Low-energy-electron transmission through epitaxial films: Cu(001) on Ni(001)

Low-energy- (<10 eV) electron transmission spectra were measured for electrons normally incident on epitaxial Cu(001) films on a Ni(001) substrate for coverages of zero to several tens of monolayers (ML). Electron transmission spectra exhibited characteristic quantum size-effect (QSE) features for discrete coverages. The QSE features in the energy gap of the substrate and its vicinity are stronger than those in the allowed energy band due to strong Bragg reflection of electrons at the film substrate interface. From the steplike increases in the transmission current, we obtain values for the upper band-gap edges ($X_1$ point) for the clean Ni(001) substrate and a thick Cu(001) overlayer of 9.5 and 7.3 eV above the Fermi level, respectively. A one-dimensional theory of electron transmission for over-layered crystals based on low-energy electron diffraction theory is presented, which incorporates the band structures of the materials in the [001] direction. The calculated transmission spectra are sensitive to the lattice spacing as well as the thickness of the film. With the use of the bulk values of lattice spacings for the film and the substrate, excellent agreement was obtained between the experimental and calculated results for coverages greater than 7 ML, but slight systematic differences were observed for 3 to 6 ML coverages. This suggests that the film is pseudomorphic up to 6 ML, after which the entire film rearranges itself to assume the bulk Cu structure.