Defect structure of epitaxial Cr_xV_1−x thin films on MgO(001)

Epitaxial thin films of Cr_xV_1−x over the entire composition range were deposited on MgO(001) by molecular beam epitaxy. The films exhibited the expected 45° in-plane rotation with no evidence of phase segregation or spinodal decomposition. Pure Cr, with the largest lattice mismatch to MgO, exhibited full relaxation and cubic lattice parameters. As the lattice mismatch decreased with alloy composition, residual epitaxial strain was observed. For 0.2 ≤ x ≤ 0.4 the films were coherently strained to the substrate with associated tetragonal distortion; near the lattice-matched composition of x = 0.33, the films exhibited strain-free pseudomorphic matching to MgO. Unusually, films on the Cr-rich side of the lattice-matched composition exhibited more in-plane compression than expected from the bulk lattice parameters; this result was confirmed with both x-ray diffraction and Rutherford backscattering spectrometry channeling measurements. The effect of thermal expansion mismatch on strain in the heterostructure was estimated. High resolution transmission electron microscopy was utilized to characterize the misfit dislocation network present at the film/MgO interface. Dislocations were found to be present with a non-uniform distribution, which is attributed to the Volmer–Weber growth mode of the films. The Cr_xV_1−x/MgO(001) system can serve as a model system to study the fundamentals of defect formation in bcc films. - Highlights: • Epitaxial Cr_xV_1−x thin films were deposited on MgO(001) by molecular beam epitaxy. • Unusually large in-plane lattice compression observed for Cr-rich films • Non-uniform misfit dislocation network attributed to Volmer–Weber growth mode • Serves as model system to study interplay between defects and radiation damage