NUSOD Blog

Connecting Theory and Practice in Optoelectronics

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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Call for Papers: Special Journal Issue on Multiscale Modeling of Solar Cells

In the last two decades, there has been an increasing interest in multiscale modeling applied to electronic devices. Several factors are driving this trend. On the one hand, device dimensions of “classical” devices like MOSFETs have continuously been scaled down in order to increase device performance. On the other hand, specific properties of quantum structures are systematically utilized in modern devices. The embedding of the active device region in its environment including access regions and contacts, and the mutual interaction between different aspects like optics, thermal heating, strain and carrier transport requires an involved multiscale/multiphysics simulation approach which can handle different physical models and different length or time scales.

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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!