Elastic properties and phonon generation in Mo/Si superlattices

Picosecond ultrasonic measurements have been carried out on molybdenum-silicon multilayers in order to study their elastic effective-medium behavior using low-frequency acoustic echoes, and their high-frequency response through localized surface modes. Taking into account the elastic properties of silicon and molybdenum thin films independently measured, we show that the long-wavelength acoustic behavior of the multilayers is well reproduced. Using an effective-medium model that includes only the two constituent layers in the periodic cell without any interfacial zone, sound velocity, echo shape, and acoustical reflection coefficient have been investigated. In contrast, the frequency of the acoustic-phonon surface modes could not be reproduced unless a modification of the surface silicon layer was introduced, in good agreement with x-ray reflectivity measurements. Finally, low-temperature experiments illustrate the potential of such wide stop-band systems for the emission of monochromatic acoustic waves, using the tunneling of a surface-localized mode or a buried nanocavity mode. The filtering and acoustic transduction properties of these artificial layered materials have also been investigated.