  Submonolayers of nickel oxide films were grown on an Ag(001) by pulsed laser
deposition, and characterized in-situ by both scanning tunneling microscopy and
X-ray photoelectron spectroscopy. We observed quasi-two-dimensional growth of
the film, and clearly identified several kinds of defects, such as embedded
metallic Ni clusters and, notably, oxygen atoms, even while looking deeply into
the substrate. These originated from Ni and O hyperthermal projectiles as well
as from NiO clusters that were formed during laser ablation of a NiO target.
Those defects played a role of nucleation sites in extending the nucleation
stage of thin film growth.
