Candidate: Jian Yin
Title: Efficiency Improvement for III-V Nitride Micro Light-Emitting Diodes
Date: August 20, 2019
Time: 2:00 PM
Place: EIT 3141
Supervisor(s): Ban, Dayan
As a solid-state lighting source with high luminance and long life-time, GaN/InGaN ì-LEDs is considered as a promising technology for many applications, including opto-genetic neuromodulation, micro-indicators and self-emissive micro-displays. The biggest technical challenge with conventional GaN/InGaN LEDs is the so-called efficiency droop, which is mainly attributed to three reasons: Auger recombination, low hole injection and carrier overflow. On the contrary, the substantial carrier loss due to SRH surface recombination is usually ignored in conventional GaN/InGaN LEDs. However, when the size of the GaN/InGaN LEDs shrinks down to a few micrometers (GaN/InGaN ì-LEDs), the increased surface area to volume ratio leads to a high carrier surface recombination rate, which significantly degrades the efficiency performance of GaN/InGaN ì-LEDs. In this case, the surface recombination velocity, which reflects the impact of surface recombination rate, becomes the most critical parameter affecting the device performance.
This thesis investigates the surface recombination impacts on the efficiency performance of size-dependent GaN/InGaN ì-LEDs. In this work, the simulation tool APSYS crosslight is employed to simulate GaN-based ì-LEDs by setting the surface recombination velocity to 3104 cm/s at room temperature. The simulation results show that the internal quantum efficiency of ì-LEDs decreases from 53% to 4% when the size shrinks down from 100ìm to 5ìm. The simulation results are then confirmed with the temperature-dependent experimental data measured on 50×50 ìm2 ì-LEDs. Further investigation shows GaN/InGaN ì-LEDs with n-doped quantum barriers significantly improved the efficiency of LEDs in common working current density range.
Based on the modeling results, a new design of GaN/InGaN ì-LEDs with n-doped quantum barriers are proposed to improve the efficiency in GaN/InGaN ì-LEDs with small sizes (<10ìm in dimension). For GaN/InGaN ì-LEDs with n-doped quantum barriers, calculation shows that the SRH recombination rate (surface recombination) in the n-GaN is much smaller than the p-GaN. By considering the tradeoff between balance of electron-hole injection and suppressing surface recombination, an optimized 5×5 ìm2 GaN/InGaN ì-LEDs with n-doped quantum barriers and single quantum well design is presented. The estimated efficiency of the new design is two times higher than that in conventional intrinsic multiple quantum wells-based ì-LEDs.
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