| Major Professsional Experience |
2006-current: Associate Professor, Dept. of Materials Science and Engineering, UVa
1994-2006: Member of the Technical Staff, Sandia National Labs
1992-1994: Post-doctoral member of the Technical Staff, Sandia National Labs
1984-86: Associate Engineer, IBM Thomas Watson Research Center
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My research engages many aspects of thin film growth, with a current focus on self-assembly at the nanoscale. Film growth continues to be a fascinating and important area of research - fascinating because of our ability to grow novel structures far from thermodynamic equilibrium by a variety of processing techniques with exquisite control, and important for the vast range of current and potential applications that employ thin films. I define "thin films" rather broadly, from the traditional two-dimensional layer-on-a-substrate, to supported arrays of quantum dots, nanorods and nanotubes. My work takes a physical metallurgist's view of electronic materials, with an emphasis on the interaction of stress, structure and surface morphology. As my interests in the fundamental materials science of nanostructures continues to grow, so too does my interest in seeing whether useful new functionalities can be achieved using the novel properties of nanostructured materials.
My central research effort is on heteroepitaxial self-assembly of nanostructures in GeSi strain-layers grown on Si (001) substrates. Alloying Ge and Si alloys allow us to take advantage of elastic strain and bandgap engineering techniques to create high performance devices, similar to III-V technology, but using Si-based materials that dominate the semiconductor industry. I employ MBE growth, combined with in situ diagnostics, to probe the detailed evolution of quantum nanostructures. My recent research manipulates the kinetics of MBE growth to produce unusual metastable structures. In particular, we've learned how to self-assemble "quantum dot molecules" (QDMs) - symmetric assemblies consisting of four proximal quantum dots that are elastically bound by a central pit in the epilayer. We're investigating pit nucleation, size selection, compositional inhomogeneities, and how to deterministically direct self-assembly using precision patterning with a focused ion beam. We've also begun to explore whether QDMs can exhibit logic-state and single-electron transistor behavior, in the scheme known as quantum cellular automata - a new approach that could keep computer CPU's on the Moore's Law curve beyond the inevitable end of CMOS-based logic. Our ultimate goal is to gain a deep scientific understanding of how QDMs self-assemble, and to employ this understanding to tailor useful structures for nanologic applications.
A rather different semiconductor system is the wide bandgap III-nitrides. These materials have risen to technological prominence with amazing rapidity, based upon their bright, efficient emission of green-to-UV light, for applications in LEDs, lasers, and detectors. My specific interest is on understanding how strain relaxation occurs in important III-nitride heterostructures grown by MOCVD, such as AlGaN/GaN and InGaN/GaN. These systems are fascinating for the unusual variety of relaxation mechanisms that have been observed depending on strain state, composition, and growth conditions. In collaboration with Sandia National Labs in New Mexico, I study processes including misfit dislocation generation, inclined threading dislocation formation, fracture, and pit or island formation. The III-nitrides offer the opportunity for materials scientists to investigate very fundamental questions in system where little is truly understood, but where the potential technological payoffs are huge.
A new research area for me is on the nucleation and growth of ZnO nanorods. Similar in many ways to GaN, ZnO is a wide bandgap semiconductor of great interest for blue light emission, and nanorods of ZnO are being studied for use in polymer-blend photovoltaics. Unlike the nitrides, monocrystalline ZnO nanorods can be grown near room temperature using chemical solution synthesis, with a seed layer of Ag to promote nanorod nucleation. Very little is known about how Ag catalyzes and controls ZnO growth, and in collaboration with Sandia we will be studying the effects of epitaxy, strain, and chemical activity on various Ag surface orientations.
