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The aging behavior of red-orange AlGaInP light-emitting diodes (LEDs) is investigated. It is found that the amount of magnesium and tellurium doping as well as of the oxygen incorporation influence the device degradation. The generation of non-radiative recombination centers in the active layer is responsible for the decrease of the LED light output during operation. The kinetics of the light aging as well as the configuration of the non-radiative recombination centers emerging during degradation depend on the doping species. High stress temperatures can accelerate the device degradation. For the aging of Te-rich device, the activation energy of 660 meV for the degradation process is determined. Deep level transient spectroscopy is used to investigate the properties of the non-radiative recombination centers. Four deep traps are detected in aged devices. One trap with activation energy of about 1 eV is found in all aged devices. In the aged O-rich device, one additional deep trap is observed with slightly lower activation energy. In the aged Te-rich device two additional shallower traps emerge during degradation.

This paper reviews the recent progress of AlGaInP high brightness light-emitting diodes. After the discussion of some basic material properties and the general problem of light extraction we will discuss several approaches of high efficiency devices.

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.

Deep defect centers in high-power AlGaAs/GaAs laser diode arrays emitting at a wavelength of 808 nm are studied by Fourier-transform photocurrent spectroscopy and by electrical deep level transient spectroscopy. Different types of deep centers with binding energies between 0.15 and 0.8 eV are found in graded-index and step-index diode structures. In all structures, the defect concentration increases with operation time. We demonstrate that different operation conditions of the devices, such as regular operation or accelerated aging at increased temperatures, cause different scenarios of deep level creation. In addition to deep centers, aging leads to the formation of shallow defects and a reshaping of the absorption band edge.

We report on micro-Raman facet temperature measurements carried out in asymmetrically coated high-power laser diode arrays. Facet temperatures of up to 600 °C are reproducibly found for high-power operation. The data are modeled using an approach based on the finite element method. Thus we are able to infer the power management of the device from the facet temperature data. Up to 7% of the total heat power is concentrated at the front facet. Furthermore. we determine the thermal load at the front facet versus the operation current, which is a key parameter for modeling high-power devices.

AlGaInP thin-film resonant cavity light-emitting diodes (RCLEDs) show an improved performance compared to standard red emitting RCLEDs. External quantum efficiencies at 650 nm of 23% and 18% with and without encapsulation, respectively, have been obtained for devices showing a maximum emission in the normal direction. Thanks to the high angle-averaged reflectivity of the bottom hybrid mirror, a strong photon recycling effect occurs in these structures. The decrease of the absorption with increasing injection level reduces photon recycling and increases extraction of lateral guided modes. The redirection of part of the emission from the vertical to the lateral direction with increasing current density is reflected in the evolution of the far-field radiation pattern.

Volatile copper(I) benzoates with variable degrees of fluorination are used for p-doping of organic hole-transport layers in single-carrier devices, charge-generation layers, and in organic light-emitting diodes. The charge-transport abilities of the doped materials correlate with the degree and position of the fluorination on the aromatic ring of the carboxylate groups.

The efficiency decrease during electrical operation of organic light-emitting diodes is a crucial issue for both applied and fundamental research. In order to investigate degradation processes, we have performed an efficiency analysis for phosphorescent state-of-the-art devices in the pristine state and after an accelerated aging process at high current density resulting in a luminance drop to less than 60% of the initial value. This loss in efficiency can be explained by a decrease of the radiative quantum efficiency of the light-emitting guest/host system from 70% to 40%, while other factors determining the efficiency are not affected.

We report new methods of identifying the effects of aging on the light–current (L–I) and current–voltage (I–V) characteristics of AlInGaP light-emitting diodes (LEDs). We believe that these methods are also applicable to other III–V compound semiconductors. We observe a broadening of the nonlinear range of the L–I characteristic accompanied by a shift to higher currents in the I–V characteristic. These features can be attributed to an increase of nonradiative recombination processes in the active layer. A second process, however, can lead to an increase of the LED output power. We conclude from an analysis of the current dependence that this process is due to a different mechanism.

High-brightness light-emitting diodes (LED) based on AlGaInP combines the possibility to achieve high efficiency with the flexibility of tuning the emission wavelength over a large range of the visible spectrum. For optimizing the device characteristics an accurate determination of the electronic properties, like e. g. the voltage drop across the semiconductor layer sequence, is desirable. We demonstrate the potential of Kelvin Force Microscopy for quantitative investigations of the voltage drop across the heterostructure layers of an operating AlGaInP LED. The surface potential was measured for external biases between -2.0 V and +1.86 V. By subtracting the zero bias result the voltage drop could be extracted quantitatively. In the low voltage regime, most of the voltage drops in the active layer. Above +1.5 V an additional voltage drop occurs on the p-side of the device, i. e. outside the active layer sequence, which reduces the efficiency of the LED. By comparing experimental data with simulations we will discuss possible mechanisms of these findings.

