Berthold Hahn

High-Power and High-Efficiency InGaN-Based Light Emitters

In this paper, we report on the latest advancements in improving AlGaInN-based visible-light-emitting-diode (LED) efficiency in epitaxy, chip, and package designs. We investigate the fundamental origin of the typical high current ¿droop¿ of efficiency observed in such LEDs. We show that this effect is most likely not caused by incomplete carrier injection or carrier escape but that it is rather a fundamental material property of InGaN/GaN-heterostructure-based light emitters. The droop can be reduced in improved epitaxial LED active-layer designs. We show how this can be achieved by lowering InGaN volume carrier density in multiple quantum wells (MQWs) and thick InGaN layers. Improved epitaxial MQW structures are then combined with a new advanced chip concept. It is optimized for high efficiency at high current operation and arbitrary scalability and can be manufactured at low cost. This is accomplished by improving light-extraction efficiency, homogenizing the emission pattern, reducing forward voltage, and lowering thermal resistance. The improved high current efficiency can be fully exploited by mounting the chip in the highly versatile new OSLON SSL package. It features very stable package materials, a small footprint, and an electrically isolated design decoupling electrical and thermal contacts.

A combined electro-optical method for the determination of the recombination parameters in InGaN-based light-emitting diodes

We present an electro-optical method for the extrapolation of the nonradiative and Auger recombination coefficients in InGaN/GaN Light-emitting diodes (LEDs). The method has the advantage of permitting the extrapolation of the recombination parameters of packaged devices, contrary to conventional techniques based on the analysis of quasibulk structures. For the analyzed devices, the average values of the nonradiative and Auger recombination coefficients have been determined to be equal to 2.3×107 s−1 and 1.0×10−30 cm6 s−1, respectively. These results are consistent with previous reports based on the analysis of quasibulk structures and on theoretical simulations. The method described in this paper constitutes an efficient tool for the analysis of the recombination dynamics in GaN-based LEDs. The results obtained within this work support the hypothesis on the importance of Auger recombination in determining the so-called efficiency droop in LED structures.

Influence of indium content and temperature on Auger-like recombination in InGaN quantum wells grown on (111) silicon substrates

High-efficiency InGaN-based light-emitting diodes have been grown on (111) silicon substrates and investigated with regard to efficiency and carrier lifetime as a function of current density. Using a single quantum well active layer ensures a well-defined active volume which enables the precise determination of the recombination coefficients in the ABC rate model for different emission wavelengths and junction temperatures. Good agreement of the resulting C values with calculated Auger coefficients is found both with respect to absolute value as well as their dependence on bandgap energy and temperature.

Experimental Determination of the Dominant Type of Auger Recombination in InGaN Quantum Wells

We investigate theoretically the influence of type and density of background carriers in the active region on the quantum efficiency of InGaN-based light emitters using an extension of the ABC rate model. A method to determine experimentally whether a certain type of Auger recombination is relevant in InGaN quantum wells is derived from these considerations. Using this approach, we show that the physical process which is the dominant cause for the efficiency droop is superlinear in the electron density and can thus be assigned to nnp-Auger recombination.

Leakage current and reverse-bias luminescence in InGaN-based light-emitting diodes

This paper reports an electro-optical analysis of the correlation between reverse-bias leakage current and luminescence in light-emitting diodes based on InGaN. The results of the analysis suggest that (i) the main mechanism responsible for leakage current conduction is tunneling, (ii) leakage current is correlated with the presence of reverse-bias luminescence, (iii) leakage current flows through preferential paths, that can be identified by means of emission microscopy, and (iv) reverse-bias luminescence could be ascribed to the recombination of electron-hole pairs in the quantum well region.

Development of high-efficiency and high-power vertical light emitting diodes

We provide an overview of the vertical chip technology and discuss recent improvements that have enabled (AlGaIn)N-based light-emitting diodes to further extend the range of their applications. In particular, the excellent scalability of chip size and low electric losses make related devices predestinated for use in high-power and high-luminance tasks. The evolution from standard vertical chips to the advanced chip design is described from a conceptual as well as from a performance point of view. Excellent stability data under demanding conditions are shown, which are the basis for the operation of devices in automotive applications requiring high reliability at current densities exceeding 3 A/mm2. As the vertical chip technology is not directly dependent on the substrate owing to its removal in the chip process, it is highly flexible with respect to the change of substrate materials to the very promising (111) silicon, for example.

Analysis of Defect-Related Localized Emission Processes in InGaN/GaN-Based LEDs

This paper reports an extensive analysis of the defect-related localized emission processes occurring in InGaN/GaN-based light-emitting diodes (LEDs) at low reverse- and forward-bias conditions. The analysis is based on combined electrical characterization and spectrally and spatially resolved electroluminescence (EL) measurements. Results of this analysis show that: (i) under reverse bias, LEDs can emit a weak luminescence signal, which is directly proportional to the injected reverse current. Reverse-bias emission is localized in submicrometer-size spots; the intensity of the signal is strongly correlated to the threading dislocation (TD) density, since TDs are preferential paths for leakage current conduction. (ii) Under low forward-bias conditions, the intensity of the EL signal is not uniform over the device area. Spectrally resolved EL analysis of green LEDs identifies the presence of localized spots emitting at 600 nm (i.e., in the yellow spectral region), whose origin is ascribed to localized tunneling occurring between the quantum wells and the barrier layers of the diodes, with subsequent defect-assisted radiative recombination. The role of defects in determining yellow luminescence is confirmed by the high activation energy of the thermal quenching of yellow emission (Ea = 0.64 eV).

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.

Degradation of High-Brightness Green LEDs Submitted to Reverse Electrical Stress

This letter describes an extensive analysis of the reverse-bias degradation of green light-emitting diodes. The analysis consists in a wide set of stress tests carried out under different negative-bias levels. The results presented in this letter indicate the following: 1) Leakage current is strongly correlated to the presence of reverse-bias luminescence; 2) reverse current flows through preferential leakage paths and is due to a soft-breakdown mechanism that is possibly correlated to the presence of structural defects; 3) reverse-bias stress can induce an increase in the leakage current, with a corresponding decrease in the breakdown voltage of the samples; and 4) the degradation rate has a linear dependence on the (reverse) stress-current level, suggesting that degradation is induced by hot carriers. On the basis of the evidence collected in this letter, degradation can be ascribed to the generation/propagation of point defects due to the injection of highly accelerated carriers.

Advanced technologies for high-efficiency GaInN LEDs for solid state lighting

Solid state lighting has seen a rapid development over the last decade. They compete and even outperform light sources like incandescent bulbs and halogen lamps. LEDs are used in applications where brightness, power consumption, reliability and costs are key parameters as automotive, mobile and display applications. In the future LEDs will also enter the market of general lighting. For all of these new applications highly efficient, scalable and cost efficient technologies are required. These targets can be matched by SiC based flip chip LEDs which enable the design of high current chips with efficiencies of up to 28 lm/W in white solderable packages. An alternative approach is the implementation of thinfilm technology for GaInN. The LED is fabricated by transferring the epilayers with laser lift off from sapphire to a GaAs host substrate. In combination with efficient surface roughening and highly reflective p-mirror metalization an extraction efficiency of 70% and wall plug efficiency of 24% at 460 nm have been shown. The chips showed 16 mW @ 20 mA with a Voltage of 3.2 V. The technology is scalable from small size LEDs to high current Chips and is being transferred to mass production.