arXiv subjects

Atsushi Oshiyama

Publications and source records attributed to Atsushi Oshiyama.

At least 19 recordsLinked to original sources

arXiv2025

Multistability of interstitial magnesium and its carrier recombined migration in gallium nitride

We present density-functional-theory calculations which provide a microscopic picture of the recombination-enhanced migration of interstitial Mg in GaN. We determine stable structures and migration pathways with accurate HSE approximation to the exchange-correlation energy, and also computed recombination rates using the obtained energy spectrum and wavefunctions. It is found that the migration between the most stable octahedral sites (Mg$_{\textrm{O}}$) via newly found interstitial complex structure shows the lowest migration energy in which one or two electrons are captured during the migration, that the most stable charge state of 2+ changes to 1+ or neutral, and that by this recombination of carriers the migration barrier is significantly reduced. Starting from Mg$_{\textrm{O}}^{2+}$, Mg captures an electron becoming the 1+ charge state and overcomes the barrier of 1.65 eV, much reduced from 2.23 eV in case of the migration with the 2+ charge state kept. Moreover, further electron capture is realized accompanied by substantial structural relaxation, thus Mg becoming neutral. Detailed HSE calculations for this second capture show that the migration barrier is 1.55 eV, thus clarifying the important role of the carrier recombination for Mg migration in GaN. These findings are corroborated by the present quantitative calculations of recombination rates based on electronic Hamiltonian constructed from our DFT-obtained energy spectrum. The timescale of the recombination is clarified to be in or under the timescale of the migration with typical electron density and the enhancement is expected to be significant.

cond-mat.mtrl-sci↗

arXiv2024

First-Principles Study of Recombination-Enhanced Migration of an Interstitial Magnesium in Gallium Nitride

The stable and metastable configurations of interstitial Mg in GaN and its migration energy barriers are studied from first-principles calculations. In addition to the conventional octahedral (O, global energy minimum) and tetrahedral (T, metastable) interstitial sites, we discover two new metastable interstitial complexes with formation energy lower than or close to that of T configuration but higher than O. Except for Mg at O site which only has +2 charge state, all other configurations also permit charge states +1 or 0. The minimum migration energy barrier for Mg$^{++}$ between O sites is found to be 1.95 eV. We further find that, when Fermi energy is close to the conduction band, the migration between O sites via metastable configurations occur through a recombination-enhanced mechanism in which the charge state changes from +2 at O site to 0 at metastable sites by consecutive capture of two electrons during the migration. This process greatly reduces the migration energy barrier to as low as 1.47 eV. This value is consistent with experiments, and we also discuss the role of intrinsic defects in the migration of Mg.

cond-mat.mtrl-sci↗

arXiv2021

Exploration of a new reconstructed structure on GaN(0001) surface by Bayesian optimization

GaN(0001) surfaces with Ga- and H-adsorbates are fundamental stages for epitaxial growth of semiconductor thin films. We explore stable surface structures with nanometer scale by the density-functional calculations combined with Bayesian optimization, and succeed to reach a single structure with satisfactorily low mixing enthalpy among hundreds of thousand possible candidate structures. We find that the obtained structure is free from any postulated high symmetry previously introduced by human intuition, satisfies electron counting rule locally, and shows new adsorbate arrangement, reflecting characteristics of nitride semiconductors.

cond-mat.mtrl-sci↗

arXiv2021

Order-$N$ orbital-free density-functional calculations with machine learning of functional derivatives for semiconductors and metals

