Ab Initio-based Approach Research Articles

An ab initio-based approach for the formation of pyramidal inversion domain boundaries in highly Mg-doped GaN

The formation of pyramidal inversion domain (PID) boundaries in highly Mg-doped GaN is investigated on the basis of ab initio calculations. Results for the absolute interface energies of PID boundaries reveal that their upper (0001) and side {113} facets hosting Mg atoms are stable owing to the electron counting rule. Furthermore, with these absolute interface energies, the formation energies for various PID boundaries of varying sizes and concentrations were estimated. Energy comparisons with those of substitutional Mg demonstrate that the formation of large PID boundaries is preferred when numerous Mg atoms are incorporated in the GaN matrix. The results suggest that the formation of PID boundaries is crucial for Mg doping and the resulting hole concentrations.

Modeling carbon coverage on polar GaN surfaces during MOVPE

Carbon coverage on polar GaN surfaces during MOVPE were calculated by a theoretical model which incorporates configurational entropy. Generally, surface reconstruction with the lowest energy was explored by an ab initio-based approach. On the other hand, appearance probability of the other surfaces can be discussed by the model. Therefore, coverage of minor elements on the surfaces can be analyzed. In this research, influence of source gas ratio, total pressure, and carrier gas ratio of MOVPE conditions on the carbon coverage on the surfaces was revealed theoretically. Furthermore, orientation dependence of carbon coverage and its incorporation mechanism were discussed.

Enhancement of the predictive power of molecular dynamics simulations for the determination of self-diffusion coefficient and viscosity demonstrated for propane

For the development and optimization of classical effective force fields (FFs) commonly used in molecular dynamics (MD) simulations, often experimental data are required. An alternative fully-predictive approach is the use of simplified pair-specific, ab initio-based FFs (AI-FFs) derived from quantum calculations in the limit of zero density. This approach, which could successfully be employed in our recent work for methane (CH4) and carbon dioxide (CO2), has not been investigated for relatively large systems including propane (C3H8). In the present study, C3H8 was selected as model system to evaluate the predictive power of MD simulations with respect to the determination of the self-diffusion coefficient and viscosity. For this aim, a semi-rigid version of our new simplified all-atom AI-FF was applied over a broad density range from the superheated vapor to the gas state and supercritical region up to the compressed liquid state. To elucidate the effects of the AI-FF on the calculated dynamical properties of C3H8, corresponding MD simulations representing the first investigations for the studied thermodynamic states were also performed using commonly employed, effective literature FFs in the form of TraPPE and OPLS. With the exception of the compressed liquid state, it was found that our ab initio-based approach improves the prediction results for the self-diffusion coefficient and viscosity at the studied thermodynamic states corresponding to pressures between (0.1 and 5) MPa and temperatures between (293.15 and 403.15) K compared to the use of effective FFs. This could be demonstrated by comparison with the ab initio-based calculations at the limit of zero density and the scarce experimental data. It could be figured out that the combination of an all-atom and a rigid or semi-rigid representation in our ab initio-based FFs for C3H8 as well as CH4 and CO2 suits well for a reliable simulation of dynamical properties over a broad density range.

Toward a Microscopic Understanding of the Magnetization Behavior of a Multimolecular Single Crystal of Radical-Bridged [DyIII4] Cubane Units: A Joint Ab Initio, Micro-Superconducting Quantum Interference Device, and Electron Paramagnetic Resonance Study

The field-dependent magnetization and magnetization blocking in single-crystalline samples of radical-bridged [DyIII4] cubane units are studied by ab initio calculations, micro-superconducting quantum interference device, and electron paramagnetic resonance. Because of the canted spin structure, a transverse method was applied to detect ferromagnetic and antiferromagnetic projections, for which remarkably different hysteresis magnetization loops were observed and investigated. Given 18 complexes in a unit cell of the crystal, a special ab initio-based approach has been developed to describe the magnetism in this material. The application of this ab initio methodology reveals a remarkable correlation between the evolution of the calculated magnetizations of each of the 18 molecules in the unit cell and the experimentally measured single-crystal hysteresis loop structure. This affords a level of insight into the microscopic magnetic properties that is unprecedented for a spin system of such complexity and h...

