Light Extraction Enhancement in GaN-Based Light-Emitting Diodes with Patterned Micro-Pillars

Abstract

A nitride-based light-emitting diode (LED) is a key device for the next-generation solid-state lighting due to its high efficiency, high reliability, and potential to offer new functionalities. However, the large contrast in the refractive index between the LED semiconductor and ambient air (i) limits the light extraction efficiency (LEE) of LEDs by means of total internal reflection, and (ii) distorts the spatial distribution of light intensity emitted from the LED. As a result, planar-surface LEDs normally have a very low LEE and an emission pattern with the peak emission intensity along the LED surface-normal. Such a pointed emission pattern is not appropriate for uniform illumination, e.g. an interior and a street lighting. These two problems have been solved separately. First, the low LEE of GaN-based LEDs has been improved by means of surface/interface roughening and chip shaping. Second, the emission pattern of LEDs has been controlled by the help of secondly optics and external cavities such as photonic crystals. However, a combined solution for both problems requires advanced manipulation capabilities for the surface of LEDs, which is not fully developed yet. Here, we present a technique that provides full control of surface properties (esp. refractive index) in the LED structure: the LEDs having patterned micro-pillars. The effect of the patterned micro-pillars is investigated and the structure of micro-pillars is optimized in order to obtain maximum LEE and also emission-pattern control.