Light Extraction Efficiency Part A. Ray Tracing for Light Extraction Efficiency (LEE) Modeling in Nitride LEDs

Abstract

We describe simulations of the light extraction efficiency (LEE) as a function of the major materials parameters and geometries in the three main LED structures used today, namely nitride LEDs on GaN substrates, on patterned sapphire substrates and flip chip nitride LEDs. We use ray tracing simulations of LEDs to show how the various extraction schemes operate. The simulations of this chapter show that both surface roughening and patterned sapphire substrates (PSS) lead to high efficiencies, although based on somewhat different mechanisms. The results appear here for the same device parameters (most of them with conservative values), which allow meaningful comparisons. Some industry results may be higher, either due to better materials quality (lower absorption in particular for ITO and metals) or more aggressive designs (smaller lossy contact areas). For LEDs on GaN substrates, the LEE is determined by substrate absorption. A desirable feature is the ability to scale the extraction efficiency for large chips in order to reduce costs. In almost all cases, the use of large surface LEDs is detrimental due to the long ray paths to reach the sidewalls except for those extraction schemes where light is continuously extracted or when substrates have very low absorption.