Crystalline GeTe-based phase-change alloys: Disorder in order

Abstract

Through the combined use of x-ray absorption and scattering experiments and ab initio simulations, we demonstrate that the metastable cubic phase of GeTe-based phase-change alloys, e.g. Ge 2 Sb 2 Te 5 , is significantly more disordered than is generally believed, with a large number of Ge atoms located off octahedral resonantly bonded sites. The stochastic off-octahedral locations of Ge atoms, that are invisible to Bragg diffraction probing the average structure, lead to disruption of the continuous resonance bonding network of the crystalline phase, resulting in localization of charge carriers. It is proposed that the degree of coherency of local rhombohedral displacements, that may be varied, e.g., by doping, can serve as means to control electrical properties of Ge-Sb-Te alloys.

Exploring the subsurface atomic structure of the epitaxially grown phase-change material Ge2Sb2Te5

“…The smallest peak width is observed for the Te-Te distance evidencing the large order in the Te layer. For the strongly discussed Ge-Te bonds [40,46], the RDF exhibits two peaks, corresponding to the well-known three shorter and three longer Ge-Te bonds [80,82]. There are no indications of a bond length d 2.7Å, which would be the hallmark of tetrahedral Ge [40,81].…”

“…Such tetrahedral bonding of Ge surrounded by Vcs has been conjectured some time ago also for the bulk of the GST-225 rocksalt structure by using electron microscopy and diffraction data in combination with DFT results [40]. However, this interpretation has not been confirmed in subsequent works using TEM [31,41,42], XRD [43,44], extended x-ray absorption fine structure (EXAFS) [45,46], and more refined DFT calculations, partially combined with molecular dynamics (MD) simulations [47,48]. Interestingly, a nuclear magnetic resonance (NMR) study provided some hints for the presence of tetrahedral Ge within a so-called nanocrystalline GST-225 phase, but not in its microcrystalline counterpart.…”

Nucleus-driven crystallization of amorphous Ge2Sb2Te5: A density functional study

“…36 A study of the stoichiometric defects involved changes in the Ge/Sb sublattice, 37 and recent x-ray absorption and scattering measurements and DF simulations suggest that the displacements of many Ge atoms led to considerable bonding disorder. 38 Our graphical software 27 has allowed us to follow all simulations in detail, and there are indeed clear indications that the total energy favors Te occupancy of one sublattice of the rock salt structure. Nevertheless, the very short crystallization times mean that other contributions to the free energy, particularly configurational entropy, play crucial roles.…”

Hard x-ray photoelectron spectroscopy study of Ge2Sb2Te5; as-deposited amorphous, crystalline, and laser-reamorphized

“…8 Recent studies have shown that a certain number of Ge sites (about 30%) are present as resonantly bonded sites in the amorphous phase 9 and, conversely, up to 30% of sites in the crystalline phase may not be resonantly bonded octahedral sites. 10,11 Hence, during the phase change process, the structure does not change between the two end states but rather a change in fraction of resonantly bonded Ge (and possibly Sb) sites takes place. For a detailed discussion on the possible coordination of Ge in resonant and non-resonant bonding in Ge 2 Sb 2 Te 5 see the work by Krbal et al 9,10 In this paper, we present a high resolution hard x-ray photoelectron spectroscopy (HAXPES) investigation of the prototypical phase change material Ge 2 Sb 2 Te 5 .…”

“…10,11 Hence, during the phase change process, the structure does not change between the two end states but rather a change in fraction of resonantly bonded Ge (and possibly Sb) sites takes place. For a detailed discussion on the possible coordination of Ge in resonant and non-resonant bonding in Ge 2 Sb 2 Te 5 see the work by Krbal et al 9,10 In this paper, we present a high resolution hard x-ray photoelectron spectroscopy (HAXPES) investigation of the prototypical phase change material Ge 2 Sb 2 Te 5 . With HAXPES, element specific information on the chemical state and bonding ionicity of constituent atoms can be obtained.…”