Compared with sapphire and SiC, Si is a promising substrate for growing GaN epi-layers due to its low cost, large size, and facilitation of the backside process. However, the GaN grown on Si substrates always has a low quality and large stress due to the large mismatch between the lattice and the thermal expansion coefficient. To ease this disadvantage, patterned Si substrates have been developed to release the stress, and the crystalline quality was also improved to some extent, which depends on the Si mesa size. However, the reduction of the critical size to sub-micron is at a very high cost in terms of the expensive sophisticated photolithography machine. In the first part of this thesis, nano-scale porous Si substrates were prepared with the help of the Al anodization technique. With optimized pore parameters, LEDs were successfully grown. Compared with LEDs grown on the micro-scale grid-patterned Si substrates, those grown on porous Si substrates exhibited a 20% improvement in light output power (LOP), and the reverse leakage current has been reduced to 10^-8A.

The reliability and characteristic changes over time of various colored LEDs are very important issues for applications. Many reports have illustrated the deteriorating performance of LEDs at extreme conditions or after a prolonged period of operation. However, there is hardly any study on the natural degradation of LEDs over time, namely, the ‘Aging’ of the device. In the second part, the natural degradation in green LEDs is investigated. Green light is very important for LED applications, because it is located at the peak of human eye’s sensitivity, and is one of the primary colors for producing white-light. However, green LEDs have undertaken a serious LOP drop from our observation, while no significant drop was observed in blue LEDs. The proposed origin of this phenomenon is discussed.