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 ${\text{Ge}}_{0.7}{\text{Sb}}_{0.2}\text{Te}$ and ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$, 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 (${\text{Ge}}_{0.7}{\text{Sb}}_{0.2}\text{Te}$, $R3m$, $a=4.237\text{Å}$, $c=10.29\text{Å}$). 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 ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$ 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. $325°\text{C}$. HRTEM of samples annealed at $400°\text{C}$ 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 $475°\text{C}$. Powder diffraction on samples of ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$ 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 $\text{cubic}\to \text{rhombohedral}$ 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 $500°\text{C}$. 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.

Figure 1

SEM image of a fragment of an ingot with the composition ${\text{Ge}}_{0.7}{\text{Sb}}_{0.2}\text{Te}$; the trigonal morphology of plate-shaped domains and their multiple twinning to form pseudocubic arrangements (see magnified inset) is easily seen.

Figure 2

Representation of the crystal structure of quenched ${\text{Ge}}_{0.7}{\text{Sb}}_{0.2}\text{Te}$ (white spheres: cation position with Ge, Sb, and vacancy occupation, dark spheres: Te): both the rhombohedral (hexagonal setting, upright) and the pseudocubic rocksalt-type unit cells are outlined, GeTe-type layers are indicated by interconnection of the atoms with the shortest interatomic distances, some of the longer interatomic distances are also indicated (gray fragmented “bonds” that highlight the pseudo-octahedral coordination of the cations).

Figure 3

Temperature-programmed powder diffraction experiment ($\text{Mo}K\alpha 1$ radiation) of a quenched sample ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$ (from bottom to top, heating and cooling ramp $5°\text{C}/\text{min}$). At $600°\text{C}$, several patterns were recorded and later added to obtain more precise lattice parameters of the high-temperature phase. One reflection from the furnace material is indicated by an asterisk.

Figure 4

HRTEM image of a ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$ specimen as quenched from $500°\text{C}$ (top, inset: Fourier transform) and the corresponding SAED pattern (bottom, indices of some Bragg positions are given).

Figure 5

HRTEM image of a ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$ specimen annealed at $400°\text{C}$ for 20 h and subsequently quenched (top, the spacings of cation defect layers are indicated, they yield an average of $54\text{Å}$) and the corresponding SAED pattern (bottom).

Figure 6

Powder diffraction patterns of fractions of ${\text{Ge}}_{0.8}{\text{Sb}}_{0.13}\text{Te}$ samples with grain sizes $0–20\mu \text{m}$, $20–59\mu \text{m}$, and $59–105\mu \text{m}$: quenched from $500°\text{C}$ (bottom, with anisotropic peak broadening depending on the grain size, inset: ${220}_c$ reflection and its full width at half maximum), slowly cooled from 500 to $250°\text{C}$ (middle, the asterisks indicate diffracted intensity from the sample support) and quenched after annealing at $400°\text{C}$ for 20 h (top). The slowly cooled and annealed samples show rhombohedral reflection splitting (indicated by arrows for the ${104}_h$ and ${110}_h$ reflections) and superstructure reflections (marked for the batches with the smallest grain sizes by dotted ellipses at an exemplary position).

Figure 7

Phase relations and transformations between the cubic high-temperature phase, the metastable pseudocubic or rhombohedral phase, and the stable trigonal layered superstructure of GeTe-rich GST materials as observed for bulk samples.