Philippe Godignon

Optical nano-imaging of gate-tunable graphene plasmons

The ability to manipulate optical fields and the energy flow of light is central to modern information and communication technologies, as well as quantum information processing schemes. However, because photons do not possess charge, a way of controlling them efficiently by electrical means has so far proved elusive. A promising way to achieve electric control of light could be through plasmon polaritons—coupled excitations of photons and charge carriers—in graphene. In this two-dimensional sheet of carbon atoms, it is expected that plasmon polaritons and their associated optical fields can readily be tuned electrically by varying the graphene carrier density. Although evidence of optical graphene plasmon resonances has recently been obtained spectroscopically, no experiments so far have directly resolved propagating plasmons in real space. Here we launch and detect propagating optical plasmons in tapered graphene nanostructures using near-field scattering microscopy with infrared excitation light. We provide real-space images of plasmon fields, and find that the extracted plasmon wavelength is very short—more than 40 times smaller than the wavelength of illumination. We exploit this strong optical field confinement to turn a graphene nanostructure into a tunable resonant plasmonic cavity with extremely small mode volume. The cavity resonance is controlled in situ by gating the graphene, and in particular, complete switching on and off of the plasmon modes is demonstrated, thus paving the way towards graphene-based optical transistors. This successful alliance between nanoelectronics and nano-optics enables the development of active subwavelength-scale optics and a plethora of nano-optoelectronic devices and functionalities, such as tunable metamaterials, nanoscale optical processing, and strongly enhanced light–matter interactions for quantum devices and biosensing applications.

A Survey of Wide Bandgap Power Semiconductor Devices

Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

Anisotropic growth of long isolated graphene ribbons on the C face of graphite-capped 6H-SiC

We present an investigation of large, isolated, graphene ribbons grown on the C-face of on-axis semi-insulating 6H-SiC wafers. Using a graphite cap to cover the SiC sample, we modify the desorption of the Si species during the Si sublimation process. This results in a better control of the growth kinetics, yielding very long (about 300 microns long, 5 microns wide), homogeneous monolayer graphene ribbons. These ribbons fully occupy unusually large terraces on the step bunched SiC surface, as shown by AFM, optical microscopy and SEM. Raman spectrometry indicates that the thermal stress has been partially relaxed by wrinkles formation, visible in AFM images. In addition, we show that despite the low optical absorption of graphene, optical differential transmission can be successfully used to prove the monolayer character of the ribbons.

SiC Schottky Diodes for Harsh Environment Space Applications

This paper reports on the fabrication technology and packaging strategy for 300-V 5-A silicon carbide Schottky diodes with a wide temperature operation range capability (between -170 °C and 300 °C). These diodes have been designed for harsh environment space applications such as inner Solar System exploration probes. Different endurance tests have been performed to evaluate the diode behavior when working at a high temperature and under severe thermal cycling conditions (ranged from -170 °C to 270 °C). The radiation hardness capability has been also tested. It has been found that the hermeticity of the package in a neutral atmosphere is a key aspect to avoid an electrical parameter drift. Moreover, the use of gold metallization and gold wire bonds on the anode allows reducing the diode surface and bonding degradation when compared to Al-containing technology. On the back-side cathode contact, the Ti/Ni/Au metallization and AuGe combination have shown a very good behavior. As a result, the manufactured diodes demonstrated high stability for a continuous operation at 285 °C.

Manufacturing and full characterization of silicon carbide-based multi-sensor micro-probes for biomedical applications

Semi-insulating silicon carbide (SiC) is a fully processable semiconductors substrate that is commonly used as an alternative to conventional silicon (Si) in high-power applications. Here we examine the feasibility of using SiC as a substrate for the development of minimally invasive multi-sensor micro-probes in the context of organ monitoring during transplantation. In particular, we make a thorough comparison of Si and SiC material mechanical and electrical properties, and we extend this analysis to life-like situations using completed devices. Our results show that SiC outperforms Si in all respects, with a four times higher modulus of rupture for SiC devices and a 10-fold increase in the frequency range for electrical measurements in SiC-based probes. These results suggest that SiC should be preferably used over Si in all biomedical applications in which device breakage must be avoided or very precise electrical measurements are required.

Membraneless glucose/O2 microfluidic enzymatic biofuel cell using pyrolyzed photoresist film electrodes

Biofuel cells typically yield lower power and are more difficult to fabricate than conventional fuel cells using inorganic catalysts. This work presents a glucose/O2 microfluidic biofuel cell (MBFC) featuring pyrolyzed photoresist film (PPF) electrodes made on silicon wafers using a rapid thermal process, and subsequently encapsulated by rapid prototyping techniques into a double-Y-shaped microchannel made entirely of plastic. A ferrocenium-based polyethyleneimine polymer linked to glucose oxidase (GOx/Fc-C6-LPEI) was used in the anode, while the cathode contained a mixture of laccase, anthracene-modified multi-walled carbon nanotubes, and tetrabutylammonium bromide-modified Nafion (MWCNTs/laccase/TBAB-Nafion). The cell performance was studied under different flow-rates, obtaining a maximum open circuit voltage of 0.54 ± 0.04 V and a maximum current density of 290 ± 28 μA cm(-2) at room temperature under a flow rate of 70 μL min(-1) representing a maximum power density of 64 ± 5 μW cm(-2). Although there is room for improvement, this is the best performance reported to date for a bioelectrode-based microfluidic enzymatic biofuel cell, and its materials and fabrication are amenable to mass production.

