Existence of tetrahedral site symmetry about Ge atoms in a single-crystal film of Ge₂Sb₂Te₅ found by x-ray fluorescence holography

The authors discuss x-ray fluorescence holography (XFH) measurements taken from an epitaxial layer of the digital versatile disk random access memory (DVD-RAM) material Ge₂Sb₂Te₅ grown on a single-crystal GaSb(100) substrate. By using fluorescent photons from the Ge atoms in the matrix, a three-dimensional atomic image was obtained around the Ge atoms in a Ge₂Sb₂Te₅ film; details of the three-dimensional atomic arrangement will aim at clarification of the high-speed writing and erasing mechanism of the laser-induced crystal-amorphous phase transition in this DVD-RAM material. Analysis of the XFH images revealed that the epitaxial layer did not possess a hexagonal structure as in the equilibrium phase of Ge₂Sb₂Te₅, but a cubic structure with tetrahedral site symmetry about Ge atoms, different from the previous powder diffraction result. The present structure may support the umbrella-flip model of the Ge atoms between the octahedral site in the distorted rocksalt crystal and the tetrahedral site in the amorphous phase on the laser-induced phase transition.