Impact of dispersion forces on the atomic structure of a prototypical network-forming disordered system: The case of liquid GeSe2

Aset of structural properties of liquid GeSe2 are calculated by using first-principles molecular dynamics and including, for the first time, van derWaals dispersion forces. None of the numerous atomic-scale simulations performed in the past on this prototypical disordered network-forming material had ever accounted for dispersion forces in the expression of the total energy. For this purpose, we employed either the Grimme-D2 or the maximally localized Wannier function scheme. We assessed the impact of dispersion forces on properties such as partial structure factors, pair correlation functions, bond angle distribution, and number of corner vs edge sharing connections. The maximally localized Wannier function scheme is more reliable than the Grimme-D2 scheme in reproducing existing first-principles results. In particular, the Grimme-D2 scheme worsens the agreement with experiments in the case of the Ge-Ge pair correlation function. Our study shows that the impact of dispersion forces on disordered chalcogenides has to be considered with great care since it cannot be necessarily the same when adopting different recipes. Published by AIP Publishing.

The role of dispersion forces on the atomic structure of glassy chalcogenides: The case of GeSe4 and GeS4

Assil Bouzid, Carlo Massobrio

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

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

We present a theoretical study of the physical characteristics of metal/semiconductor junctions. Using first principle pseudopotential calculations, we have investigated the nature of electronic states with energies within the semiconductor band gap of representative abrupt, defect-free, anion-terminated metal/III-V interfaces. Namely, we focused on Al contacts to GaAs(001), AlAs(001) and cubic GaN(001) as well as on Al, Au and Cu junctions to cubic GaN(001). Recent advances in Schottky barrier concepts emphasize the possible relationship between interface states and the formation of the Schottky barrier. We aim at understanding the atomic-scale mechanisms responsible for interface states as well as their role in the Schottky barrier formation process. At As-terminated Al/GaAs(001) and Al/AlAs(001) junctions, resonant and localized interface states occur at the J point of the interface 2D Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between the outermost cation atoms of the semiconductor and the interfacial Al atoms of the metal. These interface states derive from an interaction between localized states of the Al(001) surface and semiconductor conduction band states, mediated by localized states of the unreconstructed, As-terminated semiconductor (001) surface. Our results indicate that interface states of the intermetallic, bonding-like kind could play an important role in the transport properties of metal/AlxGa1-xAs junctions. We have also investigated the electronic structure of Al, Au and Cu junctions to cubic, N-terminated GaN(001). The localized interface state reported for As-terminated Al/GaAs(001) and Al/AlAs(001) junctions occurs also at metal/GaN interfaces under the condition that atoms on the outermost atomic plane of the metal are placed in front of the outermost semiconductor cation. This indicates that the formation mechanism of this state is a very general one. In contrast to Al/AlxGa1-xAs junctions, these states occur at energy much larger than EF for the contacts to GaN. Thus, they are not expected to contribute significantly to the electronic transport of the latter interfaces. However, a large number of interface states attributed to d-type orbitals occur over a wide energy range including EF at contacts of noble metals to GaN.

EPFL2003

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

EPFL2003