Investigation of ZnO epilayers grown under various Zn/O ratios by plasma-assisted molecular-beam epitaxy

We have investigated ZnO epilayers grown under various Zn/O ratios by plasma-assisted molecular-beam epitaxy. The growth conditions are elucidated by a relationship between growth rate and Zn/O ratios. Surface phase diagrams are obtained by investigation of reflection high-energy electron diffraction. Hexagonal-shaped two-dimensional islands with atomic steps measured by atomic force microscopy are observed on ZnO layers grown under oxygen-rich and stoichiometric flux conditions. On the other hand, ZnO layers grown under Zn-rich conditions exhibit hexagonal pits. The x-ray rocking curve of ZnO samples grown under an oxygen-rich condition is broader than that of ZnO samples grown under stoichiometric or Zn-rich flux conditions, implying a reduction in threading dislocation density. Photoluminescence spectra reveal the strongest intensity from ZnO layers grown under stoichiometric flux conditions, compared with those grown under Zn- and oxygen-rich conditions. The relation between linewidth of the x-ray rocking curve and intensity of photoluminescence suggests that threading dislocations act as nonradiative centers. In conclusion, the Zn/O flux ratio during growth has a strong influence on the quality of ZnO epilayer surfaces, crystal structures, and optical properties. (C) 2002 American Institute of Physics.