509 research outputs found
Band-edge problem in the theoretical determination of defect energy levels: the O vacancy in ZnO as a benchmark case
Calculations of formation energies and charge transition levels of defects
routinely rely on density functional theory (DFT) for describing the electronic
structure. Since bulk band gaps of semiconductors and insulators are not well
described in semilocal approximations to DFT, band-gap correction schemes or
advanced theoretical models which properly describe band gaps need to be
employed. However, it has become apparent that different methods that reproduce
the experimental band gap can yield substantially different results regarding
charge transition levels of point defects. We investigate this problem in the
case of the (+2/0) charge transition level of the O vacancy in ZnO, which has
attracted considerable attention as a benchmark case. For this purpose, we
first perform calculations based on non-screened hybrid density functionals,
and then compare our results with those of other methods. While our results
agree very well with those obtained with screened hybrid functionals, they are
strikingly different compared to those obtained with other band-gap corrected
schemes. Nevertheless, we show that all the different methods agree well with
each other and with our calculations when a suitable alignment procedure is
adopted. The proposed procedure consists in aligning the electron band
structure through an external potential, such as the vacuum level. When the
electron densities are well reproduced, this procedure is equivalent to an
alignment through the average electrostatic potential in a calculation subject
to periodic boundary conditions. We stress that, in order to give accurate
defect levels, a theoretical scheme is required to yield not only band gaps in
agreement with experiment, but also band edges correctly positioned with
respect to such a reference potential
O2 oxidation reaction at the Si(100)-SiO2 interface: A first-principles investigation
We investigated the oxidation reaction of the O2 molecule at the Si(100)-SiO2 interface by using a constrained ab initio molecular dynamics approach. To represent the Si(100)-SiO2 interface, we adopted several model interfaces whose structural properties are consistent with atomic-scale information obtained from a variety of experimental probes. We addressed the oxidation reaction by sampling different reaction pathways of the O2 molecule at the interface. The reaction proceeds sequentially through the incorporation of the O2 molecule in a Si-Si bond and the dissociation of the resulting network O2-species. The oxidation reaction occurs nearly spontaneously and is exothermic, regardless of the spin state of the O2 molecule. Our study suggests a picture of the silicon oxidation process entirely based on diffusive processe
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Band Offsets at the Si/SiO Interface from Many-Body Perturbation Theory
We use many-body perturbation theory, the state-of-the-art method for band
gap calculations, to compute the band offsets at the Si/SiO interface. We
examine the adequacy of the usual approximations in this context. We show that
(i) the separate treatment of band-structure and potential lineup
contributions, the latter being evaluated within density-functional theory, is
justified, (ii) most plasmon-pole models lead to inaccuracies in the absolute
quasiparticle corrections, (iii) vertex corrections can be neglected, (iv)
eigenenergy self-consistency is adequate. Our theoretical offsets agree with
the experimental ones within 0.3 eV
The vibrational dynamics of vitreous silica: Classical force fields vs. first-principles
We compare the vibrational properties of model SiO_2 glasses generated by
molecular-dynamics simulations using the effective force field of van Beest et
al. (BKS) with those obtained when the BKS structure is relaxed using an ab
initio calculation in the framework of the density functional theory. We find
that this relaxation significantly improves the agreement of the density of
states with the experimental result. For frequencies between 14 and 26 THz the
nature of the vibrational modes as determined from the BKS model is very
different from the one from the ab initio calculation, showing that the
interpretation of the vibrational spectra in terms of calculations using
effective potentials can be very misleading.Comment: 7 pages of Latex, 4 figure
Structure and energetics of the Si-SiO_2 interface
Silicon has long been synonymous with semiconductor technology. This unique
role is due largely to the remarkable properties of the Si-SiO_2 interface,
especially the (001)-oriented interface used in most devices. Although Si is
crystalline and the oxide is amorphous, the interface is essentially perfect,
with an extremely low density of dangling bonds or other electrically active
defects. With the continual decrease of device size, the nanoscale structure of
the silicon/oxide interface becomes more and more important. Yet despite its
essential role, the atomic structure of this interface is still unclear. Using
a novel Monte Carlo approach, we identify low-energy structures for the
interface. The optimal structure found consists of Si-O-Si "bridges" ordered in
a stripe pattern, with very low energy. This structure explains several
puzzling experimental observations.Comment: LaTex file with 4 figures in GIF forma
Structure and oxidation kinetics of the Si(100)-SiO2 interface
We present first-principles calculations of the structural and electronic
properties of Si(001)-SiO2 interfaces. We first arrive at reasonable structures
for the c-Si/a-SiO2 interface via a Monte-Carlo simulated annealing applied to
an empirical interatomic potential, and then relax these structures using
first-principles calculations within the framework of density-functional
theory. We find a transition region at the interface, having a thickness on the
order of 20\AA, in which there is some oxygen deficiency and a corresponding
presence of sub-oxide Si species (mostly Si^+2 and Si^+3). Distributions of
bond lengths and bond angles, and the nature of the electronic states at the
interface, are investigated and discussed. The behavior of atomic oxygen in
a-SiO2 is also investigated. The peroxyl linkage configuration is found to be
lower in energy than interstitial or threefold configurations. Based on these
results, we suggest a possible mechanism for oxygen diffusion in a-SiO2 that
may be relevant to the oxidation process.Comment: 7 pages, two-column style with 6 postscript figures embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#ng_sio
Isobaric first-principles molecular dynamics of liquid water with nonlocal van der Waals interactions
We investigate the structural properties of liquid water at near ambient
conditions using first-principles molecular dynamics simulations based
on a semilocal density functional augmented with nonlocal van der Waals
interactions. The adopted scheme offers the advantage of simulating
liquid water at essentially the same computational cost of standard
semilocal functionals. Applied to the water dimer and to ice I-h, we
find that the hydrogen-bond energy is only slightly enhanced compared to
a standard semilocal functional. We simulate liquid water through
molecular dynamics in the N pH statistical ensemble allowing for
fluctuations of the system density. The structure of the liquid departs
from that found with a semilocal functional leading to more compact
structural arrangements. This indicates that the directionality of the
hydrogen-bond interaction has a diminished role as compared to the
overall attractions, as expected when dispersion interactions are
accounted for. This is substantiated through a detailed analysis
comprising the study of the partial radial distribution functions,
various local order indices, the hydrogen-bond network, and the
selfdiffusion coefficient. The explicit treatment of the van der Waals
interactions leads to an overall improved description of liquid water
Pathways of bond topology transitions at the interface of silicon nanocrystals and amorphous silica matrix
The interface chemistry of silicon nanocrystals (NCs) embedded in amorphous
oxide matrix is studied through molecular dynamics simulations with the
chemical environment described by the reactive force field model. Our results
indicate that the Si NC-oxide interface is more involved than the previously
proposed schemes which were based on solely simple bridge or double bonds. We
identify different types of three-coordinated oxygen complexes, previously not
noted. The abundance and the charge distribution of each oxygen complex is
determined as a function of the NC size as well as the transitions among them.
The oxidation at the surface of NC induces tensile strain to Si-Si bonds which
become significant only around the interface, while the inner core remains
unstrained. Unlike many earlier reports on the interface structure, we do not
observe any double bonds. Furthermore, our simulations and analysis reveal that
the interface bond topology evolves among different oxygen bridges through
these three-coordinated oxygen complexes.Comment: 5 pages 6 figures 1 tabl
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