Observation of T2-like coherent optical phonons in epitaxial Ge2Sb2Te5/GaSb(001) films

Abstract

The phonon spectrum of Ge2Sb2Te5 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 Ge2Sb2Te5 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 T2 mode expected for the Oh point group, confirming that the underlying crystallographic structure is cubic. New modes are observed in both Ge2Sb2Te5 and GaSb after application of large pump fluences, and are interpreted as 1 and 2 dimensional modes associated with segregation of Sb.

Coherent phonon modes of crystalline and amorphous Ge2Sb2Te5 thin films: A fingerprint of structure and bonding

“…The effect of increased pump fluence upon the transient R and AR signals obtained from e-GST was reported previously 14 and has been investigated in the present study for p/ a-GST films. The transient R and AR signals were recorded at high fluence, over a period of about 40 min, before being remeasured at the same location on the sample using a reduced pump fluence.…”

“…The e-GST response was fitted to a function consisting of a Gaussian function to describe the initial peak, a series of Gaussian error functions to describe the multi-exponential background, and cosine functions to describe the oscillatory response, as has been described previously. 14 The fitting yielded two strong oscillatory signals and various relaxation times associated with the complicated background that were carefully assigned to carrier-carrier and carrier-phonon interaction processes, and to the decay of a space charge field within the piezoelectric GaSb.…”

“…Pumpprobe measurements revealed a mode with a slightly lower frequency of 6.77 THz in the GaSb reference sample. 14 The modes observed in GaSb by Raman and pump-probe measurements are attributed to the zone centre LO and TO phonons, respectively. 19 The broad band between 2 and 6 THz was analyzed by first subtracting a sloping background, the physical origin of which is not known, and then fitting to two Lorentzian lineshapes, as shown in Figure 5.…”

“…12,13 In a time-resolved ultrafast optical study of epitaxial cubic-Ge 2 Sb 2 Te 5 /GaSb (001), anisotropic reflectance (AR) measurements revealed a dominant phonon mode of 3.4 THz frequency. 14 The dependence of the amplitude on pump and probe polarization was well described by a theory that includes phonon excitation by both transient stimulated Raman scattering (TSRS) and a space charge (SC) field. The theory, which includes the crystal symmetry through the Raman tensor, revealed that the 3.4 THz mode has a threedimensional T 2 -like character, confirming the view that the underlying crystallographic structure is cubic.…”

“…The results are compared to the R and AR responses of an epitaxial sample described previously 14 and with Raman spectra obtained from the three samples. THz optical phonons of different frequencies have been observed in the three different phases, helping to advance understanding of the crystal structure and phase transition in GST.…”