Heteroepitaxial growth of alpha-Fe2O3, gamma-Fe2O3 and Fe3O4 thin films by oxygen-plasma-assisted molecular beam epitaxy

We have grown epitaxial alpha-Fe2O3(0001) thin films on Pt(111)/alpha-Al2O3(0001), and gamma-Fe(2)O3(001) and Fe3O4(001) thin films on MgO(001) by molecular beam epitaxy using an elemental Fe source and an electron cyclotron resonance oxygen plasma source. These iron oxide thin films are single crystals with excellent crystalline quality. Use of a Pt(111) buffer layer for growth of alpha-Fe2O3(0001) on alpha-Al2O3(0001) improves the crystal quality for epitaxial alpha-Fe2O3(0001) films. Growth of epitaxial alpha-Fe2O3 and gamma-Fe2O3 films under the same growth conditions, but on different substrates, suggests that it is possible to grow a thermodynamically unstable or metastable phase such as gamma-Fe2O3 by selecting a suitable substrate (e.g. small lattice mismatch and similar surface symmetry). The alpha-Fe2O3(0001) and gamma-Fe2O3(001) film surfaces ar; unreconstructed whereas the Fe3O4(001) film surface exhibits a (root 2x root 2)R45 degrees reconstruction. Application of a simple electron counting rule reveals that the bulk-terminated alpha-Fe2O3(0001) and gamma-Fe2O3(001) surfaces are autocompensated, but the bulk-terminated Fe3O4(001) surface is not. The reconstruction is due to the formation of an ordered array of tetrahedral iron vacancies required to achieve surface charge neutralization.