Orbital-free density functional theory (OFDFT) offers a challenging way of electronic-structure calculations scaled as $\mathcal{O}(N)$ computation for system size $N$. We here develop a scheme of the OFDFT calculations based on the accurate and transferrable kinetic-energy density functional (KEDF) which is created in an unprecedented way using appropriately constructed neural network (NN). We show that our OFDFT scheme reproduces the electron density obtained in the state-of-the-art DFT calculations and then provides accurate structural properties of 24 different systems, ranging from atoms, molecules, metals, semiconductors and an ionic material. The accuracy and the transferability of our KEDF is achieved by our NN training system in which the kinetic-energy functional derivative (KEFD) at each real-space grid point is used. The choice of the KEFD as a set of training data is essentially important, because first it appears directly in the Euler equation which one should solve and second, its learning assists in reproducing the physical quantity expressed as the first derivative of the total energy. More generally, the present development of KEDF $T[ρ]$ is in the line of systematic expansion in terms of the functional derivatives $δ^{\ell_1} T/δρ^{\ell_1}$ through progressive increase of $\ell_1$. The present numerical success demonstrates the validity of this approach. The computational cost of the present OFDFT scheme indeed shows the $\mathcal{O}(N)$ scaling, as is evidenced by the computations of the semiconductor SiC used in power electronics.

physics.comp-ph↗

arXiv2020

Screw dislocation that converts p-type GaN to n-type: Microscopic study on the Mg condensation and the leakage current in p-n diodes

Recent experiments suggest that Mg condensation at threading dislocations induce current leakage, leading to degradation of GaN-based power devices. To study this issue, we perform first-principles total-energy electronic-structure calculations for various Mg and dislocation complexes. We find that threading screw dislocations (TSDs) indeed attract Mg impurities, and that the electronic levels in the energy gap induced by the dislocations are elevated towards the conduction band as the Mg impurity approaches the dislocation line, indicating that the Mg-TSD complex is a donor. The formation of the Mg-TSD complex is unequivocally evidenced by our atom probe tomography in which Mg condensation and diffusion through [0001] screw dislocations is observed in p-n diodes. These findings provide a novel picture that the Mg being a p-type impurity in GaN diffuses toward the TSD and then locally forms an n-type region. The appearance of this region along the TSD results the reverse leakage current.

cond-mat.mtrl-sci↗

arXiv2018

Structural stability and energy levels of carbon-related defects in amorphous SiO$_2$ 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$_2$ using the melt-quench technique in molecular dynamics. Considering the position dependence of the O chemical potential near and far from the SiC/SiO$_2$ interface, we determine the most abundant forms of carbon-related defects: Far from the interface, the CO$_2$ or CO in the internal space in SiO$_2$ 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.

cond-mat.mtrl-sci↗

arXiv2017

Unfolding energy spectra of multi-periodicity materials

We propose a new unfolding scheme to analyze energy spectra of complex large-scale systems which are inherently of multi-periodicity. Considering twisted bilayer graphene (tBLG) as an example, we first show that the conventional unfolding scheme in the past using a single primitive-cell representation causes serious problems in analyses of the energy spectra. We then introduce our multi-space representation scheme in the unfolding method and clarify its validity for tBLG. Velocity renormalization of Dirac electrons in tBLG is elucidated in the present unfolding scheme.

cond-mat.mtrl-sci↗

arXiv2017

A novel intrinsic interface state controlled by atomic stacking sequence at interfaces of SiC/SiO$_2$

On the basis of ab-initio total-energy electronic-structure calculations, we find that interface localized electron states at the SiC/SiO$_2$ 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.

cond-mat.mtrl-sci↗

arXiv2017

Microscopic Mechanism of Carbon Annihilation upon SiC Oxidation due to Phosphorous Treatment: Density-Functional Calculations Combined with Ion Mass Spectrometry

We report first-principles static and dynamic calculations that clarify the microscopic mechanism of carbon annihilation due to phosphorous treatment upon oxidation of silicon carbide (SiC). We identify the most stable form of the phosphorus (P) in the oxide as the four-fold coordinated with the dangling PO unit and find that the unit attracts carbon ejected from the interface, thus operating as a carbon absorber. This finding provides a microscopic reasoning for the first time for the promotion of the oxidation reaction on one hand and the annihilation of the C-related defects at the interface on the other. Secondary ion mass spectrometry measurements are also performed and the obtained carbon profile corroborates the theoretical finding above.

