  We study how FeO w\"{u}stite films on Ru(0001) grow by oxygen-assisted
molecular beam epitaxy at elevated temperatures (800-900 K). The nucleation and
growth of FeO islands are observed in real time by low-energy electron
microscopy (LEEM). When the growth is performed in an oxygen pressure of 10-6
Torr, the islands are of bilayer thickness (Fe-O-Fe-O). In contrast, under a
pressure of 10-8 Torr, the islands are a single FeO layer thick. We propose
that the film thickness is controlled by the concentration of oxygen adsorbed
on the Ru. More specifically, when monolayer growth increases the adsorbed
oxygen concentration above a limiting value, its growth is suppressed.
Increasing the temperature at a fixed oxygen pressure decreases the density of
FeO islands. However, the nucleation density is not a monotonic function of
oxygen pressure.
