Laser-Induced Coherent Acoustic Phonons for Measuring Elastic Constants of Ultra-Thin Films

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Abstract

We determine the out-of-plane elastic constants C33 of ultra-thin films using gigahertz coherent acoustic phonons. Acoustic phonons are generated and detected using femtosecond-laser pulses, and by measuring the resonance frequency of non-propagating acoustic phonons in thin films, C33 is determined. In determining the C33, film thickness and mass density are required, and we determine them by the x-ray reflectivity measurements. We apply this method to the Fe thin films of 11-52 nm thickness. When the film thickness is 11 nm, C33 was larger than C33 of the bulk Fe. As the film thickness increases, C33 decreases, and when the film thickness is larger than 35 nm, C33 was smaller than the bulk value. However, by heating the substrate, C33 shows recovery towards the bulk value. There results are explained by the incohesive bonding at grain boundaries and residual stress.

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