NUSOD Blog

Connecting Theory and Practice in Optoelectronics

NUSOD 2017 Preview: Maximum GaN-laser power limited by Auger recombination

AugerT80Auger recombination inside the light-emitting InGaN quantum wells (QWs) was recently identified as major cause of output power limitations in GaN-based blue light-emitting diodes (LEDs) which are the core of many modern light sources. In this electron-hole recombination process, the released energy is transferred to another carrier (electron or hole) without light emission. The Auger recombination rate rises strongly with the QW carrier density and therefore intensifies with stronger current injection into the LED.

In contrast to LEDs, GaN-based blue laser diodes are expected to suffer less from Auger recombination,  based on the popular opinion that the QW carrier density does not rise with increasing current injection above lasing threshold. Shuji Nakamura, who received the 2014 Nobel Prize in physics for his pioneering work on GaN-LEDs, stated in his Nobel lecture that “Auger recombina­tion, with the resulting efficiency droop, does not appreciably occur in blue laser diodes”.  We dispute this claim based on our numerical analysis of  high-power InGaN/GaN laser measurements.

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What causes the low energy efficiency of GaN-based lasers?

CW80CThe energy efficiency is the fraction of the electrical input energy that is emitted as laser light. It is usually given as power conversion efficiency (PCE) and it is surprisingly low for GaN-based lasers. OSRAM just announced a record number of PCE=43%  at SPIE Photonics West. This is certainly a remarkable achievement, considering the struggle to break the mysterious 40% limit. However, 43% is far below the record PCE of 84% reported for GaN LEDs. The inherently low hole conductivity and large series resistance on the p-doped side of GaN lasers are usually blamed for the efficiency deficit. However, the series resistance is known to shrink with rising temperature, which can be attributed to the increasing density of free holes in p-doped layers. Thus, one would expect that the PCE improves at elevated temperatures. But the OSRAM paper reported that the measured PCE drops with higher ambient temperature despite the shrinking series resistance. Ergo, there seems to be an even stronger loss mechanism involved.

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How to get your simulation paper accepted

Looking back at 2016, I just realized that my yearly load of peer reviews has increased to almost 80 journal papers, mainly in the field of optoelectronic device simulation. The rising number of such paper submissions to top journals is certainly good news, but the paper quality is often insufficient. Unfortunately, I have to propose rejection of most papers after a detailed assessment of essential mistakes. A fundamental mistake in my view is the unproven assumption that simulations represent the real world. Authors often don’t seem to understand that computer simulations lead us into a virtual reality in which many unreal effects can happen – depending on their choice of mathematical models and  material parameters.

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A fresh look at high-power laser analysis

nthOne of the key rules of semiconductor laser physics relates to the carrier density inside the active layer. As long as I can remember, this rule states that the carrier density remains constant when the injection current rises above the lasing threshold. The reason lies in the stimulated emission of photons which consumes all additional carriers injected above threshold. The threshold carrier density delivers the threshold optical gain that compensates for the optical loss, which is usually not dependent on the injection current. Thus, the threshold carrier density should also remain constant. However, my recent analysis of high-power lasers yields different results (see picture). Read more of this post

NUSOD 2016 Conference breaks paper record

NUSOD-16 delegates

NUSOD 2016 participants

Thanks to all participants and organizers for a very stimulating conference in Sydney which featured a record number of 103 papers, two more than in Rome 2011. Summaries of all presentations are now available online. For the first time, we conducted a poster competition which contributed to two quite vibrant poster sessions. Prizes were awarded to posters MP01, MP31, and TuP32. Our rump session on Thursday evening started some interesting and important discussions which may continue on this blog. The 17th NUSOD conference will take place at the Technical University of Denmark, July 24-28, 2017. I very much hope to see you there!