Reflection High Energy Electron Diffraction (RHEED) Study of Nanostructures: From Diffraction Patterns to Surface Pole F
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Reflection High Energy Electron Diffraction (RHEED) Study of Nanostructures: From Diffraction Patterns to Surface Pole Figures Fu Tang, Toh-Ming Lu, and Gwo-Ching Wang Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute 110, 8th Street, Troy, NY 12180
ABSTRACT In this report we present a brief overview of the growth of nanostructures by the oblique angle deposition where the nanostructures possess both out-of-plane and in-plane preferred orientations or a biaxial texture. The degree of preferred crystal orientations can be quantitatively determined from a method called “RHEED surface pole figure analysis” that we developed recently.
INTRODUCTION Many thin film materials and nanostructures are not single crystals but polycrystallines with a preferred orientation of the crystalline grains. There are various categories of common textures: fiber texture with one-degree orientation and biaxial texture with two-degree orientation. Extreme cases of texture are random orientations and single crystals. Many physical properties of materials depend on grain orientations. Therefore it is critical to understand the science of the texture evolution process in order to control and design its texture. It is well known that reflection high-energy electron diffraction (RHEED) has been an important tool to monitor the growth rate of epitaxial films on single crystal substrates through “intensity oscillations” and to determine the structure of the films through the diffraction patterns. The questions are: what information can we obtain from RHEED of nanostructures grown on non-single crystal substrates such as amorphous substrates? Can we use this common laboratory electron diffraction technique to study the initial to the final stages of nanostructure growth in terms of their structure and texture? By understanding the science of growth, can we develop a method to grow single crystal nanostructures on an amorphous substrate that would change the ways of growing single crystal films? Very often, the initial growth of nanostructures on an amorphous substrate exhibits a completely random crystal orientation which gives a ring structure in the RHEED pattern. From the ring radius and the full-width-at-half-maximum of the ring the lattice constant and the average size of nanocrystals (as small as ~2 nm) can be determined, respectively [1]. If these nanocrystals evolve into a preferred orientation the rings break and arcs are formed. An example of in situ RHEED pattern measurements of the growth of Cu nanocrystals on an amorphous substrate has been demonstrated in our lab. [1, 2] A particularly interesting case is the growth of the nanostructures by the oblique angle deposition where the nanostructures possess both out-of-plane and in-plane preferred orientations or a biaxial texture. The degree of preferred crystal orientations can be quantitatively determined from a method called “RHEED surface pole figure analysis” that we developed recently [3, 4].
EXPERIMENT Oblique angle deposition a
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