Enhanced external quantum efficiency of light emitting diodes by fabricating two-dimensional photonic crystal sapphire substrate with holographic technique

Investigation in fabricating two-dimensional (2D) photonic crystal (PC) on sapphire substrates for enhancing external efficiency of GaN based light emitting diodes (LEDs) is presented. 2D-PC was fabricated on a sapphire substrate using holographic lithography and inductively coupled plasma ( ICP) dry etching. LEDs with 2 mu m thick n-GaN layer, four pairs of InGaN/GaN quantum well structures and 200 nm thick p-GaN layer were grown on the patterned sapphire substrate (PSS) by metal organic chemical vapor deposition ( MOCVD). The PC fabricated on PSS has 2D hexagonal lattice pattern, with 3.8 mu m lattice constant and 800 nm depth. LED output measurement shows 100% increase in the average luminous intensity of PSS-LEDs compared with that of conventioanI LEDs. The measured X-ray rocking curves of (0002) diffraction for GaN layers grown on patterned and non-patterned sapphire substrates indicate that the quality of GaN crystal grown on PSS is not improved, which implies that the large enhancement of external quantum efficiency of PSS-LED is not caused by the increase in internal efficiency but possibly by the increase in extraction efficiency, which results from the scattering of the PSS.