Enhancement Of Light Output Power From LEDs Based On Micro/nanostructures Derived From Monolayer Colloidal Crystals

Light emitting diodes(LEDs) have found wide applications in areas such as traffic signal and full back light in liquid crystal display, due to their advantages of energy-saving, environmental protection, and long life. The invention of Ga N based LEDs enables bright white light sources, thus Ga N based LEDs are expected to replace the traditional illumination sources. One of the major challenges for their real application in solid-state lighting is the low light output power(LOP). In order to enhance the LOP of the Ga N LEDs, micro and nanoscale features with various structures and diverse functions have been incorporated into Ga N LEDs. Micro/nanofabrication technology based on monolayer colloidal crystals(MCCs) has proved to be a low-cost, parallel and high-throughput technique to make features in nanoscale. This thesis focuses on the enhancement of light output power from LEDs based on micro/nanostructures derived from monolayer c olloidal crystals.High-quality MCCs are the foundation of the fabrication of micro/nanostructure. First, according to the material nature of different colloidal spheres we have developed two facile approaches to fabricate high-quality colloidal monolayer mask. The mechanical strength of the polystyrene MCCs was enhanced because the latex spheres were fused together. As a result, extra cracks induced by the water evaporation could be reduced to a maximum degree when the polystyrene colloidal mask was transferred to a desired solid substrate for nanofabrication.Nanopatterned sapphire substrates(NPSS) have been fabricated using a wet chemical etching methodbased on monolayer colloidal crystals. By manipulating the interstice size between the colloidal spheres, NPSS with volcano shape and concave pyramid shape have been fabricated. It has been proved that the effective height of the silica etching masks have an important effect on the morphology of the resultant NPSS. It’s worth mentioning that the fabrication process of NPSS with concave pyramid was a fully wet etching approach and eliminated the traditional semiconductor processes. Both the crystalline quality of epitaxial Ga N films on concave-pyramid-NPSS and the LOP of the resultant LED devices have been improved. Besides, the fabrication processes almost had no detrimental effects on the electrical properties of the NPSS LEDs.Finally, MCCs have been utilized as micro-lens arrays to expose thick photoresist, and 3D complex ordered nanostructures havebeen achieved. Nanostructures based on thick photoresist have been employed as the masks to prepare the NPSS for deep UV LEDs. Finite difference time domain method has been performed to model the thin photoresist exposure process. The dimension of the spheres used in the simulation ranged from the nanoscale to the micron scale in size. The focusing ability of colloidal spheres with different diameters and the distribution of the focused light have be en investigated. The validation of simulation has been proved by comparing the simulation results with the experiment results. By using the thin photoresist as the masks, photonic crystalstuctures have been incorporated into Ga N LEDs to improve the performance of the LEDs.