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

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

Fibre coupled micro-light emitting diode array light source with integrated band-pass filter for fluorescence detection in miniaturised analytical systems

In this work, a new type of miniaturized fibre-coupled solid-state light source is demonstrated as an excitation source for fluorescence detection in capillary electrophoresis. It is based on a parabolically shaped micro- light emitting diode (µ-LED) array with a custom band-pass optical interference filter (IF) deposited at the back of the LED substrate. The GaN µ-LED array consisted of 270 individual µ-LED elements with peak emission at 470nm, each about 14µm in diameter and operated as a single unit. Light was extracted through the transparent substrate material, and coupled to an optical fibre (400µm in diameter, numerical aperture NA = 0.37), to form an integrated µ-LED-IF-OF light source component. This packaged µ-LED-IFOF light source emitted approximately 225µW of optical power at a bias current of 20mA. The bandpass IF filter was designed to reduce undesirable LED light emissions in the wavelength range above 490 nm . Devices with and without IF were compared in terms of optical power output, spectral characteristics as well as LOD values. While the IF consisted of only 7.5 pairs (15 layers) of SiO2/HfO2 layers it resulted in an improvement of the baseline noise as well as the detection limit measured using fluorescein as test analyte, both by approximately one order of magnitude, with a LOD of 1×10-8 mol/L obtained under optimised conditions. The µ-LED-IF-OF light source was then demonstrated for use in capillary electrophoresis with fluorimetric detection. Limits of detection obtained by this device were compared to those obtained with a commercial fibre coupled LED device

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

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

InAlN-based LEDs emitting in the near-UV region

Fully functional InAlN-based ultraviolet LEDs emitting at 340–350 nm were demonstrated for the first time; detailed electrical and optical characterization is presented and discussed. Results from the measurements at pulsed conditions are in agreement with the attribution of the dominant electroluminescence peak to near-band-edge emission. The composition of the AlGaN barriers was chosen to give the same internal polarization field as that of the InAlN wells. A simulation study of this polarization-matched heterostructure shows a significant increase in the electron-hole overlap integral if compared with a standard AlGaN/AlGaN active region having the same level of carrier confinement. Limitations and problems of these preliminary devices are also presented, and possible future work aimed at increasing their efficiency is discussed

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

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

Low defect large area semi-polar (11[Formula: see text]2) GaN grown on patterned (113) silicon.

We report on the growth of semi-polar GaN (11[Formula: see text]2) templates on patterned Si (113) substrates. Trenches were etched in Si (113) using KOH to expose Si {111} sidewalls. Subsequently an AlN layer to prevent meltback etching, an AlGaN layer for stress management, and finally two GaN layers were deposited. Total thicknesses up to 5 [Formula: see text]m were realised without cracks in the layer. Transmission electron microscopy showed that most dislocations propagate along [0001] direction and hence can be covered by overgrowth from the next trench. The defect densities were below [Formula: see text] and stacking fault densities less than 100 cm [Formula: see text]. These numbers are similar to reports on patterned r-plane sapphire. Typical X-ray full width at half maximum (FHWM) were 500" for the asymmetric (00.6) and 450" for the (11.2) reflection. These FHWMs were 50 % broader than reported for patterned r-plane sapphire which is attributed to different defect structures and total thicknesses. The surface roughness shows strong variation on templates. For the final surface roughness the roughness of the sidewalls of the GaN ridges at the time of coalescence are critical.This work was supported by EU- FP7 ALIGHT No. NMP-2011-280587 and the UK En- gineering and Physical Sciences Research Council No. EP/I012591/1.This is the final version. It was first published by http://onlinelibrary.wiley.com/doi/10.1002/pssb.201451591/abstrac