We present the results on aging mechanisms acting in high-power laser diode arrays (LDAs) employing Fourier-transform (FT) spectroscopy. The FT spectrometer was used as an excitation source for performing photocurrent (PC) measurements in two sets of aged LDA samples. The PC spectra reveal both the evolution of a defect band located in the optically active layer as well as modifications of the interband part of the spectrum upon aging. Such changes represent sensitive quantitative measures of the aging status and provide insight into the microscopic changes of the device structure upon aging.

The use of Germanium as an alternative substrate for the growth of AlInGaP LEDs provides several technical advantages such as lower substrate costs and the possibility of fabricating As-free AlInGaP devices. The LED layer structures are grown in a multiwafer MOVPE reactor on 4 inch Ge substrates. The growth conditions, such as temperature and substrate orientation, influence the LED external efficiency and its degradation behavior. In particular, it is found that during growth Ge is incorporated into the layers, which strongly affects the LED efficiency. Moreover a defect annealing occurs during regular operation resulting in an increased efficiency. Electrical characterization as well as deep level transient spectroscopy are performed in order to characterize the nonradiative recombination centers. In addition a quantitative analysis of the external quantum efficiency, before and after degradation, is carried out and the relative change in the nonradiative recombination rate is evaluated.

High brightness AlGaInP thin-film resonant cavity LEDs with an emission wavelength around 650 nm are presented. The combination of a thin-film waveguide structure and a resonant cavity with an omnidirectional reflector (ODR) leads to significantly higher efficiencies compared to standard resonant cavity LED (RCLED) structures. Preliminary devices based on this configuration show external quantum efficiencies of 23% and 18% with and without encapsulation, respectively, despite a non-ideal detuning. These devices exhibit a narrow far-field pattern and are therefore adapted for applications requiring high brightness emitters such as for example plastic optical fiber communications. By opting for a negative detuning, i.e. a cavity resonance that is red-shifted compared to the intrinsic emission spectrum, even higher efficiencies should be achievable.

The internal quantum efficiency of light emitting diodes (LEDs) based on the AlGaInP material system was analysed. This study formulated a comparatively simple rate equation model to gain a quantitative understanding of the contributions of the different loss mechanisms. On the experimental side a series of LED structures was grown with multiple (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P quantum wells embedded in In/sub 0.5/Al/sub 0.5/P confining layers. The emission wavelength was adjusted varying the aluminum content, from 650 nm down to 560 nm in the green spectral range. Simple test structures were processed and the optical output power measured over a certain range of operating currents and temperatures. On the other hand, in the computation of the external quantum efficiency it is important to note that due to re-absorption and re-emission of light in the active layer ('photon recycling') the extraction efficiency becomes dependent on the internal quantum efficiency. This effect was accounted for by performing ray-tracing simulations for these specific LED structures. For small internal efficiencies the extraction efficiency decreases by a factor of 3. Including this effect in the model a good fit to the experimental data over a wide temperature and current range could be achieved.

Operation-induced degradation of internal quantum efficiency of high-brightness (Al x Ga 1-x ) 0.5 In 0.5 P 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.

High-Power Laser Diode Arrays (LDA) are of increasing interest as pump sources for solid-state lasers or for printing. For increasing reliability and lifetime of LDAs detailed knowledge about the microscopic effects accompanied with gradual aging under operating conditions is required. It will be demonstrated that in lateral direction spatially resolved electroluminescence (EL) and photocurrent (PC) spectroscopy can be used as sensitive tools for analysing strain and creation of defects in the active region of the device.

High-power laser diode arrays (LDA) are of utmost importance for a number of applications such as for pumping solid state lasers, laser frequency conversion, material processing and printing. For these applications device operation conditions include heavy thermal load, resulting in aging processes which limit lifetime and reliability. In order to overcome these limits microscopic insight into the aging processes is needed. Device aging is well-known to be accompanied by deep level creation, however, the mechanism trough in which specific deep levels are associated with a failure event must be separately proved for each device design. Thus analytical work and the development of new analytical techniques are required. Here we introduce Fourier-transform photo-current (FTPC) spectroscopy as a new method for the spectroscopy of deep levels in devices, e.g. for monitoring changes in the deep level concentration within the optically active layer. The basic idea of this technique is to measure the spectral sensitivity of the LDA for front facet illumination by the light coming from a FT spectrometer. The electrical signal generated within the LDA is taken from the power supply contacts of the device and is fed into the signal input port of the spectrometer. We find that the spectra obtained in this way are extremely sensitive to the aging processes and in particular to the increase of deep level contributions.

