Graduate SeminarsSubhash Mahajan Regents Professor and Fulton Technical Fellow Ira A. Fulton School of Engineering Arizona State University
Friday, october 30, 2009 - 9:30 am 1175 Benedum Hall
"Self-assembled Nanostructures in Mixed III-V and III-N Layers and Their Influence on Devices" Abstract: The mixed III-V and III-N layers are technologically relevant and scientifically interesting materials. They find applications in light emitters, detectors, photovoltaics, and high speed devices. The mixed III-V layers crystallize in the zinc-blende structure which consists of two interpenetrating FCC sub-lattices. The group III atomic species reside on one sub-lattice, whereas the group V species occupy the second sub-lattice. An interesting question is whether or not the atomic species are distributed at random on their respective sub-lattices in the mixed layers. We will show experimentally that the layers which contain atomic species differing in their covalent tetrahedral radii exhibit two types of deviations from randomness: phase separation and atomic ordering. Phase separation occurs on the surface while the layer is growing and the resulting composition modulations are aligned along the soft directions lying tin the growth plane. On the other hand, the occurrence of (2x4) reconstruction on the surface biases the occupation of sub-surface sites by the atomic species differing in tetrahedral radii. This leads to CuPt type ordering on two of the four {111} planes, resulting in doubling of the periodicity along the <111> directions. Furthermore, the nature of ordering is dependent on the type of surface reconstruction.. The group III-N layers crystallize in the wurtzitic structure, which consists of two interpenetrating HCP sub-lattices. As in the case of mixed III-V layers, the group III species reside on one sub-lattice and the N atoms occupy the second sub-lattice. We will demonstrate that the mixed layers containing group III species which have different covalent tetrahedral radii also phase separate and atomically order. Phase separation occurs along the <10-10> and <11-20> directions lying in the (0001) growth plane. Ordering occurs on the (0001) planes, resulting in doubling of the periodicity along the [0001] direction. We will show that the occurrence of phase separation and ordering reduces carrier mobility and band gaps. We will argue that the presence of these microstructural features enhances the reliability of light emitters. The author gratefully acknowledges the support of NSF and DOE for the above research. Biography: Dr. Subhash Mahajan is a Regents’ Professor and a Fulton Technical Fellow in the Ira A. Fulton School of Engineering, Arizona State University (ASU). He obtained his B.E. in metallurgy with the highest honors from the Indian Institute of Science, Bangalore, India. He completed his Ph.D. in materials science and engineering, the University of California, Berkeley. Subsequently, he held positions at the University of Denver; The Atomic Energy Research Establishment, Harwell, England; Bell Telephone Laboratories, Murray Hill; and Carnegie Mellon University. He joined ASU in 1997. He was the Chair of the Department of Chemical and Materials Engineering from 2000 to 2006. He was the Founding Director of the School of Materials from 2006 to 2009. Professor Mahajan’s research focuses on the interrelationship between structure and properties. He has published extensively on this topic and has received numerous awards for excellence in research and education of electronic materials. He is a Member of the National Academy of Engineering. Professor Mahajan is the Coordinating of Acta Materialia, Scripta Materialia and Acta Biomaterialia.
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