Yeeu-Chang Lee

The Influence of Nanoimprinting Surface Structures on the Optical Efficiency of GaN-Based LEDs

Spin-on glass structures are built directly on an indium tin oxide surface by imprinting lithography to improve the optical efficiency of GaN-based blue LEDs. Two different mold materials, sapphire and silicon, are fabricated via conventional UV lithography and a wet etching process. In the UV lithography, the 750-nm linewidth mask directly contacts the thin photoresist to make a good resolution pattern. Our study shows that the periodic surface structures made via imprinting enhance the light-extraction efficiency-surface texturing reduces the total internal reflection and Fresnel reflection when light is generated by active layers of the LEDs. The light output power values of the conventional LED and imprinting nanohole-array LEDs are 21.7 and 27.3 mW, respectively, under 150-mA current injections.

Experimental and simulation studies of anti-reflection sub-micron conical structures on a GaAs substrate.

In order to reduce surface reflection, anti-reflective (AR) coatings are widely used on the surfaces of solar cells to improve the efficiency of photoelectric conversion. This study employed colloidal lithography with a dry etching process to fabricate sub-micron anti-reflection structures on a GaAs substrate. Etching parameters, such as RF power and etching gas were investigated to determine their influence on surface morphology. We fabricated an array of conical structures 550 nm in diameter and 450 nm in height. The average reflectance of a bare GaAs wafer was reduced from 35.0% to 2.3% across a spectral range of 300 nm - 1200 nm. The anti-reflective performance of SWSs was also calculated using Rigorous Coupled Wave Analysis (RCWA) method. Both simulation and experiment results demonstrate a high degree of similarity.

Performance Analysis of Reflection Reduction in Submicrometer Structures Using Rigorous Coupled Wave Analysis

The broadband antireflective (AR) properties of submicrometer structures (SMSs) have attracted considerable attention in recent years as a means to reduce the surface reflectance of optical and optoelectronic devices. This study employed polystyrene sphere lithography in conjunction with dry etching to fabricate SMSs on an Si substrate. We fabricated an array of conical structures 600 nm in diameter and 760 nm in height. Experimental results demonstrate that SMSs suppress the average reflectance to below 1% across a spectral range of 300-1200 nm. The AR performance of SMSs was then simulated using rigorous coupled wave analysis, the results of which are very close to those obtained in the experiment. To investigate the mechanism underlying the AR properties of SMSs, we calculated the reflectance spectra using structures of various heights, diameters, and refractive indices.

Fabrication of high-refractive-index microstructures and their applications to the efficiency improvement of GaN-based LEDs

This study employed a UV-assisted roller imprinting technique using PDMS soft molds to imprint periodically inverted-pyramid structures on the surface of GaN-based LEDs. The refractive indices of the structures were 1.5, 1.7, and 1.9, which enhanced light output power by 26%, 43%, and 50%, respectively, compared to conventional LEDs. Materials with a greater refractive index indicate a larger critical angle of total internal reflection from the semi-conductor to the imprinted material. Once photons are extracted into the imprinted structure, they are prone to be extracted into the air through the structured surface. The numerical simulation performed using the ray tracing method proved useful for identifying the microstructure with optimal light extraction efficiency. Simulation results showed that LEDs with imprinted structures of varying refractive indices enhance optical efficiency in a manner similar to that demonstrated in these experiments.

2-inch full wafer nanoimprinting for GaN-based LEDs

A rapid and cost effective method for nanostructure fabrication in a large area was proposed in this paper. The integration of conventional ultra-violate lithography and self-developed technique was adopted to fabricate 700 nm nanocasts. Polydimethysiloxane (PDMS) was used to be a flexible mold material for the 2-inch wafer area uniform imprinted structures. This rapid structure fabrication technique can be applied to commercialized light emitting diode as a texturing surface to raise the light extraction efficiency significantly.