Growth and electronic structure of ultrathin epitaxial Pd(111) films on Fe(110) and Co(0001) substrates

Epitaxial Pd(111) layers on Fe(110) and Co(0001) thick-film substrates atop W(110) are studied using low-energy electron diffraction (LEED), Auger electron spectroscopy, and angle-resolved photoemission with He i, Ne i, and synchrotron-radiation photon sources. Clear LEED patterns obtained for Pd coverages between 0 and 10 atomic layers (AL's) indicate commensurate Pd(111) overlayer structures in both systems. Satellite reflexes for films in the monolayer coverage range indicate long-range coincidences of 10 Pd atoms on 11 substrate atoms along specific directions. Auger peak intensities of overlayer and substrate as a function of film thickness show straight-line segments separated by breaks at the completion of full atomic layers, suggesting layer-by-layer growth up to a Pd coverage of at least 2 AL and no interdiffusion with the substrates. Interface states are identified for monolayer coverages in both systems by observing a saturation of their intensity as a function of film thickness near 1 AL and a vanishing dispersion with the electronic wave-vector component perpendicular to the surface (k_⊥). In contrast to bulklike features at higher Pd coverage, the interface states are insensitive to the incident-light polarization (s or p), possibly because of the almost incommensurate structure of the Pd overlayer. The total Pd-induced 4d bandwidth of the monolayer seen in photoemission spectra taken at photon energies near 60--90 eV is approximately 3.5 eV and exhibits noble-metal character, i.e., little intensity near the Fermi level EF. Monolayer spectral features in both systems show considerable ({≥}1 eV) dispersion with the electronic-wave-vector component parallel to the surface, k_∥, indicating interactions within the overlayer. All Pd-induced features have mainly overlayer character, i.e., all exhibit a Pd-like photon-energy dependence in the region of the Pd 4d Cooper minimum.The transition to a bulklike Pd(111) electronic structure is virtually complete at a Pd coverage of 5 AL, although dispersion with k_⊥ starts at 3 AL and a final-state resonance typical of the bulk is even discernible at 1.5 AL. A ``second-layer'' interface state for Pd coverages between 1.5 and 2.5 AL probably arises through the changed potential caused by the second Pd layer.