Investigation On Light Extraction Efficiency Enhancement Of GaN-based Light-emitting Diodes By Nanostructure And Microstructure

Due to energy saving and environment friendly, light-emitting diodes (LEDs) gradually replace the traditional light sources as the next-generation light souce, having led to a worldwide illumination revolution. GaN-based LEDs not only have widely employed in various applications, such as outdoor display, liquid crystal display (LCD) backlighting, city decoration, traffic signal light and so on, but also are the core component of the white LED. Therefore, the research for GaN-based LEDs has an essential importantance. As a result of a high refractive index difference between GaN expitaxial layer and ambient medium, light extraction efficiency (LEE) of the GaN-based LEDs is severely limited. Much attention has been paid to LEE enhancement of GaN-based LEDs, to realize high efficieny and high brightness GaN-based LEDs.In this work, fabrication of specific microstructure and nanostructure on GaN-based LEDs was proposed to enhance the LEE. The formation mechanism of micro/nano-structrure and their role on LEE enhancement for LEDs was theoretically and experimentally investigated and analyzed. The detailed results obtained are shown as the following.(1) An approach for enhancing LEE of GaN-based LEDs by growing ZnO nanorod arrays (ZNAs) on ITO transparent electrode was proposed. Corresponding 3-dimensional (3D) numerical model of the GaN-based LED with ZNAs was built. The Mento Carlo ray tracing method was utilized to simulate the light extraction property of the LED model and evaluate the light extraction effect of the ZNAs. Compared with the LED without ZNAs, the simulation results showed an improvement factor of 69.4% for the light output of the LED with ZNAs, which suggested that the ZNAs should be helpful to enhance LEE of the GaN-based LEDs.(2) Growth of ZNAs using hydrothermal mothod was investigated. Experimental results showed influences of seed layer, pH value of reaction solution and surface quality on the morphology, coverage and optical property of the ZNAs grown. Using zinc acitate solution as reaction solution, ZNAs was hydrothermally grown on the surface of the ITO electrode of the GaN-based LEDs, stably heated at 85℃or 90℃by water bath. With the ZNAs grown on the transparent electrode surface, the light ouput of the LEDs could be enhanced by 60.6%. At the same time, by growing ZNAs on the patterned ITO electrode of the GaN-based LEDs, influence of the morphology and alignments of the ZNAs on LEE was experimentally and theoretically discussed.(3) Enhacing LEE for GaN-based LEDs by fabricating 3D back reflector on the surface of sapphire substrate was proposed.3D numerical model of the GaN-based LED with 3D reflector was built. The simulation was conducted by Mento Carlo ray tracing method to evaluate the effect of 3D reflector for enhancing LEE of the GaN-based LEDs. Compared to the the LED model with planar reflector, the simulated light output of the LED model with 3D reflector was improved 29.6%, suggesting that 3D reflector should be better for enhancing LEE of the GaN-based LEDs than planar reflector.(4) Processing and mechanism of fabricating sapphire microstructures by wet etching method were investigated and discussed. Based on these experimental results, the 3D reflector was fabricated on GaN-based LEDs. With the help of SiO2 mask with triangle periodic pattern, after etched in the mixed etchant (H3PO4:H2SO4=1:3) at high temperature, sapphire microstructure array with a good alignment was formed on the surface of sapphire substrate of the GaN-based LEDs, which was attributed to the anisotropic etching of the sapphire crystal. With the 3D reflector, the light output flux of the GaN-based LEDs was improved 11%, compared to the LEDs with tranditional planar reflector.