Atomic-scale structure of the glassy Ge2Sb2Te5 phase change material: A quantitative assessment via first-principles molecular dynamics

Assil Bouzid, Carlo Massobrio
Amer Physical Soc, 2017
Journal paper

Abstract

The amorphous structure of the phase change material Ge2Sb2Te5 (GST) has been the object of controversial structural models. By employing first-principles molecular dynamics within density functional theory, we are able to obtain quantitative agreement with experimental structural findings for the topology of glassy GST. To this end, we take full advantage of a thoughtful, well established choice of the exchange-correlation (XC) functional (Becke-Lee-Yang-Parr, BLYP), combined with appropriate options for the nonlocal part in the pseudopotential construction for Ge. Results obtained by using the Perdew-Burke-Ernzerhof (PBE) XC functional and a similar strategy for the Ge pseudopotential constructions are also presented, since they are very valuable and worthy of consideration. The atomic structure of glassy GST is characterized by Ge atoms lying in a predominant tetrahedral network, albeit a non-negligible fraction of Ge atoms are also found in defective octahedra.

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Assil Bouzid, Carlo Massobrio
Amer Physical Soc, 2017
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/234093?ln=en

About this result

Related concepts (11)

Molecular dynamics (MD) is a computer simulation method for analyzing the physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a fixed period of time, giv

Density-functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (

Car–Parrinello molecular dynamics or CPMD refers to either a method used in molecular dynamics (also known as the Car–Parrinello method) or the computational chemistry software package used to impleme

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Recent first-principles molecular dynamics simulations (FPMD) results on two chalcogenide glasses (glassy GeS4 and GeSe4) are revisited by accounting explicitly for van der Waals (vdW) dispersion forces. This effort is motivated by the observation that such contributions were found to be important in the case of glassy GeTe4 while they were negligible for another disordered chalcogenide system, liquid GeSe2. With the present results, we provide additional evidence intended to establish under which conditions and for which systems the consideration of dispersion forces plays a role in determining the atomic structure. For these purposes we employ two different dispersion schemes used in conjunction with the BLYP (Becke, Lee, Yang and Parr) exchange correlation functional. The first is the DFT-D2 vdW correction introduced by Grimme et al. (Grimme (2006) [1]) while the second is the vdW(w) approach based on the Wannier functions analysis (Silvestrelli, 2008 [2]). The atomic structures obtained agree well with the ones obtained by Bouzid et al. [3] without considering dispersion forces. Due to the vdW interactions, glassy GeSe4 features a higher number of Ge fourfold coordinated. The two vdW approaches also agree to a large extent, exceptions occurring for some moderate differences in the intensity of the peaks in the Ge-Ge pair correlation function.

ELSEVIER SCIENCE BV2018

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Structural and electronic properties of an abrupt 4H-SiC(0001)/SiO2 interface model: Classical molecular dynamics simulations and density functional calculations

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Using a density functional approach, we study structural and electronic properties of the 4H(0001)-SiC/SiO2 interface. Through the sequential use of classical and ab initio simulation methods, we generate an abrupt model structure which describes the transition between crystalline SiC and amorphous SiO2 without showing any coordination defect. The first step in our generation procedure consists in identifying suitable interfacial bonding patterns which account for the bond density reduction across the interface. In the second step, the connection to amorphous SiO2 is achieved through classical molecular dynamics. The atomic positions are then relaxed within a generalized gradient approximation of density functional theory. The final model structure shows good structural parameters and an oxide density typical of amorphous SiO2. We investigate the electronic structure of the generated model interface through the local density of states obtained with hybrid density functionals. In our atomically abrupt interface model, the full oxide band gap is recovered at a distance of similar to 5 A from the interface. This extent is in good agreement with estimates derived from internal photoemission measurements and provides support for the abrupt nature of the interface. We obtained band offsets through two different procedures: by evaluating the local density of states and by aligning the band extrema through the local electrostatic potential. Band offsets calculated with various functionals are compared to experimental values. The best agreement is achieved for a hybrid functional in which a screened Coulomb potential is used in the Hartree-Fock exchange term. For this case, the calculated band offsets underestimate the experimental values by similar to 25%, but agree with experiment within a few percent when expressed with respect to the oxide band gap.

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