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

Abstract

Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge2Sb2Te5 (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T2-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe4 tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe6 octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3–4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb2Te3 or GeTe4 tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.

Far-Infrared and Raman Spectroscopy Investigation of Phonon Modes in Amorphous and Crystalline Epitaxial GeTe-Sb2Te3 Alloys

“…In the case of c-GST326, the IR phonon mode centered at ~70 cm −1 , according to the m m point group of the cubic symmetry, is attributed to a T 1u . However, as already mentioned, vacancies, defects and distortion of bonds could break the inversion symmetry predicted by the space group, leading to a mixed nature of the phonon modes 19 . The broad mode T 1u of c-GST326 is composed by the superposition of the phonon modes of the binary constituent compounds, E and A 1u for GeTe and Sb 2 Te 3 , respectively, [see Fig.…”

Enhanced reliability of phase-change memory via modulation of local structure and chemical bonding by incorporating carbon in Ge2Sb2Te5

“…15,[29][30][31] Another peak at 125 cm À1 (C peak) is attributed to the A 1 mode of the GeTe (4Àn) Ge n (n ¼ 1, 2, eventually 0) corner-sharing tetrahedral structure and is based on the assignment for the amorphous GeTe layer and lighter Ge 2 Te 3 complexes. 15,[28][29][30] The main peak (D peak) at 165 cm À1 is the result of the Sb-Te bond vibrations of the defective-octahedral units in Sb 2 Te 3 . 15,[29][30][31] In addition, the weak broad peak (E peak) observed at 220 cm À1 can be attributed to the F 2 mode of the rhombohedral deformed rock-salt structure and bending modes of the GeTe 4 tetrahedral structure.…”

“…Cu-K a radiation was maintained at 9 and 400 mA for the XRD measurements. The Raman spectra [28][29][30] were recorded using a Horiba Jobin-Yvon LabRam Aramis spectrometer with the 514.5 nm line of an Ar-ion laser as the excitation source. The Raman-scattered light signal was collected in a backscattering geometry using a 50Â microscope objective lens.…”

Flux periodic oscillations and phase-coherent transport in GeTe nanowire-based devices

“…Note that the peak labeled by the symbol (*) at around 140 cm −1 arises from surface oxidation 29 . Other peaks/shoulders may originate from the breaking of inversion symmetry due to vacancies, defects, and distortion of bonds 30 .…”