Ab initio-based approach to novel behavior in semiconductor hetero-epitaxial growth

Novel behavior in semiconductor hetero-epitaxial growth such as InAs/GaAs is systematically investigated using ab initio-based approach incorporating temperature T and beam equivalent pressure p. The calculated In adsorption-desorption boundary curve elucidates that In atoms can adsorb on the InAs(111)A-(2×2) with In-vacancy wetting layer (WL) surface with simultaneous As adsorption similarly to homo-epitaxial growth. On the InAs(001)-(2×4)α2 WL surface, however, In atoms hardly adsorb on the (001) WL in contrast with In adsorption on homo-epitaxial surface without strain. These results suggest that strain accumulated in the WL is not significant for the hetero-epitaxial growth on the InAs(111)A but strongly affects the growth process on the InAs(001) at the initial growth stage. It is also found that the InAs(001)-(2×3) WL surface fully covered by As dimers is unstable while the (n×3) surfaces (n=4, 6, and 8) with (n−1) dimers and one missing dimer is more stable at the conventional molecular beam epitaxial growth conditions. It should be noted that In atoms are incorporated through In-As dimer formation with As desorption below InAs coverage θ∼0.9 monolayer (ML) on the (n×3) surfaces. This suggests that the growth dose not proceed at θ>0.9 ML without strain relaxation on the InAs/GaAs(001).

Simulating the Radio-Frequency Dielectric Response of Relaxor Ferroelectrics: Combination of Coarse-Grained Hamiltonians and Kinetic MonteCarlo Simulations.

The radio-frequency dielectric response of the lead-free Ba(Zr_{0.5}Ti_{0.5})O_{3} relaxor ferroelectric is simulated using a coarse-grained Hamiltonian. This concept, taken from real-space renormalization group theories, allows us to depict the collective behavior of correlated local modes gathered in blocks. Free-energy barriers for their thermally activated collective hopping are deduced from this abinitio-based approach, and used as input data for kinetic MonteCarlo simulations. The resulting numerical scheme allows us to simulate the dielectric response for external field frequencies ranging from kHz up to a few tens of MHz for the first time and to demonstrate, e.g., that local (electric or elastic) random fields lead to the dielectric relaxation in the radio-frequency range that has been observed in relaxors.

Theoretical approach to surface reconstruction of InN(0001) during raised-pressure metalorganic vapor-phase epitaxy

We investigated the surface reconstruction of InN(0001) during raised-pressure metalorganic vapor-phase epitaxy using an ab initio-based approach. We observed that the reconstructed structure changes from In-rich surfaces such as In bilayer and monolayer surfaces to an ideal surface with increasing growth temperature. In addition, we investigated the effects of surface reconstruction on the growth process using a newly improved thermodynamic analysis method. Although no barrier is present in the growth reaction when the In-rich surfaces appear, the results suggest that the surface phase acts as a barrier in the growth reaction when the ideal surface appears. Furthermore, we discuss the growth conditions that enable high-temperature growth with a smooth reaction path.

<i>Ab Initio</i>-Based Approach to Structural Change in InAs (001)-(2×3) Wetting Layer Surfaces during MBE Growth<i><sup> </sup></i>

<i>Ab Initio</i>-Based Approach to Structural Change in InAs (001)-(2×3) Wetting Layer Surfaces during MBE Growth<i><sup> </sup></i>

Surface Stability and Growth Kinetics of Compound Semiconductors: An Ab Initio-Based Approach.

We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed.

Ab initio-based approach to incorporation of N atoms on GaAs(001) surfaces

The incorporation of N atoms on GaAs(001)-(2×4)α2 and -(3×3) surfaces under the growth conditions is theoretically investigated on the basis of surface phase diagrams which are obtained by comparing the adsorption energy given by ab initio calculations with the gas-phase chemical potentials. The calculated surface phase diagrams demonstrate that a single N–As dimer is formed on GaAs(001)-(2×4) and -(3×3) surfaces. Furthermore, two N atoms of surface are found to be substituted for As atoms located at the third layer of the surface. The analysis of stable structures reveals that the stability originates from the competition between energy profit in Ga–N bond formation and energy deficit in strain accumulation due to N located at the third layer. These results suggest that the bond energy as well as lattice strain are crucial for N incorporation processes on GaAs(001) substrate.

Ab initio-based approach to initial incorporation of Bi on |GaAs(001)-c(4×4)α surface

The initial incorporation processes of bismuth on GaAs(001)-c(4×4) surface are investigated by using our ab initio-based approach taking chemical potentials in the gas phase into account. The calculated surface phase diagrams obtained by comparing the adsorption energy with the gas-phase chemical potential demonstrate that the form of incorporated Bi depends on the growth conditions. The Ga–Bi dimer easily adsorbs on the missing dimer region of the c(4×4)α surface where as the Bi–Bi dimer adsorption occurs after the Ga–As dimer desorption on the surface with a Ga–Bi dimer. These results imply that the adsorption of Bi–Bi dimer accompanied with Ga–As dimer desorption is crucial for the growth process of GaAsBi on GaAs(001) substrates.

