Laser picosecond ultrasonic measurement of longitudinal sound velocity in nanometer thin films

We demonstrate applications of ultrafast-laser-based picosecond ultrasonic techniques to sound velocity measurement in nanometer thin films. The longitudinal sound velocities in various thin films (100 similar to 300 nm thick) were directly determined by pulse-echo technique. Many of the measured velocities deviate from the bulk values significantly and show strong dependence on the growth conditions. To overcome the problems such as echo overlapping and reduced accuracy in much thinner films, we further propose a method capable of measuring the sound velocity in films thinner than 10 run. By measuring the effective sound velocities of periodic multilayer stacks with different thickness ratios, the velocities of individual constituents can be extracted using the superlattice phonon dispersion relation. The longitudinal sound velocities in ion-beam sputtered Mo and amorphous Si films of 2 similar to 5 nm thickness have been determined using this method. We have also performed theoretical calculations on the laser generation and detection of acoustic pulses in multilayers. The pulse shapes and spectra of acoustic waves were analyzed, and the results agree well with the experimental results.