Abstract
This research contains two major topics. Firstly, we analyze the efficiency enhancement of blue light-emitting diodes (LEDs), 400-nm and 380-nm ultraviolet (UV) LEDs by incorporating an air-hole layer within the epitaxial structure. As compared to the conventional patterned sapphire substrate (C-PSS) LEDs and flat sapphire substrate (FSS) LEDs with air-hole, the fabricated patterned sapphire substrate (PSS) LEDs with air-hole exhibit the lowest full width at half maximum (FWHM) of (002) and (102) diffraction peaks, the better light output power, and the highest external quantum efficiency. Remarkable performance improvement in the PSS LED with air-hole is attributed to the better epitaxial quality with threading dislocations terminated by the air-hole structure and the higher scattering at interface between GaN and air-void. In terms of the reliability test after 72 hrs, the decay of the light output power would be outstandingly improved for the 400-nm and 380-nm UV-LEDs. It is well proposed that this methodology provides a promising alternative to C-PSS LEDs. Secondly, the performance improvement of 380-nm InGaN/AlGaN UV-LEDs are investigated by incorporating an undoped Al(In)GaN thin interlayer between the InGaN well and AlGaN barrier in multi-quantum-well (MQW) active region. By inserting the graded-composition AlGaN and AlInGaN thin interlayers, the light output powers of UV-LEDs are significantly increased by 70% and 105% at 20 mA, respectively, as compared to the LED without the interlayer. Remarkable efficiency enhancement in the UV-LEDs with graded-composition AlGaN and AlInGaN interlayers is mainly attributed to the further improvement of the electron confinement and hole injection with more uniform distribution in the MQW active region. Besides, photoluminescence (PL) and atomic force microscope (AFM) analyses indicate that the MQW quality can be enhanced by incorporating a graded-composition AlInGaN thin interlayer in the MQW active region of UV-LEDs.