New Mechanisms in Epitaxial Crystal Growth: The Role of Incorporated Defects Revealed By In-Situ X-ray Scattering

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Understanding the fundamental mechanisms of epitaxial crystal growth is of great interest, both because the growth kinetics is a window to atomic-scale processes and because of the desire to control and exploit growth behavior in order to create novel nanoscale materials. This individual investigator award supports a project to investigate new mechanisms of epitaxial growth where vacancies and stacking faults that are incorporated at the growing surface influence the surface morphology. Conventional experimental surface tools, such as scanning-probe techniques (STM, AFM) or electron and atom scattering, are sensitive only to the surface and, therefore, have overlooked the role of incorporated defects, which are subsequently buried below the surface. This project exploits the simultaneous surface and subsurface sensitivity of x-ray scattering using bright x-ray beams from the Advanced Photon Source. The objective is to understand the mechanisms of defect incorporation and how these defects are related to the evolving surface morphology. Because vacancies and stacking faults begin their incorporation locally at the surface, such systems are amenable to theoretical description and, thus, these experimental studies provide an essential intellectual foundation to explore more complex defect systems. Collaborations with theoretical and experimental groups will help achieve a comprehensive understanding that could not otherwise be achieved by a single technique or research group. The project involves graduate students, postdoctoral researchers as well as undergraduates, thereby leading to the integration of research and education though training and mentoring. Their educational experience is significantly enhanced through collaborative research, which strengthens communication skills, and by direct experience with a state-of-the-art x-ray source.With new device technologies moving towards significantly smaller length-scales, the need for fundamental science to explore how atoms move and arrange themselves on surfaces is becoming imperative for understanding how materials grow. Although there has been significant recent progress towards understanding some of the mechanisms that dictate atomic assemblies on surfaces, the role of defects (meaning atoms that do not occupy expected positions) has been overlooked. Because defects can be initially incorporated at the surface and then become buried below it, conventional experimental tools which are sensitive only to the surface cannot see the defects. This individual investigator award supports research that will utilize a beam of x-rays from the Advanced Photon Source, which is the brightest source of x-rays in the United States, to interrogate both the surface and the subsurface structure as atoms are deposited onto a surface. The objective of the project is to understand the mechanism by which defects are incorporated and how the defect incorporation is related to the evolving atomic-scale features on the surface. It is expected that these experimental studies will yield essential new insight to growth mechanisms and, thus, provide a comprehensive framework for understanding how materials grow at surfaces. The project will train and mentor graduate students and postdoctoral researchers as well as undergraduate students. Extensive collaborative work with other research groups will both strengthen the objectives of this project as well as help the development of the students' communication skills. Their research activities at the Advanced Photon Source will enhance their educational experience while these students will become part of a national resource of future scientists who can utilize these new state-of-the-art facilities.