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Understanding Phase-Change Memory Alloys from a Chemical Perspective.

Kolobov AV, Fons P, Tominaga J - Sci Rep (2015)

Bottom Line: At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown.In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons.Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

View Article: PubMed Central - PubMed

Affiliation: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan.

ABSTRACT
Phase-change memories (PCM) are associated with reversible ultra-fast low-energy crystal-to-amorphous switching in GeTe-based alloys co-existing with the high stability of the two phases at ambient temperature, a unique property that has been recently explained by the high fragility of the glass-forming liquid phase, where the activation barrier for crystallisation drastically increases as the temperature decreases from the glass-transition to room temperature. At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown. In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons. Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

No MeSH data available.


Related in: MedlinePlus

Upper row: schematic of the formation of a tetrahedral Ge configuration.As in Fig. 2, Ge atoms are shown in green and Te atoms are shown in orange. When a Ge atom with a protruding LP-orbital (marked A) comes close to another Ge atom (B) and is aligned with the neighbouring Ge-Te bond (between atoms B–C) (left panel), a three-center A–B–C bond is established (middle), whose subsequent rupture at the opposite arm results in the formation of a GeTd–GePy configuration (between atoms A–B), leaving behind a two-fold coordinated Te atom (C) (right). Middle row: evolution of CDD clouds during the in-silico amorphisation process using DFT simulations substantiating the schematic shown in the upper panel (see text for details). CDD clouds corresponding to the LP-electrons of an sp3 hybridised Ge orbital and a Te lone-pair p-orbital can be seen in the left and right panels respectively in addition to increased CDD midway between Ge and Te atoms that are signatures of covalent bonds. The presence of CDD clouds on both sides of the Ge atom (marked B in the figure) in the central panel is evidence of the formation of a transient three-center Ge-Ge-Te bond. Lower row: zooms into vicinities of the atoms that participate in the formation of TCB.
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f4: Upper row: schematic of the formation of a tetrahedral Ge configuration.As in Fig. 2, Ge atoms are shown in green and Te atoms are shown in orange. When a Ge atom with a protruding LP-orbital (marked A) comes close to another Ge atom (B) and is aligned with the neighbouring Ge-Te bond (between atoms B–C) (left panel), a three-center A–B–C bond is established (middle), whose subsequent rupture at the opposite arm results in the formation of a GeTd–GePy configuration (between atoms A–B), leaving behind a two-fold coordinated Te atom (C) (right). Middle row: evolution of CDD clouds during the in-silico amorphisation process using DFT simulations substantiating the schematic shown in the upper panel (see text for details). CDD clouds corresponding to the LP-electrons of an sp3 hybridised Ge orbital and a Te lone-pair p-orbital can be seen in the left and right panels respectively in addition to increased CDD midway between Ge and Te atoms that are signatures of covalent bonds. The presence of CDD clouds on both sides of the Ge atom (marked B in the figure) in the central panel is evidence of the formation of a transient three-center Ge-Ge-Te bond. Lower row: zooms into vicinities of the atoms that participate in the formation of TCB.

Mentions: The process of the formation of a tetrahedrally coordinated Ge site is illustrated in Fig. 4. In the upper panel, we show a schematic of the process. When a Ge atom (A) possessing an extended s-p mixed LP-orbital becomes aligned with a covalent Ge-Te bond between atoms B and C (left panel), a Ge-Ge-Te (A–B–C) TCB can be generated (center), with the subsequent formation of a Ge-Ge (A–B) bond, concomitant with the rupture of the Ge-Te arm of the TCB (right). The additional Ge-Ge bond is thus indeed created at the expense of the Ge-Te bond. As in the case of chalcogenide glasses, the key point is that the formation of the homopolar Ge-Ge bonds does not require precursory rupture of strong two-center covalent Ge-Te bonds. The subsequent destruction of the Ge-Te arm of a TCB actually serves to strengthen the remaining Ge-Ge bond and thus stabilises the amorphous phase. Schematically this process can be described as:


Understanding Phase-Change Memory Alloys from a Chemical Perspective.

