Coexistence of tetrahedral- and octahedral-like sites in amorphous phase change materials

Abstract

Chalcogenide alloys are materials of interest for optical recording and non-volatile memories. We perform ab-initio molecular dynamics simulations aiming at shading light onto the structure of amorphous Ge2Sb2Te5 (GST), the prototypical material in this class. First principles simulations show that amorphous GST obtained by quenching from the liquid phase displays two types of short range order. One third of Ge atoms are in a tetrahedral environment while the remaining Ge, Sb and Te atoms display a defective octahedral environment, reminiscent of cubic crystalline GST.

Structure of amorphousGe8Sb2Te11:<

“…The Te-Te and Sb-Sb pair-correlation functions show medium range order, and the dihedral angle distributions show that cubic arrangements of Te and, in particular, Sb atoms are favored. The Sb coordination can also be described as "distorted octahedral" 17 or "Peierls distorted" 34 since the integration of the Sb-Te PDF up to the first minimum ͑3.9 Å͒ gives a total coordination number close to 6. XPS measurements of the valence electron densities of states in GST-225 and GST-8,2,11 are reproduced well in both amorphous and crystalline forms, as are the differences between the two phases.…”

“…As in GST-225, most of the four-membered rings in Ge 8 Sb 2 Te 11 show AB alternation, confirming that the basic building blocks in the amorphous and crystalline ͑rocksalt͒ phases are ABAB squares. 15,17,18 A good example is the ABAB column shown in Fig. 1.…”

“…In GST-225, for example, it has been proposed that a Ge atom flips between tetrahedral and octahedral sites. 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.…”

Microscopic properties of liquid water from combined ab initio molecular dynamics and energy decomposition studies

“…The propagation scheme in second-generation CPMD relies on the appropriate usage of the information about the electrons from the previous MD steps and, in contrast to CPMD, does not require a fictitious mass parameter to maintain the accuracy of the Born-Oppenheimer dynamics. The superior efficiency of this new approach, which, depending on the system, varies between one to two orders of magnitude, has been demonstrated for a large variety of different systems [15][16][17][18][19][20] .…”

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

“…There are many four-membered rings and few triangular configurations in all cases. As in GST-225, most of the four-membered rings in Ge 8 Sb 2 Te 11 show AB alternation, confirming that the basic building blocks in the amorphous and crystalline phases are ABAB squares [16,24,25]. A good example is the ABAB column shown in Fig.…”