Phenomenological Explanation of Elastic Anomalies in Superlattices

Brillouin Light Scattering Investigation of the Elastic Properties of Ta/Al Metallic Superlattices

The Brillouin light scattering technique has been exploited for investigating the elastic properties of periodic superlattices made by alternating layers of Ta and Al. These multilayers, deposited by d.c. sputtering on glass and Si substrates, present a polycrystalline structure with (110) and (111) texture for the Ta and Al layers, respectively. They have total thicknesses of about 0.5 μm and periods ranging from 4 to 10 nm. Measurement of the phase velocities of the Rayleigh and Sezawa acoustic modes from the frequency position of the corresponding Brillouin peaks, yielded informaton on the effective elastic constants of the superlattices. for large periods (8-10 nm) the values determined experimentally are in good agreement with those calculated from the elastic constants of the bulk materials, while for lower periods (4-6 nm) the estimated elastic constants exhibit a marked increase. This anomalous behavior has been attributed to the presence of a transition layer at each interface, where Ta and Al interdiffuse, as observed by x-ray and electron microscopy experiments.