Influence of strain relaxation on the relative orientation of ZnO and ZnMnO wurtzite lattice with respect to sapphire substrates

ZnO and Zn1−x Mn x O () films with 2 μm thickness were grown on (0001) sapphire substrate by molecular beam epitaxy. X-ray, electronic and optical studies show that films have a single crystalline columnar structure with unevenly distributed impurities and defects at the interfaces and boundaries of columns. ZnO and Zn1−x Mn x O films shows a high-quality hexagonal crystal structure with ZnO cells rotated by 30° relative to the sapphire substrate. We establish that the lateral coherence length obtained from x-ray analysis of Zn1−x Mn x O films is decreased from 900 nm to 400 nm at Mn variation from x = 0 to 0.07, which corresponds to variation of an average column diameter in these films. We find that in Zn1−x Mn x O films the area sizes of coherent phonon decaying are determined by the coherent areas of concentration homogeneity of Mn distributions which are much smaller then the dimensions of the columns. Modeling of ZnO/Al2O3 interface structure and properties was performed by means of first-principle density functional theory calculations. We employ an approach based on the use of large supercells (up to 460 atoms) which makes the simulation of interfaces with very large lattice mismatch possible. In this case amorphization of crystal structure in the vicinity of the interface is appears as a natural result of calculations leading to reduction of internal strains that that originate from the ZnO/Al2O3 lattice mismatch. In all cases the double ZnO layer next closest to the to interface (as well as the upper layers) maintains a nearly perfect wurtzite crystal structure. Based on calculations we propose a new model of interface microstructure which includes Zn- or O-monolayers located between conventional ZnO and Al2O3 surfaces. Adhesion energies of ZnO films to sapphire substrate were calculated for unrotated as well as for 30° rotated domains in the cases of Zn- and O-faced ZnO surfaces both with and without additional Zn- or O-monolayers. Comparison of these quantities suggests that energy gain at interface formation is somewhat larger for 30°- rotated domains than for unrotated ones.