Broadband quantum dot micro-light-emitting diodes with parabolic sidewalls

Arrays of long wavelength, self-organized InGaAs quantum dot micron sized light-emitting diodes (mu-LEDs) with parabolic sidewalls are introduced. The parabolic profiles of the mu-LEDs produced by resist reflow and controlled dry etching improve the extraction efficiency from the LEDs by redirection of the light into the escape cone by reflection from the sidewalls. A fourfold increase in the substrate emitted power density compared to a reference planar LED is measured. The reflected light is verified to be azimuthally polarized. The spectral width of the emission can be greater than 200 nm. (C) 2008 American Institute of Physics. (DOI: 10.1063/1.2898731

Mixed-Mode Sensitivity Analysis of a Combined Differential and Common Mode Active Receiving Antenna Providing Near-Hemispherical Field-of-View Coverage

A theoretical framework for a mixed differential and common mode sensitivity analysis of active receiving antennas is presented, which includes the derivation of a novel set of noise parameters for dual-mode balanced amplifiers. The analysis is applied to an example of a mixed-mode active wire antenna design, consisting of an integrated monopole and dipole structure. Results of numerical simulations and experimental measurements are presented which show that, for a single-polarized design, the judicious use of both differential and common modes enables the field-of-view coverage to be extended over the entire hemisphere with a variation in receiving sensitivity of less than 3 dB in the E-plane

Quad-Mode Antenna for Wide-Scan Sparse Arrays

A conical quad-mode antenna excited through four orthogonal transverse electromagnetic modes is presented. The radiation characteristics of each mode are validated through measurements, illustrating the complimentary nature of the four far-field radiation patterns through which near-hemispherical field-of-view coverage can be achieved

Low-resistance Ni-based Schottky diodes on freestanding n-GaN

Schottky diodes formed on a low doped (5 x 10(16) cm(-3)) n-type GaN epilayer grown on a n(+) freestanding GaN substrate were studied. The temperature dependent electrical characteristics of Ni contacts on the as-grown material are compared with an aqueous, potassium hydroxide (KOH) treated surface. In both cases the diodes are dominated by thermionic emission in forward bias, with low idealities (1.04 at room temperature) which decrease with increasing temperature, reaching 1.03 at 413 K. The Schottky barrier height is 0.79 +/- 0.05 eV for the as-grown surface compared with 0.85 +/- 0.05 eV for the KOH treated surface at room temperature. This is consistent with an inhomogeneous barrier distribution. The specific on-state resistance of the diodes is 0.57 m Omega cm(2) The KOH treatment reduces the room temperature reverse leakage current density at -30 V to 1 x 10(-5) A cm(-2) compared to 6 x 10(-2) A cm(-2) for the as-grown samples. (C) 2007 American Institute of Physics. (DOI:10.1063/1.2799739

A scalable optoelectronic neural probe architecture with self-diagnostic capability

There is a growing demand for the development of new types of implantable optoelectronics to support both basic neuroscience and optogenetic treatments for neurological disorders. Target specification requirements include multi-site optical stimulation, programmable radiance profile, safe operation, and miniaturization. It is also preferable to have a simple serial interface rather than large numbers of control lines. This paper demonstrates an optrode structure comprising of a standard complementary metal-oxide-semiconductor process with 18 optical stimulation drivers. Furthermore, diagnostic sensing circuitry is incorporated to determine the long-term functionality of the photonic elements. A digital control system is incorporated to allow independent multisite control and serial communication with external control units

Equivalent circuit of a quadraxial feed for ultra-wide bandwidth quadruple-ridged flared horn antennas

Abstract-An equivalent circuit model of a quadraxial feed for ultra-wide bandwidth quadruple-ridged flared horn (QRFH) antennas is presented. The circuit is synthesised by only 3 unknowns and achieves an accurate input impedance for a wide range of dimensions. This model allows fast synthesis of optimal feeding designs that would ensure the excitation of the fundamental mode at the input of a QRFH

Carrier distribution in InGaN/GaN tricolor multiple quantum well light emitting diodes

Carrier transport in InGaN light emitting diodes has been studied by comparing the electroluminescence (EL) from a set of triple quantum well structures with different indium content in each well, leading to multicolor emission. Both the sequence and width of the quantum wells have been varied. Comparison of the EL spectra reveals the current dependent carrier transport between the quantum wells, with a net carrier flow toward the deepest quantum well. (C) 2009 American Institute of Physics. (doi:10.1063/1.3244203

Metal contacts to p-type GaN by electroless deposition

Initial results are presented on the electroless deposition of metal contacts to p-type gallium nitride (GaN). Deposition procedures were developed for the deposition of both nickel and tungsten-cobalt (W-Co) contacts onto p-type GaN. Attempts to deposit platinum on p-type GaN failed, despite the fact that electroless platinum deposition was successfully achieved on other substrate types. Nickel contacts were overlaid with gold and annealed in oxygen ambient to form ohmic contacts with specific contact resistivity values down to 2x10-2 &OHgr;cm2. Measurements at elevated temperatures up to 140 degrees C showed that the specific contact resistivity was almost independent of temperature. The tungsten-cobalt contacts showed rectifying behaviour even after annealing at 650 degrees C. This makes this contact type a possible candidate for Schottky contacts in high temperature applications

High power surface emitting InGaN superluminescent light-emitting diodes

A high power InGaN superluminescent light-emitting diode emitting normal to the substrate is demonstrated. The device uses a structure in which a monolithically integrated turning mirror reflects the light at both ends of the in-plane waveguide to direct amplified spontaneous emission downward through the transparent GaN substrate. Record optical peak powers of >2 W (both outputs) are reported under pulsed operation at 1% duty cycle. A broad, smooth emission spectrum with a FWHM of 6 nm centered at 416 nm is measured at peak output and ascribed to very low feedback associated with the turning mirror and antireflection coating

Aperture array development for future large radio telescopes

We present the design of a phased array system for future radio telescopes. This includes a system overview and recent results of the designed and implemented system, the Electronic Multi-Beam Radio Astronomy Concept (EMBRACE). Furthermore, simulations with a full-EM antenna simulator, combined with measurements on actual hardware, will provide information for the next design step, the Aperture Array Verification System (AAVS). With AAVS, we will prove design readiness of this novel array technology