Electronic structure of strained copper overlayers on Pd(110)

The electronic structure of strained copper thin films on Pd(110) (lattice mismatch: 7.8%) has been probed as a function of film thickness by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. While the copper films grow epitaxially, an extremely slow convergence of the film electronic structure to that typical of a Cu(110) single crystal occurs, with distinct changes occurring in the film electronic structure for films thicknesses up to 60 ML. A new peak at 1.6 eV binding energy, absent in either Pd or Cu(110), which exhibits only weak polarization dependence and little dispersion as a function of photon energy, is shown to be due to emission from an impurity state arising from Pd in a dilute CuPd alloy formed by intermixing during film growth at the Pd-Cu interface. Large changes in the binding energy and peak shape of the lower Cu d and sp bands are observed as a function of film thickness, Self-consistent KKR electronic structure calculations in the Gamma K, UX direction have been performed for a range of geometries, including distorted films adopting in-plane lattice parameters of the underlying Pd(110) substrate, and a range of interlayer spacings. The coverage-dependent changes in the film electronic structure are shown to be due to the film initially adopting a highly strained pseudomorphic geometry and a slightly distorted fee structure (c/a=0.93+/-0.03). A gradual anisotropic relaxation of the film geometry towards that of bulk fee copper, initially along the [1 (1) over bar 0] direction and at higher coverages along the [001] azimuth, occurs as the film thickness increases. The binding energy of the lower d and sp bands, which shift up to 0.5 eV per 0.1 Angstrom change in the interlayer separation, is shown to be an accurate monitor of the degree of distortion in pseudomorphic thin Cu films. (C) 2000 Elsevier Science B.V. All rights reserved.