Martensitic Transformation in Ge₂Sb₂Te₅ Alloy

Abstract

Ge 2 Sb 2 Te 5 (hereafter GST) chalcogenide is of great importance as a key material for the emerging nonvolatile phasechange random access memory (PRAM) [1][2][3] and the optical data storage. [4,5] This chalcogenide exhibits three phases: amorphous, metastable rocksalt face-centered cubic (FCC) and stable hexagonal (Hex) crystalline phases. The crystalline FCC and Hex phases are found to be electrically distinguishable in GST. [6] A switching memory was demonstrated by using (Ge 1 Sb 2 Te 4 ) 0.8 (Sn 1 Bi 2 Te 4 ) 0.2 alloy recently, based on its reversible phase changes between FCC and Hex phases. [7] The whole switching operation in PRAM and phase change optical disk is based on the following cyclic process: amorphous solid → crystalline FCC → crystalline Hex → liquid (melt) → amorphous solid. Therefore, sound understanding of the phase-change between the two GST crystalline phases will lead to improved phase-change devices. Stimulated by the fact that FCC and hexagonal phases of GST are with identical composition and of independent existence, one may think the possibility of martensitic transformation between them, just as the martensitic transformation between FCC and hexagonal close-packed phases in metallic cobalt. [8] Known as rapid diffusionless phase transitions, martensitic transformations are found in many metals, alloys and ceramics. [9][10][11] Moreover, the rapid phase-change in GST is well consistent with the ultra-fast nature of martensitic transformation. Acknowledging these strong implications, it is exciting to conduct theoretical investigation of the applicability of martensitic transformation concept to GST system. In this paper, we will prove that the martensitic transformation from FCC to Hex occurs favorably in Ge 2 Sb 2 Te 5 alloy during phase transition from FCC to Hex, by taking both structure and energy into consideration. Crystallographic Model of Martensitic Transformation in GSTIn terms of the metastable FCC phase (space group: Fm 3 m) with a lattice parameter a FCC = 6.02 Å, 4(a) sites are fully occupied by Te atoms as one FCC sublattice backbone with Ge, Sb and 20 % vacancies at the 4(b) sites forming the other FCC sublattice (see Fig. 1(a)). [6] The stable hexagonal phase shown in Figure 1(c) consists of cycling 9 layers in one unit cell with a space group of P 3 m 1, e.g., -Te-Ge-Te-Sb-TeTe-Sb-Te-Ge-stacking order with lattice parameters a Hex = 4.2 Å and c Hex = 16.96 Å. [12] This hexagonal structure can be divided into 3 components: the middle Te-rich -Sb-TeTe-Sb-, the mirror-symmetrical lower -Te-Ge-Te-Sb-and upper -Sb-Te-Ge-Te-blocks. The outlined triangle in Figure 1(c) represents a (113) plane.Recently, the ab initio total energy calculation showed that the vacancies are concentrated, [1] highly ordered and layered in FCC owing to the cubic structure symmetry. [13] Thus the ordered distribution of Ge, Sb and vacancies can be formed, more advantageous than their random positions, at least as an intermediate state during the phase transition from FCC to Hex phases as tempe...

Surface Energy Driven Cubic-to-Hexagonal Grain Growth of Ge2Sb2Te5 Thin Film

“…In view of the similar configuration and minor energy discrepancy between the 100% VOC- and h-GST lattices, a non-diffusion controlled slide of the building blocks was proposed to understand the f-to-h transformation 18 . Analogous research achievement stated that a shearing martensitic transformation from {200} planes of f-GST to {} planes of h-GST should be energetically favorable during f-to-h transition 19, 20 . In addition, the discovery of twin crystals consisted by one f- and one h-grain led to an “epitaxial growth model” to interpret the structure evolution manner as f-GST approaching the h-phase 21, 22 .…”

“…We also speculate that the vacancy-ordering process into {111} planes 11, 12, 17 may or may not occur in every f-grain, or to say, it is not an indispensable way for f-grains evolving into h-ones especially in the growth period. The adjustments by sliding the building blocks between similar f- and h-atomic configurations 17, 19, 21 would be more likely to involve the incubation of h-seeds from the f-matrix. The present scenarios may offer a more comprehensive perspective to understand the phase transition physics of this key material, and be essential for optimizing GST-based commercialized phase change materials to boost the performances of high density PCM device.…”

Local atomic arrangements and lattice distortions in layered Ge-Sb-Te crystal structures

“…However, ordering of Ge cations in GST326 in the middle of building block was assumed. Although different GST compounds (stable and metastable) are very close in terms of structure174548495051, it is not clear why the cation layers in the GST124 structure should not possess intermixing. As an explanation, it was proposed that the Te, Sb and Ge atoms in the (0001) plane have to satisfy the 3Ge-Te-3Sb rule, in which Te atoms are surrounded by three Ge and three Sb atoms located in opposite corners of the octahedron structures45.…”

“…An explanation could be that the theoretical calculations do not take into account the influence of temperature on cation intermixing. Furthermore, the transformation mechanism between metastable and stable GST phases is considered to occur by preferential ordering of vacancies randomly distributed in the metastable GST into vacancy layers without long range diffusion of Ge and Sb and without pronounced change in local composition at cation sites of the parent metastable GST phase2450. It was also shown that the layered GST225 structure with intermixed cation layers is thermodynamically stable up to the melting point19.…”

How important is the {103} plane of stable Ge2Sb2Te5 for phase‐change memory?

“…Firstly, {103} crystal plane group constitutes new unit cell of Hex phase of GST; secondly, {103} is correlated to the rapidness of the phase transition. In order to unravel the importance of {103} plane in achieving the fast phase transition of GST, our previous work concerning with the phase transition from FCC to Hex in GST (Zhang et al ., ) and some important research are analyzed.…”