Optically functional surface structures for GaN-based light-emitting diodes

The unrestricted control of the surface structure and refractive index would allow for new and improved functionalities in optoelectronic devices. Specifically, micro-patterned graded-refractive-index (GRIN) coatings can enable control of emission pattern and promote light extraction in GaInN light-emitting diodes (LEDs). We design and demonstrate coatings that are patterned into arrays of GRIN micro-pillars, each composed of five dielectric layers of (TiO₂)_x(SiO₂)_1-x with the bottom layer (adjacent to semiconductor) having the highest refractive index and the top layer (adjacent to air) having the lowest one. The GRIN micro-pillars, including their planar geometric shape and size, are structured for emission pattern control and maximum light-extraction efficiency. It is shown that the peak emission intensity of the GRIN LEDs is controllable from ±20° to ±50° off the surface-normal. In addition, LEDs patterned with an array of four-pointed-star-shaped GRIN micro-pillars with a pillar-size of 4 μm and spacing between neighboring pillars of 4 μm show a 155% enhancement in light-output power over an uncoated planar reference LED.