Success in the laboratories of OSRAM in Regensburg
Researchers at OSRAM Opto Semiconductors, the high-tech company based in Regensburg, Germany, have been able to demonstrate the physical effect responsible for the decrease in efficiency (droop) at typical operating current densities. In LEDs, based on the indium gallium nitride (InGaN) material system, the “bipolar Auger effect” reduces the efficiency with which charge carriers are converted into light. This research provides semiconductor experts in academic institutes and in industry with a plausible roadmap for finding ways to suppress this effect.
“Droop” is the name given to the drop in luminous efficacy (efficiency) of an LED at current densities above a few amps per square centimeter, particularly at a typical operating current of 350 milliamps (mA) for a 1mm² chip. Up to now, this effect could be measured, but none of the various hypotheses could be definitively proved by experiment. Drawing on its many years of technological know-how, OSRAM Opto Semiconductors has now conducted a specially designed experiment to show that the bipolar Auger effect is a major contributor to droop. The results of the experiment have been published in the “Applied Physics Letters” journal.
OSRAM Opto Semiconductors demonstrates Auger recombination for holes as well
The generation of light in an LED is based on radiative recombination of an electron with a hole. In the case of Auger recombination in semiconductors, the energy released, is not converted into light but is transferred to a third charge carrier – in other words either an electron or a hole receives additional energy. The high-energy charge carrier then “relaxes” by releasing its energy as heat.
The University of Santa Barbara (UCSB) recently provided evidence of high-energy Auger electrons in a blue LED under typical operating conditions. The experiment conducted by OSRAM Opto Semiconductors has now shown that holes behave in a similar way. This is particularly relevant for the design of LEDs, because electrons and holes contribute equally to the generation of light.
The UCSB recently detected the emission of high-energy electrons which they associated with Auger recombination from a blue LED under typical operating conditions. However, alternative explanations concerning the origin of these hot carriers are discussed among researchers. Scientists at the Rensselaer Polytechnic Institute presented indication that a decreasing fraction of electrons reaching the layers intended for light generation causes the droop. The experiment conducted by OSRAM Opto Semiconductors has now shown directly that the Auger processes, where an electron and a where a hole acts as the third particle, both occur at a relevant rate and therefore lead to a significantly reduced efficiency.
The bipolar Auger effect is a major contributor to the droop
In this experiment the detector for the Auger effect in the form of ultraviolet quantum wells is directly incorporated in the structure to be investigated. This modified setup enabled to detect orders of magnitude more Auger charge carriers than UCSB was able to show. This in turn led researchers to the conclusion that Auger recombination is the main reason for droop. “Our project has identified the principal cause of the droop effect. The scope of future research projects can now be narrowed, with the focus particularly on measures to eliminate the Auger effect”, said Dr. Roland Zeisel, head of Analytics and Metrology at OSRAM Opto Semiconductors.
Years of know-how provide the platform
Behind the insight into the bipolar Auger effect lie years of accumulated know-how at OSRAM Opto Semiconductors in the field of epitaxy. The expertise of the semiconductor company in terms of analytics and metrology, combined with its knowledge of crystal growth, were the keys to success in this research project. After a way has been found to suppress droop, it will be possible to produce even more efficient light emitting diodes that will be able to offer the same or greater light output from smaller, and therefore less expensive, chips.
Information on the experiment and a detailed presentation of the results, that have been made in the context of PhD thesis, can be found in the “Applied Physics Letters” (Applied Physics Letters 103, 071108 (2013).