Integration of Photonic Crystals on GaN-Based Blue LEDs Using Silicon Mold Substrates

In this paper, we demonstrate a novel method to integrate photonic crystals (PhCs) on GaN-based blue light-emitting diodes (LEDs) using a silicon substrate as a mold for forming the PhCs. This method starts with fabricating a 2D grooved Si substrate as that mold. Subsequently, GaN-based epitaxial layers are grown on the Si mold-substrate, which effectively reduces the dislocation density in GaN by enhanced lateral epitaxial growth. After the epitaxial layers are bonded onto a highly reflective substrate, the Si mold-substrate is removed. This substrate-transfer technique replicates PhC from the mold-substrate on the LED surface free from processing damages. The resultant LEDs with PhC have outperformed the LEDs without PhC in the optical output power by 80%, taking advantage of the enhanced light extraction by PhC.

I. Introduction

With the wide spread of blue–white GaN-based light-emitting diode (LED) markets, including backlighting and illumination [1], there arises a strong demand for the increase of the external quantum efficiency. The efficiency is limited by low-light extraction efficiency due to total internal reflection (TIR) at LED surfaces which leads to undesired optical absorption by crystal defects and–or metal electrodes [2]. Light can be extracted only when the incident angle is smaller than the critical value, which is about 23 deg at the interface between GaN and the air. The TIR limits the light extraction efficiency up to approximately 4% per surface in the conventional GaN-based LEDs. Fig. 1.

Schematic of the InGaN blue LED with surface photonic crystal.