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Title: Local Bonding Arrangements in Ge2Sb2Te5: Importance of Ge and Te Bonding in Optical Memory Materials

Abstract

Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching grants researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Applications of these theories to switching in a-chalcogenides holds the promise of finding the best composition suited for switching applications. EXAFS spectroscopy is an ideally suited technique to investigate the switching properties of these materials. Results of previous EXAFS experiments will be presented and viewed through the lens of bond constraint theory.

Authors:
 [1]
  1. Physics Department, North Carolina State University, Raleigh, NC 27695-8202 (United States)
Publication Date:
OSTI Identifier:
21054593
Resource Type:
Journal Article
Resource Relation:
Journal Name: AIP Conference Proceedings; Journal Volume: 882; Journal Issue: 1; Conference: XAFS13: 13. international conference on X-ray absorption fine structure, Stanford, CA (United States), 9-14 Jul 2006; Other Information: DOI: 10.1063/1.2644423; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ABSORPTION SPECTROSCOPY; ANTIMONIDES; BINDING ENERGY; CHALCOGENIDES; FINE STRUCTURE; GERMANIUM; GERMANIUM TELLURIDES; GLASS; SEMICONDUCTOR MATERIALS; TELLURIUM; X-RAY SPECTROSCOPY

Citation Formats

Baker, D. A. Local Bonding Arrangements in Ge2Sb2Te5: Importance of Ge and Te Bonding in Optical Memory Materials. United States: N. p., 2007. Web. doi:10.1063/1.2644423.
Baker, D. A. Local Bonding Arrangements in Ge2Sb2Te5: Importance of Ge and Te Bonding in Optical Memory Materials. United States. doi:10.1063/1.2644423.
Baker, D. A. Fri . "Local Bonding Arrangements in Ge2Sb2Te5: Importance of Ge and Te Bonding in Optical Memory Materials". United States. doi:10.1063/1.2644423.
@article{osti_21054593,
title = {Local Bonding Arrangements in Ge2Sb2Te5: Importance of Ge and Te Bonding in Optical Memory Materials},
author = {Baker, D. A.},
abstractNote = {Studies of amorphous (a-) semiconductors have been driven by technological advances as well as fundamental theories. Observation of electrical switching, for example, fueled early interest in a-chalcogenides. More recently a-chalcogenide switching has been applied quite successfully to DVD technology where the quest for the discovery of better-suited materials continues. Thus, switching grants researchers today with an active arena of technological as well as fundamental study. On the theoretical front, bond constraint theory and rigidity theory provide a powerful framework for understanding the structure and properties of a-materials. Applications of these theories to switching in a-chalcogenides holds the promise of finding the best composition suited for switching applications. EXAFS spectroscopy is an ideally suited technique to investigate the switching properties of these materials. Results of previous EXAFS experiments will be presented and viewed through the lens of bond constraint theory.},
doi = {10.1063/1.2644423},
journal = {AIP Conference Proceedings},
number = 1,
volume = 882,
place = {United States},
year = {Fri Feb 02 00:00:00 EST 2007},
month = {Fri Feb 02 00:00:00 EST 2007}
}
  • Ge 2Sb 2Te 5 and related phase change materials are highly unusual in that they can be readily transformed between amorphous and crystalline states using very fast melt, quench, anneal cycles, although the resulting states are extremely long lived at ambient temperature. These states have remarkably different physical properties including very different optical constants in the visible in strong contrast to common glass formers such as silicates or phosphates. This behavior has been described in terms of resonant bonding, but puzzles remain, particularly regarding different physical properties of crystalline and amorphous phases. Here we show that there is a strongmore » competition between ionic and covalent bonding in cubic phase providing a link between the chemical basis of phase change memory property and origins of giant responses of piezoelectric materials (PbTiO 3, BiFeO 3). This has important consequences for dynamical behavior in particular leading to a simultaneous hardening of acoustic modes and softening of high frequency optic modes in crystalline phase relative to amorphous. As a result, this different bonding in amorphous and crystalline phases provides a direct explanation for different physical properties and understanding of the combination of long time stability and rapid switching and may be useful in finding new phase change compositions with superior properties.« less
  • The electronic structure of the layered telluride Nb[sub 3]Ge[sub 0.9]Te[sub 6] was examined by performing tight-binding band calculations on the stoichiometric composition Nb[sub 3]GeTe[sub 6]. Due to the short interlayer Te[center dot][center dot][center dot]Te contacts, the top part of the Te p[sub z]-block bands of Nb[sub 3]GeTe[sub 6] is raised above the Fermi level. This gives rise to a partial p [yields] d electron transfer and to positive overlap populations for the interlayer Te[center dot][center dot][center dot]Te contacts, as in the case of the CdI[sub 2]-type ditellurides MTe[sub 2] (M = Ti, Nb). The Fermi surfaces calculated for Nb[sub 3]GeTe[submore » 6] suggest that Nb[sub 3]Ge[sub 0.9]Te[sub 6] is a three-dimensional metal, with its electrical conductivity increasing along the b < c < a axis directions. 12 refs., 4 figs., 1 tab.« less
  • The preparation of several salts of the Me{sub 3}Te{sup +} and Ph{sub 3}Te{sup +} cations is reported along with {sup 125}Te static and CP/MAS solid-state NMR spectra of the Me{sub 3}Te{sup +} salts and {sup 125}Te CP/MAS spectra of the Ph{sub 3}Te{sup +} salts. In addition, crystal structures of the salts Me{sub 3}Te{sup +}Cl{sup {minus}} {times} H{sub 2}O (1), Me{sub 3}Te{sup +}I{sup {minus}} (2), Me{sub 3}Te{sup +}NO{sub 3}{sup {minus}} (3), Ph{sub 3}Te{sup +}NO{sub 3}{sup {minus}} (4), Ph{sub 3}Te{sup +}Cl{sup {minus}} {times}{1/2}CHCl{sub 3} (5), and (Ph{sub 3}Te){sub 2}SO{sub 4} {times} 5H{sub 2}O (6) have been determined to help in the interpretationmore » of some of the above spectra. It is shown that secondary bonding interactions with the anions have significant and substantial influence on the geometries of the cations and their crystal packings and on the resulting NMR spectra in terms of the tellurium shielding tensors and isotropic chemical shifts and in the presence of log-range spin {1/2} to spin 3/2 couplings. These couplings are indicative of long-range covalent interactions between the anions and cations in this series of compounds. In particular, the {sup 125}Te CP/MAS spectrum of Me{sub 3}Te{sup +}Cl{sup {minus}} {times} H{sub 2}O consists of a septet due to coupling to two equivalent chlorine atoms. In the case of 4 and 5 the presence of 4 and 2 crystallographically independent Te atoms is confirmed in the NMR spectra. 38 refs., 11 figs.« less