Observation of T(2)-like coherent optical phonons in epitaxial Ge(2)Sb(2)Te(5)/GaSb(001) films

The phonon spectrum of Ge(2)Sb(2)Te(5) is a signature of its crystallographic structure and underlies the phase transition process used in memory applications. Epitaxial materials allow coherent optical phonons to be studied in femtosecond anisotropic reflectance measurements. A dominant phonon mode with frequency of 3.4 THz has been observed in epitaxial Ge(2)Sb(2)Te(5) grown on GaSb(001). The dependence of signal strength upon pump and probe polarization is described by a theory of transient stimulated Raman scattering that accounts for the symmetry of the crystallographic structure through use of the Raman tensor. The 3.4 THz mode has the character of the 3 dimensional T(2) mode expected for the O(h) point group, confirming that the underlying crystallographic structure is cubic. New modes are observed in both Ge(2)Sb(2)Te(5) and GaSb after application of large pump fluences, and are interpreted as 1 and 2 dimensional modes associated with segregation of Sb.