Kolobov AV, Fons P, Tominaga J - Sci Rep (2015)

Upper row: schematic of the formation of a tetrahedral Ge configuration.As in Fig. 2, Ge atoms are shown in green and Te atoms are shown in orange. When a Ge atom with a protruding LP-orbital (marked A) comes close to another Ge atom (B) and is aligned with the neighbouring Ge-Te bond (between atoms B–C) (left panel), a three-center A–B–C bond is established (middle), whose subsequent rupture at the opposite arm results in the formation of a GeTd–GePy configuration (between atoms A–B), leaving behind a two-fold coordinated Te atom (C) (right). Middle row: evolution of CDD clouds during the in-silico amorphisation process using DFT simulations substantiating the schematic shown in the upper panel (see text for details). CDD clouds corresponding to the LP-electrons of an sp3 hybridised Ge orbital and a Te lone-pair p-orbital can be seen in the left and right panels respectively in addition to increased CDD midway between Ge and Te atoms that are signatures of covalent bonds. The presence of CDD clouds on both sides of the Ge atom (marked B in the figure) in the central panel is evidence of the formation of a transient three-center Ge-Ge-Te bond. Lower row: zooms into vicinities of the atoms that participate in the formation of TCB.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4555180&req=5

f4: Upper row: schematic of the formation of a tetrahedral Ge configuration.As in Fig. 2, Ge atoms are shown in green and Te atoms are shown in orange. When a Ge atom with a protruding LP-orbital (marked A) comes close to another Ge atom (B) and is aligned with the neighbouring Ge-Te bond (between atoms B–C) (left panel), a three-center A–B–C bond is established (middle), whose subsequent rupture at the opposite arm results in the formation of a GeTd–GePy configuration (between atoms A–B), leaving behind a two-fold coordinated Te atom (C) (right). Middle row: evolution of CDD clouds during the in-silico amorphisation process using DFT simulations substantiating the schematic shown in the upper panel (see text for details). CDD clouds corresponding to the LP-electrons of an sp3 hybridised Ge orbital and a Te lone-pair p-orbital can be seen in the left and right panels respectively in addition to increased CDD midway between Ge and Te atoms that are signatures of covalent bonds. The presence of CDD clouds on both sides of the Ge atom (marked B in the figure) in the central panel is evidence of the formation of a transient three-center Ge-Ge-Te bond. Lower row: zooms into vicinities of the atoms that participate in the formation of TCB.
Mentions: The process of the formation of a tetrahedrally coordinated Ge site is illustrated in Fig. 4. In the upper panel, we show a schematic of the process. When a Ge atom (A) possessing an extended s-p mixed LP-orbital becomes aligned with a covalent Ge-Te bond between atoms B and C (left panel), a Ge-Ge-Te (A–B–C) TCB can be generated (center), with the subsequent formation of a Ge-Ge (A–B) bond, concomitant with the rupture of the Ge-Te arm of the TCB (right). The additional Ge-Ge bond is thus indeed created at the expense of the Ge-Te bond. As in the case of chalcogenide glasses, the key point is that the formation of the homopolar Ge-Ge bonds does not require precursory rupture of strong two-center covalent Ge-Te bonds. The subsequent destruction of the Ge-Te arm of a TCB actually serves to strengthen the remaining Ge-Ge bond and thus stabilises the amorphous phase. Schematically this process can be described as:

Bottom Line: At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown.In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons.Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

View Article: PubMed Central - PubMed

Affiliation: Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8562, Japan.

ABSTRACT
Phase-change memories (PCM) are associated with reversible ultra-fast low-energy crystal-to-amorphous switching in GeTe-based alloys co-existing with the high stability of the two phases at ambient temperature, a unique property that has been recently explained by the high fragility of the glass-forming liquid phase, where the activation barrier for crystallisation drastically increases as the temperature decreases from the glass-transition to room temperature. At the same time the atomistic dynamics of the phase-change process and the associated changes in the nature of bonding have remained unknown. In this work we demonstrate that key to this behavior is the formation of transient three-center bonds in the excited state that is enabled due to the presence of lone-pair electrons. Our findings additionally reveal previously ignored fundamental similarities between the mechanisms of reversible photoinduced structural changes in chalcogenide glasses and phase-change alloys and offer new insights into the development of efficient PCM materials.

No MeSH data available.


Related in: MedlinePlus