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Connecting Theory and Practice in Optoelectronics
It’s time again to reflect on my peer review experience over the past year. Supported by the availability of high-end commercial software, the number of journal paper submissions on optoelectronic device simulation keeps rising. However, authors often seem to view such software as magic tool that instantaneously delivers realistic results. Mathematical models always simplify reality. But how simple is too simple? Some papers don’t even discuss the underlying theory. There are different levels of simplification possible, which are all based on specific assumptions. Certain assumptions may be inappropriate in the given situation. That is why high-end software packages offer some alternative modeling approaches and let the user decide. In other words, the user should have a detailed understanding of internal device physics and of the models provided by the software.
However, this is only the first step of a successful simulation strategy. The next step is the evaluation of material parameters used in the software. Initial simulation results are typically far off measured characteristics because key parameters are inaccurate. Literature values are quite scattered in some cases. If crucial parameters cannot be measured directly on the device, they should be varied in the simulation until quantitative agreement with measurements is achieved. The model itself may be inadequate if such effort fails or if the fit value is outside the published range. On the other hand, competing models could deliver nearly identical results (see picture) so that more decisive measurements are needed. Such calibration process is often difficult and time-consuming, but in my view, it is the only way to accomplish realistic simulations. Otherwise, calculated results are unreliable and may lead to wrong conclusions. Read more of this post
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
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.
Much 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
Ever 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