Pleun Maaskant

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.

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.

On the specific contact resistance of metal contacts to p-type GaN

Extraction of the specific contact resistance, ρc, using the established circular transmission line measurement (c-TLM) and a series resistance measurement across mesa-isolated diodes is compared for a non-alloyed Pd/Ag contact to p-GaN. The limitations of the c-TLM technique are discussed and it is shown that for ρc values below 10−4 Ω cm2 both unintentional submicron errors in the actual radii and uncertainty in the resistance measurements can lead to order of magnitude changes in the extracted ρc. An additional current–voltage measurement across a mesa-isolated diode is proposed. The accuracy of the extracted ρc in this case requires consistency of the intrinsic diode characteristic, namely the ideality, at the bias voltages used for extraction, which is in turn related to the carrier transport and recombination properties. From a comparison with the c-TLM, we conclude that the resistance should be extracted at current densities <10 A cm−2 as junction heating changes the diode ideality.

Fabrication of GaN-Based Resonant Cavity LEDs

This article describes a fabrication process for GaN based resonant cavity (RC) LEDs. The devices emit through the transparent sapphire substrate. An AlGaN/GaN DBR forms the bottom mirror, while a PdAg-based p-contact metallisation serves as top mirror. The PdAg mirrors have been characterized optically by reflectometry, and electrically by circular TLM measurements. A specific contact resistivity of 1.0 × 10 -2 Ωcm 2 has been measured, without any annealing of the contacts. For the GaN to PdAg interface a reflectivity of 60% has been measured for a wavelength of 510 nm. At 20 mA bias the RC-LEDs show a forward voltage of 3.5 V and an optical output of 330 μW through the back of the wafer.

Experimental Characterisation of GaN-Based Resonant Cavity Light Emitting Diodes

For the first time, 2λ and 3λ GaN-based microcavities are demonstrated, consisting of a monolithically grown distributed Bragg reflector (DBR) as the first mirror and a metal as the second mirror. The metal acts both as a mirror and electrical contact, emission being through the sapphire substrate. No epitaxial lift-off or dielectric mirrors are required. Reflectance measurements and angle-resolved electroluminescence measurements clearly show the presence of a resonant cavity at wavelengths around 520 nm for the samples presented. At measurement angles off axis, the resonance moves to shorter wavelengths, in agreement with simple theory. Reflectance measurements indicate an effective penetration depth of the optical field into the DBR of approximately 3λ. Design and epitaxy were developed within the EU-funded AGETHA consortium.

Failure mechanisms associated with the fabrication of InGaN-based LEDs

In this paper, we report on the device fabrication and characterization of InGaN/GaN multiple quantum well light-emitting diodes (LEDs) on sapphire substrates. Devices that showed nonrectifying behavior have been investigated and it is suggested that metal migration along defect tubes is the likely cause of this behavior, It is recommended to use lower alloying temperatures for the p-contact metallization to avoid this type of failure. Deposition and subsequent removal of sputtered oxide was also found to have a highly detrimental effect on the p-contact quality.

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

Arrays of long wavelength, self-organized InGaAs quantum dot micron sized light-emitting diodes (μ-LEDs) with parabolic sidewalls are introduced. The parabolic profiles of the μ-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 200nm.

Development of optics with micro-LED arrays for improved opto-electronic neural stimulation

The breakthrough discovery of a nanoscale optically gated ion channel protein, Channelrhodopsin 2 (ChR2), and its combination with a genetically expressed ion pump, Halorhodopsin, allowed the direct stimulation and inhibition of individual action potentials with light alone. This work reports developments of ultra-bright elec tronically controlled optical array sources with enhanced light gated ion channels and pumps for use in systems to further our understanding of both brain and visual function. This work is undertaken as part of the European project, OptoNeuro. Micro-LED arrays permit spatio-temporal control of neuron stimulation on sub-millisecond timescales. However they are disadvantaged by their broad spatial light emission distribution and low fill factor. We present the design and implementation of a projection and micro-optics system for use with a micro-LED array consisting of a 16x16 matrix of 25 μm diameter micro-LEDs with 150 μm centre-to-centre spacing and an emission spectrum centred at 470 nm overlapping the peak sensitivity of ChR2 and its testing on biological samples. The projection system images the micro-LED array onto micro-optics to improve the fill-factor from ~2% to more than 78% by capturing a larger fraction of the LED emission and directing it correctly to the sample plane. This approach allows low fill factor arrays to be used effectively, which in turn has benefits in terms of thermal management and electrical drive from CMOS backplane electronics. The entire projection system is integrated into a microscope prototype to provide stimulation spots at the same size as the neuron cell body (μ10 pm).

Free-standing gallium nitride Schottky diode characteristics and stability in a high-temperature environment

Schottky diodes have been fabricated using low-resistivity n-type free-standing GaN substrates with a reduced defect density lowly doped n-type epi-layer and an Ni/Ti/Pt/Au Schottky contact metalization. A thermionic field emission current transport mechanism was identified with a Schottky barrier height of about 0.75 eV and a diode ideality of 1.1 measured at 25 ?C, both of which increase with measurement temperature up to 200 ?C. The diodes were subjected to long-term testing under forward current (1.3 A cm?2) or reverse voltage (?3.5 V) biased storage at 300 ?C in N2 for 466 h and were also monitored under non-biased storage conditions for up to 1000 h at 350 ?C and 400 ?C in N2 or at 300 ?C for 1500 h in air. Except for the non-biased storage test at 400 ?C, the diodes show <10% drift in ideality and barrier height during the long-term storage tests. For the 400 ?C test, there is a significant increase in both barrier height and ideality over a relatively short storage period (48 h). This to be the first reported study on the long-term stability of Schottky diodes on free-standing GaN and while no catastrophic (e.g. thermal runaway) degradation of any of the diodes was observed, it is proposed that optimized thermal annealing of the Ni-based Schottky contact metalization in the temperature range 350?400 ?C is necessary for stable long-term operation at high temperature.

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.