Enhanced Light Output of GaN-Based Vertical-Structured Light-Emitting Diodes With Two-Step Surface Roughening Using KrF Laser and Chemical Wet Etching

A two-step roughening process that uses a KrF excimer laser and KOH chemical etching for the n-GaN layer surface of vertically structured GaN-based light-emitting diodes (VLEDs) to yield circular protrusions with hexagonal cones atop for light extraction enhancement is demonstrated. A possible mechanism of the formation of the circular protrusions commenced by laser irradiation with nonuniform etching rates at sites with various dislocation densities was investigated. An improvement in light output power of about 95% at 350-750 mA compared to that of flat VLEDs was obtained for the two-step roughened VLEDs, which is attributed to the increase in surface emission area and dimensions of roughness, and, in particular, the decrease in the n-GaN layer thickness.