When a free electron fills the shell, a x-ray photon with energy characteristic of the target material is emitted. The high energy electrons also eject inner shell electrons in atoms through the ionization process. As electrons collide with atoms in the target and slow down, a continuous spectrum of x-rays are emitted, which are termed Bremsstrahlung radiation. In a x-ray tube, which is the primary x-ray source used in laboratory x-ray instruments, x-rays are generated when a focused electron beam accelerated across a high voltage field bombards a stationary or rotating solid target. X-rays are produced generally by either x-ray tubes or synchrotron radiation. The energetic x-rays can penetrate deep into the materials and provide information about the bulk structure. Because the wavelength of x-rays is comparable to the size of atoms, they are ideally suited for probing the structural arrangement of atoms and molecules in a wide range of materials. For diffraction applications, only short wavelength x-rays (hard x-rays) in the range of a few angstroms to 0.1 angstrom (1 keV - 120 keV) are used. X-rays are electromagnetic radiation with typical photon energies in the range of 100 eV - 100 keV. Principles of Protein X-ray Crystallography, by Jan Drenth, Springer, 1994 (Crystallography).Brumberger, Editor, Kluwer Academic Publishers, 1993 (SAXS techniques) Modern Aspects of Small-Angle Scattering, by H.Tanner, Taylor & Francis, Ltd., 1998 (Semiconductors and thin film analysis) High Resolution X-ray Diffractometry and Topography, by D.Cullity, Addison-Wesley, 1978 (Covers most techniques used in traditional material characterization) Elements of X-ray Diffraction,2nd Ed., by B.D.Warren, General Publishing Company, 1969, 1990 (Classic x-ray physics book) Elements of Modern X-ray Physics, by Jens Als-Nielsen and Des McMorrow, John Wiley & Sons, Ltd., 2001 (Modern x-ray physics & new developments).Extensive and authoritative discussions can be found in the numerous books and journal articles on this subject. It is designed for people who are novices in this field but are interested in using the techniques in their research. This is intended as a (very) brief introduction to some of the common x-ray diffraction techniques used in materials characterization. Mitsubishi Chemical - Center for Advanced Materials.IRG 3: Resilient Multiphase Soft Materials.IRG-1: Magnetic Intermetallic Mesostructures.In these cases, beam rocking allows precise variation of the incidence angle. The beam rocking option is very helpful in critical cases where the sample position cannot be adjusted easily during the growth. The electron gun can be mounted on the vacuum chamber without using a bellows for mechanical adjustment. In this way, the incidence angle can be precisely controlled electronically without either modifications of the geometry of the gun or motion of the sample. When equipped with the beam rocking option, specially designed optics shift the electron path off axis in the gun and refocus it onto the sample, maintaining the spot position. Electron sources from STAIB Instruments are uniquely designed to allow this precise electronic control of the beam position using sophisticated electron beam deflection optics. The unique STAIB feature, beam rocking, allows precise adjustment and variation of the incidence angle using electronic controls, without moving the sample. RHEED is the premium choice technique for growth monitoring since multiple different parameters can be captured simultaneousley in situ, and in real time.įor RHEED analysis, the electron beam must impinge onto the surface at grazing incidence. The intensity of diffraction spots or streaks is used to monitor the deposition at a layer-by-layer sensitivity (technique named RHEED oscillations). When used in a deposition device, the diffraction diagram instantly displays the surface modifications. RHEED delivers real time detailed information about the crystal structure of the bulk and the smoothness and crystalline quality of the surface layer with a sub-monolayer resolution. The ability to monitor variations of the diagram during deposition is most valuable. The diffracted electrons build a diagram consisting of a superposition of spots at Bragg angles for the bulk material, streaks from the surface diffraction, and Kikuchi lines from channeling effects. High energy electrons have a wavelength much smaller than the lattice spacing and diffraction conditions can be reached simply by setting the incident beam at a very small incidence angle to the surface. Low energy electrons have a wavelength that is in the order of atomic spacing and can be diffracted by lattice atoms. Electrons can be diffracted by a surface in a similar way as visible light, except that the associated wavelength is much smaller.
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