Elastic constants of Mo/Ta superlattices measured by Brillouin scattering

The elastic properties of Mo/Ta superlattices and the dependences on bilayer wavelength over the range 7–200 Å are investigated. These are the first such measurements for a bcc-bcc system, and significant differences from previously studied metal superlattices are found. Surface velocities of as many as 12 acoustic modes guided by the thin-film samples have been measured using Brillouin spectroscopy. The elastic constants $c_{11}$,$c_{13}$,$c_{33}$ and $c_{44}$ for the superlattices are determined by fitting the data to an acoustic model of supported hexagonal films. We compare these stiffnesses with those predicted from the bulk constituent properties. The relative Brillouin cross section is found to be accurately represented by a pure ripple scattering theory with no photoelastic contribution.