Application of Bond Constraint Theory to the Switchable Optical Memory MaterialGe2Sb2Te5

Abstract

A new extended x-ray-absorption fine structure spectroscopy study of local bonding identifies for the first time significant concentrations of Ge-Ge bonds in amorphous Ge2Sb2Te5. The study provides a new understanding of the local molecular structure of this phase-change material. Application of bond constraint theory indicates that the amorphous phase is an ideal network structure in which the average number of constraints per atom equals the network dimensionality. Analysis within this framework imparts new and significant insights concerning the nature of the reversible optically driven amorphous-crystalline phase transition of Ge2Sb2Te5.

Amorphous structures of Ge/Sb/Te alloys: Density functional simulations

“…The AD structure of a-GST (648 atoms, over 200 ps) differs from MQ (460 atoms, [27]) in essential ways: (1) The environment of Ge atoms is predominantly tetrahedral in AD and disordered octahedral in MQ, and our results for the Ge-Te bond lengths agree with EXAFS measurements [8,29,30]. This resolves the contradiction between measured and bond lengths and those found in earlier DF calculations, (2) homopolar and Ge-Sb bonds are more common and reduce the number of ABAB squares, which are needed for rapid crystallization.…”

“…EXAFS measurements [8,29,30] tetrahedral Ge atoms, and the Ge-Te maximum shifts to 2.69 Å . Ge-Ge bonds (2.52 Å ), which correspond mainly to tetrahedral Ge atoms here, agree well with experiment (2.47 AE 0.03 Å ) [29]. Sb-Te bond lengths change little upon amorphization, and the first maximum of AD and MQ is at 2.89 Å .…”

Structure of amorphousGe8Sb2Te11:<

“…11 Recent numerical simulations of the amorphous phases [15][16][17][18][19] have provided new information and a more consistent picture of both systems: ͑a͒ octahedral bond angles predominate, 12,15,17,19 ͑b͒ tetrahedrally coordinated Ge atoms do occur, 15,17,19 ͑c͒ evennumbered rings are characteristic, 12 particularly "ABAB squares" ͑A: Sb, Ge; B: Te͒, 15,18,19 and ͑d͒ Te atoms ͑and Sb in GST-225͒ are overcoordinated; i.e., they do not obey the "8 − N" rule, where N is the number of valence electrons. 13,15,17,19 One might speculate that GST-8,2,11 has similar features to GST-225, or that its GeTe-rich nature ͑x = 1 9 ͒ could lead to a close resemblance to GeTe ͑x =0͒. We show that the first assumption is valid but the latter is not.…”

“…The usage of PBEsol functional reduces bond lengths by 1%-2%, and the values of 2.54, 2.73, and 2.88 Å for Ge-Ge, Ge-Te, and Sb-Te bonds are closer to the EXAFS results ͑2.48, 2.63, and 2.85 Å͒ for GST-225. 13 As in a-GST-225, 15,16 both Sb and Te have higher coordination than expected from the "8 − N rule," where N is the number of valence electrons. The Ge-Ge coordination number ͑0.7͒ is significantly less than in GeTe.…”

Structural phase transitions on the nanoscale: The crucial pattern in the phase-change materialsGe2Sb2Te5and GeTe

“…Extended x-ray absorption fine structure ͑EXAFS͒ measurements on GST have found significant concentrations of Ge-Ge bonds and indications that overcoordinated Te atoms play a role, 12 and there has been renewed focus on the role of vacancies. 13,14 Density functional ͑DF͒ calculations suggest that the metastable structure is consistent with rocksalt symmetry but comprises two units ordered along the ͓111͔ direction.…”