NUSOD Blog

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Tag Archives: high power

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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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

GaN-based lasers for droop-free lighting?

droop2Ever since Shuji Nakamura mentioned in his Nobel lecture in Stockholm that lasers are the future of lighting, I am puzzled by this claim, especially by the now widely circulated statement that laser diodes are free from efficiency droop. It suggests an advantageous energy efficiency of laser diodes, as shown in the last figure 17 of his lecture (which is actually invalidated by his own reference). If you are familiar with the much debated efficiency droop burdening GaN-LED lighting, you will agree that the underlying carrier loss mechanisms are also  present in laser diodes. Even worse, laser diodes require a higher carrier density in the active layers and therefore exhibit stronger Auger recombination and possibly also electron leakage already at lasing threshold. Read more of this post

Blue LED with more than 100% quantum efficiency

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Energy band diagram (red) and light emission profile (blue) of the proposed LED design.

Quantum efficiency is the ratio of emitted photons to injected electrons. So, how can an electron generate more than one photon ?  One possible solution was demonstrated numerically by inserting tunnel junctions into the multi-quantum well (MQW) active region of a GaN-based LED (details). With one tunnel-junction (TJ), each electron gets two chances to generate a photon, because it can tunnel back into the conduction band after the first generation process. Inspired by the numerical demonstration, such GaN-based TJ-MQW LED was recently fabricated for the first time (details).  With three tunnel-junctions (picture), each electron gets four chances to generate a photon, enabling quantum efficiencies up to 400%. Read more of this post

Experts still divided over cause of GaN-LED efficiency droop

I conducted a little poll among the attendeeLED efficiency droops of the LED conference at Photonics West in San Francisco last week. They still turn out to be divided over the microscopic mechanism behind the high-power efficiency loss in GaN-based LEDs. This droop phenomenon threatens the success of solid-state lighting applications since the energy efficiency of LED light bulbs remains close to that of traditional fluorescent lamps. I found a similar division in research publications on this topic, details are given in this review. Read more of this post