56 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
Charge state of the O molecule during silicon oxidation through hybrid functional calculations
We study the charge state of the diffusing O molecule during silicon
oxidation through hybrid functional calculations. We calculate charge
transition levels of O in bulk SiO and use theoretical band offsets to
align these levels with respect to the Si band edges. To overcome the band-gap
problem of semilocal density fuctionals, we employ hybrid functionals with both
predefined and empirically adjusted mixing coefficients. We find that the
charge transition level in bulk SiO occurs at 1.1 eV
above the silicon conduction band edge, implying that the O molecule
diffuses through the oxide in the neutral charge state. While interfacial
effects concur to lower the charge transition level, our estimates suggest that
the neutral charge state persists until silicon oxidation.Comment: 4 pages, 3 figure
First-principles theory of the luminescence lineshape for the triplet transition in diamond NV centre
In this work we present theoretical calculations and analysis of the vibronic
structure of the spin-triplet optical transition in diamond nitrogen-vacancy
centres. The electronic structure of the defect is described using accurate
first-principles methods based on hybrid functionals. We devise a computational
methodology to determine the coupling between electrons and phonons during an
optical transition in the dilute limit. As a result, our approach yields a
smooth spectral function of electron-phonon coupling and includes both
quasi-localized and bulk phonons on equal footings. The luminescence lineshape
is determined via the generating function approach. We obtain a highly accurate
description of the luminescence band, including all key parameters such as the
Huang-Rhys factor, the Debye-Waller factor, and the frequency of the dominant
phonon mode. More importantly, our work provides insight into the vibrational
structure of nitrogen vacancy centres, in particular the role of local modes
and vibrational resonances. In particular, we find that the pronounced mode at
65 meV is a vibrational resonance, and we quantify localization properties of
this mode. These excellent results for the benchmark diamond nitrogen-vacancy
centre provide confidence that the procedure can be applied to other defects,
including alternative systems that are being considered for applications in
quantum information processing
Defect Energy Levels in Density Functional Calculations: Alignment and Band Gap Problem
For materials of varying band gap, we compare energy levels of atomically
localized defects calculated within a semilocal and a hybrid density-functional
scheme. Since the latter scheme partially relieves the band gap problem, our
study describes how calculated defect levels shift when the band gap approaches
the experimental value. When suitably aligned, defect levels obtained from
total-energy differences correspond closely, showing average shifts of at most
0.2 eV irrespective of band gap. Systematic deviations from ideal alignment
increase with the extent of the defect wave function. A guideline for comparing
calculated and experimental defect levels is provided.Comment: 4 pages, 3 figure
Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride
Hexagonal boron nitride (h-BN) is a tantalizing material for solid-state
quantum engineering. Analogously to three-dimensional wide-bandgap
semiconductors like diamond, h-BN hosts isolated defects exhibiting visible
fluorescence, and the ability to position such quantum emitters within a
two-dimensional material promises breakthrough advances in quantum sensing,
photonics, and other quantum technologies. Critical to such applications,
however, is an understanding of the physics underlying h-BN's quantum emission.
We report the creation and characterization of visible single-photon sources in
suspended, single-crystal, h-BN films. The emitters are bright and stable over
timescales of several months in ambient conditions. With substrate interactions
eliminated, we study the spectral, temporal, and spatial characteristics of the
defects' optical emission, which offer several clues about their electronic and
chemical structure. Analysis of the defects' spectra reveals similarities in
vibronic coupling despite widely-varying fluorescence wavelengths, and a
statistical analysis of their polarized emission patterns indicates a
correlation between the optical dipole orientations of some defects and the
primitive crystallographic axes of the single-crystal h-BN film. These
measurements constrain possible defect models, and, moreover, suggest that
several classes of emitters can exist simultaneously in free-standing h-BN,
whether they be different defects, different charge states of the same defect,
or the result of strong local perturbations
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Dangling Bonds in Hexagonal Boron Nitride as Single-Photon Emitters.
Hexagonal boron nitride has been found to host color centers that exhibit single-photon emission, but the microscopic origin of these emitters is unknown. We propose boron dangling bonds as the likely source of the observed single-photon emission around 2Â eV. An optical transition where an electron is excited from a doubly occupied boron dangling bond to a localized B p_{z} state gives rise to a zero-phonon line of 2.06Â eV and emission with a Huang-Rhys factor of 2.3. This transition is linearly polarized with the absorptive and emissive dipole aligned. Because of the energetic position of the states within the band gap, indirect excitation through the conduction band will occur for sufficiently large excitation energies, leading to the misalignment of the absorptive and emissive dipoles seen in experiment. Our calculations predict a singlet ground state and the existence of a metastable triplet state, in agreement with experiment
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