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

Abstract

Rewritable data-storage media and promising nonvolatile random-access memory are mainly based on phase-change materials ͑PCMs͒ which allow reversible switching between two metastable ͑amorphous and crystalline͒ 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 materials͒ is partially retained in metastable states obtained by quenching of bulk samples. We focus on compositions such as Ge 0.7 Sb 0.2 Te and Ge 0.8 Sb 0.13 Te, 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 ͑Ge 0.7 Sb 0.2 Te, R3m, a = 4.237 Å, c = 10.29 Å͒. 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 ͑HRTEM͒ on quenched samples of Ge 0.8 Sb 0.13 Te 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°C. HRTEM of samples annealed at 400°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°C. Powder diffraction on samples of Ge 0.8 Sb 0.13 Te 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°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.

Precession Electron Diffraction – a versatile tool for the characterization of Phase Change Materials

“…Frequently, the crystals are characterized by a coherent and nanoscale intergrowth of rock salt type slabs with distinct thicknesses which are separated by neighboring Te-layers [30]. Such inhomogeneities were also observed in all our samples, cf.…”

Increasing Seebeck Coefficients and Thermoelectric Performance of Sn/Sb/Te and Ge/Sb/Te Materials by Cd Doping

“…[ 5,12 ] The existence range of a trigonal phase when heating quenched samples becomes smaller with increasing n as the transition temperature to the cubic HT phase decreases, whereas the transition temperature from the metastable quenched phase to the trigonal phase varies between 375 °C ( n = 3) and 300 °C ( n = 17). [ 5,12,19,29 ] The compositions of all metastable GST and SST samples investigated were confi rmed by EDX measurements (cf. Figure 3 and Tables S5 and S6 in the Supporting Information) clearly reveal NaCl-type average structures.…”

Festkörperchemie 2010

“…In einer Untersuchung an GeSbTe‐Bulk‐Materialien,35 die ein wesentlicher Bestandteil heutiger Bluray‐Disks sind, deutet sich eine Stabilisierung von metastabilen Phasen durch Stresseffekte an. Diese beruhen auf Zwillingsbildung und anisotroper Verzerrung.…”