Accelerated ab initio Molecular Dynamics of III/V Semiconductor Thin-Film Epitaxy

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TECHNICAL SUMMARYThis award supports computational and theoretical research and education on a continuing challenge in materials simulation: Conducting simulations of structural evolution over long time and length scales, while retaining accurate detail at the atomic level. The PI intends to advance capabilities of rare-event simulations to enable simulations of a wide range of condensed-matter systems. The PI aims to develop methods to advance the current capabilities of accelerated molecular dynamics. An accelerated molecular dynamics protocol will be developed for both classical and ab initio simulations based on the Bond-Boost method. In this algorithm, parallel computing is exploited to parameterize the Bond-Boost potential on the fly, as well as to combine this hyperdynamics-based algorithm with parallel replica dynamics for large acceleration with efficient parallel scaling. To extrapolate accelerated molecular dynamics simulations to larger length and time scales, they will be integrated into kinetic Monte Carlo models. The combination of accelerated molecular dynamics with kinetic Monte Carlo will allow the quantitative application of this technique to multi-scale problems, where the length and time scales range from atomic scales to mesoscopic scales.The Adaptive Bond-Boost method will be applied in first-principles-based simulations of thin-film growth on GaAs(001) -- both homoepitaxy and InAs heteroepitaxy will be studied. Thin-film growth in these systems is technologically significant; there are applications in electronic, optoelectronic, and spintronic devices. From a fundamental perspective, recent experimental work has shown that GaAs(001) substrates can transform and play an active role in diffusion and the morphologies that form during homoepitaxy. This system breaks the conventional paradigm that the substrate is a static template. The way that adatom diffusion, island nucleation, and multi-layer growth occur in such a dynamical environment may be considerably different than that envisioned in the conventional picture of thin-film growth. The PI aims to resolve these phenomena and their role in thin-film growth in this compound semiconductor system. For studies of InAs heteroepitaxy on GaAs(001), existing semi-empirical potentials will be tested against experiment and first-principles density-functional theory for an extensive list of properties. A combination of classical and ab initio accelerated molecular dynamics simulations will be used to probe deposition and diffusion on an InAs wetting layer on GaAs(001). InAs forms self-assembled quantum dots via the Stranski-Krastanov growth mode on GaAs substrates and the proposed studies will be the first real-space, atomic scale studies to resolve the nucleation of quantum dots from the wetting layer.This project includes graduate student and postdoctoral training and involvement of undergraduate students. Aspects of the research will be included in graduate and undergraduate coursework. The state of the art in understanding multi-scale modeling will be advanced through organization of symposia at conferences, as well as international workshops.NON-TECHNICAL SUMMARYThis award supports computational and theoretical research and education on a continuing challenge in materials simulation: Using the newest theoretical and algorithmic developments, which can describe a small number of atoms for only a very short time, on the order of one millionth of one millionth of a second, to describe how materials with many atoms evolve over usual time scales of seconds to hours during their fabrication. The PI aims to advance capabilities of accelerated molecular dynamics. In ordinary molecular dynamics, the motion of each atom is simulated on a computer. The computer steps through very small time increments that are set by the need to include processes that occur from the interaction of individual atoms. At this rate, simulations are too slow to reach the, relatively speaking, long times needed to describe, for example the growth of a thin film of one material on the surface of another. Accelerated molecular dynamics applies ideas from statistical mechanics to enable computer simulations to access longer time scales and to include physical and chemical processes that happen infrequently but play an important role in determining, for example, the structure of a thin film. The PI will use new simulation methods that are developed to simulate the growth of thin-films of materials on the surfaces of the semiconductor material gallium arsenide. This will serve as model for understanding thin film growth on other materials, many being important for applications in electronic, optoelectronic, and spintronic devices, such as lasers, solar cells, and computers of the future. Technological bottle-necks arise from a lack of understanding of how the structure of thin films evolves as they are grown by molecular-beam epitaxy, in which atoms are deposited onto the surface from a vapor. By simulating this materials fabrication technique, the PI aims to develop new insight into how films of materials can be fabricated more effectively for applications. Aspects of the research will be included in graduate and undergraduate coursework and the state of the art in understanding multi-scale modeling will be advanced through organization of symposia at conferences, as well as international workshops. Students will be trained in advanced methods for simulating materials and materials growth.