Layer-by-layer epitaxial growth of polar MgO (111) films with atomically flat surfaces

Abstract

We have succeeded in layer-by-layer epitaxial growth of MgO (111) films using single-crystalline NiO buffer films by pulsed laser deposition along the [111] polar growth direction despite a strong electrostatic instability. Layer-by-layer growth was clearly observed up to ~ 10 MgO layers with monolayer step and terrace structure. Further thicker films of 10 - 500 MgO layer present the root mean square surface roughness as small as ~ 0.2 nm with high crystalline quality comparable to the single-crystal NiO buffer layers although the step and terrace structure disappeared. Transmission electron microscopy study indicates formation of an atomic smooth MgO/NiO interface although low-quality domains with dislocations and strains and high-quality domains coexisted in the regin away from the interface. Thus the electrostatic instability of thicker MgO films induced the switching from layer-by-layer to three dimensional growth mode by introducing the double domain structures.