A two-dimensional self-consistent laser model has been used for the simulation of the facet heating of red emitting
AlGaInP lasers. It solves in the steady-state the complete semiconductor optoelectronic and thermal equations in the
epitaxial and longitudinal directions and takes into account the population of different conduction band valleys. The
model considers the possibility of two independent mechanisms contributing to the facet heating: recombination at
surface traps and optical absorption at the facet. The simulation parameters have been calibrated by comparison with
measurements of the temperature dependence of the threshold current and slope efficiency of broad-area lasers. Facet
temperature has been measured by micro-Raman spectrometry in devices with standard and non absorbing mirrors
evidencing an effective decrease of the facet heating due to the non absorbing mirrors. A good agreement between
experimental values and calculations is obtained for both devices when a certain amount of surface traps and optical
absorption is assumed. A simulation analysis of the effect of non absorbing mirrors in the reduction of facet heating in
terms of temperature, carrier density, material gain and Shockly-Read-Hall recombination rate profiles is provided.
Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone
according to Bragg´s law. The extraction angle of a single mode is determined by the phase match between the guided
mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the
number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs)
with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed
MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the
n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice
type and lattice constant of the PhC. Two- six- and twelve-fold symmetry was observed in the azimuthal plane from 1D
lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was
enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by
80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external
quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the
enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations.
Keywords: light-emitting diodes, photonic crystal, cavity, InGaN
Photonic crystals are known to enhance the extraction efficiency of LEDs and simultaneously shaping the emission
pattern. In order to determine the radiation pattern we developed a model based on coupled mode theory that takes into
account the lattice pattern, etch depth and the mode distribution. From a basic geometrical consideration a fundamental
limit for the directionality is predicted. The calculations fit well to experimental data obtained from green InGaN LEDs
incorporating a hexagonal PhC revealing a maximum directionality of 31% within 30°. Additional FDTD simulations
were performed for determining quantitatively the extraction efficiency of PhC LEDs compared to LEDs with a
roughened surface. Despite its lower overall extraction efficiency, the PhC LED outperforms a standard LED with
surface roughening within an acceptance angle of 34° due to the higher directionality of the extracted light.
The measurement of the bias and temperature dependent photoluminescence, photocurrent and their decay times allows
to deduce important physical properties such as barrier height, electron-hole overlap and the magnitude of the
piezoelectric field in InGaN quantum wells. However the analysis of these experiments demands for a detailed physical
model based on a realistic device structure which is able to predict the measured quantities. In this work a selfconsistent
model is presented based on a realistic description of the alloy and doping profile of a green InGaN single
quantum well light emitting diode. The model succeeds in the quantitative prediction of the quantum confined Stark
shift and the associated change in the electron-hole overlap measured via the change in the bimolecular decay rate using
literature parameters for the piezoelectric constants. The blue shift of the emission under forward current conditions can
be attributed to the carrier induced screening of the piezoelectric charges as predicted by the model. The photocurrent is
calculated via thermionic tunneling through the barriers using a WKB-approximation and the calculated potential profile
for the tunneling barrier. From the fact that the bias and temperature dependence of the experimentally observed
photocurrent cannot be described by the thermionic tunneling model even though the theoretical potential profile fits
excellent to the luminescence data, we conclude that the carrier escape is dominated by a different mechanism such as
defect- or phonon-assisted tunneling.
The internal quantum efficiency as a function of the internal electric field was studied in InGaN/GaN based quantumwell
heterostructures. Most striking, we find the IQE to be independent of the electron hole overlap for a standard green-emitting
single quantum-well LED structure. In standard c-plane grown InGaN quantum wells, internal piezo-fields are
responsible for a reduced overlap of electron and hole wavefunction. Minimization of these fields, for example by
growth on non-polar m- and a-planes, is generally considered a key to improve the performance of nitride-based light
emitting devices. In our experiment, we manipulate the overlap by applying different bias voltages to the standard c-plane
grown sample, thus superimposing a voltage induced band-bending to the internal fields. In contrast to the IQE
measurement, the dependence of carrier lifetime and wavelength shift on bias voltage could be explained solely by the
internal piezo-fields according to the quantum confined Stark effect. Measurements were performed using temperature
and bias dependent resonant photoluminescence, measuring luminescence and photocurrent simultaneously.
Furthermore, the doping profile in the immediate vicinity of the QWs was found to be a key parameter that strongly
influences the IQE measurement. A doping induced intrinsic hole reservoir inside the QWs is suggested to enhance the
radiative exciton recombination rate and thus to improve saturation of photoluminescence efficiency.
Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.5P light-emitting devices (LEDs) is analysed experimentally and theoretically. A test series of LEDs was grown by MOCVD with identical layer sequence but different Aluminum content x in the active AlGaInP layer resulting in devices emitting light between 644 nm and 560 nm. The analysis yields the wavelength dependence of both the nonradiative recombination constant A as well as the carrier leakage parameter C of devices before and after aging. While test devices with λ>615 nm are very stable, LEDs with shorter emission wavelengths exhibit both an increase of A and a slight decrease of C upon aging. Possible degradation mechanisms are discussed.
The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.
There is a large number of new applications in lighting and display technology where high-brightness AlGaInP-LEDs can provide cost-efficient solutions for the red to yellow color range. Osram Opto Semiconductors has developed a new generation of MOVPE-grown AlInGaP-LEDs to meet these demands. Our structures use optimized epitaxial layer design, improved contact geometry and a new type of surface texturing. Based on this technology we achieve luminous efficiencies of more than 30 lm/W and wallplug efficiencies exceeding 10% of LEDs on absorbing GaAs substrates. The epitaxial structure does not require the growth of extremely thick window layer and standard processes are used for the chip fabrication. This allows for high production yields and cost-efficient production.
We report on a novel electro-optic modulator structure based on the two-dimensional Franz- Keldysh effect (2D-FKE) in multiple quantum well (MQW) structures. Due to the increased electron-hole interaction in these quasi-two-dimensional systems, strong excitonic resonances are observed even at room temperature. If an electric field is applied parallel to the layers of a MQW structure, very low electric fields (10 - 30 kV/cm) are sufficient to cause field ionization of the excitons, because of their weak in-plane confinement. Large absorption changes as high as 7000 cm-1 with field changes of only 30 kV/cm have been observed in GaAs/AlGaAs-MQWs. In addition, an increase of the absorption below and oscillations of the absorption coefficient above each subband transition are obtained due to the two-dimensional Franz-Keldysh effect. These features have been applied in our novel electro- optic modulator structure. Using interdigitated metal-semiconductor-metal (MSM) contacts, high in-plane electric fields can be generated with moderate voltages. Furthermore the low capacitance of these MSM structures is particularly favorable for high speed applications. In a MSM-modulator structure, consisting of 75 GaAs/AlGaAs quantum wells with a distributed Bragg-reflector (DBR) below the MQW-layers, a maximum contrast ratio of 5:1 without using any cavity effects has been achieved with a voltage swing of 20 V.
We have studied the field dependence of the absorption coefficient of three GaAs-AlAs superlattices using a new modulation technique, the wavelength modulated photocurrent spectroscopy. At small applied electric fields we observe transitions corresponding to the edges of the joint miniband density of states between electrons and heavy holes as well as light holes. At intermediate fields Franz-Keldysh oscillations appear at the lower and upper band edges of the heavy and light hole joint miniband. With further increasing electric field these oscillations transform gradually into Wannier-Stark ladder transitions. The experimentally observed features are well reproduced by numerical calculations.
We performed room temperature photo- and electro-modulation measurements on MBE-grown GaAs pnp and
pin structures with large layer thicknesses. In these crystals the optical properties are expected to be dominated
by the local field-induced changes of the dielectric function rather than by subband-transitions. The dominating
effect in the pnp-structure turns out to be the spatial dependence of the varying refractive index, resulting in a
characteristic interference pattern. For the case of the pin structure we clearly observe Franz-Keldysh oscillations
changing in amplitude and width with the internal field. Theoretical calculations using effective mass theory are in
very good quantitative agreement with the experimental results. This agreement can be achieved only by the inclusion
of excitonic effects in the calculation of the field-dependent absorption.
We report results of photoreflectance (PR) investigations of GaAs/Alo.3Gao.7As hetero-n-i-p-i crystals with either
GaAs-QWs ("type II") in the intrinsic region or GaAs-QWs interspersed in the n-region ("type I"). To understand
the complex PR-spectra of these samples we changed several measurement parameters such as ac-pump-intensity,
dc-pump-intensity, pump frequency and temperature. Especially the spectra of type II showed a strong temperature
dependence. We compare these spectra to spectra calculated with the model of an infinite quantum well in an electric
field. Because of the dielectric function being periodic in space the z-dependence of the dielectric function is taken