Anomalous modulus and work function at the interfaces of thin films

Abstract

We use Rayleigh waves and a Kelvin probe to measure the elastic modulus and the work function of thin polycrystalline metallic films prepared by electron beam evaporation. A high-resolution in situ measurement of Rayleigh-wave velocities enables us to determine the modulus of the topmost layer of the film (while it is being grown). Using this technique, we can measure the modulus at the interfaces between Ir, Pd, Pt, Ag, and Au. For many metal-pair combinations (e.g., Pt, Ag, or Ir on Pd), each film's modulus equals that of the bulk from the onset of deposition. However, the first couple of nanometers of Pd deposited onto Ir cause an abnormal increase (up to threefold) in the modulus. These same films show an unexpected lower work function. To explain these unusual effects, we present a model based on structural defects in the underlying film. The dependence of modulus and work function on film thickness within a single layer suggests an explanation for previous inconsistent reports of modulus changes in metallic multilayers (i.e., the supermodulus effect).