Patterned LED Substrates
by Mike Watts
Patterning in LEDs continues to get plenty of research attention, with no less than 11 papers at Photonics West 2011. This is an interesting example of the challenges of technology and business forecasting. For several years it looked like improving light extraction using a surface photonic crystal would be implemented, but to date the improvement in light extraction has not offset the additional process cost.
At this year’s conference the focus was on using patterned surfaces to reduce dislocation density in the growth of GaN layers, buried photonic crystal layers for light extraction, and the synergistic combination of both.
Gallium Nitride (GaN) is expensive and slow to grow. As a result, a thin GaN layer is grown on another cheaper substrate, a process known as “heteroepitaxy.” The challenge is to find a lattice match. Neither sapphire or silicon carbide is a perfect match, so the GaN that is grown has many growth imperfections or dislocations.
It turns out that the dislocations start at the growth surface and make their way to the surface. The next idea was to cover parts of the surface and allow the epitaxy to “overgrow,” reducing the dislocations in the area that was overgrown (shown in the figure below.) The drawings show how the dislocations grow from the surface.
In this example, the facets divert the dislocations parallel to the surface. Using patterned sapphire substrates has two effects: it reduces the dislocation density in GaN and enhances the light extraction efficiency from the LED chip by scattering the light confined in the GaN layer.
This all looks very intriguing; however, multiple papers at the conference suggest that the final commercial solution is still unclear. There were all sorts of detailed reports on
- The effect of the angle of the pattern for an LED grown on a patterned Si substrate
- The effect to crystal orientation for an on Si (112) patterned substrates
- Improved light extraction improved the crystal quality through nano-epitaxial lateral overgrowth.
- Improved light extraction due to light scattering effects, using nano-cavity arrays
- Improved light extraction where the patterned layer is positioned much closer to the active layer
- Improved external quantum efficiency from a unique pyramidal pattern on the patterned sapphire substrate.
- Growing a nonpolar free-standing GaN single crystal on a closely lattice-matched LiAlO2 or LiGaO2 substrate.
- Growing a high quality GaN LED structure on a sapphire substrate patterned with SiO2 strips
- InGaN LEDs device structure grown with a graded-growth-temperature profile
- Optimization of side wall angle and side wall curvature for an LED with a photonic crystal lattice etched into the underlying sapphire substrate.
My take away from the conference is that patterning is going to be part of the ultimate LED solution. Placing a bet on which particular implementation will win is still anyone’s guess.
About the Author
Mike Watts has been patterning since 1 um was the critical barrier, in other words for a longtime. I am a tall limey who is failing to develop a Texas accent here in Austin. I have a consulting shingle at www.impattern.com.
My blog “ImPattering” will focus on the latest developments in the business and technology of patterning. I am particularly interested in trying to identify how the latest commercial applications evolve.