Experimental observation of X-ray diffraction from a thin crystalline filmat a 90 degrees Bragg reflection

The first experimental observation of diffraction from a thin surface layer at a 90 degrees Bragg reflection is reported. A thin (< 1 mu m) InGaAs fil m deposited on a GaAs(800) substrate was studied near the 90 degrees Bragg position. Slight, less than 0.1%, difference in the lattice spacing between the layer and the substrate, has allowed, for the first time, a direct and exclusive observation of the diffraction profile from a thin layer as if i t was a "free-standing" thin crystal.