Thickness-dependent relaxation of NiO(001) overlayers on MgO(001) studied by X-ray diffraction

Under equilibrium conditions, coherent growth on a substrate having the same crystal structure as the overlayer will break down above a critical thickness as a result of the stable formation of misfit dislocations. In practice, however, owing to the existence of kinetic barriers, much thicker coherent layers may often be grown. We present a systematic study of the relaxation behaviour of NiO(001) layers (from 160 up to 1600 Angstrom in thickness), grown by molecular beam epitaxy on MgO(001) substrates. Ex situ X-ray specular reflectivity measurements determined the layer thickness and interface roughness. Relaxation of the NiO lattice parameter was detected with X-ray diffraction via reciprocal-space maps about non-specular diffraction peaks common to the substrate and overlayer. The Poisson ratio for NiO was experimentally determined to be 0.21. A comparison with theoretical predictions of the relaxation behaviour for NiO on MgO is made. (C) 1999 Elsevier Science B.V. All rights reserved.