industry LED manufacturing
the growth conditions for the last few layers of the epistructures so that they form a rough surface (Figure 1 i). The downside of this approach is that it can compromise electronic and optical performance, and we believe that it is better to insert a crystalline scattering layer inside the p-type GaN layer (Figure 1 h). Take this route and a flat surface can be formed on the top of the chip, simplifying subsequent processing steps.
Gains are also possible by tailoring the transparent contact material by chemical treatments to create scattering objects on the contact surface (see Figure 2 c and d). In addition, it is possible to use a similar technology with chips employing a flip-chip geometry, with light extracted from the sapphire side of the device. In this case, scattering objects are formed on the sapphire surface (see Figure 2 e and f).
Figure 1. A basic InGaN-based LED chip can include many features for improving light extraction: a)bottom mirror, b) scribing area, c) ultra low dislocation density GaN buffer, d) light scattering epitaxial layer, e) chi sidewalls, g) metal contacts, h) internal light scattering p-layer, i) contact material and j) chip coating
acquired with a scanning electron microscope reveal that it is possible to control the size and shape of these features by judicious choice of the growth regime (see Figure 2 a and b). The shape of these voids can be controlled from nearly vertical to fully inclined. Thanks to this versatility, it is possible to produce an optimised dispersion structure with excellent crystal quality through careful selection of growth modes and the thickness and composition of the layers.
Extracting light through the top Internal reflection at the chip’s top surface, which reduces LED output, can be cut with either antireflective optical coatings (see Figure 1 j) or objects that are highly dispersive. According to theory, objects are most effective at dispersing light when the ratio of the wavelength of this radiation inside the material is between one-tenth and twice its physical dimension. Dispersion efficiency peaks when this ratio is between one-third and one.
Increasing dispersion by tailoring the chip’s surface is a widely adopted approach for boosting light extraction. There are numerous highly sophisticated, very robust methods that can be adopted, but traditional photo-resist technologies are far from ideal because it is challenging to scale this approach to dimensions comparable to the very short emission wavelengths of GaN-based materials.
Quite often mask-less approaches are more suitable, from both a cost and yield perspective. A well-known, very efficient method for increasing dispersion involves altering
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www.compoundsemiconductor.net June 2011
Traditional scribing technology for chip separation tends to create visible damage on our substrates and their epilayers near the scribing area. Low damage scribing techniques combined with post scribing chemical treatment is an effective way to solve this problem.
Although the sidewalls of the chip account for a very small proportion of its surface area, they play a pivotal role in determining the LED’s extraction efficiency. That’s because emission from the active region transgresses equally in all directions, and due to the high degree of total internal reflection within the device, a significant portion of this light is guided towards the sidewalls. We have found that extraction efficiency can be improved with various etching methods that either taper the sidewalls
Figure 2. Scanning electron microscopy images reveal: the microstructures to reduce tension in GaN layer (a, b); the tailoring of transparent contact materials to increase light extraction (c,d); and the structuring of the sapphire surface for flip-chip technologies (e,f)
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