PhD subject proposed at CEA-LETI in Grenoble in the Optics and Photonics Departement (DOPT)
Impact of InGaN quantum wells design and composition on the performance of µLEDs
The growth and fabrication of InGaN/GaN blue LEDs have reached a high degree of maturity thanks to their extensive use in the field of LED lighting. Because of their robustness, thermal stability and high efficiency, these LEDs are seen as promising candidates to make high-luminance and high resolution micro-displays for the field of augmented reality. The main difference is that the dimensions of LEDs are now of a few µm instead of a few hundreds. Over the last 10 years, the CEA-LETI has developed a strong expertise in the realization of such InGaN-based microdisplays1. In addition, the ideal way of making color micro-displays is to use µ-LEDs emitting in the three fundamental colors: Blue, Green and Red (RGB) by changing the Indium composition in the quantum well (the option of using color conversion has inherent efficiency limitations). Blue and green-emitting microdisplays have already been realized at LETI and continue to be improved. They have taken advantage of the maturity of LED epi-structure developed for large LEDs. For the realization of future red emitting micro-display, an extensive work has been done at LETI to realize full InGaN red LED on InGaN-based buffer, that allows strain relaxation necessary for a higher Indium incorporation2. The use of the same nitride semi-conductor could ultimately ease the realization of a full color µdisplay.
However contrary to the large LEDs used in the lighting industry, the µLEDs composing the pixels of µdisplays suffers from a loss of efficiency due to non-radiative recombination happening at the sidewalls. Many process-related defects lies at this interface (impurities, vacancies, dangling bonds etc.) and are expected to act as non-radiative defects. The negative impact of the sidewalls on LED performance increases with the perimeter-over-surface ratio, and hence becomes critical for µLEDs3. The passivation is the usual way to recover a high efficiency for these µLEDs. It mainly consists in chemical etching of defects and in binding dangling bonds at the interface with an oxide or a nitride. But diffusion length of electrons and holes in the InGaN quantum wells should also play a role in the performance of µLEDs. Indeed a large diffusion of carrier should increase the probability of non-radiative recombination at the sidewalls and hence decrease the efficiency of the LEDs.
It is well known that the Indium atoms random distribution in the InGaN quantum wells induces potential fluctuations that can reduce the lateral diffusion of carriers. It is expected that this effect depend on the Indium concentration in the quantum wells. This would prevent non-radiative recombination at the sidewalls of µLEDs from happening. Indeed, it was reported by Smith and coworkers that that the degradation of quantum efficiency with the size reduction of µLEDs was not clearly observed in green quantum wells but it was in the blue ones4. The Indium concentration in the quantum wells seems to have a beneficial impact on the realization of µLEDs. On the other side, it was recently emphasized that, contrary to the previous believes, electron-hole pairs can diffuse over long distances in some very good quality InGaN quantum wells5. The author measured a diffusion length ranging from 1 to 50µm depending on the carrier concentration in the quantum well. These apparently conflicting results call for an investigation.
We propose to study the impact of quantum well design and composition on the performance of µLEDs. Over the last 10 years, researchers have gained a much better understanding of InGaN quantum wells in large LEDs6. However, the requirements for µLEDs developed for micro-displays applications are different from those for large LEDs used in the field of lighting. For example, in the case of large LEDs, it is possible to get blue quantum wells with a very high Internal Quantum Efficiency (IQE~95%) as for green quantum wells IQE cannot go beyond 40% and for red beyond just a few %. However, for µLED, because of the impact of non-radiative recombination at the sidewalls and possible different diffusion coefficients, it is not clear that the performance ranking will remain the same (or at least not with the same ratio). The optimum design and composition of the active region for the fabrication of blue, green and red µLEDs will likely be different from the optimum design of large LEDs.
The proposed PhD thesis will consist in designing and growing InGaN quantum wells by Metal Organic Chemical Vapor Deposition (MOCVD) varying the Indium composition and the active region design and by characterizing them thoroughly with various spectroscopic technics. We are particularly interested by the variations of IQE and the related carrier diffusion length between different designs of quantum wells. In addition, µLEDs will be realized on selected epi-structures to test the performances of devices made with optimized quantum wells. This last step may or may not be realized directly by the student, as the focus of the PhD should be in priority the design, the growth and, for a large part, the characterization of the InGaN quantum wells in order to identify the best structure to make efficient blue, green and hopefully red µLEDs. For this reason, the PhD candidate will be based at the LCEM (emissive devices laboratory) where part of the electro-optical characterization will take place. The growth and part of the structural characterization will be done at the LMP (materials for photonics laboratory). Both laboratories are part of CEA/LETI (Electronics, Technology and Instrumentation Laboratory) in Grenoble.
The candidate must have a solid background in solid-state physics and be interested in technological developments related to innovation. He must also have good reporting capabilities given the multidisciplinarity of the work and the interactions between the different laboratories.
1. Templier, F. GaN-based emissive microdisplays: A very promising technology for compact, ultra-high brightness display systems: GaN-based emissive microdisplays. J. Soc. Inf. Disp. 24, 669–675 (2016).
2. Dussaigne, A. et al. Full InGaN red (625 nm) micro-LED (10 µm) demonstration on a relaxed pseudo-substrate. Appl. Phys. Express (2021) doi:10.35848/1882-0786/ac1b3e.
3. Olivier, F. et al. Influence of size-reduction on the performances of GaN-based micro-LEDs for display application. J. Lumin. 191, 112–116 (2017).
4. Smith, J. M. et al. Comparison of size-dependent characteristics of blue and green InGaN microLEDs down to 1 μ m in diameter. Appl. Phys. Lett. 116, 071102 (2020).
5. David, A. Observation of long-range carrier diffusion in InGaN quantum wells, and implications from fundamentals to devices. ArXiv210407199 Cond-Mat Physicsphysics (2021).
6. David, A., Young, N. G., Lund, C. & Craven, M. D. Review—The Physics of Recombinations in III-Nitride Emitters. ECS J. Solid State Sci. Technol. 9, 016021 (2020).