Improving the Light-Emitting Efficiency of GaN LEDs Using Nanoimprint Lithography

Abstract

A light-emitting diode (LED) is an electroluminescent device with a broad selection of emission wavelengths (colors). The unique properties of LEDs, such as compactness, low power consumption, long lifetime, and fast turn-on time have made LEDs an indispensable component in modern traffic lighting, display, car lighting, and cell phones applications. In recent years, LED usage has grown more rapidly due to the application of backlights in large-size flat panel displays, a market previously dominated by CCFL. In addition, wide speculation foresees that the next boom for LEDs could arise from the interior/exterior lighting market. As shown in U.S. energy consumption statistics conducted by the U.S. government, the energy consumption for interior/exterior lighting occupies 22%~25% of the total electrical energy produced in the U.S. Using LEDs to replace all traditional interior/exterior lighting used today can save at least an estimated amount of 20 billion U.S. dollars in annual energy costs while also significantly reducing carbon emission. Therefore, LEDs have attracted a large amount of investments from corporations in Asia, North America, and Europe, aiming to develop reliable processes to improve the production yield and optical efficiency of LEDs. The nanoimprint lithography is a nanoscale structural formation technique with highly reproducible patterns, and is therefore suitable for LED fabrication. The typical length scale of structures for applications of LEDs ranges from a few hundred nanometers to a few micrometers, which is roughly on par with the resolution limit of the traditional optical lithography technology. Though a higher resolution is attainable using the stepper projection lithography method, the corresponding higher process cost renders the fabricated LEDs less economical. Unlike the integrated circuit, the microand nanostructures of an LED are often simple 2D periodic patterns. Once an imprinting mold with a high accuracy and precision is made from techniques such as the stepper projection lithography (or alternatively the electron beam, the ion beam, and the interference lithography), the LED patterns can be massively reproduced. This article discusses the role of the precision nanoimprint lithography for improving the optical efficiency of LEDs.