Marcus Scherer

Extraction Efficiency of GaN-Based LEDs

In this work, we investigate the extraction efficiency for UV emitting rectangular 300 x 300 μm 2 gallium nitride (GaN) based light emitting diodes (LEDs) by simulation with a raytracer tool. It is shown that the extraction efficiency depends strongly on slight variations of the absorption in the GaN layers. Furthermore, the influence of the substrate shape is studied. For standard rectangular sapphire substrate based LEDs the calculated extraction efficiency is 12.4%, whereas for silicon carbide substrate based devices the higher refractive index causes a lower efficiency (4.5%). Using a shaped SiC substrate the extraction efficiency can be improved to 17.2% and 19.9% for a sapphire substrate. The influence of geometric design parameters like sidewall angle are analyzed as well.

Absorption and light scattering in InGaN-on-sapphire- and AlGaInP-based light-emitting diodes

Different experimental and simulation techniques aiming at a better understanding of lateral mode absorption in light-emitting diodes (LEDs) are presented in this paper. A measurement of transmitted power versus propagation distance allows us to derive the absorption losses of LED layer structures at their emission wavelength. Two models for the observed intensity distribution are presented: one is based on scattering, whereas the other relies on selective absorption. Both models were applied to InGaN-on-sapphire-based LED structures. Material absorption losses of 7 cm/sup -1/ for the scattering model and 4 cm/sup -1/ for the absorbing-layer model were obtained. Furthermore, these values are independent of the emission wavelength of the layer structure in the 403-433-nm range. The losses are most likely caused by a thin highly absorbing layer at the interface to the substrate. In a second step, interference of the modal field profile with the absorbing layer can be used to determine its thickness (d=75 nm) and its absorption coefficient (/spl alpha/ /spl ap/ 3900 cm/sup -1/). This method has also been tested and applied on AlGaInP-based layer structures emitting at 650 nm. In this case, the intensity decay of /spl alpha/=30 cm/sup -1/ includes a contribution from the absorbing substrate.

High-efficiency red and infrared light-emitting diodes using radial outcoupling taper

In this paper, we give an overview of light-emitting diodes (LEDs) with radial tapers. Light is generated in the very center of a circularly symmetrical structure and is outcoupled at a tapered ring. Encapsulated devices with an emission wavelength of 980 nm achieve wallplug efficiencies of 48%. Non-encapsulated InGaAlP-based red-emitting LEDs show quantum efficiencies of 13%. A new device design combines the taper with a wafer-scale soldering technique promising a feasible fabrication method.

Efficient light-emitting diodes with radial outcoupling taper at 980- and 630-nm emission wavelength

We have investigated efficient light outcoupling from light- emitting diodes (LEDs) by introducing lateral tapers. The concept is based on light generation in the very central area of a circularly symmetric structure. After propagating between two highly reflecting mirrors light is outcoupled in a tapered mesa region. By proper processing we achieve quantum efficiencies of almost 40% for outcoupling via a planar surface or quantum and wallplug efficiencies of 52% and 48%, respectively, for encapsulated devices. Neglecting reabsorption, approximative equations yield optimum design parameters.

Substrates for wide bandgap nitrides

Different substrates for gallium nitride growth are discussed. The commercially relevant substrates, silicon carbide and sapphire, and the two most promising alternatives, silicon and gallium nitride, are compared in terms of suitability for epitaxial processes and in their effects on devices. An estimation on future market success is given.

Low resistive p-type GaN using two-step rapid thermal annealing processes

Two-step thermal annealing processes were investigated for electrical activation of magnesium- doped galliumnitride layers. The samples were studied by room-temperature Hall measurements and photoluminescence spectroscopy at 16 K. After an annealing process consisting of a short-term step at 960 °C followed by a 600 °C dwell step for 5 min a resistivity as low as 0.84 Ω cm is achieved for the activated sample, which improves the results achieved by standard annealing (800 °C for 10 min) by 25% in resistivity and 100% in free hole concentration. Photoluminescence shows a peak centered at 3.0 eV, which is typical for Mg-doped samples with high free hole concentrations.

Absorption of guided modes in light emitting diodes

The absorption of lateral guided modes in light emitting diodes is determined by the photocurrent measurement method. A theory for waveguide dispersion is presented and extended by ray-tracing simulations. Absorption coefficients of InGaN-on-sapphire and AlGaInP-based structures is evaluated by comparison with simulation curves. For nitride-based samples with emission wavelengths of 415 nm and 441 nm an absorption of 7 cm-1 is obtained. It is found that scattering is present in the buffer layer and influences the lateral intensity distribution. The investigated AlGaInP-based sample exhibits an absorption of α = 30 cm-1 at 650 nm emission wavelength.

Multiple-step annealing for 50% enhanced p-conductivity of GaN

In this article, multiple-step rapid thermal annealing (RTA) processes for the activation of Mg doped GaN are compared with conventional single-step RTA processes. The investigated multiple-step processes consist of a low temperature annealing step at temperatures between 350 C and 700 C with dwell times up to 5 min and a short time high temperature step. With optimized process parameters, and multiple-step processes, we achieved p-type free carrier concentrations up to 1-2 × 10 18 cm 3 . The best achieved conductivity, so far, lies at 1.2Ω 1 cm 1 This is a 50% improvement compared to conventional single-step process at 800°C, 10 min.

Hydride Vapour Phase Epitaxy Growth of GaN Layers under Reduced Reactor Pressure

In this paper, Hydride Vapour Phase Epitaxy (HVPE) of GaN layers under reduced pressures is reported. First results show that the HVPE grown GaN layers exhibit excellent electrical, crystallographic and optical quality. By reducing the reactor pressure from 950 to 250 mbar, improvements in background doping (down to 2 × 1016 cm—3) and Hall mobility (up to 300 cm2/Vs) are observed. Experiments on MOVPE overgrowth on HVPE GaN layers show excellent results. Low temperature PL spectra of the overgrowth MOVPE layer reveal all three free exciton levels (FE A, FE B, FE C) without any visible bound excitons.

Efficient InAlGaP light-emitting diodes using radial outcoupling taper

We present results on efficient InGaAlP light-emitting diodes using lateral outcoupling taper. This concept is based on light generation in the very central area of a circularly symmetric structure and, after light propagation between two mirrors, outcoupling in a tapered mesa region. We have demonstrated the suitability of this concept on As-based Light-Emitting Diodes emitting at 980 nm. Since the idea is not limited to a certain material system, we fabricated InGaAlP-based LEDs emitting in the red wavelength regime. By adjusting the process flow to the new material system we were able to achieve external quantum efficiencies in the range of 13% for unencapsulated devices. Additionally we present a new concept combining the idea of outcoupling tapers with a waferscale soldering technique. First samples show external quantum efficiencies in the range of 11%.