Oxygen-dosage effect on the structure and composition of ultrathin NiO layers reactively grown on Ag(001)

NiO ultrathin films have been prepared under UHV conditions on Ag~001! substrate by metal deposition in O2 atmosphere. The films were prepared by deposition of the amount of metal correspondent to 1 ML ~monolayer! of NiO in the presence of different oxygen-to-metal flux ratios, to investigate the oxygen dosage effect on the structure and composition of the growing layer and on the stoichiometry of Ni oxide. Thicker films ~up to 20 ML! were also prepared. The structure has been monitored both in reciprocal and direct space by low-energy electron diffraction and primary-beam diffraction-modulated electron emission. Core-level x-ray photoemission spectroscopy has been used to study film chemistry and composition. At low coverage ~below 2 ML! a dramatic dependence of structure and composition on the oxygen-to-nickel flux ratio has been observed. Low oxygen dosage induces a (231) reconstruction in the 1-ML films that evolves to a (131) phase as the dosage and/or the film thickness increases. In the layers prepared at low oxygen dosage a large fraction of metallic Ni coexists with the Ni oxide, but the oxidized fraction largely prevails for high oxygen dosage. The oxygen dosage during the growth also affects thicker films. The mosaic formation, which has been ascribed to misfit strain relaxation, is related to a low oxygen-to-nickel flux ratio.