Different diagnostic methods were investigated in order to evaluate their potential as aging sensitive tool for the analysis of high-power laser diode arrays. The well-known method of photocurrent spectroscopy, i.e. the measurement of the spectral sensitivity of a laser diode acting as a detector, was found to monitor aging properties of high power laser diode arrays in a convenient way. Photocurrent spectra of high power laser diode arrays emitting at 808 nm (1.53 eV) were investigated in the 0.8 - 3.0 eV photon energy range. Aging induced changes in different spectral regions reveal the influence of difference mechanisms affecting the structure. One aging effect--the growth of the defect concentration within the optically active layers of the devices--is monitored. Conclusions on the microscopic nature of the changes are drawn and several applications are discussed.

Optical probes are used for the investigation of aging properties of high-power laser diode arrays. Two methods - micro-Raman and laser beam induced current (LBIC) scans are discussed: Micro-Raman spectroscopy - a laboratory based method - was applied in order to investigate facet temperature distributions for fresh and aged high-power laser diode arrays (LDA) under regular operation conditions. Furthermore, facet temperatures were measured for operation close to the catastrophic optical damage (COD) level.For these experimental situation facet temperatures of more than 600 degrees C were obtained. In order to conclude from pure temperature data to the thermal management of the LDA we performed modeling work based on the finite element method. Close to the COD up to 14 percent of the total thermal load of the LDA is concentrated to a very thin region close to the facet. Photocurrent spectra and LBIC were found to indicate aging induced effects such as defect creation as well as strain effects. LBIC - method which also could be used as test method for fabrication processes - allows to map aging induced changes of defect distributions in LDAs. Furthermore, the potential of LBIC as quick strain inspection method is discussed.

High-power laser diode arrays (LDA) are important radiation sources for a number of applications. Device operation usually happens under heavy thermal load. Thus for reducing failure sensitivity, detailed insight into the power balance of the devices is of great importance. We report on new facet temperature measurements carried out in 808-nm emitting high-power LDA based on InAlGaAs/GaAs double quantum well structures. We determined temperature distributions along the line of the emitters in arrays as well as across the optical active layer of single emitters. A comparison of facet temperatures for different laser waveguide architectures such as graded index, step index, and large optical cavity is provided. Together with the bulk temperature, as well as the temperature of the submount close to the laser chip, we obtain extensive information on the temperature distribution in devices during high-power operation. A second series of experiments was aimed at getting insight into thermal processes accompanied with catastrophic optical damage. Using the set of experimental temperature data, FEM simulations enable us to consistently describe the power management of the device for various operation conditions.

The efficiency of organic light-emitting diodes (OLEDs) is still limited by poor light outcoupling. In particular, the excitation of surface plasmon polaritons (SPPs) at metal-organic interfaces represents a major loss channel. By combining optical simulations and experiments on sim-plified luminescent thin-film structures we elaborate the conditions for the extraction of SPPs via coupling to high-index media. As a proof-of-concept, we demonstrate the possibility to extract light from wave-guided modes and surface plasmons in a top-emitting white OLED by a high-index prism.

The authors present a detailed investigation of defects generated during catastrophic optical damage (COD) in high-power 650nm AlGaInP lasers using microphotoluminescence (μ-PL) mapping, focused ion beam (FIB) microscopy, and deep-etching techniques. High-resolution μ-PL images demonstrated that during COD, nonradiative dark line defects (DLDs) originate from the front mirror of the laser and propagate in several branches into the laser perpendicular to the output facet. Furthermore, FIB microscopy identified the epitaxial layers affected by COD, revealing that DLDs are confined to the active region. In addition, deep etching confirmed that these defects have a noncrystalline nature.

The authors report on quantitative investigations of the voltage drop across the heterostructure layer sequence of an operating AlGaInP light-emitting diode via Kelvin probe force microscopy for different external biases between −2.0 and +1.86V. In the low voltage regime, most of the voltage drops in the active layer. For bias voltages above +1.5V, however, they found an additional voltage drop on the p side of the device, which reduces the power efficiency of the light-emitting diode.

The internal quantum efficiency of (AlxGa1−x)0.5In0.5P light-emitting devices (LEDs), with an emission wavelength ranging from 650 to 560 nm, is determined by means of a model that takes into account the radiative and nonradiative recombination in the active layer, the diffusive leakage of carriers into the confining layers, and the influence of photon recycling on the light extraction efficiency. The evaluation is based on measurements of the external quantum efficiency of the LEDs as a function of the operating current and temperature. The analysis provides the wavelength dependence of both the nonradiative recombination as well as the carrier leakage.