Sven-Silvius Schad

GaN/AlN-based quantum-well infrared photodetector for 1.55 μm

We report optical absorption and photocurrent measurements on a GaN/AlN-based superlattice. The optical absorption has a full width at half maximum of 120 meV and takes place at an energy of 660 meV (5270 cm−1); this corresponds to a wavelength of 1.9 μm. While the optical absorption remained unchanged up to room temperature, the photocurrent signal could be observed up to 170 K. With respect to the optical absorption, the photocurrent peak was slightly blueshifted (710 meV/5670 cm−1) and had a narrower width of 115 meV. Using this quantum-well infrared photodetector, we were able to measure the spectrum of a 1.55 μm superluminescent light-emitting diode.

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.

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.

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.

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%.