Morphology evolution of ZnO(000 1̄) surface during plasma-assisted molecular-beam epitaxy

Morphology evolution of ZnO(000 1) surface during plasma-assisted molecular-beam epitaxy was studied. In a step-flow growth mode, terraces bounded by anisotropic steps along 〈11 20〉 characterize the surface of ZnO epilayers. Adatoms favorably incorporate at upsteps, which translates hexagonal islands to regular terraces with equalized width. Surface morphology is sensitive to the Zn/O ratio. Oxygen stabilizes mobile Zn adatoms hence oxygen rich conditions result in rough step edges and irregular hexagonal terraces. Steps become smoother with increasing Zn/O ratio until the stoichiometric condition is achieved. As the Zn flux exceeds the stoichiometry, faceted hexagonal pits form on the ZnO(000 1) surface. Rather than the lack of surface mobility, the lack of stabilization of adatoms is responsible to the formation of pit.