Philippe Bove

Output power density of 5.1/mm at 18 GHz with an AlGaN/GaN HEMT on Si substrate

Microwave frequency capabilities of AlGaN/GaN high electron mobility transistors (HEMTs) on high resistive silicon (111) substrate for power applications are demonstrated in this letter. A maximum dc current density of 1 A/mm and an extrinsic current gain cutoff frequency (F/sub T/) of 50 GHz are achieved for a 0.25 /spl mu/m gate length device. Pulsed and large signal measurements show the good quality of the epilayer and the device processing. The trapping phenomena are minimized and consequently an output power density of 5.1 W/mm is reached at 18 GHz on a 2/spl times/50/spl times/0.25 /spl mu/m/sup 2/ HEMT with a power gain of 9.1dB.

Amplified piezoelectric transduction of nanoscale motion in gallium nitride electromechanical resonators

A fully integrated electromechanical resonator is described that is based on high mobility piezoelectric semiconductors for actuation and detection of nanoscale motion. We employ the two-dimensional electron gas present at an AlGaN/GaN interface and the piezoelectric properties of this heterostructure to demonstrate a resonant high-electron-mobility transistor enabling the detection of strain variation. In this device, we take advantage of the polarization field divergence originated by mechanical flexural modes for generating piezoelectric doping. This enables a modulation of carrier density which results in a large current flow and thus constitutes a motion detector with intrinsic amplification.

AlGaN-GaN HEMTs on Si with power density performance of 1.9 W/mm at 10 GHz

AlGaN-GaN high electron mobility transistors (HEMTs) on silicon substrate are fabricated. The device with a gate length of 0.3-/spl mu/m and a total gate periphery of 300 /spl mu/m, exhibits a maximum drain current density of 925 mA/mm at V/sub GS/=0 V and V/sub DS/=5 V with an extrinsic transconductance (g/sub m/) of about 250 mS/mm. At 10 GHz, an output power density of 1.9 W/mm associated to a power-added efficiency of 18% and a linear gain of 16 dB are achieved at a drain bias of 30 V. To our knowledge, these power results represent the highest output power density ever reported at this frequency on GaN HEMT grown on silicon substrates.

Study of Au coated ZnO nanoarrays for surface enhanced Raman scattering chemical sensing

Eight 1 cm2 samples of self-organising zinc oxide (ZnO) nanopillar arrays with preferential vertical orientation were grown by pulsed laser deposition and then coated with 30 nm of Au using either thermal or electron-beam evaporation. Each sample had a different set of ZnO and Au growth conditions. The Au/ZnO nanoarrays were then tested for use in surface enhanced Raman scattering (SERS) detection of thiophenol molecules. The ratio of ISERS/IRaman was adopted as a measure of the SERS sensitivity and was found to vary from 1.7 to 23.7 within the 8 samples. The impact of the density, width, filling factor, orientation, homogeneity and shape of the nanostructures on the average SERS intensity and the within-wafer reproducibility of the SERS response were considered for 9 paired comparisons based on fixing all but one of the growth parameters for each pairing. Overall, smaller nanopillar width was found to correlate with stronger average SERS signal while more vertically aligned arrays with higher filling factors showed better within-wafer reproducibility.

Photoreflectance study of GaAsSb/InP heterostructures

Photoreflectance (PR) spectroscopy experiments are reported on GaAsSb∕InP heterostructures. The GaAsSb PR spectrum is studied as a function of temperature and the transition nature is shown to change from Franz-Keldysh oscillations (FKO) at room temperature to a third derivative functional form (TDFF) line shape at low temperatures. Combining both analysis (FKO and TDFF) in the same sample, we derive internal electric field and phase values of the PR transition, together with accurate values for alloy band gap energy on the whole temperature range. Type II interface recombination is shown to reduce photovoltage effects as a function of temperature. FKO are found to appear for a very weak electric field (8kV∕cm) in the GaAsSb∕InP heterostructure, contrary to usual observations. This point is discussed in relation with the broadening parameter of the transition.

Surface Fermi level in GaAsSb structures grown by molecular beam epitaxy on InP substrates

Photoreflectance (PR) spectroscopy is performed to investigate the Fermi level pinning at the surface of GaAsSb, in a series of epitaxial structures with different Sb concentration. PR spectra exhibit Franz–Keldysh oscillations, originating from the built-in electric field in the GaAsSb layer. Experimental results indicate that the surface Fermi level is pinned in the lower half bandgap. The surface Fermi level is determined versus the Sb concentration between 38% and 52%.

