| AVS 54th International Symposium | |
| Nanometer-scale Science and Technology | Wednesday Sessions |
| Session NS2+EM-WeA |
| Session: | Nanoscale Devices and Nanowires II |
| Presenter: | A. Fujimoto, Toshiba Corporation, Japan |
| Authors: | A. Fujimoto, Toshiba Corporation, Japan K.A. Asakawa, Toshiba Corporation, Japan |
| Correspondent: | Click to Email |
Light-Emitting Diodes (LEDs) are widely used for cell phones, display panels, LCD backlights, traffic signals, and automobile rear lamp. They are expected to be used for automobile head lamps, and interior illumination. Higher luminance LEDs are required for these purposes. The total efficiency of LEDs is determined by the product of the internal quantum efficiency and the extraction efficiency. The internal quantum efficiency has been improved more than 80 %. But, the extraction efficiency remains 10 %, because of the large difference of the refractive index between substrates (n=3-3.5) and the air. Therefore, to improve the extraction efficiency is the key for higher luminance LEDs. In order to improve the extraction efficiency, we fabricated nanostructures having the antireflection and the diffraction effect onto the semiconductor surface. Nanostructures have the cone, cylinder, and the mesa part. Since nanostructures have the cone and the mesa part and the refractive index from the semiconductor to the air changes smoothly within the critical angle, the incident light within the critical angle can be extracted without a loss, and the transmittance increases about 30 % compared with that of the flat surface. Furthermore, since nanostructures have the cylinder and the cylinder has the diffraction effect, the incident light over the critical angle can be extracted as the -1st order light. Nanostructures having two functions can be expected to improve the light-extraction efficiency greatly. Such structures are usually fabricated by electron beam (EB) lithography, but are too costly for mass production. To overcome this challenge, block copolymer lithography was employed. The polystyrene (PS) - polymethyl methacrylate (PMMA) diblock copolymer was used in this study. The PMMA was removed by reactive ion etching (RIE) since the PMMA has a much faster etch rate than the PS by RIE. The gallium phosphide (GaP) substrate was dry-etched by chlorine-based inductively coupled plasma RIE using the remaining PS dots as a mask. The optical extraction efficiency of the substrates with subwavelength columnar structures improved 2.6 times compared to unprocessed flat substrates; the pillars' height was 350 nm, diameter was 130 nm, and pitch was 180 nm. We also manufactured a real LED and increased light emission volume 1.8 times compared with conventional LED at the same energy consumption.