60 research outputs found
Structure and energetics of carbon-related defects in SiC (0001)/SiO systems revealed by first-principles calculations: Defects in SiC, SiO, and just at their interface
We report first-principles calculations that reveal the atomic forms,
stability, and energy levels of carbon-related defects in SiC (0001)/SiO systems. We clarify the stable position (SiC side, SiO side, or
just at the SiC/SiO interface) of defects depending on the oxidation
environment. Under an O-rich condition, the di-carbon antisite ((C)) in the SiC side is stable and critical for -channel
MOSFETs, whereas the di-carbon defect (Si-C-C-Si) at the interface becomes
critical under an O-poor condition. Our results suggest that the oxidation of
SiC under a high-temperature O-poor condition is favorable in reducing the
defects, in consistent with recent experimental reports.Comment: 17 pages, 10 figure
Structural stability and energy levels of carbon-related defects in amorphous SiO and its interface with SiC
We report the density-functional calculations that systematically clarify the
stable forms of carbon-related defects and their energy levels in amorphous
SiO using the melt-quench technique in molecular dynamics. Considering the
position dependence of the O chemical potential near and far from the
SiC/SiO interface, we determine the most abundant forms of carbon-related
defects: Far from the interface, the CO or CO in the internal space in
SiO is abundant and they are electronically inactive; near the interface,
the carbon clustering is likely and a particular mono-carbon defect and a
di-carbon defect induce energy levels near the SiC conduction-band bottom, thus
being candidates for the carrier traps.Comment: 8 figures, to be published in Japanese Journal of Applied Physic
Analysis of single and composite structural defects in pure amorphous silicon: a first-principles study
The structural and electronic properties of amorphous silicon (-Si) are
investigated by first-principles calculations based on the density-functional
theory (DFT), focusing on the intrinsic structural defects. By simulated
melting and quenching of a crystalline silicon model through the Car-Parrinello
molecular dynamics (CPMD), we generate several different -Si samples, in
which three-fold (), five-fold (), and anomalous four-fold ()
defects are contained. Using the samples, we clarify how the disordered
structure of -Si affects the characters of its density of states (DOS). We
subsequently study the properties of defect complexes found in the obtained
samples, including one that comprises three defects, and we show the
conditions for the defect complexes to be energetically stable. Finally, we
investigate the hydrogen passivation process of the defects in -Si and
show that the hydrogenation of is an exothermic reaction and that the
activation energy for a H molecule to passivate two sites is
calculated to be 1.05 eV
Interstitial Channels that Control Band Gaps and Effective Masses in Tetrahedrally Bonded Semiconductors
We find that electron states at the bottom of the conduction bands of
covalent semiconductors are distributed mainly in the interstitial channels and
that this floating nature leads to the band-gap variation and the anisotropic
effective masses in various polytypes of SiC. We find that the channel length,
rather than the hexagonality prevailed in the past, is the decisive factor for
the band-gap variation in the polytypes. We also find that the floating nature
causes two-dimensional electron and hole systems at the interface of different
SiC polytypes and even one-dimensional channels near the inclined SiC surface.Comment: 5 pages, 6 figure
Quantum Singwi-Tosi-Land-Sjoelander approach for interacting inhomogeneous systems under electromagnetic fields: Comparison with exact results
For inhomogeneous interacting electronic systems under a time-dependent
electromagnetic perturbation, we derive the linear equation for response
functions in a quantum mechanical manner. It is a natural extension of the
original semi-classical Singwi-Tosi-Land-Sjoelander (STLS) approach for an
electron gas. The factorization ansatz for the two-particle distribution is an
indispensable ingredient in the STLS approaches for determination of the
response function and the pair correlation function. In this study, we choose
an analytically solvable interacting two-electron system as the target for
which we examine the validity of the approximation. It is demonstrated that the
STLS response function reproduces well the exact one for low-energy
excitations. The interaction energy contributed from the STLS response function
is also discussed.Comment: 5 figure
A novel intrinsic interface state controlled by atomic stacking sequence at interfaces of SiC/SiO
On the basis of ab-initio total-energy electronic-structure calculations, we
find that interface localized electron states at the SiC/SiO interface
emerge in the energy region between 0.3 eV below and 1.2 eV above the bulk
conduction-band minimum (CBM) of SiC, being sensitive to the sequence of atomic
bilayers in SiC near the interface. These new interface states unrecognized in
the past are due to the peculiar characteristics of the CBM states which are
distributed along the crystallographic channels. We also find that the electron
doping modifies the energetics among the different stacking structures.
