- 8,834 visits since January 2015
Optoelectronic Device NewsMy Tweets
Connecting Theory and Practice in Optoelectronics
Thus far, the highest output power measured on GaN-based lasers is about 7W, as shown in the picture.  In comparison, some GaAs-based lasers emit more than 30W in continuous-wave operation at room temperature. A key reason for this difference is the inherently large p-side electrical resistance of GaN-based laser diodes. It leads to strong Joule heating which lowers the gain and boosts various loss mechanisms that eventually cause the typical power roll-off at high currents.  Read more of this post
Auger 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.
Roughly four years ago researchers at the École Polytechnique and UCSB reported that III-Nitride (III-N) LEDs exhibit hot carrier effects in a strong correlation with the efficiency droop. These new measurements added fuel to the already actively ongoing discussion in the III-N LED device simulation community about the development of more accurate simulation models than the presently used quasi-equilibrium models. In particular, the measurements and subsequent works suggested that hot electrons and holes created in the process of Auger recombination might even affect the operating voltage of LEDs. However, full LED device simulations have so far lacked detailed models of hot carrier effects and primarily relied on drift and diffusion currents of carriers within the Fermi-Dirac distribution. Read more of this post
The 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.
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.