Mar 5, 2009
Nanoporous layer brightens LED performance
GaN-based light-emitting diodes (LEDs) have attracted considerable attention as components in solid-state lighting. For solid-state lighting, the light-extraction efficiency of an LED structure must be increased. There are two main methods for improving the light-extraction efficiency of a gallium nitride (GaN) LED: the use of an LED structure configuration on a patterned sapphire substrate (PSS); and roughing the surface of an LED structure by nanolithography followed by a dry or wet etching process.
The former can improve the crystallography of the LED structure and its reliability except for the light-extraction efficiency. However, for the former, the strip (or hole) with micron-grade dimensions was hardly generated by a high-yield standard photolithography process; thus, the LED structure configuration on PSS requires a thick GaN epitaxial layer (typically 6–8 µm) to coalesce the strip (or hole), and as a result growth time and processing costs increase.
Recent reports have demonstrated that anodic aluminum oxide (AAO) films could be prepared in the form of spatially regular and hexagonally ordered arrays of nanoscale pores using a cost-effective and reproducible process of anodizing of aluminum in acidic electrolytes. These AAO films have also been extensively adopted in the development of various functional nanostructures, such as filters, two-dimensional photonic crystals and surface nanopatterns on LEDs.
In a recent GaN-based LED study, which was published in Nanotechnology, the authors investigated a nanoporous aluminum nitride (AlN) layer prepared using an AAO process and its application as a buffer layer for a GaN-based LED fabricated on sapphire substrate. Following this AAO process, the average pore spacing and pore diameter of the nanoporous AlN layer were in the ranges 180–200 nm and 100–150 nm, respectively. The light output power of the GaN-based LED with a nanoporous AlN layer was about 53% higher than that of a GaN-based LED without a nanoporous AlN layer at an injection current of 20 mA. At an injection current of 80 mA, the light output power was increased by about 34%.
About the author
The work was performed at the National Taipei University of Technology and the National Taiwan University of Science and Technology. It was supported by the National Science Council of Taiwan. Prof. Lung-Chien Chen is the head of the Solid-State Optoelectronics Group at the National Taipei University of Technology. Chih-Kai Wang is a masters student studying materials science at the National Taipei University of Technology. Jenn-Bin Huang is a PhD student studying materials science at the National Taiwan University of Science and Technology and he also work at Nova Technology Inc. Prof. Lu-Sheng Hong is the head of the Photonic Materials and Devices Researching Group at the National Taiwan University of Science and Technology.