Single-crystal sapphire microstructure for high-resolution synchrotron X-ray monochromators

We report on the growth and characterization of sapphire single crystals for X-ray optics applications. Structural defects were studied by means of laboratory double-crystal X-ray diffractometry and white-beam synchrotron-radiation topography. The investigations confirmed that the main defect types are dislocations. The best quality crystal was grown using the Kyropoulos technique. Therein the dislocation density was 10²-10³ cm^-2 and a small area with approximately 22 mm² did not show dislocation contrast in many reflections. This crystal has suitable quality for application as a backscattering monochromator. A clear correlation between growth rate and dislocation density is observed, though growth rate is not the only parameter impacting the quality. We assessed the quality of five sapphire single crystals grown by known techniques. The main defect type is curve shaped dislocations. Synchrotron white beam topography revealed a very high quality wafer grown by Kyropoulos technique with dislocation density of 10²-10³ cm^-3. The minimum full-width at half maximum of the rocking curve and the resolution for this wafer are about 2.4′′ and 1.7 meV obtained from double-crystal diffractometry and backscattering geometry reflectivity, respectively.