Abstract
We propose a method to measure the longitudinal sound velocity in thin films of a few nanometer thickness using laser-based picosecond ultrasonics. In periodic multilayer structures, picosecond pulse-echo techniques were used to measure the effective sound velocity, which is related to the velocities of individual constituents through the superlattice phonon dispersion relation. The individual sound velocities can then be extracted, provided two or more effective velocities are obtained from multilayers of different thickness ratios. Longitudinal sound velocities in ion-beam sputtered Mo and amorphous Si films of 2 to 5 nm thickness have been determined to be 98 and 94% of the bulk speed, respectively. We believe this technique has general applicability to sound velocity measurement in ultra-thin films.
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