Misfit dislocation network at the Ag/MgO(001) interface: A grazing-incidence x-ray-scattering study

The misfit dislocation network at the Ag/MgO(001) interface has been investigated by grazing incidence x-ray diffraction on Ag films of different thicknesses, between 100 and 1500 Angstrom deposited at room temperature by molecular beam epitaxy on MgO(001) surfaces with different step densities. They were analyzed after growth and after annealing at increasing temperatures. A square network of edge dislocations is found to release the 3% lattice misfit between Ag and MgO. The dislocation lines are found oriented along [110] directions, with a/2[110] Burgers vector. Quantitative analysis shows that the substrate is deformed according to the elasticity theory. The as-grown Ag films contain stacking faults and twins, in amounts that increase with increasing substrate surface step density. Annealing the film at low temperature 300 degrees C eliminates the stacking faults. Annealing at higher temperatures induces a recrystallization of the Ag epilayers, as well as an improved ordering of the dislocation network. For increasing temperatures, the period of the dislocation network instantaneously adapts to the actual lattice parameter misfit. After cooling, a large residual deformation is found in Ag, which is smaller when the substrate step density is larger. This is interpreted in terms of an energetic barrier for the nucleation of misfit dislocations.