Carrier dynamics analysis for efficiency droop in GaN-based light-emitting diodes with different defect densities using time-resolved electroluminescence

• Published in: Journal of physics. D, Applied physics Vol. 49; no. 9; pp. 95101 - 95108
• Main Authors: Yoo, Yang-Seok, Na, Jong-Ho, Son, Sung Jin, Cho, Yong-Hoon
• Format: Journal Article
• Language: English
• Published: IOP Publishing 09.03.2016
• Subjects: Carrier density; carrier dynamics; Current injection; efficiency droop; Electroluminescence; Gallium nitrides; Light-emitting diodes; Sapphire; Spectra; Substrates; time-resolved electroluminescence
• ISSN: 0022-3727; 1361-6463
• DOI: 10.1088/0022-3727/49/9/095101

We developed a direct experimental approach for investigating the correlation between efficiency droop and recombination rate variation under current injection conditions by using time-resolved electroluminescence (EL) technique. We applied this approach to understand the droop phenomenon of GaN-based light-emitting diodes grown on patterned sapphire substrates (LED-on-PAT) and planar sapphire substrates (LED-on-PLA). Because of lower dislocation density and current leakage in LED-on-PAT compared to LED-on-PLA, it was found that the effective carrier density injected into quantum wells (QWs) in LED-on-PAT was higher than that of the LED-on-PLA under the same current injection conditions, based on the analysis of spectral broadening of EL spectra with varying current injection and photoluminescence experiments under resonant and non-resonant excitation conditions. The efficiency droop in LED-on-PAT was found to be much more severe than that of LED-on-PLA, despite the higher overall quantum efficiency of LED-on-PAT. From the time-resolved EL analysis, we could separate radiative and non-radiative recombination contributions and directly observe (i) the decrease and saturation of radiative recombination time and (ii) the increase and following decrease in behavior of non-radiative recombination time with increasing current injection level, showing a strong correlation between efficiency droop and recombination rate variation.