Optical phonons of GeSbTe alloys: Influence of structural disorder

Phase-change materials such as pseudobinary GeSbTe (GST) alloys are widely used for fabrication of rewritable and nonvolatile memory devices and also reveal perspective thermoelectric parameters. Their electron spectrum demonstrates the topological insulator behavior. These promising properties are related to unique structural features of a lattice, study of which attracts much attention since full clarity was not achieved so far. Here, we study Raman spectra of monocrystalline GST alloys to examine influence of lattice structure on phonon spectrum. In the Raman spectra, the broad bands typical for the amorphous state and sharp lines of the crystalline one are observed. The use of a one-dimensional harmonic chain model of atoms composing the lattice unit cell allows us to explain the origin of these features. It is shown that the first component is related to dispersion of phonon frequencies in the different lattice unit cells, which is caused by Ge/Sb disorder. The sharp spectral lines are attributed to phonons originated from those of Sb2Te3 as the structural basis of GST. Their frequencies are invariants relative to Ge/Sb disorder, and GST alloy is an ordered crystal for these modes.