Improved Light Extraction Efficiency in GaN-Based Light Emitting Diodes

Abstract

The improvement in light extraction efficiency (LEE) of GaN-based LEDs is one of the most important areas for increasing the external quantum efficiency for solid-state lighting applications. We summarize the advances in this field. GaN-based light-emitting diodes (LEDs) have made a lot of progress with regard to growth, doping, and p-ohmic contacts [1–3]. The external quantum efficiency (EQE) of LEDs is equal to IQE (internal quantum efficiency) ×LEE (light extraction efficiency). Although the internal quantum efficiency (IQE) has been improved by advances in epitaxial growth techniques, the methods used to extract the photons from the quantum wells (QWs) still need to be improved. It has been reported that the LEE has to be over 90% to achieve 200 lm/W in white LEDs.According to Snell’s law, the light extraction angle (escape angle or escape cone) between GaN (n ∼ 2. 5) and air (n ∼ 1) was about 23. 5^∘. Snell’s law states that the escape angle (θ) can be calculated by $\theta {=\sin }^{-1}({n}_{\mathrm{air}}/{n}_{\mathrm{GaN}})$ where n _air and n _GaN are the index of air and GaN, respectively. Therefore, the total internal reflection (TIR) resulting from the narrow escape cone prevents the protons from escaping from the semiconductor. In other applications such as optical fibers and optical waveguides, TIR is required. If Indium tin oxide, which is a common transparent conducting oxide (n ∼ 2. 19), is used, the escape angle can be increased up to 27^∘, which is still narrow. The photons outside of the escape angle will be re-absorbed after multiple reflections or increase the operating temperature by dissipation (Fig. 5.1).