Signature of Tetrahedral Ge in the Raman Spectrum of Amorphous Phase-Change Materials

We computed the Raman spectrum of amorphous GeTe by ab initio simulations and empirical bond polarizability models. The calculated spectrum is in very good agreement with experimental data and contains the signatures of all the peculiar local structures of the amorphous phase revealed by recent ab initio simulations, namely, tetrahedral Ge and defective octahedral sites for a fraction of Ge (mostly 4-coordinated) and for all Te (mostly 3-coordinated) atoms. In particular, the spectrum above 190  cm-1 is dominated by tetrahedral structures, while the most prominent peaks around 120 and 165  cm-1 are mainly due to vibrations of atoms in defective octahedral sites. Finally, the peak around 75  cm-1, which dominates the spectrum in HV scattering geometry, is mostly due to vibrational modes involving threefold coordinated Te atoms.