Thin Films of Ge–Sb–Te-Based Phase Change Materials: Microstructure and in Situ Transformation

The ternary compounds GeSb 2 Te 4 and Ge 2 Sb 2 Te 4 are promising phase change materials for application in the fields of data storage and recording. In the present study, thin films of the materials were deposited on inert substrates using the magnetron sputter technique. The microstructures of the amorphous, metastable, and stable crystalline states were investigated by X-ray powder diffractometry and high resolution transmission electron microscopy (HRTEM). In the as-deposited state, the materials are amorphous. Upon heating, they crystallize first into a metastable phase with a NaCl-type structure, followed by a transformation into a layered structure (stable phase) at higher temperatures. HRTEM studies reveal that nanosized particles with a granular morphology are present in all materials for the crystalline metastable state. In the case of GeSb 2 Te 4, these nanocrystals do not grow significantly when the sample transforms to the stable phase, which contains layers of structural vacancy. Ge 2 Sb 2 Te 4 behaves differently. In particular, the structural transition to the crystalline stable phase is accompanied by a pronounced increase in the crystallite size up to several micrometers. The crystals are built from nanoscale lamellae, which do not contain vacancy layers. Rietveld refinement of the X-ray data of Ge 2 Sb 2 Te 4 in the metastable crystalline state reveals that the compound consists of two phases of which one is Ge rich and the other Ge poor. The size of the coherent scattering domains in the former phase was about 20 nm, whereas the value for the second phase was determined to be about 2 nm.