Surfactant effects of lithium dopant during molecular beam epitaxy of ZnO films

Surfactant effects of lithium dopant during epitaxial growth of ZnO films were investigated by using reflection high-energy electron diffraction, atomic force microscopy, and x-ray photoemission spectroscopy (XPS). Three-dimensional ZnO epitaxial growth was found to be changed into the two-dimensional layer-by-layer mode after the onset of Li doping, resulting in atomically flat ZnO surfaces. Li atoms, adsorbed on the growth front, induced 12 x 12 and 6 x 6 reconstructions on the polar ZnO( 0001) surface. Further XPS depth profiling investigations of these reconstructed surfaces demonstrated that most of the Li atoms were segregated within the outermost 1 nm ZnO surface. It was proposed that the spontaneous depolarization of the polar ZnO surface caused by the enrichment of Li dopant would reduce the charge accumulation on the polar surface to decrease the surface energy and finally result in the transition of surface morphologies and surface structures.