High Power Vertical-structure GaN-based LEDs with Improved Current Spreading and Blocking Designs
ABSTRACT Based on experimental results which reveal that the contact of indium-zinc-oxide (IZO) and IZO/Ti to n-GaN layer is Schottky and ohmic, respectively, localized Ti deposition associated with a transparent IZO layer is proposed to serve as both current blocking and current spreading layer. In addition, an anisotropic mesa etching on the surface layer (n-GaN) of regular vertical-conducting metal-substrate GaN-based light-emitting diodes (VM-LEDs) is also proposed to further decrease the resistance difference between the outside path and the inner one. The effectiveness of the proposed schemes were verified by a two-dimensional device simulator (ISE-TCAD), which indicates that significant immune of current crowding under cathode contact pad would be possible once an optimal combination of the n-GaN layer etching depth and width, IZO thickness, and Schottky blocking width has been achieved. In experiments, 40-mil LEDs with an anisotropic mesa etching of 400 mum in width and 2 mum in depth, 200 mum in Schottky blocking width, and a 300-nm-thick IZO layer have been successfully fabricated. Typical improvement in light output power by about 25% at an injection current of 350 mA as compared to the regular VM-LEDs has been obtained.