Last but not least, I am interested in the growth and manipulation of nanoparticles, and their use to study a variety of fundamental issues in nanoscience. "Manipulation" for me primarily involves learning how to crystallographically orient nanoparticles by a variety of methods, how to prevent aggregation of clean nanoparticles, and how to controllably etch nanoparticles to reduce their size or modify their morphology. "Uses" I intend to make of modified nanoparticle arrays include catalyzed CVD growth of carbon nanotubes, Ostwald ripening studies, and investigations of plasticity and interdiffusion in strained core-shell structures.
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Chair of the Materials Research Society Public Outreach Committee, 2005-current
Elected to Materials Research Society Board of Directors, 2002-2004.
Co-Chair of the Materials Research Society Fall Meeting, 2001.
Member of team winning the DOE Basic Energy Sciences Award for Outstanding Sustained Research in Metallurgy and Ceramics, 1994.
AT&T Bell Labs Graduate Fellowship, 1990-1992.
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BEYOND THE HETEROEPITAXIAL QUANTUM DOT: SELF-ASSEMBLING COMPLEX NANOSTRUCTURES CONTROLLED BY STRAIN AND GROWTH KINETICS, J. L. Gray, R. Hull, Chi-Hang Lam, P. Sutter, J. Means, and J. A. Floro, Phys. Rev. B 72, 155323 (2005).
KINETIC SIZE SELECTION MECHANISMS IN HETEROEPITAXIAL QUANTUM DOT MOLECULES, J. L. Gray, N. Singh, D. M. Elzey, R. Hull, and J. A. Floro, Phys. Rev. Lett. 92, 135504 (2004).
MISFIT DISLOCATION FORMATION IN THE AlGaN/GaN HETEROINTERFACE, J.A. Floro, D. M. Follstaedt, P. Provencio, S. J. Hearne, and S. R. Lee, J. Appl. Phys. 96, 7087 (2004).
THE ORIGINS OF GROWTH STRESSES IN AMORPHOUS SEMICONDUCTOR THIN FILMS, J. A. Floro, P. G. Kotula, S. C. Seel, and D. J. Srolovitz, Phys. Rev. Lett. 91, 096101 (2003).
THE DYNAMIC COMPETITION BETWEEN STRESS GENERATION AND RELAXATION MECHANISMS DURING VOLMER-WBER THIN FILM GROWTH, J. A. Floro, S. J. Hearne, J. A. Hunter, P. Kotula, E. Chason, S. C. Seel, and C. V. Thompson, J. Appl. Phys. 89, 4886 (2001).
CURVATURE-BASED TECHNIQUES FOR REAL-TIME STRESS MEASUREMENT DURING THIN FILM GROWTH, Jerrold A. Floro and Eric Chason, in In Situ and Real Time Characterization of Thin Films, Orlando Auciello and Alan R. Krauss, eds. (John Wiley and Sons, NY 2001), pp. 191-216.
BRITTLE-DUCTILE RELAXATION KINETICS OF STRAINED ALGAN/GAN HETEROSTRUCTURES, S. J. Hearne, J. Han, S. R. Lee, J. A. Floro, and D. M. Follstaedt, Appl. Phys. Lett. 76, 1534 (2000).
NOVEL SIGE ISLAND COARSENING KINETICS: OSTWALD RIPENING AND ELASTIC INTERACTIONS, J. A. Floro, M. B. Sinclair, E. Chason, L. B. Freund, R. D. Twesten, R. Q. Hwang, and G. A. Lucadamo, Phys. Rev. Lett. 84, 701 (2000).
SiGe ISLAND SHAPE TRANSITIONS INDUCED BY ELASTIC REPULSION, J. A. Floro, G. A. Lucadamo, E. Chason, L. B. Freund, M. Sinclair, R. D. Twesten, and R. Q. Hwang, Phys. Rev. Lett. 80, 4717 (1998).
SiGe COHERENT ISLANDING AND STRESS RELAXATION IN THE HIGH MOBILITY REGIME, J. A. Floro, E. Chason, R. D. Twesten, R. Q. Hwang, and L. B. Freund, Phys. Rev. Lett. 79, 3946 (1997).
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