Disorder, defects, and band gaps in ultrathin (001) MgO tunnel barrier layers

We report scanning tunneling spectroscopy studies of the electronic structure of 1.5-3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers exhibit a bulklike band gap, similar to 5-7 eV, and sparse, localized defect states with characteristics attributable to oxygen and, in some cases, Mg vacancies. Thin MgO layers exhibit an electronic structure indicative of interacting defect states forming band tails which in the thinnest case extend to similar to +/- 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.