cond-mat.mtrl-sci↗

arXiv2017

Comprehensive study on band-gap variations in $sp^3$-bonded semiconductors: roles of electronic states floating in internal space

We have performed electronic structure calculations to explore the band-gap dependence on polytypes for $sp^3$-bonded semiconducting materials, i.e., SiC, AlN, BN, GaN, Si, and diamond. In this comprehensive study, we have found that band-gap variation depending on polytypes is common in $sp^3$-bonded semiconductors; SiC, AlN, and BN exhibit smallest band gaps in $3C$ structure, whereas diamond does in $2H$ structure. We have also clarified that the microscopic mechanism of the band-gap variations is attributed to peculiar electron states $floating$ in internal channel space at the conduction-band minimum (CBM), and that internal channel length and the electro-static potential in channel affect the energy level of CBM.

cond-mat.mtrl-sci↗

arXiv2016

Mechanisms of initial oxidation of 4H-SiC (0001) and (000$\bar{1}$) surfaces unraveled by first-principles calculations

We have performed electronic state calculations to clarify the initial stage of the oxidation of the Si- and C-faces in 4H-SiC based on the density-functional theory. We investigate how each Si and C atomic site is oxidized on C- and Si-face, and explore most probable reaction pathways, corresponding energy barriers, and possible defects generated during the oxidation. We have found that carbon annihilation process is different between on Si- and on C-face, and this difference causes different defects in interface; In C-face case, (1), carbon atoms are dissociated directly from the substrate as CO molecules. (2), after CO dissociation, 3-fold coordinated oxygen atoms (called Y-lid) are observed at the interface. (3), high density of C-dangling bonds can remain at the interface. In Si-face case, (1), C atoms inevitably form carbon nano clusters (composed of a few atoms) in interface to reduce the number of dangling bonds there. Moreover, we have found that the carbon nano clusters are composed of not only single but also double chemical bonds. (2), We have observed that CO molecules are dissociated from the carbon nano clusters in MD simulations. Furthermore, we have investigated whether H$_2$ and NO molecules react with the defects found in this study.

cond-mat.mtrl-sci↗

arXiv2016

Microscopic mechanisms of initial formation process of graphene on SiC(0001) surfaces

We report total-energy calculations based on the density-functional theory that clarify microscopic mechanisms of initial stage of graphene formation on the SiC(0001) surface. We explore favorable reactions for desorption of either Si or C atoms from the stepped surface by determining the desorption and the subsequent migration pathways and calculating the corresponding energy barriers for the first time. We find that the energy barrier for the desorption of an Si atom at the step edge and the subsequent migration toward stable terrace sites are lower than that of a C atom by 0.75 eV, indicative of the selective desorption of Si from the SiC surface. We also find that the subsequent Si desorption is an exothermic reaction. This exothermicity comes from the energy gain due to the bond formation of C atoms being left near the step edges. This is certainly a seed of graphene flakes.

cond-mat.mtrl-sci↗

arXiv2016

First-principles calculations that clarify energetics and reactions of oxygen adsorption and carbon desorption on 4H-SiC ($11\bar20$) surface

We report static and dynamic first-principles calculations that provide atomistic pictures of the initial stage of the oxidation processes occurring at the ($11\bar20$) surface of 4H-SiC. Our results unveil reaction pathways and their associated free-energy barriers for the adsorption of oxygen and the desorption of carbon atoms. We find that oxygen adsorption shows structural multi-stability and that the surface-bridge sites are the most stable and crucial sites for subsequent oxidation. We find that an approaching O$_2$ molecule is adsorbed, then dissociated and finally migrates toward these surface-bridge sites with a free-energy barrier of 0.7 eV at the ($11\bar20$) surface. We also find that a CO molecule is desorbed from the metastable oxidized structure upon the overcoming of a free-energy barrier of 2.4$\sim$2.6 eV, thus constituting one of the annihilation process of C during the oxidation. The results of the CO molecule desorption on the ($11\bar20$) surface are compared with the ($000\bar1$) surface. A catalytic effect of dangling bonds at the surface, causing a drastic reduction of the CO desorption energy, is found on the ($000\bar1$) surface and the microscopic picture of the effect is ascribed to an electron transfer from the Si to C dangling bonds. The intrinsic ($11\bar20$) surface does not show this catalytic effect, and this is because the surface consists of an equal amount of Si and C dangling bonds and the electron transfer occurs before the desorption.

