Influence of Solute Incorporation Mechanisms on the Properties of Highly Mismatched Alloys

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Non-technical Description: Highly mismatched alloys are materials that contain chemical elements with very different atomic sizes and different ability to attract nearby electrons. When a few atoms with larger or smaller sizes are added to an alloy, its electrical and optical properties often change dramatically. The goal of this research is to examine the impact of the atoms' local surroundings on the electronic and optical properties of alloy films and multi-layer structures, with an eye towards engineering structures for temperature-insensitive lasers, high-selectivity spin valves, and high efficiency photovoltaics. The project provides training to graduate, undergraduate, and high school students, engaging them in collaborations with investigators from several universities and laboratories throughout the world. The new knowledge acquired is broadly disseminated through publications and presentations, and graduate and undergraduate curriculum development. Outreach activities emphasize the mentoring of women and underrepresented minorities. Technical Description: The project is focused on fundamental understanding of solute incorporation mechanisms and their influence on the electronic states, transport, and optoelectronic properties of highly mismatched alloys. The primary goal of the project is to develop a predictive framework for the influence of substitutional N and Bi atoms, as well as N-As, N-N, or N-Bi pairs, on the properties of both random and ordered GaAs(BiN) alloy films and heterostructures. The GaAs(BiN) alloys are prepared on a variety of substrates using plasma-assisted molecular-beam epitaxy, followed by post-epitaxy rapid-thermal annealing sequences. The interplay between surface reconstruction, surface stress and local electronic states is monitored in real-time using in-situ reflection high-energy electron diffraction, multi-beam optical stress sensing and scanning tunneling microscopy/spectroscopy. The alloy stoichiometry and local atomic environments are examined using atom-probe tomography and novel channeling ion beam analyses. To determine the correlations between solute incorporation mechanisms and electronic states, the experimental transport and spectroscopic data is compared with both atomistic and density-functional theory computations. In addition to determining the composition dependence of band offsets of quaternary alloys with respect to GaAs, signatures for nested versus staggered band offsets are examined using scanning tunneling spectroscopy and time-resolved photoluminescence. The project at the University of Michigan involves collaborations with scientists at national laboratories and international universities, providing broad range of opportunities for the participating students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.