For 365-nm ultraviolet light-emitting diodes (UV LEDs), an electron blocking layer (EBL) is usually utilized to mitigate electron overflow. However, using EBL might obstruct holes from injecting into the active region. Moreover, the large polarization field in conventional EBL might also pull down the effective barrier height for electrons, and thus the electrons could easily overflow to the p-side region. To solve the above drawbacks, in this study, the Al content and p-doping concentration of the EBL in typical 365-nm UV LEDs are investigated systematically. Specifically, designs of AlGaN/GaN superlattice EBL and Al-content-graded EBL are explored in detail.
In this paper, we investigated the failure mechanisms of blue InGaN LEDs grown on patterned sapphire substrates and demonstrated the influence of patterned sapphire substrates on the reliability of GaN LED by comparing with conventional LEDs grown on planar sapphire substrates. From experimental results, we found that InGaN LEDs grown on patterned substrates had a higher turn-on voltage but a smaller series resistance compared with conventional LEDs owing to rough inner patterns and small threading dislocation density. Both samples were then acceleratedly aged under a high DC current for two hours. Failure modes were studied with various measurements taken before and after aging. From the power evolution performance, we found that output power of LEDs with patterned substrates increased slightly due to fewer defects while output power of conventional LEDs decayed. This can be inferred from small reverse leakage currents and tunneling currents observed from Log I-V characteristics and EMMI measurement of P-LEDs. A slight redshift in emission wavelength was also found during aging because of possible leakage shunt paths caused by defect generation. Moreover, operation voltage increased slightly after aging which was caused by contact degradation induced by thermal annealing.
In this study, a specific design on the electron blocking layer (EBL) by band engineering is investigated numerically with an aim to improve the output performance and to reduce the efficiency droop in green LEDs. Systematic analyses including the energy band diagrams, carrier distributions in the active region, and electron leakage current are given and the simulation results show that the proposed lattice-compensated superlattice-AlGaN/InGaN EBL can provide better optical and electrical output performances when compared to the conventional rectangular AlGaN EBL. The output power of the green LED can be enhanced by a factor of 52% and the applied voltage can be reduced from 5.08 V to 4.53 V at an injection current of 1500 mA. The internal quantum efficiency is improved and the percentage of the efficiency droop can also be reduced from 58% to 37%, which is mainly attributed to the successful suppression of electron leakage current and improvement in hole injection efficiency.
The blue shift of viewing angle in the top-emitting organic light-emitting devices is discussed in this study. For the
single-mode cavity, the device of anode metal/ m-MTDATA (40 nm)/ α-NPD (10 nm)/ Alq3 (47.6 nm)/ LiF (1 nm)/ Ag
(20 nm) with the metal phase difference of 1.30 π has the minimum blue shift of viewing angle. For the double-mode
cavity, the recombination area must be away from the cathode for the device with better performance. However, the
double-mode cavity with only one recombination area still has worse FWHM and gets more serious the blue shift of
viewing angle than the single-mode cavity. Therefore, the double-mode cavity with a recombination area at each
antinode is performed, and the results prove that the blue shift of viewing angle and the FWHM are improved. Finally,
we replace the emission layer with Alq3:DCM (0.01%) and adjust the main peak wavelength in the double-mode cavity
by adjusting the thicknesses of the cavity. The results show that the FWHM and the blue shift of viewing angle obtain
further improvement for the double-mode cavity with a recombination area at each antinode.
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