  We report scanning tunneling spectroscopy studies of the electronic structure
of 1.5 to 3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers
exhibit a bulk-like band gap, approximately 5-7 eV, and sparse, localized
defect states with characteristics attributable to oxygen and, in some cases,
Mg vacancies. Thin MgO layers exhibit electronic structure indicative of
interacting defect states forming band tails which in the thinnest case extend
to approximately 0.5 V of the Fermi level. These vacancy defects are ascribed
to compressive strain from the MgO/Fe lattice mismatch, accommodated as the MgO
grows.
