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...

Phase-change-induced martensitic deformation and slip system in GeSbTe

“…The formation of a new crystalline phase by shear stress, with slight atomic rearrangement, indicates that this structural change can arise from martensitic transformation without any diffusion of atoms. 20 A previous study predicted martensitic transformations in GeSbTe materials; 21 although that study was concerned with the transformation from the FCC phase to a hexagonal phase in GeSbTe materials, it suggests that GeSbTe materials can be deformed via diffusionless transformation.…”

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.…”

Structural deformation and void formation driven by phase transformation in the Ge2Sb2Te5 film

“…Since the stacking sequence along the [111] direction in the cubic structure is partly the same as that along the [0001] direction in the hexagonal phase, the cubic phase is easily transformed into a hexagonal structure by rotating the counterpart of the structural unit. 36 In this context, the phase transition from cubic to hexagonal is a martensitic transformation (known as a rapid diffusion-less transformation), 37 which typically shows high coherency at the interfaces of the parent and transformed regions. The TEM result indicates that the invariant plane of the martensitic transformation may be the (111) plane of the cubic phase, which is also well explained by the similarity between the (111) plane of the cubic phase and the (0001) plane of the hexagonal phase.…”