Mahbub Akhter

High-Speed Substrate-Emitting Micro-Light-Emitting Diodes for Applications Requiring High Radiance

InGaN-based micro-light-emitting diodes (µ-LEDs) emitting at 470 nm and composed of micropixels each with controlled shaping achieves directed light output with an angular full width at half maximum of 48°. The reflected light from the mesa sidewalls is azimuthally polarized. The small signal bandwidth of an individual µ-LED is >500 MHz. A cluster of 14 µ-LEDs is used to achieve a large signal data transfer rate of 500 Mbps in a form which is compatible with communication over plastic optical fibre.

High Bandwidth Freestanding Semipolar (11–22) InGaN/GaN Light-Emitting Diodes

Freestanding semipolar (11-22) indium gallium nitride (InGaN) multiple-quantum-well light-emitting diodes (LEDs) emitting at 445 nm have been realized by the use of laser lift-off (LLO) of the LEDs from a 50-μm-thick GaN layer grown on a patterned (10-12) r-plane sapphire substrate (PSS). The GaN grooves originating from the growth on PSS were removed by chemical mechanical polishing. The 300 μm × 300 μm LEDs showed a turn-on voltage of 3.6 V and an output power through the smooth substrate of 0.87 mW at 20 mA. The electroluminescence spectrum of LEDs before and after LLO showed a stronger emission intensity along the [11-23]InGaN/GaN direction. The polarization anisotropy is independent of the GaN grooves, with a measured value of 0.14. The bandwidth of the LEDs is in excess of 150 MHz at 20 mA, and back-to-back transmission of 300 Mbps is demonstrated, making these devices suitable for visible light communication (VLC) applications.

Semipolar (202̅3) nitrides grown on 3C–SiC/(001) Si substrates

Heteroepitaxial growth of GaN buffer layers on 3C–SiC/(001) Si templates (4°-offcut towards [110]) by metalorganic vapour phase epitaxy has been investigated. High-temperature grown Al0.5Ga0.5N/AlN interlayers were employed to produce a single (203) GaN surface orientation. Specular crack-free GaN layers showed undulations along [10] with a root mean square roughness of about 13.5 nm (50 × 50 μm2). The orientation relationship determined by x-ray diffraction (XRD) was found to be [20]GaN ∥[10] and [034]GaN ∥[110]3C − SiC/Si . Low-temperature photoluminescence (PL) and XRD measurements showed the presence of basal-plane stacking faults in the layers. PL measurements of (203) multiple-quantum-well and light-emitting diode structures showed uniform luminescence at about 500 nm emission wavelength. A small peak shift of about 3 nm was observed in the electroluminescence when the current was increased from 5 to 50 mA (25–250 A cm−2).

Red and green resonant cavity LEDs for datacom applications

We report on the development of resonant cavity LEDs (RCLEDs) for use in short distance datacommunication applications using the IEEE 1394 standard where plastic optical fibre (POF) is the physical medium. The devices are designed for 650nm and 500nm emission where POF has low attenuation. The red devices based on InGaAlP/GaAs are optimised for room temperature operation and 90μm diameter devices have a maximum coupled power to 1mm diameter POF of 1mW. At 10mA the coupled power is 0.4mW with a quantum efficiency of 2%. Current spreading is shown to be critical in optimising the output power. The devices function as resonant cavity detectors with a response FWHM of 4.2nm centred at 650nm. The blue-green RCLEDs are based on InGaN/GaN and use a hybrid metal-epitaxial mirror cavity. This wavelength is preferable for longer links. The substrate emitting devices have fibre-coupled powers of 200μW at 20mA. A datarate of 250Mb/s is measured. The resonant cavity is confirmed by angularly resolved spectral measurements. The tradeoff between green and red devices is discussed.

250-nm emitting LED optimized for optical fibre coupling

In this study we report the growth, fabrication and packaging of AlGaN-based light emitting diodes (LEDs) operating at 250 nm for space application [1]. The device fabrication and packaging processes were optimized to ensure a reliable optical fibre coupling. Electrical and optical characterization is presented and discussed; in particular a four-fold increase of the integrated ultraviolet power is demonstrated compared with a non-optimized device. A potential expansion of this approach to InAlN-based devices will also be discussed.

MHz operation of 250 nm ultra-violet micro-light emitting diodes

Deep Ultra Violet Light Emitting diodes have undergone increasing research interest due to their application for sterilisation of water and air for example [1]. They also can potentially provide compact efficient sources for fluorescence excitation [2] and food preservation [3]. A key advantage of such LEDs over the alternative Hg lamp sources is their ease of control and switching, in addition to the benefits to the environment, where Hg disposal is a critical issue.

Enhanced efficiency of near-UV emitting LEDs for solid state lighting applications

The performance of a series of near-UV (~385 nm) emitting LEDs, consisting of high efficiency InGaN/AlInGaN QWs in the active region, was investigated. Significantly reduced roll-over of efficiency at high current density was found compared to InGaN/GaN LEDs emitting at a similar wavelength. The importance of optical cavity effects in flip-chip geometry devices has also been investigated. The light output was enhanced by more than a factor of 2 when the light-emitting region was located at an anti-node position with respect to a high reflectivity current injection mirror. A power of 0.49 mW into a numerical aperture of 0.5 was obtained for a junction area of 50 micrometers in diameter and a current of 30 mA, corresponding to a radiance of 30 W/cm2/str.

Packaging technology for high power blue-green LEDs

High brightness LEDs (HBLEDs) have been fabricated on GaN semiconductor material grown on sapphire substrate. These devices provide an optical output power in excess of 50 mW at a driving current of 1 amp. For this high current application, large size (1.8 mm × 0.6 mm) GaN LEDs are flip-chip mounted onto a heat sink to provide a low thermal resistance path from the junction to the ambient. For the flip-chip mounting, a Au/Sn/Au solder and a Au/Au thermal compression bonding process have been optimized. The bond strength of the Au/Sn solder joints and the Au-Au bonds is measured through shear testing. Good bond strength results of 224 g/f for the Au/Sn/Au solder and 288 g/f for the solid Au bonds have been achieved. The thermal modeling of the assembly is done with a finite element analysis and the optimum design has been adopted for this high current application. At present these assemblies are under lifetime test and so far nearly 6000 hours of continuous operation has been achieved.