Work function changes induced by deposition of ultrathin dielectric films on metals: A theoretical analysis

Ultrathin dielectric films deposited on a metal surface induce variations in the work function that can be relevant for the final properties of the metal/oxide interface. In this work we analyze with the help of density-functional theory calculations the effect of depositing three-layer films of insulating materials such as LiF, NaCl, MgO, CaS, and BaO on various (001) metal surfaces. We found that the change in work function Delta Phi is due to three main contributions: an electrostatic "compression" effect which dominates for highly ionic films such as LiF, a charge-transfer effect which is largest for less ionic films such as BaO, and the surface relaxation induced by the formation of the interface bond which largely depends on the lattice mismatch between the dielectric film and the metal. Finally, we propose a universal correlation between the work function change and the energy difference between the position of the Fermi level of the metal surface and the top of the valence band of the dielectric film.