Implication for performance of electron devices fabricated on different SiC
surfaces are discussed.Comment: 5 pages, 4 figure
Implementation of quantum imaginary-time evolution method on NISQ devices: Nonlocal approximation
The imaginary-time evolution method is widely known to be efficient for
obtaining the ground state in quantum many-body problems on a classical
computer. A recently proposed quantum imaginary-time evolution method (QITE)
faces problems of deep circuit depth and difficulty in the implementation on
noisy intermediate-scale quantum (NISQ) devices. In this study, a nonlocal
approximation is developed to tackle this difficulty. We found that by removing
the locality condition or local approximation (LA), which was imposed when the
imaginary-time evolution operator is converted to a unitary operator, the
quantum circuit depth is significantly reduced. We propose two-step
approximation methods based on a nonlocality condition: extended LA (eLA) and
nonlocal approximation (NLA). To confirm the validity of eLA and NLA, we apply
them to the max-cut problem of an unweighted 3-regular graph and a weighted
fully connected graph; we comparatively evaluate the performances of LA, eLA,
and NLA. The eLA and NLA methods require far fewer circuit depths than LA to
maintain the same level of computational accuracy. Further, we developed a
``compression'' method of the quantum circuit for the imaginary-time steps as a
method to further reduce the circuit depth in the QITE method. The eLA, NLA,
and the compression method introduced in this study allow us to reduce the
circuit depth and the accumulation of error caused by the gate operation
significantly and pave the way for implementing the QITE method on NISQ
devices.Comment: 9 pages, 3figure
Periodicity-free unfolding method of electronic energy spectra: Application to twisted bilayer graphene
We propose a novel periodicity-free unfolding method of the electronic energy
spectra. Our new method solves a serious problem that calculated electronic
band structure strongly depends on the choice of the simulation cell, i.e.,
primitive-cell or supercell. The present method projects the electronic states
onto the free-electron states, giving rise to the {\it plane-wave unfolded}
spectra. Using the method, the energy spectra can be calculated as a completely
independent quantity from the choice of the simulation cell. We have examined
the unfolded energy spectra in detail for three models and clarified the
validity of our method: One-dimensional interacting two chain model, monolayer
graphene, and twisted bilayer graphene. Furthermore, we have discussed that our
present method is directly related to the experimental ARPES (Angle-Resolved
Photo-Emission Spectroscopy) spectra.Comment: 10 pages, 5 figure
Comparison of Green's functions for transition metal atoms using self-energy functional theory and coupled-cluster singles and doubles (CCSD)
We demonstrate in the present study that self-consistent calculations based
on the self-energy functional theory (SFT) are possible for the electronic
structure of realistic systems in the context of quantum chemistry. We describe
the procedure of a self-consistent SFT calculation in detail and perform the
calculations for isolated transition metal atoms from V to Cu as a
preliminary study. We compare the one-particle Green's functions (GFs) obtained
in this way and those obtained from the coupled-cluster singles and doubles
(CCSD) method. Although the SFT calculation starts from the spin-unpolarized
Hartree--Fock (HF) state for each of the target systems, the self-consistency
loop correctly leads to degenerate spin-polarized ground states. We examine the
spectral functions in detail to find their commonalities and differences among
the atoms by paying attention to the characteristics of the two approaches. It
is demonstrated via the two approaches that calculations based on the density
functional theory (DFT) can fail in predicting the orbital energy spectra for
spherically symmetric systems. It is found that the two methods are quite
reliable and useful beyond DFT.Comment: 7 figure
Quasiparticle energy spectra of isolated atoms from coupled-cluster singles and doubles (CCSD): Comparison with exact CI calculations
In this study, we have calculated single-electron energy spectra via the
Green's function based on the coupled-cluster singles and doubles (GFCCSD)
method for isolated atoms from H to Ne. In order to check the accuracy of the
GFCCSD method, we compared the results with the exact ones calculated from the
full-configuration interaction (FCI). Consequently, we have found that the
GFCCSD method reproduces not only the correct quasiparticle peaks but also
satellite ones by comparing the exact spectra with the 6-31G basis set. It is
also found that open-shell atoms such as C atom exhibit Mott gaps at the Fermi
level, which the exact density-functional theory (DFT) fails to describe. The
GFCCSD successfully reproduces the Mott HOMO-LUMO (highest-occupied molecular
orbital and lowest-unoccupied molecular orbital) gaps even quantitatively. We
also discussed the origin of satellite peaks as shake-up effects by checking
the components of wave function of the satellite peaks. The GFCCSD is a novel
cutting edge to investigate the electronic states in detail.Comment: 9 pages, 4 figure
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