Twin Domains in 111 oriented {CdO/MgO} superlattices: homoepitaxy versus heteroepitaxy

The structural properties of (111)-oriented {CdO/MgO} superlattice structures grown on c-sapphire and cubic MgO substrates have been studied by high resolution X-ray diffraction. The growth was performed in a plasma-assisted molecular beam epitaxy system. Although both superlattices are (111)-oriented and the {CdO/MgO} structure has 3m symmetry. It was shown that the superlattice on c-sapphire consists of misoriented domains, whereas no such domains were observed on (111) MgO. The twin domains are rotated by 180° with respect to each other and by 30° with respect to the sapphire substrate. We show that the crucial phenomena based on the formation of rotation domains and their number in heteroepitaxy depend fundamentally on the relationship between substrate and epilayer symmetries.

💡 Research Summary

This study presents a comparative structural analysis of (111)-oriented CdO/MgO superlattices (SLs) grown on two different substrates: c-plane sapphire (Al2O3) and (111) MgO. Using plasma-assisted molecular beam epitaxy (MBE), a series of SLs with fixed MgO thickness (4 monolayers, ML) and varying CdO thickness (1-10 ML) were fabricated under identical growth conditions. The primary investigative tool was high-resolution X-ray diffraction (HRXRD), employed in multiple geometries to obtain a complete structural picture.

Conventional symmetric and asymmetric XRD scans, along with reciprocal space mapping (RSM), confirmed the (111) orientation of all SLs. The analysis showed that the average out-of-plane lattice constant (a⊥) of the SLs increased systematically with CdO thickness, a trend observed on both substrates and consistent with theoretical predictions. This indicated that the intrinsic strain state of the SL period was independent of the substrate.

The critical finding emerged from non-coplanar skew-symmetric XRD geometry, specifically through φ-scans (azimuthal scans) measured at an inclination angle (χ~54°). For SLs grown on (111) MgO substrates, the φ-scan of the SL (111) reflection showed three peaks separated by 120°, confirming single-domain epitaxial growth with threefold symmetry. In stark contrast, the same measurement for SLs grown on c-sapphire revealed six peaks separated by 60°. This pattern is a definitive signature of two families of domains, rotated by 180° with respect to each other, known as twin domains. Pole figure measurements corroborated this result.

The authors attribute this fundamental difference to the symmetry relationship at the film-substrate interface. While both the SL (with rocksalt structure) and the c-sapphire substrate possess a 3m point group symmetry, the actual atomic alignment at the heterointerface involves a 30° in-plane rotation of the SL lattice relative to the sapphire lattice. This rotation breaks the point symmetry at the interface, creating two energetically equivalent nucleation sites for the epilayer. These sites subsequently grow into domains that are rotated by 180°, forming twins. In the case of the MgO substrate, the epilayer and substrate share the same cubic crystal structure and orientation (approaching homoepitaxy), leading to a coherent interface with matched symmetry and thus single-domain growth without twin formation.

In conclusion, this work demonstrates that the formation of rotational twin domains in heteroepitaxial systems is governed not merely by lattice mismatch but fundamentally by the symmetry relationship between the substrate and the epilayer. Using advanced XRD methodologies, it provides clear experimental evidence that seemingly similar superlattice structures can possess vastly different microscopic domain structures based on the substrate choice, with significant implications for defect engineering and the optimization of electronic and optical properties in oxide-based heterostructures.