Improvement Of External Quantum Efficiency Of AlGaInP-Based LEDs By Wet Chemical Etching GaP Window Layers

As a new type of lighting source,light emitting diodes?LEDs?have found wide applications due to their high brightness,low power consumption and long service life.So far,although the internal quantum efficiency of LEDs has reached more than 90%,the external quantum efficiency is only about 5%.The reason is that the majority of light emitted from the active layer suffers reflection loss at the semiconductor-air interface,resulting in low light extraction efficiency of LED chips.To address this problem,many solutions have been proposed,including flip-chip structures,Distributed Bragg reflector?DBR?layer structures,LED geometry design,photonic crystals,nanoparticles on surface,surface roughening,etc.Among them,wet surface roughening is commonly adopted by industry due to its low cost,easy operation and large-scale processability.The selection of etching solution is the key of wet surface roughening.The motivation of our work is to explore the ideal etching solution on the roughening of the GaP window layers of red LED chips based on AlGaInP.Firstly,we used various diluted acid solutions to etch the GaP window layers.The solutions include: HCl : HNO3 : H2O?3 : 1 : 4?,HCl : HNO3 : CH3COOH?3 : 1 : 4?and HCl : HNO3 : CH3 COOH : H2O?3 : 1 : 4 : 8?.The results show that the GaP window layers of LED chips exhibited different surface morphologies after etching.These three etching solutions effectively roughened the GaP window layers,thus reducing the reflection of emitted light at the interface and improving the external quantum efficiency.The weighted average brightness of LED chips was 120 mcd before etching and increased to 165 mcd after being etched.The external quantum efficiency of the LED chips was increased by 37.5%.Secondly,mixed solutions comprised of saturated citric acid?C6H8O7?and 30 wt% of H₂O₂ were used as the etching solution.We explored the effects of etching time,temperature and composition of the solutions on the surface morphology of the GaP window layers and photoelectric performance of LED chips.The results show that the solution can effectively etch the GaP window layers.Rough surfaces were formed on the GaP window layers.The optimal performance of LED chips was achieved after echting the window layers with a solution of C6H8O7 : H₂O₂?1 : 1?at 60 oC for 1 h.The luminous intensity increased from 96 mcd to 117.5 mcd and the external quantum efficiency increased by 22.4%.Finally,mixed solutions comprised of saturated oxalic acid?C2H2O4?,succinic acid?C4H6O4?or malic acid?C4H6O5?mixed with H₂O₂?30 wt%?with a volume ratio of 1 : 1 were used as the etching solution.It was found that all of the etching solutions could etch the GaP window layers of the red LED chips at 60 oC.Among them,saturated oxalic acid mixed with H₂O₂ exhibited the fastest etching rate and effectively reduced the processing time.