  Using $\approx$40 fs ultrashort laser pulses, we investigate the picosecond
acoustic response from a prototypical phase change material, thin
Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST) films with various thicknesses. After excitation
with a 1.53 eV-energy pulse with a fluence of $\approx$ 5 mJ/cm$^{2}$, the
time-resolved reflectivity change exhibits transient electronic response,
followed by a combination of exponential-like strain and coherent acoustic
phonons in the gigahertz (GHz) frequency range. The time-domain shape of the
coherent acoustic pulse is well reproduced by the use of the strain model by
Thomsen et al. (Phys. Rev. B 34, 4129, 1986). We found that the decay rate (the
inverse of the relaxation time) of the acoustic phonon both in the amorphous
and in the crystalline phases decreases as the film thickness increases. The
thickness dependence of the acoustic phonon decay is well modeled based on both
phonon-defect scattering and acoustic phonon attenuation at the GST/Si
interface, and it is revealed that those scattering and attenuation are larger
in crystalline GST films than those in amorphous GST films.

Materials Science