Nanofabrication of normally-off GaN vertical nanowire MESFETs.

Gallium nitride (GaN) all-around (wrap) gate vertical nanowire (V-NW) field-effect transistors (FETs) are favorable for enhanced electrostatic control of the gate and selectivity for normally on/off operation. In this work, GaN V-NW FETs with a Schottky barrier gate (V-NW MESFETs), were fabricated for the first time. A nanofabrication process with comprehensive description of all processing steps is reported. It was validated with the demonstration of GaN V-NW MESFETs consisting of an array of 900 (30 × 30) GaN NWs with the narrowest until now reported diameter of 100 nm and all-around gate length of 250 nm. The GaN NWs were formed by a top-down approach, which combines conventional nanopatterning techniques and anisotropic wet etching of an initial GaN epilayer, grown by plasma assisted molecular beam epitaxy on a sapphire (0001) substrate. DC I-V characteristics exhibited normally-off operation and threshold voltage of +0.4 V, due to electron depletion region from the all-around Schottky barrier. A maximum drain-source current density (J ds) of 330 A cm-2 and maximum transconductance (g m) of 285 S cm-2 were obtained from I-V measurements. The results and directions for further optimization were discussed.

Ultra-low threshold polariton lasing at room temperature in a GaN membrane microcavity with a zero-dimensional trap.

Polariton lasers are coherent light sources based on the condensation of exciton-polaritons in semiconductor microcavities, which occurs either in the kinetic or thermodynamic (Bose-Einstein) regime. Besides their fundamental interest, polariton lasers have the potential of extremely low operating thresholds. Here, we demonstrate ultra-low threshold polariton lasing at room temperature, using an all-dielectric, GaN membrane-based microcavity, with a spontaneously-formed zero-dimensional trap. The microcavity is fabricated using an innovative method, which involves photo-electrochemical etching of an InGaN sacrificial layer and allows for the incorporation of optimally-grown GaN active quantum wells inside a cavity with atomically-smooth surfaces. The resulting structure presents near-theoretical Q-factors and pronounced strong-coupling effects, with a record-high Rabi splitting of 64 meV at room-temperature. Polariton lasing is observed at threshold carrier densities 2.5 orders of magnitude lower than the exciton saturation density. Above threshold, angle-resolved emission spectra reveal an ordered pattern in k-space, attributed to polariton condensation at discrete levels of a single confinement site. This confinement mechanism along with the high material and optical quality of the microcavity, accounts for the enhanced performance of our polariton laser, and pave the way for further developments in the area of robust room temperature polaritonic devices.

Strained GaAs/InGaAs Core-Shell Nanowires for Photovoltaic Applications.

We report on the successful growth of strained core-shell GaAs/InGaAs nanowires on Si (111) substrates by molecular beam epitaxy. The as-grown nanowires have a density in the order of 10(8) cm(-2), length between 3 and 3.5 µm, and diameter between 60 and 160 nm, depending on the shell growth duration. By applying a range of characterization techniques, we conclude that the In incorporation in the nanowires is on average significantly smaller than what is nominally expected based on two-dimensional growth calibrations and exhibits a gradient along the nanowire axis. On the other hand, the observation of sharp dot-like emission features in the micro-photoluminescence spectra of single nanowires in the 900-1000-nm spectral range highlights the co-existence of In-rich enclosures with In content locally exceeding 30 %.

Extraction of absorption coefficients from as-grown GaN nanowires on opaque substrates using all-optical method.

We demonstrate a new all-optical method to measure absorption coefficients in any family of as-grown nanowires, provided they are grown on a substrate having considerable difference in permittivity with the nanowire-air matrix. In the case of high crystal quality, strain-free GaN nanowires, grown on Si (111) substrates, the extracted absorption coefficients do not exhibit any enhancement compared to bulk GaN values, unlike relevant claims in the literature. This could be attributed to the relatively small diameters, short heights, and high densities of our nanowire arrays.

Graphene-like metal-on-silicon field-effect transistor.

This paper presents a field-effect transistor with a channel consisting of a two-dimensional electron gas located at the interface between an ultrathin metallic film of Ni and a p-type Si(111) substrate. The gate length is L = 2 µm, its width is W = 180 µm, and the source-drain separation is 188 µm, the role of the gate dielectric being played by the surface states of the ultrathin metal layer. We have demonstrated that the two-dimensional electron gas channel is modulated by the gate voltage. The dependence of the drain current on the drain voltage has no saturation region, similar to a field-effect transistor based on graphene. The drain current is 2 mA at a drain voltage of 3 V and a gate voltage of 1.07 V, while the transconductance is 0.6 mS for a drain voltage of 6 V and a gate voltage of 1 V. However, the transport in this transistor is not ambipolar, as in graphene, but unipolar.

InGaN nanopillars grown on silicon substrate using plasma assisted molecular beam epitaxy.

Single crystalline and single phase In(x)Ga(1-x)N nanopillars were grown spontaneously on (111) silicon substrate by plasma assisted molecular beam epitaxy. The surface morphology, structural quality, and optoelectronic properties of InGaN nanopillars were analyzed using scanning electron microscopy (SEM), energy dispersive x-ray (EDXA) analysis, high resolution x-ray diffraction (HR-XRD), and both room and low temperature photoluminescence spectra. The EDXA results showed that these nanopillars were composed of InGaN and the amount of indium incorporation in In(x)Ga(1-x)N NPs could be controlled by changing the growth temperature. The room temperature and low temperature PL spectra revealed that the emission wavelength could be tuned from a blue to green luminescent region depending on the growth temperature. The wavelength tuning was attributed to a higher amount of In incorporation at a lower growth temperature which was consistent with the EDXA and HR-XRD results.

Gastric ulcer bleeding: diagnosis by computed tomography.

A case of CT demonstration of a bleeding gastric ulcer is presented, in a patient with confusing clinical manifestations. Abdominal CT was performed without oral contrast medium administration, and showed extravasation of intravenous contrast into a gastric lumen distended with material of mixed attenuation. It is postulated that if radiopaque oral contrast had been given, peptic ulcer bleeding would probably have been masked. CT demonstration of gastric ulcer bleeding, may be of value in cases of differential diagnostic dilemmas.