AES and HREELS studies of the Cu/MgO(100) interface

We have studied the formation of the Cu/MgO interface with Auger electron spectroscopy and high-resolution electron energy-loss spectroscopy (HREELS). Copper overlayers ranging in thickness from 1 to 20 A were formed on an MgO single- crystal substrate by evaporation from a Knudsen effusion cell. Direct Auger spectra for the O KLL, Cu LMM and Mg KLL lines were collected. The evolution of the Auger peak intensities shows that the copper initially forms a complete monolayer, followed by island formation (i.e. Stransky–Krastanov growth). Low energy electron diffraction patterns recorded after Cu deposition confirmed the ordering of the first monolayer. A significant decrease in energy of the surface MgO phonon peak was observed by HREELS following deposition of 20 A of copper. Simultaneously with this shift of the phonon energy, we observed a decrease of the intensity ratio of the phonon-loss and elastic peaks. These results have been interpreted using the dielectric theory. Theoretical spectra of MgO covered by a layer of Cu2O obtained by using dielectric constants taken from the literature were able to reproduce qualitatively the decreases in energy and intensity of the phonon-loss peak with increasing copper thickness.