90-degree Bragg Reflection

One of the potentially important features of a 90-degree Bragg reflection from a crystal is its application to novel analytical-experimental techniques for x-ray characterization of crystals, e.g. The extremely high sensitivity of the 90 degree reflection to the incident radiation energy, e.g. [1-2], the crystal angular and azimuthal orientation, e.g. (Nikulin, Davis and Cookson, Phys. Stat. Sol. (a), 1998), and crystal-lattice spacing changes, make this experimental environment very attractive for use as an optical element and as an enhanced analytical technique.

Recently we reported the first experimental observation of synchrotron radiation diffraction from a thin crystalline film deposited on a single-crystal substrate (Nikulin, Davis, Jones, Usher, Souvorov and Freund, Phys. Stat. Sol. (a), 2000). A 1.0 micron thick film deposited on a GaAs(100) substrate was studied near the 90-degree Bragg position. The experiment was performed on a bending magnet beamline BM05 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Slight, less than 0.1%, difference in the lattice spacing between the layer and the substrate, allowed, for the first time, direct observation of the diffraction profile from a thin layer as if it was a "free-standing" thin crystal (Nikulin, Davis, Jones, Usher, Souvorov and Freund, Phys. Stat. Sol. (a), 2000). Relative intensity of the layer diffraction peak compared to the substrate diffraction peak was found to be unexpectedly low. We conjectured that this effect might be due to interference suppression, within the surface layer, of the back-reflected radiation (Nikulin, Davis, Jones, Usher, Souvorov and Freund, Phys. Stat. Sol. (a), 2000).

More recent experimental studies of a thin film reflecting at 90-degrees were performed on an undulator beamline at the Advanced Photon Source (APS), Chicago, USA. That experiment confirmed that our conjecture that the layer might work as a Fabry-Perot interferometer (Nikulin, Davis, Jones, Usher, Souvorov and Freund, Phys. Stat. Sol. (a), 2000).

This page presents new, high energy-resolution, data collected using an extremely highly monochromatic synchrotron (undulator) radiation source. We also discuss here experimentally observed diffraction phenomena that occur when synchrotron radiation is diffracted from a thin surface layer at almost exactly 90-degree condition. (Nikulin, Tamasaku, Ishikawa and Usher, Japanese J. Appl. Phys., 2000 ).

[1] K. Kohra and T. Matsushita, Z. Naturforsch. Teil A27, 484 (1972).
[2] W. Graeff and G. Materlik, Nuclear Instruments and Methods 195, 97 (1982).

	The experimental set-up for observation of the diffraction from a thin film at a 90-degree Bragg reflection performed at the SPring-8, beamline BL29XU, Harima Institute, Japan. The sketch is not to scale.
	Photograph of the sample-detector arrangement
	The two-dimensional experimental intensity profile measured in the scintillation detector as a function of the angular deviation of the InGaAs/GaAs(800) crystal (layer thickness was 1.5 micron, x=0.2%) from the 90-degree Bragg position. Higher intensity peak corresponds to the diffraction from the GaAs substrate and lower peak corresponds to the diffraction from the InGaAs surface layer. "Fringes" on the top of the substrate peak are due to the limited radiation energy step.
	The two-dimensional experimental intensity profile measured in the scintillation detector as a function of the angular deviation of the InGaAs/GaAs(800) crystal (layer thickness was 0.5 micron, x=0.5%) from the 90-degree Bragg position. Higher intensity peak corresponds to the diffraction from the GaAs substrate and lower peak corresponds to the diffraction from the InGaAs surface layer. "Fringes" on the top of the substrate peak are due to the limited radiation energy step.
AFTER: Nikulin, Tamasaku, Ishikawa and Usher, Japanese J. Appl. Phys., 2000 (submitted)