Characterization Methods and Systems X-Ray Diffraction XRD char.ppt 4 Apr 2007
Methods of Characterization Characterization methods can be categorized into several general categories. These include (but aren’t limited to): 1. Physical--determination of the structural, topographical, morphological state of the sample. Includes SEM, TEM, AFM. 2. Chemical/compositional--characterization of the chemical elements or compounds present in the sample, possibly including location on the surface or with depth into the sample. Includes XRD, EDX, SIMS, XRF, XPS. 3. Electrical--measurement of the resistivity/conductivity of a sample, carrier concentration, mobility of charge carriers. Includes 4PP. 4. Optical--measurement of the optical properties or behavior of the sample. Includes ellipsometry, interference spectroscopy, FTIR, PL. 5. Magnetic--measurement of the magnetic properties or behavior of the sample. Includes VSM, Hall. 6. Mechanical--determination of the mechanical properties (strength, elastic modulus, etc) of the sample.
X-ray Diffraction X-ray diffraction can be used to characterize film structure (degree of crystallinity), intermixing of multiple films, & epitaxial composition, among others. X-rays are electromagnetic radiation with a very short wavelength. They are produced when electrically charged particles of sufficient energy are decelerated. In an X-ray tube, the high voltage maintained across the electrodes draws electrons toward a metal target (the anode). X-rays are produced at the point of impact, and radiate in all directions. Tubes with copper targets, which produce their strongest characteristic radiation (K1) at a wavelength of about 1.5Å. (0.15 nm).
X-ray Diffraction If an incident X-ray beam encounters a crystal lattice, general scattering occurs. Most scattering is destructively interfered and is lost (destructive interference). Diffraction occurs when scattering in a certain direction from one atomic plane is in phase with scattered rays from other atomic planes. Under this condition the reflections combine to form enhanced wave fronts that mutually reinforce each other (constructive interference). The relation by which diffraction occurs is known as the Bragg law. Each crystalline material has a characteristic atomic structure and it will diffract X-rays in a unique characteristic pattern. Bragg’s Law: 2dsin = n = X-ray wavelength = X-ray incident angle d = lattice spacing of atomic planes (assuming a crystal)
X-ray Diffraction The basic geometry of an X-ray diffractometer includes a. a source of monochromatic radiation and b. an X-ray detector situated on the circumference of a graduated circle centered on the powder specimen. c. divergent slits, located between the X-ray source and the specimen, and divergent slits, located between the specimen and the detector, limit scattered (non-diffracted) radiation, reduce background noise, and collimate the radiation. d. the detector and specimen holder are mechanically coupled with a goniometer giving a detector rotation of 2 for a specimen rotation of .
X-ray Diffraction A curved-crystal monochromator containing a graphite crystal is used to ensure that the detected radiation is monochromatic. When positioned properly just in front of the detector, only K radiation is directed into the detector, and the Kß radiation is directed away. The signals from the detector are filtered by pulse-height analysis, scaled to measurable proportions, and sent to a linear ratemeter for conversion into a continuous current.
X-ray Diffraction For crystalline specimens, XRD can determine the lattice spacing d. A crystalline specimen will produce a characteristic diffraction pattern of spots. This diffraction pattern can be analyzed to give the crystal structure. Since the method can determine the lattice parameter d, it can also be used in mechanical characterization such as to characterize strain, d/d0, inside an epitaxial sample.
X-ray Diffraction XRD can be used to identify crystal structures. Individual atomic planes (families) can be identified. X-ray diffraction patterns of (a) BaLa2ZnO5 and (b) BaNd2ZnO5.
These patterns can be used for compositional analysis in an unknown sample.
XRD can be used to investigate the crystal environment of particular atoms. Ta displaying cubic symmetry in the mineral microlite. Ta showing hexagonal symmetry in the mineral calciotantite.
Can use XRD to generate a time series to follow changes in a sample under experimentation. This sample undergoes a phase transition as a function of T between 150°C and 170°C. high T phase low T phase
X-ray Diffraction For polycrystalline or noncrystalline materials, X-rays may be diffracted into a number of different directions.