On the atomic structure of thin amorphous Ge-Sb-Te films

The structure of thin films of Ge2Sb2Te5 was studied by high-resolution transmission electron microscopy (HRTEM) and by X-ray diffraction (XRD). The as-evaporated films prepared by flash evaporation were amorphous. Nearly, all the atoms were randomly distributed but even in as-evaporated-films small areas with some ordering were noticeable. The number and size of the ordered areas increased with thermal treatment of films to temperatures similar to 80 degrees C, i.e., similar to 70 degrees C below the crystallization temperature of amorphous Ge2Sb2Te5. After thermal treatment, small embryos of nucleation centers (size <8 nm) were formed. According to XRD measurements the films as a whole were still fully amorphous. The crystallite model of the structure of amorphous solids is then partly valid, at least for Ge2Sb2Te5. We expect that it can be applied also for other Ge-Sb-Te-based films, as well as for the other amorphous chalcogenides. The possible impact of the formation of nucleation "embryos" on storage time of data in optical and electrical phase-change memory materials and devices is discussed. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim