Study of defects in shaped sapphire crystals by synchrotron X-ray phase contrast imaging

Single crystalline sapphire ribbons grown by the Stepanov method exhibit relatively high dislocation densities and often contain slightly misoriented grains. In order to understand the formation of dislocation structures during growth, we studied neck portions cut off perpendicular to the growth axis [ ​\(\overline{1}\)​ 010] of basal-plane-faceted ribbons. The samples have been characterized using phase-contrast and Bragg-diffraction imaging (topography) with synchrotron radiation. It has been found that in the growth direction from the neck towards the main body of the ribbon the dislocation density increases due to multiplication of dislocations. Combining the both imaging techniques, the dislocations were shown to be located around gas voids in sapphire crystals. Computer simulations of the phase-contrast images were carried out to obtain the correct size of the voids.