Interface and electronic characterization of thin epitaxial Co3O4 films

The interface and electronic structure of thin (∼20-74 nm) Co3O4(1 1 0) epitaxial films grown by oxygen-assisted molecular beam epitaxy on MgAl2O4(1 1 0) single crystal substrates have been investigated by means of real and reciprocal space techniques. As-grown film surfaces are found to be relatively disordered and exhibit an oblique low energy electron diffraction (LEED) pattern associated with the O-rich CoO2 bulk termination of the (1 1 0) surface. Interface and bulk film structure are found to improve significantly with post-growth annealing at 820 K in air and display sharp rectangular LEED patterns, suggesting a surface stoichiometry of the alternative Co2O2 bulk termination of the (1 1 0) surface. Non-contact atomic force microscopy demonstrates the presence of wide terraces separated by atomic steps in the annealed films that are not present in the as-grown structures; the step height of ≈2.7 Å corresponds to two atomic layers and confirms a single termination for the annealed films, consistent with the LEED results. A model of the (1×1) surfaces that allows for compensation of the polar surfaces is presented.