Kinks and Surface Potentials

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9814055SpenceThis project addresses dislocation kink dynamics, and measurement of the Coulomb potential normal to a surface by quantitative convergent-beam reflection high energy diffraction (Micro-probe RHEED). Dislocation kinks will be observed by atomic resolution TEM in materials such as sapphire, SiC and MgO, where it has been found possible to prepare atomically flat terraced surfaces for thin TEM samples under moderate (10-7 Torr) vacuum conditions by annealing in oxygen or vacuum. "Forbidden" Bragg reflections, arising from the stacking fault between partial dislocations, will be used to form lattice images, allowing images to be formed without surface noise. The approach is to determine which process (kink formation, kink migration or obstacles along the dislocation line) limits kink (and hence dislocation) velocity, for given conditions of temperature and stress. By extending prior quantitative convergent-beam TEM measurements of bonding in crystals to the RHEED geometry, it is planned to refine the electrostatic potential extending into the vacuum from ceramic surfaces. This potential is critical for chemical reactions at surfaces, for adsorption, catalysis, epitaxy, the diffusion bonding process, oxidation, and semiconductor crystal growth. The surface potential barrier is also critical in electronic processes, such as field emission and secondary electron emission. Measurements of the modification to this potential which result from the deposition of a monolayer or more, of atoms can provide a direct experimental understanding of how bonds are formed at surfaces, and of the modification of atomic charge densities.%%%The project addresses basic research issues in a topical area of materials science having high potential technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new aspects of electronic/photonic devices. Experimental tools are now available to allow atomic level observation of elementary surface processes which when better understood allow advances in fundamental science and technology. The basic knowledge and understanding gained from the research is expected to contribute to improving the performance and stability of advanced devices and circuits by providing a fundamental understanding and a basis for designing and producing improved materials, and materials combinations. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area.***9814055SpenceThis project addresses dislocation kink dynamics, and measurement of the Coulomb potential normal to a surface by quantitative convergent-beam reflection high energy diffraction (Micro-probe RHEED). Dislocation kinks will be observed by atomic resolution TEM in materials such as sapphire, SiC and MgO, where it has been found possible to prepare atomically flat terraced surfaces for thin TEM samples under moderate (10-7 Torr) vacuum conditions by annealing in oxygen or vacuum. "Forbidden" Bragg reflections, arising from the stacking fault between partial dislocations, will be used to form lattice images, allowing images to be formed without surface noise. The approach is to determine which process (kink formation, kink migration or obstacles along the dislocation line) limits kink (and hence dislocation) velocity, for given conditions of temperature and stress. By extending prior quantitative convergent-beam TEM measurements of bonding in crystals to the RHEED geometry, it is planned to refine the electrostatic potential extending into the vacuum from ceramic surfaces. This potential is critical for chemical reactions at surfaces, for adsorption, catalysis, epitaxy, the diffusion bonding process, oxidation, and semiconductor crystal growth. The surface potential barrier is also critical in electronic processes, such as field emission and secondary electron emission. Measurements of the modification to this potential which result from the deposition of a monolayer or more, of atoms can provide a direct experimental understanding of how bonds are formed at surfaces, and of the modification of atomic charge densities.%%%The project addresses basic research issues in a topical area of materials science having high potential technological relevance. The research will contribute basic materials science knowledge at a fundamental level to new aspects of electronic/photonic devices. Experimental tools are now available to allow atomic level observation of elementary surface processes which when better understood allow advances in fundamental science and technology. The basic knowledge and understanding gained from the research is expected to contribute to improving the performance and stability of advanced devices and circuits by providing a fundamental understanding and a basis for designing and producing improved materials, and materials combinations. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area.***