Epitaxy and bonding of Cu films on oxygen-terminated α-Al2O3(0001) surfaces

Transmission electron microscopy studies of oxygen-terminated Cu(111)/α-Al2O3(0001) interfaces possessing two different orientation relationships are reported. The oxygen-terminated α-Al2O3(0001) surfaces were generated by utilizing an oxygen plasma at different temperatures in ultrahigh vacuum, and then Cu thin films were grown by molecular beam epitaxy on these surfaces. A clean hydroxylated α-Al2O3(0001) surface could be obtained by room temperature oxygen plasma cleaning. On this surface Cu films grew following the conventional face-centered cubic/hexagonal close-packed orientation relationship at room temperature, i.e. (111)Cu-bar (0001)α and +/-[1-bar 10]Cu-bar [101-bar 0]α, and were nearly incoherent to the substrate lattice. The high-temperature (∼750 deg. C) oxygen plasma treatment after a dehydroxylation process of α-Al2O3(0001) generated an oxygen-rich surface. On this surface, the Cu films showed a thermally activated wetting by forming Cu2O-like bonding at the interface in an orientation relationship rotated by 30o from the conventional orientation relationship around the interface normal: (111)Cu-bar (0001)α and +/-[21-bar 1-bar ]Cu-bar [101-bar 0]α