Researchers at Sandia National Laboratories in Albuquerque, N.M., have developed a process for depositing layers of GaN that span etched trenches in sapphire substrates. The process, which they call cantilever epitaxy, may improve the performance of LEDs by reducing the number of dislocations by an order of magnitude compared with material grown on planar sapphire substrates.

The researchers begin by creating a pattern of posts and trenches in the sapphire with plasma-assisted etching. They then deposit GaN on the substrate at 500 to 600 °C, creating nucleation sites atop the posts for subsequent deposition. Further growth at 950 °C produces facets over the posts that turn vertical dislocations to horizontal when lateral growth is continued at 1100 °C, which leads the layers to coalesce over the trenches. This results in the formation of a uniform GaN layer with low vertical dislocation density.