Abstract
In this paper, different efficiency behaviors of blue LEDs fabricated on the conventional sapphire substrate (C-LEDs) and patterned sapphire substrate (PSSLEDs) by metal organic chemical vapor deposition are investigated. Maximum external quantum efficiency (EQE) of PSSLEDs shows about 23.5 % improvement compared with that of C-LEDs. However, PSSLEDs have lower peak-efficiency-current at which the EQE reaches maximum, and suffer more serious efficiency droop. Besides, slight changes of efficiency droop for these two type LEDs in aging experiments indicate that structure defects do not play major roles for efficiency droop, and faster broadenings of the full width at half maximum of the electro-luminescence (EL) spectrum when injection current increases suggest stronger band filling effects in PSSLEDs. From the rate equation analyses, the value of coefficient C' is found to be two orders larger than Auger coefficient, and PSSLEDs have a greater coefficient C', which implies more carriers overflow from QWs in PSSLEDs. Therefore, it is suggested that, compared to Auger recombination, carrier overflow from QWs would play a more important part in the efficiency droop of blue LEDs.
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