Semiconductor Today recently published a new article about sapphire quality, Marked advancement in sapphire crystal quality from improved process control, written by John Ciraldo of Rubicon Technology. The article examines sapphire quality and how sapphire makers strive to keep up with the pace of advances in LED technology. Sapphire is a very important part of LEDs. Sapphire is the substrate, or foundation, for more than 85% of LEDs. And quality starts with that foundation.
According to Ciraldo, sapphire is the most suitable material for a substrate in LEDs because, in addition to its availability, favorable optical properties and relatively low cost of use, it has superior lattice (arrangement of atoms) matching to GaN, the material that is layered on sapphire during epitaxial growth to make an LED. The industry calls it a “lattice mismatch” because the layers don’t line up perfectly due to differences in crystalline structure of the two materials. The mismatch between GaN and sapphire can be further exacerbated by defects in the sapphire crystal such as surface bubbles, dislocations and impurities. The quality of the sapphire and this “mismatch” ultimately impacts the performance of an LED that provides the light source for an LED light bulb.
Ciraldo notes that LED producers continue to push the limits of power and efficiency in their devices making substrate quality an increasingly important consideration. He notes that “as a result, substrate producers need to continue to innovate and find new ways to enhance their material.”
He explains how Rubicon Technology takes a holistic approach to improving the quality of its sapphire using vertical integration throughout crystal growth. Rubicon controls every aspect of the crystal growth process from the raw material all the way through finishing in order to gain greater consistency and uniformity and has earned a reputation for overall sapphire material quality.
How do they know they produce quality sapphire? Ciraldo details how Rubicon uses x-ray diffraction (XRD) rocking curves and x-ray topography images to evaluate the quality of sapphire. For research related to the article, Ciraldo partnered with scientists at The Advanced Photon Source at Argonne National Laboratories, Dr. Albert Macrander and Dr. Naresh Kujala, to evaluate sapphire samples from Rubicon and two competitors. This work is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.
Rocking curve data shows that material from Rubicon exhibits a greater overall intensity with a significantly narrower peak, both of which are indicators of superior crystal quality. In addition, the rocking curve data shows higher symmetry in the Rubicon sample, indicative of a very low stress gradient within the material. Low stress is another very important characteristic of a high quality crystal. X-ray topography measures crystalline quality. The team examined the same samples using X-ray topography. The images show defects in the lattice structure represented by dark spots and streaks. The Rubicon sample demonstrates fewer defects and streaks.
Ciraldo concludes that, “by exercising more control over the production of our sapphire material through a vertically integrated approach, Rubicon Technology has demonstrated vast improvements in the overall quality of sapphire crystals that make them much more suited to advanced applications, including high-efficiency LEDs.”
For Further Reading
Semiconductor Today, Marked advancement in sapphire crystal quality from improved process control, http://ow.ly/xZcGO