  Using scanning tunneling microscopy (STM), we investigate oxide-induced
growth pits in Si thin films deposited by molecular beam epitaxy. In the
transition temperature range from 2D adatom islanding to step-flow growth,
systematic controlled air leaks into the growth chamber induce pits in the
growth surface. We show that pits are also correlated with oxygen-contaminated
flux from Si sublimation sources. From a thermodynamic standpoint, multilayer
growth pits are unexpected in relaxed homoepitaxial growth, whereas oxidation
is a known cause for step pinning, roughening, and faceting on elemental
surfaces, both with and without growth flux. Not surprisingly, pits are
thermodynamically metastable and heal by annealing to recover a smooth periodic
step arrangement. STM reveals new details about the pits' atomistic origins and
growth dynamics. We give a model for heterogeneous nucleation of pits by
preferential adsorption of {\AA}-sized oxide nuclei at intrinsic growth
antiphase boundaries, and subsequent step pinning and bunching around the
nuclei.
