220 research outputs found
Multi-step atomic reaction enhanced by an atomic force microscope probe on Si(111) and Ge(111) surfaces
We present first-principles total-energy electronic-structure calculations
that provide the microscopic mechanism of the adatom interchange reaction on
the Sn- and Pb-covered Ge(111)-(2x8) and the Sb-covered Si(111)-(7x7) surfaces
with and without the tip of the atomic force microscope (AFM). We find that,
without the presence of the AFM tip on the Ge surface, the adatom interchange
occurs through the migration of the adatom, the spontaneous formation of the
dimer structures of the two adatoms, the dimer-dimer structural transitions
that induce the exchange of the positions of the two adatoms, and then the
backward migration of the adatom. We also find that the dimer structure is
unfeasible at room temperature on the Si surface and the adatom interchange are
hereby unlikely. With the presence of the tip, we find that the reaction
pathways are essentially the same for the Ge surface but that the energy
barriers of the migration and the exchange processes are substantially reduced
by the AFM tip. We further find that the AFM tip induces the spontaneous
formation of the dimer structure even on the Si surface, hereby opening a
channel of the interchange of the adatoms. Our calculations show that the bond
formation between the AFM tip atom and the surface adatom is essential for the
atom manipulation using the AFM tip.Comment: 9 pages, 7 figure
Structural stability and energy-gap modulation through atomic protrusion in freestanding bilayer silicene
We report on first-principles total-energy and phonon calculations that
clarify structural stability and electronic properties of freestanding bilayer
silicene. By extensive structural exploration, we reach all the stable
structures reported before and find four new dynamically stable structures,
including the structure with the largest cohesive energy. We find that atomic
protrusion from the layer is the principal relaxation pattern which stabilizes
bilayer silicene and determines the lateral periodicity. The hybrid-functional
calculation shows that the most stable bilayer silicene is a semiconductor with
the energy gap of 1.3 eV
New Identification of Metallic Phases of In Atomic layers on Si(111) Surfaces
We report first-principles calculations that clarify atomic structures and
coverage of the metallic phases of In overlayers on Si (111) surfaces.
Calculated energy bands and scanning tunneling microscopy images along with the
obtained energetics of various phases reveal that the two metallic phases with
the periodicity observed experimentally are single
and double In overlayers, as opposed to prevailing assignments
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
Structural Tristability and Deep Dirac States in Bilayer Silicene on Ag(111) Surfaces
We report on total-energy electronic-structure calculations in the
density-functional theory performed for both monolayer and bilayer silicene on
Ag(111) surfaces. The rt3 x rt3 structure observed experimentally and argued to
be the monolayer silicene in the past [Chen et al., Phys. Rev. Lett. 110,
085504 (2013)] is identified as the bilayer silicene on the Ag(111) surface.
The identification is based on our accurate density-functional calculations in
which three approximations, the local density approximation, the
generalized-gradient approximation, and the van-der-Waals-density-functional
approximation, to the exchange-correlation energy have been carefully examined.
We find that the structural tristability exists for the rt3 x rt3 bilayer
silicene. The calculated energy barriers among the three stable structures are
in the range of 7 - 9 meV per Si atom, indicating possible flip-flop motions
among the three. We have found that the flip-flop motion between the two of the
three structures produces the honeycomb structure in the STM images, whereas
the motion among the three does the 1 x 1 structure. We have found that the
electron states which effectively follow Dirac equation in the freestanding
silicene couple with the substrate Ag orbitals due to the bond formation, and
shift downwards deep in the valence bands. This feature is common to all the
stable or metastable silicene layer on the Ag(111) substrate.Comment: 7 pages, 4 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
Crossover between Silicene and Ultra-Thin Si Atomic Layers on Ag(111) Surfaces
We report on total-energy electronic structure calculations in the
density-functional theory performed for the ultra-thin atomic layers of Si on
Ag(111) surfaces. We find several distinct stable silicene structures:
, , with the
thickness of Si increasing from monolayer to quad-layer. The structural
bistability and tristability of the multilayer silicene structures on Ag
surfaces are obtained, where the calculated transition barriers infer the
occurrence of the flip-flop motion at low temperature. The calculated STM
images agree well with the experimental observations. We also find the stable
existence of -bonded chain and dimer-adatom-stacking
fault Si(111)-surface structures on Ag(111), which clearly shows the crossover
of silicene-silicon structures for the multilayer Si on Ag surfaces. We further
find the absence of the Dirac states for multilayer silicene on Ag(111) due to
the covalent interactions of silicene-Ag interface and Si-Si interlayer.
Instead, we find a new state near Fermi level composed of orbitals
locating on the surface layer of multilayer silicene,
which satisfies the hexagonal symmetry and exhibits the linear energy
dispersion. By examining the electronic properties of -bonded
chain structures, we find that the surface-related states of multilayer
Si structures are robust on Ag surfaces.Comment: 24 pages, 11 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
A precaution for the hybrid density functional calculation of open-shell systems
We show that a naive treatment of open-shell systems in hybrid density
functional calculations ignoring the spin dependence causes significant errors
due to a kind of self interaction that is not emerged in spin-dependent
calculations. As numerical examples, we compare the results of the LDA, GGA,
and PBE0 calculations on the ionization potential and electron affinity of
C molecule and the GGA and HSE calculations on the singly charged
monovacancy in crystalline Si
Spontaneous Appearance of Low-dimensional Magnetic Electron System on Semiconductor Nanostructures
We find that spin-polarized ground states emerge in nanofacets which are
self-organized on SiC (0001) surfaces. Our large-scale density-functional
calculations reveal that the nanofacet formed by bunching of single bilayer
steps generates peculiar carbon dangling bond states localized at but extended
along step edges. The flat-band characteristics of those C states cause either
ferromagnetic or anti-ferromagnetic chains on covalent semiconductors
- β¦