cond-mat.mtrl-sci↗

arXiv2016

Atomic Force Microscope manipulation of Ag atom on the Si(111) surface

We present first-principles total-energy electronic-structure calculations that provide the microscopic mechanism of the Ag atom diffusion between the half unit cells (HUCs) on the Si(111)-(7x7) surface with and without the tip of the atomic force microscope (AFM). We find that, without the presence of the AFM tip, the diffusions between the two HUCs are almost symmetric with the energy barrier of about 1 eV in the both directions. The diffusion is a two-step process with an intermediate metastable configuration in which the Ag atom is at the boundary of the HUCs. With the presence of the tip, we find that the reaction pathways are essentially the same, but the energy barrier in one direction is substantially reduced to be 0.2 - 0.4 eV by the tip, while that of the diffusion in the reverse direction remains larger than 1 eV. The Si tip reduces the energy barrier more than the Pt tip due to the flexibility of the tip apex structure. In addition to the reduction of the barrier, the tip traps the diffusing adatom preventing the diffusion in the reverse direction. Also we find that the shape of the tip apex structure is important for the trapping ability of the adatom. When the tip apex structure is blunt, the adatom interacts with the tip atom other than the tip apex atom. The bond formation between the AFM tip atom and the surface adatom is essential for the atom manipulation using the AFM tip. Our results show that the atom manipulation is possible with both the metallic and semiconducting AFM tips.

cond-mat.mtrl-sci↗

arXiv2016

Band-unfolding approach to Moirè-induced band-gap opening and Fermi-level-velocity reduction in twisted bilayer graphene

We report on the energy spectrum of electrons in twisted bilayer graphene (tBLG) obtained by the band-unfolding method in the tight-binding model. We find the band-gap opening at particular points in the reciprocal space, that elucidates the drastic reduction of the Fermi-level velocity with the tiny twisted angles in tBLGs. We find that Moirè pattern caused by the twist of the two graphene layers generates interactions among Dirac cones, otherwise absent, and the resultant cone-cone interactions peculiar to each point in the reciprocal space causes the energy gap and thus reduced the Fermi-level velocity.

cond-mat.mtrl-sci↗

arXiv2016

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.

cond-mat.mtrl-sci↗

arXiv2016

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$_{60}$ molecule and the GGA and HSE calculations on the singly charged monovacancy in crystalline Si.

cond-mat.mtrl-sci↗

arXiv2015

Structural Evolution and Optoelectronic Applications of Multilayer Silicene

Despite the recent progress on two-dimensional multilayer materials (2DMM) with weak interlayer interactions, the investigation on 2DMM with strong interlayer interactions is far from its sufficiency. Here we report on first-principles calculations that clarify the structural evolution and optoelectronic properties of such a 2DMM, multilayer silicene. With our newly developed global optimization algorithm, we discover the existence of rich dynamically stable multilayer silicene phases, the stability of which is closely related to the extent of sp3 hybridization that can be evaluated by the average bonds and effective bond angles. The stable Si(111) surface structures are obtained when the silicene thickness gets up to four, showing the critical thickness for the structural evolution. We also find that the multilayer silicene with pi-bonded surfaces present outstanding optoelectronic properties for the solar cells and optical fiber communications due to the incorporation of sp2-type bonds in the sp3-type bonds dominated system. This study is helpful to complete the picture of structure and related property evolution of 2DMM with strong interlayer interactions.

cond-mat.mes-hall↗