modeling indicates that this high rectification coefficient arises due to broadband enhancement of heat transfer between metallic VO2 and doped Si surfaces, as compared to narrower-band exchange that occurs when VO2 is in its insulating state. This work demonstrates the feasibility of accomplishing near-field-based rectification of heat, which is a key component for creating nanoscale radiation-based information processing devices and thermal management approaches.

© 2016 Author(s).We report on statistical analysis and consistency of electrical performances of devices based on a large scale passivated graphene platform. More than 500 graphene field effect transistors (GFETs) based on graphene grown by chemical vapor deposition and transferred on 4 in. SiO2/Si substrates were fabricated and tested. We characterized the potential of a two-step encapsulation process including an Al2O3 protection layer to avoid graphene contamination during the lithographic process followed by a final Al2O3 passivation layer subsequent to the GFET fabrication. Devices were investigated for occurrence and reproducibility of conductance minimum related to the Dirac point. While no conductance minimum was observed in unpassivated devices, 75% of the passivated transistors exhibited a clear conductance minimum and low hysteresis. The maximum of the device number distribution corresponds to a residual doping below 5 × 1011 cm−2 (0.023 V/nm). This yield shows that GFETs integrating low-doped graphene and exhibiting small hysteresis in the transfer characteristics can be envisaged for discrete components, with even further potential for low power driven electronics.

Due to its remarkable properties, graphene-based devices are particularly promising for optoelectronic applications. Thanks to its compatibility with standard silicon technology, graphene could compete III-V compounds for the development of low cost and high-frequency optoelectronic devices. We present a new optoelectronic device that consists in a coplanar waveguide integrating a commercially-available CVD graphene active channel. With this structure, we demonstrate high-frequency (30 GHz) broadband optoelectronic mixing in graphene, by measuring the response of the device to an optical intensity-modulated excitation and an electrical excitation at the same time. These features are particularly promising for RADAR and LIDAR applications, as well as for low-cost high-speed communication systems.

The magnetic microstructure and domain wall distribution of antiferromagnetic α-Fe2O3 epitaxial layers is determined by statistical image analyses. Using dichroic spectromicroscopy images, we demonstrate that the domain structure is statistically invariant with thickness and that the antiferromagnetic domain structure of the thin films is inherited from the ferrimagnetic precursor layer one, even after complete transformation into antiferromagnetic α-Fe2O3. We show that modifying the magnetic domain structure of the precursor layer is a genuine way to tune the magnetic domain structure and domain walls of the antiferromagnetic layers.

International audience; We demonstrate a stabilized black phosphorus (BP) 2D platform thanks to an ultrathin MgO barrier, as required for spintronic device integration. The in-situ MgO layer deposition is achieved by using a large-scale atomic layer deposition process with high nucleation density. Raman spectroscopy studies show that this layer protects the BP from degradation in ambient conditions, unlocking in particular the possibility to carry out usual lithographic fabrication steps. The resulting MgO/BP stack is then integrated in a device and probed electrically, confirming the tunnel properties of the ultrathin MgO contacts. We believe that this demonstration of a BP material platform passivated with a functional MgO tunnel barrier provides a promising perspective for BP spin transport devices.