Raman scattering study of GeTe and Ge2Sb2Te5 phase-change materials

Structural details of the amorphous binary GeTe and ternary Ge2Sb2Te5 (GST) phase-change materials are investigated with the aid of Raman scattering. In the case of the a-GeTe, a plethora of Raman bands have been recorded and assigned on the basis of a network structure consisting of corner- and edge-sharing tetrahedra of the type GeTe4−nGen (n=0, 1, 2, 3, 4). Significant temperature-induced structural changes take place in this material even at temperatures well below the crystallization temperature. These changes tend to organize the local structure, in particular the coordination number of Ge atoms, so as to facilitate the amorphous-to-crystal transformation. The much simpler Raman spectrum of GST, characterized by one vibrational band, is accounted for by the dominance of the Sb2T3 component in Raman scattering; reasons about this explanation, as well as for the lack of any Te–Te bonds are briefly described.