Thermomechanical Assessment of Die-Attach Materials for Wide Bandgap Semiconductor Devices and Harsh Environment Applications

Currently, the demand by new application scenarios of increasing operating device temperatures in power systems is requiring new die-attach materials with higher melting points and suitable thermomechanical properties. This makes the die-attach material selection, die-attaching process, and thermomechanical evaluation a real challenge in nowadays power packaging technology. This paper presents a comparative analysis of the thermomechanical performance of high-temperature die-attach materials (sintered nano-Ag, AuGe, and PbSnAg) under harsh thermal cycling tests. This study is carried out using a test vehicle formed by four dice (considering Si and SiC semiconductors) and Cu substrates. Thermally cycled test vehicles have been thermomechanically evaluated using die-shear tests and acoustic microscopy inspections. Besides, special attention is paid to set up a nano-Ag sintering process, in which the effects of sintering pressure or substrate surface state (roughness and surface activation) on the die-attach layer are analyzed. As a main result, this study shows that the best die-attach adherence is obtained for nano-Ag when pressure is applied on the dice (using a specifically designed press) during the sintering process (11 MPa provided die-shear forces of 53 kgf). However, this die-attach presents a faster thermomechanical degradation under harsh thermal cycling tests than other considered high-temperature die-attach materials (AuGe and PbSnAg) and PbSnAg shows the best thermomechanical performances.

Accurate modeling and parameter extraction for 6H-SiC Schottky barrier diodes (SBDs) with nearly ideal breakdown voltage

We have fabricated Ni Schottky rectifiers on 2.7/spl times/ 10/sup 16/ cm/sup -3/ n-type 6H-SiC epilayer using an effective edge termination based on an oxide ramp profile around the Schottky contact. Several anneals of the Schottky contacts were experimented. In particular the diodes annealed at 900/spl deg/C showed excellent reverse characteristics with a nearly ideal breakdown at about 800 V. Forward characteristics follow the thermionic emission theory with the ideality factor close to one at low biases. An accurate analytical model and complete parameter extraction of the forward characteristics of the Ni/6H-SiC Schottky barrier diodes (SBDs) for low and high-level current densities are presented. The model takes into account the high-level injection effects and the current dependence of the series resistance. Direct extraction of the SBD parameters is carried out. A very good agreement between the simulated forward curves using extracted parameters and measured data up to 500 A/cm/sup 2/ is obtained.

Current status of self-organized epitaxial graphene ribbons on the C face of 6H–SiC substrates

The current status of long, self-organized, epitaxial graphene ribbons grown on the (0 0 0 −1) face of 6H–SiC substrates is reviewed. First, starting from the early stage of growth it is shown that on the C face of 6H–SiC substrates the sublimation process is not homogeneous. Most of the time it starts from defective sites, dislocations or point defects, that define nearly circular flakes surrounded by bare SiC. These flakes have a volcano-like shape with a graphite chimney at the centre, where the original defect was located. At higher temperatures a complete conversion occurs, which is not yet homogeneous on the whole sample. This growth process can be modified by covering the sample with a graphite cap. It changes the physics of the surface reconstruction during the Si-sublimation process and, on the C face, makes more efficient the reconstruction of few selected terraces with respect to the others. The net result is the formation of strongly step-bunched areas with, in between, long and large reconstructed terraces covered by graphitic material. Despite the low intrinsic optical absorption of a few graphene layers on SiC, micro-transmission experiments, complemented by micro-Raman spectroscopy, demonstrate that most of this graphitic coverage is made of one or two homogeneous graphene layers. We show also that most of the thermal stress between the graphene layer and the 6H–SiC substrate is relaxed by pleats or wrinkles which are clearly visible on the AFM images. Finally, the results of transport experiments performed on the graphitic ribbons reveal the p-type character of the ribbons.

High frequency CV measurements of SiC MOS capacitors

MOS capacitors have been fabricated on a p-type 6H-SiC substrate with (0001)Si orientation. The oxide was thermally grown under wet conditions. High frequency CV measurements have been carried our with different sweep rates and stress times in darkness and under different illumination conditions. The bump which appears in the CV curves is most likely a 6H-SiC manifestation of nonequilibrium effects. A hook which occurs in deep depletion when the structure is measured with UV light is observed. The density of states has been determined using the Terman method and the conductance technique including band bending fluctuations. Due to nonequilibrium effects in the semiconductor occurring during CV measurements, the Terman method induces errors in the extraction of the interface trap density of SiC MOS devices. The low value of the standard deviation of the surface potential was assumed to be rather due to the large interface state density than to an improvement of the SiC/SiO2 interface.The extracted D-it values at the SiC/SiO2 interface are in the range of 10(12) cm(-2) eV(-1) similar to those available in literature for this oxidation temperature.