Improving Light Extraction Efficiency Of GaN-based LEDs By Oxide Micro&Nano Structures

At present, light-emitting diode (LED) is the most promising solid-state light source, which has been widely used in plenty of fields. High efficiency and high brightness LEDs depend on high external quantum efficiency that relies on the arithmetic product of internal quantum efficiency and light extraction efficiency. Currently, LEDs have high internal quantum efficiency (greater than70%), but low light extraction efficiency severely restricts the external quantum efficiency. Therefore, improving the light extraction efficiency to enable high external quantum efficiency of LEDs has become a hot research field.In this paper, the main work was focused on improving light extraction efficiency of GaN-based LEDs. Zinc oxide (ZnO) nanorods/nanorones array, ZnO micro&nano composite structures and titanium dioxide (TiO2) microclusters were designed and fabricated to improve the light extraction efficiency of GaN-based LEDs. The main contents are as follows:1. The influence of the geometry on light extraction efficiency was studied, and ZnO nanostructures arrays were used to improve the light extraction efficiency of LEDs. It was proved that light extraction efficiency of isosceles triangle is higher than that of rectangular by geometrical optics method. Extending to three-dimensional case, an assumption that light extraction efficiency of cone was higher than that of rod was the proposed. ZnO nanorods array was grown on indium tin oxide (ITO) of LEDs by aqueous deposition method. Light output power of LEDs is improved by about60%. ZnO nanocones array was prepared by regulating growth condition. Light output power of LEDs with ZnO nanocones array is increased by110%. Theoretical and experimental results show that ZnO nanocones array enable higher light extraction enhancement of LEDs than ZnO nanorods array.2. Hybrid ZnO micro-mesh&nanorod arrays was designed on quartz glass by templates and solution deposition, and incident angle-resolved light transmission was studied. Two kinds of ZnO micro&nano composite structures were fabricated to improve light extraction efficiency of LEDs, and light extraction enhancement mechanism was studied. Experimental results show that two-dimensional diffraction grating effect of ZnO micro-mesh and light scattering effect of ZnO nanorods array can synergistically enhanced the transmittance from a high refractive index material to air. Hybrid ZnO micro-mesh&nanorod arrays can improve light output power of LEDs by95.9%. Scanning confocal electroluminescence microscopy imaging shows hybrid ZnO micro-mesh&nanorod arrays can integrated light output from holes’ side wall of ZnO micro-mesh and light scattering effect of ZnO nanorod. Hybrid ZnO micro-cylinders and nanorods array can increase light power of LEDs by86.4%. Confocal scanning electroluminescence microscopy imaging shows it can integrate light output from side walls of ZnO micro-cylinders and light scattering effect of ZnO nanorods.3. Embedded patterned TiO2microclusters were designed and prepared to improve light extraction efficiency of LEDs, and light extraction enhancement mechanism of TiO2microclusters was studied. Firstly, a new LED structure with TiO2microclusters periodically embedded between ITO and p-GaN was designed. Secondly, in experiment LEDs with embedded hexagonal and square arranged TiO2microclusters were prepared, and the electrical properties of LEDs maintained good. Embedded hexagonal arranged TiO2microclusters can increase light output power of LEDs by171.1%, Embedded hexagonal arranged TiO2microclusters combining with roughened sapphire substrate can increase light output power of LEDs by225.3%. The light output power of encapsulated LEDs with embedded hexagonal arranged TiO2microclusters is greater than that of encapsulated blank LEDs by82.1%. Optical microscopic imaging and confocal scanning electroluminescence microscopic imaging visualize light extraction enhancement of TiO2microclusters. Light-guide and light-scattering effect of TiO2microclusters are the main reasons for light extraction efficiency enhancement of LEDs. Ultra-low temperature PL spectra measurement shows embedded hexagonal and square arranged TiO2microclusters combined with roughened sapphire substrate can increase the absolute light extraction efficiency of encapsulated LEDs from45.8%to71.4%. Experimental results show that the structural design of embedded patterned TiO2microclusters capably the high light extraction efficiency advantages of TiO2micro&nano structures, as well as good electrical properties of LEDs and high light-emitting efficiency of active region.