Influence of stress and strain on the kinetic stability and phase transitions of cubic and pseudocubic Ge-Sb-Te materials

Rewritable data-storage media and promising nonvolatile random-access memory are mainly based on phase-change materials (PCMs) which allow reversible switching between two metastable (amorphous and crystalline) modifications accompanied by a change in physical properties. Although the phase-change process has been extensively studied, it has not been elucidated how and why the metastable crystalline state is kinetically stabilized against the formation of thermodynamically stable phases. In contrast to thin-film investigations, the present study on bulk material allows to demonstrate how the cubic high-temperature phase of GeTe-rich germanium antimony tellurides (GST materials) is partially retained in metastable states obtained by quenching of bulk samples. We focus on compositions such as Ge0.7Sb0.2Te and Ge0.8Sb0.13Te, which are important materials for Blu-ray disks. Bulk samples allow a detailed structural characterization. The structure of a multiply twinned crystal isolated from such material has been determined from x-ray diffraction data (Ge0.7Sb0.2Te, R3m, a=4.237 angstrom, c=10.29 angstrom). Although the metrics is close to cubic, the crystal structure is rhombohedral and approximates a layered GeTe-type atom arrangement. High-resolution transmission electron microscopy (HRTEM) on quenched samples of Ge0.8Sb0.13Te reveal nanoscale twin domains. Cation defects form planar domain boundaries. The metastability of the samples was proved by in situ temperature-dependent powder diffraction experiments, which upon heating show a slow phase transition to a trigonal layered structure at ca. 325 degrees C. HRTEM of samples annealed at 400 degrees C shows extended defect layers that lead to larger domains of one orientation which can be described as a one-dimensionally disordered long-periodical-layered structure. The stable cubic high-temperature modification is formed at about 475 degrees C. Powder diffraction on samples of Ge0.8Sb0.13Te with defined particle sizes reveal that the formation of the stable superstructure phase is influenced by stress and strain induced by the twinning and volume change due to the cubic. rhombohedral phase transition upon quenching. The associated peak broadening is larger for small crystallites that allow relaxation more readily. Consequently, the degree of rhombohedral distortion as well as the appearance of superstructure reflections upon annealing is more pronounced for small crystallites. The same is true for samples which were slowly cooled from 500 degrees C. Hence, the lattice distortion accompanying the phase transition toward a stable trigonal superstructure is, to a certain degree, inhibited in larger crystallites. This kinetic stabilization of metastable states by stress effects is probably relevant for GST phase-change materials.