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

抄録

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.