Structure of Cr film epitaxially grown on MgO(001)

Cr(0 0 1) films of thickness ∼100 nm were epitaxially grown on MgO(0 0 1) using molecular beam epitaxy at both room temperature and 550 °C. The structure of the films was characterized using high-resolution transmission electron microscopy, electron diffraction and electron energy-loss spectroscopy. Room temperature growth leads to highly oriented columnar structured grains in the film, while growth at 550 °C yields high-quality single-crystal Cr film, possessing a 45° rotational epitaxial relationship between the film and the substrate. For the single-crystal film, the interface exhibited coherent regions separated by equally spaced misfit dislocations. When imaged from the MgO[1 0 0]//Cr[1 1 0] direction, the dislocation spacing is 5.38 and 4.64 nm for the films grown at room temperature and 550 °C, respectively, which is slightly smaller than the expected value of 6.25 nm calculated from the differences in lattice spacing based on known lattice constants. The results presented here indicate that the lattice mismatch between the film and the substrate is accommodated mainly by interface misfit dislocations above some critical thickness. The electronic structure of the interface between the film and the substrate is also explored using electron energy-loss spectroscopy.