In the ongoing quest to make LEDs more efficient, LED chip manufacturers have developed patterned sapphire substrates (PSS). In fact, most high-brightness LEDs are made using PSS. There are very few resources online that explain patterned sapphire substrates. Here’s a brief explanation.
PSS helps extract more light from LEDs. A lot of light bounces back into the LED when using a polished sapphire substrate. Researchers discovered that patterning the surface of the substrate by etching nano-scale patterns helps more light, in the form of photons, escape, improving the light generated or extracted by the LED. It is reported that patterning can improve the extraction of light by as much as 30%.
A second important point is that patterning also improves the epitaxial growth process. The nano-patterned surface can have a positive effect on the nitride semiconductor growth process by promoting growth of the GaN in parallel to the substrate surface, called lateral growth. This also helps reduce the number of dislocations, the dislocation density, that can degrade performance.
LED chip manufacturers originally developed PSS. The patterns are quite proprietary and helped the LED chip companies differentiate themselves. Today, the sapphire industry has joined in and sapphire wafer manufacturers have begun to put patterns on sapphire wafers in partnership with the LED chip manufacturers. The patterning work is concentrated with smaller wafers in the 2 to 4-inch diameter range, but manufacturers of large diameter wafers like Rubicon Technology are beginning to develop PSS for larger wafers.
Most patterning is based on a proprietary design from the LED chip manufacturer. The patterns can vary from cones, pyramids and flat tops and can be organized in hexagonal or trigonal patterns. Some basic design rules based on height and height/pitch ratio have emerged, but so far, no standards exist. Currently the most popular pattern is a cone shape, but these patterns change frequently. Here are some sample patterns.
For Further Reading
Semiconductor Today, Patterned sapphire for nitride enhancements, http://www.semiconductor-today.com/features/SemiconductorToday_SeptOct_PatternedSapphire.pdf
Compound Semiconductor, New Wet Process For LEDs On Patterned Sapphire Boosts Efficiency, http://www.compoundsemiconductor.net/csc/news-details.php?cat=news&id=19734296
Compound Semiconductor, Rubicon Orders Multiple Profilers For Sapphire Production, http://www.compoundsemiconductor.net/csc/news-details.php?cat=news&id=19735318