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ABSTRACT: We investigated the structural, electronic and vibrational properties of amorphous and cubic Ge(2)Sb(2)Te(5) doped with N at 4.2 at.% by means of large scale ab initio simulations. Nitrogen can be incorporated in molecular form in both the crystalline and amorphous phases at a moderate energy cost. In contrast, insertion of N in the atomic form is very energetically costly in the crystalline phase, though it is still possible in the amorphous phase. These results support the suggestion that N segregates at the grain boundaries during the crystallization of the amorphous phase, resulting in a reduction in size of the crystalline grains and an increased crystallization temperature.
Journal of Physics Condensed Matter 07/2011; 23(26):265801. · 2.55 Impact Factor
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ABSTRACT: Based on ab initio molecular dynamics simulations, we investigated the structural, electronic and vibrational properties of cubic and amorphous Ge(2)Sb(2)Te(5) (GST) phase change material, focusing in particular on the effects of defects in stoichiometry on the electronic properties. It turned out Ge/Sb deficiencies (excess) in the cubic phase induce a shift of the Fermi level inside the valence (conduction) bands. In contrast, the amorphous network is flexible enough to accommodate defects in stoichiometry, keeping the Fermi level pinned at the center of the bandgap (at zero temperature). Changes in the structural and electronic properties induced by the use of hybrid functionals (HSE03, PBE0) instead of gradient corrected functionals (PBE) are addressed as well. Analysis of vibrational spectra and Debye-Waller factors of cubic and amorphous GST is also presented.
Journal of Physics Condensed Matter 06/2009; 21(25):255501. · 2.55 Impact Factor
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ABSTRACT: Based on ab initio molecular dynamics simulations, we identify the atomistic mechanism of the pressure induced amorphization of Ge(2)Sb(2)Te(5). The simulations reveal that homopolar Ge/Sb bonds appear in cubic Ge(2)Sb(2)Te(5) under pressure, giving rise to square rings rotated by 45 degrees with respect to the crystalline axis whose formation is induced by the displacement of Te atoms filling the voids of neighboring Ge/Sb stoichiometric vacancies. The concentration of these topological defects increases with pressure up to 21 GPa at which the system is destabilized and transforms into an amorphous phase in agreement with experiments.
Physical Review Letters 06/2009; 102(20):205502. · 7.37 Impact Factor
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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. Comment: 6 pages, 10 figures including supplementary material
08/2007;
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I F W Kuo,
C J Mundy,
M J McGrath,
I J Siepmann,
J VandeVondele,
M Sprik,
J Hutter,
B. Chen,
M. L. Klein,
F Mohamed, M Krack,
M Parrinello
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ABSTRACT: A series of first principles molecular dynamics and Monte Carlo simulations were carried out for liquid water to investigate the reproducibility of different sampling approaches. These simulations include Car-Parrinello molecular dynamics simulations using the program CPMD with different values of the fictitious electron mass in the microcanonical and canonical ensembles, Born-Oppenheimer molecular dynamics using the programs CPMD and CP2K in the microcanonical ensemble, and Metropolis Monte Carlo using CP2K in the canonical ensemble. With the exception of one simulation for 128 water molecules, all other simulations were carried out for systems consisting of 64 molecules. Although the simulations yield somewhat fortuitous agreement in structural properties, analysis of other properties demonstrate that one should exercise caution when assuming the reproducibility of Car-Parrinello and Born-Oppenheimer molecular dynamics simulations for small system sizes in the microcanonical ensemble. In contrast, the molecular dynamics and Monte Carlo simulations in the canonical ensemble appear to be more reliable. Furthermore, in the case of canonical Car-Parrinello molecular dynamics simulations the application of Nose-Hoover chain thermostats allows the use of larger fictitious electron masses. For the Becke-Lee-Yang-Parr exchange and correlation energy functionals and norm-conserving Troullier-Martins or Goedecker-Teter-Hutter pseudopotentials, these simulations at a fixed density of 1.0 g/cm(3) and a temperature close to 315 K point to an overstructured liquid with a height of the first peak in the oxygen-oxygen radial distribution function of about 3.0, an underestimated value of the classical constant-volume heat capacity of about 70 J/(mol K), and an underestimated self-diffusion constant of about 0.04 Angstrom(2)/ps.}
