Rainer Butendeich

Investigation of Efficiency-Droop Mechanisms in Multi-Quantum-Well InGaN/GaN Blue Light-Emitting Diodes

Efficiency-droop mechanisms and related technological remedies are critically analyzed in multi-quantum-well (QW) InGaN/GaN blue light-emitting diodes by means of numerical device simulations and their comparison with experimental data. Auger recombination, electron leakage, and incomplete QW carrier capture can separately produce droop effects in quantitative agreement with experimental data, but “extreme” values, at the limit of or outside their generally accepted range, must be imposed for related droop-controlling parameters. Less stringent conditions are needed if combinations of the aforementioned mechanisms are assumed to act jointly. Applying technological/structural modifications like QW thickness or number increase and barrier p-type doping leads to distinctive effects on droop characteristics depending on the assumed droop mechanism. Increasing the QW number appears, in particular, to be the most effective droop remedy in case the phenomenon is induced by Auger recombination. Possible technology-dependent variation of droop-controlling parameters and/or multiple droop mechanisms can, however, make discrimination of droop origin on the basis of the effects of applied technological remedies very difficult.

Soft and Hard Failures of InGaN-Based LEDs Submitted to Electrostatic Discharge Testing

This letter reports an extensive analysis of the degradation mechanisms of InGaN-based light-emitting diodes (LEDs) submitted to reverse-bias electrostatic discharge (ESD). The results of this analysis indicate that two different failure modes, namely, “soft” and “hard” degradations, can be induced by ESD pulses. The “soft” failure mode takes place as a consequence of ESD events with moderate voltage/current levels and consists in a decrease in the reverse-bias leakage current of LEDs. This effect is due to the annihilation of some of the defective paths responsible for leakage-current conduction, possibly triggered by the injection of relatively high reverse-bias current densities. “Hard” failure takes place when high-voltage/current ESD pulses are applied to an LED. After hard failure, LEDs behave as short circuits. This process is due to the high voltage levels reached by the junction during an ESD event (with subsequent dielectric rupture) or to the injection of extremely high current densities through one of the localized paths responsible for reverse-current conduction.

Far-field radiation pattern of red emitting thin-film resonant cavity LEDs

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.

A study of the failure of GaN-based LEDs submitted to reverse-bias stress and ESD events

This paper describes an extensive analysis of the degradation of InGaN-based LEDs submitted to reverse-bias stress and Electrostatic Discharge events. Results described within the paper indicate that: (i) reverse-bias current flows through localized leakage paths, related to the presence of structural defects; (ii) the position of these paths can be identified by means of emission microscopy; (iii) reverse-bias stress can induce a degradation of the electrical characteristics of the devices (decrease in breakdown voltage); (iv) degradation is due to the injection of highly accelerated carriers through the active region of the LEDs, with the subsequent generation/propagation of point defects; (v) the localized leakage paths responsible for reverse-current conduction can constitute weak regions with respect to reverse-bias ESD events.

100-lm/W InGaAlP Thin-Film Light-Emitting Diodes With Buried Microreflectors

Thin-film light-emitting diodes (LEDs) belong to the most successful LED concepts for achieving high efficiencies. The incorporation of buried microreflectors with inclined facets prevents the light generation under the top contact and bondpad and offers an additional light extraction scheme. As a result, an external quantum efficiency of 50% could be demonstrated at a wavelength of 650 nm, and a luminous efficiency of more than 100 lm/W could be achieved in the wavelength range from 595 to 620 nm

InGaAlP thin film LEDs with high luminous efficiency

In Thinfilm LEDs, the substrate absorption of the generated light is avoided by a metal reflector between the light emitting layer and the substrate. The light extraction can be further enhanced by buried microreflectors or surface texturing. We demonstrate that the combination of these technologies gives prospects equal or superior to all other known approaches in terms of luminous efficiency and luminance. At a peak wavelength of 617 nm, we have obtained a luminous efficiency of 95.7 lm/W at 20 mA. We further analyze the internal and light extration efficiencies of our LEDs using raytracing simulations as well as a theoretical model for the internal efficiency. This analysis shows quantitatively that the efficient light extraction from InGaAlP thinfilm LEDs becomes more and more difficult when approaching shorter wavelengths.

High-brightness red-emitting AlGaInP thin film RCLEDs

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.

Analog Modulation of 650-nm VCSELs

Small-signal properties of 650-nm vertical-cavity surface-emitting lasers (VCSELs) with different oxide aperture sizes were measured. A small diameter VCSEL of 3.5 mum has a maximum resonance frequency of 5.7 GHz. The photon density determines the maximum resonance frequency. Modeling also indicates higher photon densities in the small VCSEL due to better thermal behavior

Red surface emitters: powerful and fast

Vertical cavity surface emitting lasers (VCSEL) in the GaInP/AlGaInP material system have experienced a rapid development in their short history. In general lasers from that material system are suitable for a huge number of applications beginning with TV lasers and high power lasers for edge emitters, continuing with optical data storage, medical applications as well as data communication in cars, air planes, offices and between computers as application field for VCSELs. Especially automotive applications show the highest requirements on a laser with respect to operation temperature and power. In this talk we draw out the problems of the material system AlGaInP and its implications for laser applications. We discuss the epitaxial and technological solutions to overcome at least a part of these inherent problems. We will discuss the possible power that we can expect from VCSELs emitting in the range between 650 nm to 670 nm. We got from our lasers 5 mW, CW @ RT, 670nm and 2.5mW, CW@RT, 650 nm. We emphasize the role of doping, Bragg mirror grading, suitable detuning of cavity mode and gain, and optimisation of the contact layer and control of the oxide aperture in the VCSEL structure to get improved operation characteristics at higher temperatures. From the analysis of high frequency measurements, we could evaluate modulation bandwidths between 4 GHz and 10 GHz. The application of polyimide as a dielectric isolation material shows the potential to obtain modulation bandwidths beyond 10 GHz. For the intrinsic modulation bandwidth we get a value of 25 GHz, which is near the value edge emitters show. A more detailed discussion on photon lifetimes and carrier transport times will be given in the talk. Red light emitting VCSELS driven with short current pulses showed laser emission up to + 160°C case temperature. Thus, a CW operation up to +120°C can be expected after further improvement of power generation (decrease of series resistance) and heat spreading (optimized contacts and mounting). From these characteristics we can conclude that AlGaInP-surface emitting lasers have a real potential as low cost lasers for automotive applications as we all as data communication applications up to 10 GHz.

Red VCSEL for automotive applications

In this paper we discuss the problems of the AlGaInP material system and its consequences for the laser applications in vertical-cavity surface-emitting lasers (VCSEL). The epitaxial and technological solutions to overcome at least parts of the inherent problems were presented. Measured power-current curves of 660nm AlGaInP-based oxide-confined VCSEL are compared with calculated data by a cylindrical heat dissipation model to improve heat removal out of the device. Pulsed lasing operation of a 670nm VCSEL at +120°C heat sink temperature is demonstrated, where we exceeded 0.5mW and at +160°C still 25μW output power were achieved. We also studied the modulation bandwidth of our devices and achieved 4GHz and calculations lead to a maximum possible intrinsic -3dB frequency of 25GHz.