Optically functional structures on GaN-based light-emitting diodes for light-extraction efficiency enhancement and emission pattern control

Abstract

In this dissertation, we present a technique that provides full control of refractive index and surface structure at the LED surface: LEDs having patterned graded-refractive-index (GRIN) coatings (GRIN LEDs). Micro-patterned GRIN coatings that enhance light extraction and enable control of the emission pattern in GaN-based LEDs are designed and demonstrated. The coatings are patterned into arrays of GRIN micro-pillars, which are composed of five dielectric layers made of (TiO_2)_x(SiO_2)_1-x with the bottom coating (adjacent to semiconductor) having the highest refractive index and the top coating (adjacent to air) having the lowest refractive index. The GRIN micro-pillars, including their planar geometric shape and size, are structured for maximum LEE and emission-pattern control. LEDs patterned with an array of four-pointed-star-shaped GRIN micro-pillars show a 155% enhancement in light-output power over an uncoated planar reference LED. In addition, the peak emission intensity of the GRIN LEDs is shown to be controllable from ±20° to ±50° off the surface-normal, demonstrating GRIN LEDs with high LEE and emission-pattern control. These two characteristics can be tuned to match specific target applications of the LEDs.

Light-emitting diodes (LEDs) are the next-generation lighting source due to their high luminous efficacy, high reliability, environmental benignity, and the potential to offer new functionalities such as a controllable emission pattern and polarized emission. However, the large refractive-index contrast between the LED semiconductor and air (i) limits the light-extraction efficiency (LEE) of LEDs, and, at the same time, (ii) affects the emission directionality of LEDs. A solution to both problems will require advanced control of refractive index and surface structure at the LED semiconductor surface.