  Femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$
superlattices (SLs), a new class of semiconductor SLs with three different
states, have been investigated by using a reflection-type pump-probe technique
at various lattice temperatures. The time-resolved transient reflectivity (TR)
obtained in as-grown SLs exhibits the coherent A$_{1}$ optical modes at 5.10
THz and 3.78 THz, while only the single A$_{1}$ mode at 3.68 THz is observed in
annealed SLs. The decay rate of the A$_{1}$ mode in annealed SLs is strongly
temperature dependent, while that in as-grown SLs is not temperature dependent.
This result indicates that the damping of the coherent A$_{1}$ phonons in
amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than
the anharmonic phonon-phonon coupling.
