Revisiting the Growth of Large (Mg,Zr):SrGa12O19 Single Crystals: Core Formation and Its Impact on Structural Homogeneity Revealed by Correlative X-ray Imaging

We demonstrate the growth of large (Mg,Zr):SrGa12O19 (SGMZ) single crystals and use a combination of X-ray imaging techniques to analyze their structural and chemical homogeneity. Single-crystal cylinders with lengths and diameters up to about 2.5 cm are achieved. Our characterization of polished sections reveals a localized (0001) facet that is typically formed at the center of the growth interface. Such facets are seen as the key factor limiting the growth of large-area crystals with excellent structural quality due to local deviations in the segregation behavior of the dopants. We developed a lab-based X-ray diffraction imaging technique with high sensitivity that exposes subtle variations in lattice parameters and lattice tilts, which are attributed to changes in the chemical composition and the resulting elastic deformation. The relationship between unit-cell dimensions and composition is verified by micro X-ray fluorescence mapping. In this way, we find a Ga-rich center region with a reduced unit-cell volume that is surrounded by a ring of increased tilt and elastic strain. Furthermore, we observe a 6-fold in-plane anisotropy of dopant incorporation and tree-ring-shaped structures caused by macrosteps. With rocking curve widths below 23 arcsec in similar to 90% of the crystal, SGMZ crystals are largely homogeneous and hence suitable for the preparation of high-quality substrates. For most applications, the substantially enhanced crystal size enabled by very high Mg and Zr codoping levels outweighs the issues related to concentration variations arising from their addition.