NUSOD Blog

Connecting Theory and Practice in Optoelectronics

Category Archives: LED

Full Monte Carlo perspective on hot carrier effects in III-N LEDs

Figure_3_blogRoughly 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

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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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NUSOD 2016 Preview: Electroluminescent Cooling of GaN-LEDs

PeltierMuch attention is paid to the efficiency droop of GaN-based light-emitting diodes (GaN-LEDs) but another phenomenon observed on industry-grade devices is still hardly investigated in the literature: the astonishingly low bias measured even at higher current. Hurni et al. report that photons emitted from their blue LEDs have a higher energy than the injected electrons, i.e., the electrical efficiency exceeds unity up to a current density of 75 A/cm2. The suspected reason is the absorption of thermal energy by injected electrons, which is subsequently removed by blue light emission. We have recently reproduced this phenomenon by advanced numerical simulation.  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

Efficiency droop caused by carrier-induced strain shift?

srep17227-f2We still don’t fully understand the efficiency droop observed in GaN-based light-emitting diodes (LEDs). That is why I keep looking for new ideas and I just found one at nature.com [1]. The authors of this paper performed Raman measurements and observed that the GaN strain turns from compressive to tensile with increasing current density while the efficiency droops by 80% (see figure). Based on first-principle calculations, they claim that the strain reversal is caused by carrier accumulation within the InGaN quantum wells. But how can the rising density of electrons and holes expand the InGaN lattice constant in such dramatic way? Read more of this post