Diffraction can occur whenever Braggs law is tisfied. With monochromatic radiation. an XRD: "Rocking" Curve Scan arbitrary setting of a single crystal in an x'ray beams. There would therefore be very littl(ed am will not generally produce any diffrac mple normal information in a single crystal diffraction pattern from using monochromatic radiation This problem can be by continuously A or 0 over a range of values, to satisf Rock"Sample Braggs law. Practically this is done by e Vary orientation of Ak relative to sample normal while (Dusing a range of x ray wavelengths (i.e. white Rock"sample over a very small angular range. Dby rotating the crystal or, using a powder or b Resulting data of Intensity vs theta (0, sample angle) polycrystalline specimen. shows detailed structure of diffraction peak being XRD: Rocking Curve Example Rocking Curves assessing crystal quality 002) Reflection T through 0 with the counter set at a known bragg angle, 20. The resulting intensity versus 0 curve is known as a rocking curve. The width of the rocking curve is a direct measure of the range of orientation on mosaic s read present in the irradiated area of the crystal, as each sub grain of is rotated. For a film that isn't truely epitaxial the width of a rocking curve of the layer peak will b infraction peak showing its detailed structure ment of the quality of the layer Rotating Crystal Method Laue method In the rotating crystal method. The laue method is mainly used to determin reflected from or transmitted throt rotated about the chosen axis. As the crvstal rotates, sets of lattice point make the Iw wn correct Bragg angle for the radiation that satisfies the bragg law for the values of d monochromatic incident beam and be formed aThe reflected beams are located surface of an imaginary cone whose axis is the out flat Experimental two practical variants of the laue method, the horizontal lines back-reflection and the transmission Laue method.5 ß Diffraction can occur whenever Bragg's law is satisfied. With monochromatic radiation, an arbitrary setting of a single crystal in an x-ray beam will not generally produce any diffracted beams. There would therefore be very little information in a single crystal diffraction pattern from using monochromatic radiation. ß This problem can be overcome by continuously varying l or q over a range of values, to satisfy Bragg's law. Practically this is done by: (1)using a range of x-ray wavelengths (i.e. white radiation), or (2)by rotating the crystal or, using a powder or polycrystalline specimen. XRD: “Rocking”Curve Scan Vary orientation of Dk relative to sample normal while maintaining its magnitude. How? “Rock”sample over a very small angular range. Resulting data of Intensity vs. theta (q, sample angle) shows detailed structure of diffraction peak being investigated. i k f k Dk “Rock”Sample Dk Sample normal XRD: Rocking Curve Example Rocking curve of single crystal GaN around (002) diffraction peak showing its detailed structure. 16.995 17.195 17.395 17.595 17.795 0 8000 16000 GaN Thin Film (002) Reflection Intensity (Counts/s) theta (deg) Rocking Curves ¾ assessing crystal quality To estimate the crystal quality a crystal is rotated through q with the counter set at a known Bragg angle, 2q. The resulting intensity versus q curve is known as a rocking curve. The width of the rocking curve is a direct measure of the range of orientation on mosaic spread present in the irradiated area of the crystal, as each sub grain of the crystal will come into orientation as the crystal is rotated. For a film that isn't truely epitaxial, the width of a rocking curve of the layer peak will be a measurement of the quality of the layer. Rotating Crystal Method In the rotating crystal method, a single crystal is mounted with an axis normal to a monochromatic x￾ray beam. A cylindrical film is placed around it and the crystal is rotated about the chosen axis. As the crystal rotates, sets of lattice planes will at some point make the correct Bragg angle for the monochromatic incident beam, and at that point a diffracted beam will be formed. The reflected beams are located on the surface of imaginary cones. When the film is laid out flat, the diffraction spots lie on horizontal lines. Laue Method ß The Laue method is mainly used to determine the orientation of large single crystals. White radiation is reflected from, or transmitted through, a fixed crystal. ß The diffracted beams form arrays of spots, that lie on curves on the film. The Bragg angle is fixed for every set of planes in the crystal. Each set of planes picks out and diffracts the particular wavelength from the white radiation that satisfies the Bragg law for the values of d and q involved. Each curve therefore corresponds to a different wavelength. The spots lying on any one curve are reflections from planes belonging to one zone. Laue reflections from planes of the same zone all lie on the surface of an imaginary cone whose axis is the zone axis. ß Experimental ß There are two practical variants of the Laue method, the back-reflection and the transmission Laue method