Featuring: Dr. Subhash Mahajan
Professor, Department of Chemical Engineering and Materials Science
University of California, Davis
The mismatch between GaN and sapphire is ~ 16%. If we were to grow it directly on sapphire at high temperatures, it would form widely separated pillars because radius of critical sized nuclei is very large. To obviate this problem, Akasaki and co-workers developed a growth protocol that is referred to as two- step epitaxy. In this process, a GaN layer is first deposited at a low temperature, followed by growth at a high temperature. The low temperature growth is termed as a nucleation layer (NL). To discern the origins of threading dislocations (TDs), we deposited GaN layers on (0001) sapphire substrates by metalorganic chemical vapor deposition. We examined as-deposited NLs, NLs after ramping to high temperature and layers grown at high temperatures for different durations by plan-view and cross-sectional transmission electron microscopy. Results indicate that as-deposited NLs are rough, but continuous, and consist of sub-grains, ~20 nm in size, that are rotated about the  direction. During ramping to high temperature, sub-grains evolve into islands that are separated from each other . During high temperature growth, deposition occurs preferentially on those islands that are well aligned with the underlying substrate. Subsequently, these islands grow laterally and vertically, leading to a continuous film. We will also demonstrate that there are two sources for threading dislocations (TDs): highly defective NLs and condensation of point defects.
We also engineered a growth protocol for reducing the density of TDs. This entailed the deposition of a thin film of silicon nitride on as-grown NLs, followed by high temperature growth. We will show that this approach favors vertical growth over lateral growth. There are two ramifications of this approach: formation of voids in the layer, and delayed formation of a continuous film. TDs tend to terminate on the voids, resulting in their reduction.
The support of above work by NSF and AFOSR is gratefully acknowledged.
Dr. S. Mahajan is a Distinguished Professor in the Department of Chemical Engineering and Materials Science and a Special Advisor to the Chancellor at the University of California. Davis. Prior to joining UC, Davis, in 2011 , he was associated with Arizona State University; Carnegie Mellon University; Bell Telephone Laboratories, Murray Hill ; The Atomic Energy Research Establishment, Harwell, England; and the University of Denver. Mahajan’s research focus is on structure/property relations in materials. He received numerous awards for excellence in research and education in electronic materials. He is a Fellow of ASM, MRS, and TMS, and is a Member of the National Academy of Engineering.