Novel process for direct bonding of GaN onto glass substrates using sacrificial ZnO template layers to chemically lift-off GaN from c-sapphire

GaN was grown on ZnO-buffered c-sapphire (c-Al2O3) substrates by Metal Organic Vapor Phase Epitaxy. The ZnO then served as a sacrificial release layer, allowing chemical lift-off of the GaN from the c-Al2O3 substrate via selective wet etching of the ZnO. The GaN was subsequently direct-wafer-bonded onto a glass substrate. X-Ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray microanalysis, Room Temperature Photoluminescence & optical microscopy confirmed bonding of several mm2 of crack-free wurtzite GaN films onto a soda lime glass microscope slide with no obvious deterioration of the GaN morphology. Using such an approach, InGaN based devices can be lifted-off expensive single crystal substrates and bonded onto supports with a better cost-performance profile. Moreover, the approach offers the possibility of reclaiming and reusing the substrate.

Wafer-scale epitaxial lift-off of optoelectronic grade GaN from a GaN substrate using a sacrificial ZnO interlayer

Full 2 inch GaN epilayers were lifted off GaN and c-sapphire substrates by preferential chemical dissolution of sacrificial ZnO underlayers. Modification of the standard epitaxial lift-off (ELO) process by supporting the wax host with a glass substrate proved key in enabling full wafer scale-up. Scanning electron microscopy and x-ray diffraction confirmed that intact epitaxial GaN had been transferred to the glass host. Depth-resolved cathodoluminescence (CL) analysis of the bottom surface of the lifted-off GaN layer revealed strong near-band-edge (3.33 eV) emission indicating a superior optical quality for the GaN which was lifted off the GaN substrate. This modified ELO approach demonstrates that previous theories proposing that wax host curling was necessary to keep the ELO etch channel open do not apply to the GaN/ZnO system. The unprecedented full wafer transfer of epitaxial GaN to an alternative support by ELO offers the perspective of accelerating industrial adoption of the expensive GaN substrate through cost-reducing recycling.

Improved LEDs and photovoltaics by hybridization & and nanostructuring

Since the development of p-type doping of gallium nitride (GaN) in the early 1990s,1–4 there has been rapid industrial development for optoelectronic devices based on alloys of GaN with aluminum and indium (AlInGaN), which span a direct bandgap from deep UV to IR, and are currently widely used in commercial white, UV, blue, and green LEDs.5 This alloy system is now projected to provide a platform for the development of novel multi-junction photovoltaics (PVs) with an unprecedented fit to the solar spectrum.6 However, improving the efficiency of InGaN-based p-n junctions is a very complex and multifaceted task for a number of reasons. One concern is the losses due to strain fields linked to the polar nature of GaN.7 A second problem is that the high refractive index of GaN ( 2.5) and the intrinsically planar surface of LED/PV structures creates light extraction and reflection issues (<15% of light escapes from a planar GaN/air interface as a result of total internal reflection, and a planar PV surface reflects >10% of incident light). Third, there is a lack of native substrates. Cheap, large-area, epitaxially matched substrates are not commercially available, so alternative, but relatively expensive, small, insulating sapphire substrates are used. The electrical insulation imposes lateral device geometries (fewer devices per wafer) and causes power and efficiency limitations. The thermal insulation is a problem because LEDs/PVs dissipate most of their heat through conduction, and overheating severely impacts the output power, efficiency, and lifetime.8 Fourth, asymmetric doping (p-GaN carrier concentration and mobility are lower than that of n-GaN) leads to efficiency issues at elevated junction currents.9 And Figure 1. Efficacy versus wavelength for state-of-the-art LEDs, showing exponential efficacy fall-off in the green.14 InGaN: Indium gallium nitride. AlInGaP: Aluminum indium gallium phosphide.

High-radiance extreme-ultraviolet light source for actinic inspection and metrology

Actinic mask defect inspection and metrology requires high-brightness extreme-ultraviolet (EUV) sources. The self-absorption of radiation limits the in-band EUV radiance of the source plasma and makes it difficult to attain the necessary brightness and power from a conventional single unit EUV source. One possible solution is through multiplexing of multiple low etendue sources. NANO-UV is delivering a new generation of EUV light source, the CYCLOPS, in which a micro-plasma-pulsed discharge source is integrated to a photon collector based on an in situ active plasma structure. The source module is characterized by high brightness, low etendue, and high irradiance at moderate output power without the use of external physical optics. Such a source could form the basic building block of EUV source through spatial-temporal multiplexing of several units to deliver the brightness and power required for actinic mask metrology. We report on the EUV source development including the extensive numerical modeling, which provided the basic parameters required for high irradiance operating regimes. A new Sn-alloy cathode material enhances the output. Based upon the multiplexing concept, a family of specially configured multiplexed source structures, the HYDRA design, is being introduced to address the mask metrology needs.