Ab initio-based approach to novel behavior of InAs wetting layer surface grown on GaAs(001)

The novel behavior of the InAs(001) wetting layer(WL) surfaces grown on GaAs(001)−c(4×4)α are systematically investigated by our ab initio-based approach and Monte Carlo (MC) simulations under the growth conditions. The ab initio-based approach incorporates the chemical potentials of In atom and As4 molecule in the gas phase as functions of temperature T and beam equivalent pressure p. The calculated adsorption–desorption boundary curve of In elucidates that In adsorption does not occur on the (2×4)α2, (1×3), and (2×3) WLs but is allowed on the fully relaxed (FR) surface at equilibrium. These results suggest that In adsorption proceeds InAs growth on the initial GaAs(001) and the final InAs(001) FR surfaces, while the novel growth process should be considered on the InAs(001) WL surfaces. The MC simulation results suggest that In adatoms do not reside in the lattice sites on the (1×3)/(2×3) to quickly desorb from the surfaces under the growth conditions, whereas In adatoms can adsorb and migrate during a certain period on the (2×4)α2. On the basis of these results, the novel behavior of InAs(001) WL surfaces is found in In adsorption with As desorption from the (2×3) WL surfaces during InAs growth on GaAs(001).

Ab initio-based approach to elemental growth process on the InAs wetting layer grown on GaAs substrate

The surface reconstructions and migration behavior on InAs(001) and InAs(111)A wetting layer on GaAs substrate are systematically investigated by our ab initio-based approach and Monte Carlo simulations. The ab initio-based approach incorporates the chemical potentials of In atom and As4 molecule in the gas phase as functions of temperature T and beam equivalent pressure pAs4. The calculated surface phase diagrams imply that the respective stable surface structures of InAs(001) and InAs(111)A are the (2×4)α2 and the In-vacancy surfaces at conventional growth conditions at T∼700–750K and pAs4∼10−7–10−6Torr. On these surfaces, adsorption–desorption boundary for In atom appears below the growth temperatures. Therefore, In adatoms tend to desorb from the InAs(001) and the InAs(111)A surfaces at equilibrium. Kinetic behavior of In adatoms obtained by the Monte Carlo simulations reveal that life time τ and diffusion length L of In adatoms are small such as τ=1.59×10−3s and L=618nm on the InAs(001) and τ=4.35×10−14s and L=0.204nm on the InAs(111)A. On the basis of these results, the growth process on the InAs(111)A is discussed in terms of self-surfactant effect where simultaneous As adsorption makes In atom adsorb on the surface.

Ab initio-based approach to elemental nitridation process of α-Al2O3

The elemental nitridation process of α-Al2O3 surface is theoretically investigated by our ab initio-based approach incorporating the pregrowth conditions such as temperature and pressure. The calculated results for adsorption diagram imply that N atoms cannot adsorb on the α-Al2O3(0001) surface at all temperatures beyond 240K supplying N2 and N source gases. This suggests that the nitridation process cannot be interpreted only by N itself but should involve another contribution. To this end, we investigate stable surface structures as functions of N and O chemical potentials. The calculated surface diagram reveals that N atoms are easily incorporated on the α-Al2O3(0001) surface under pregrowth conditions such that O atoms tend to desorb from the surface. Moreover, the N adatoms are found to be easily substituted for O atoms located beneath the surface to form the AlN layer. Consequently, O desorption is crucial for nitridation process of α-Al2O3. This is consistent with some experimental findings reported previously.

Ab initio-based approach to reconstruction, adsorption and incorporation on GaN surfaces

Reconstruction, adsorption and incorporation on various GaN surfaces are systematically investigated using an ab initio-based approach that predicts the surface phase diagram as functions of temperature and beam-equivalent pressure (BEP). The calculated results for GaN surface reconstructions with polar (0 0 0 1), nonpolar (1 1 −2 0), semipolar (1 −1 0 1) and semipolar (1 1 −2 2) orientations imply that reconstructions on GaN surfaces with Ga adlayers generally appear on the polar and the semipolar surfaces, while the stable ideal surface without Ga adsorption is found on the nonpolar GaN(1 1 −2 0) surface because it satisfies the electron counting rule. The hydrogen adsorption on GaN(0 0 0 1) and GaN(1 1 −2 0) realizes several surface structures forming N–H and Ga–NH2 bonds on their surfaces that depend on temperature and Ga BEP during metal-organic vapor-phase epitaxy (MOVPE). In contrast, the stable structures due to hydrogen adsorption on the semipolar GaN(1 −1 0 1) and GaN(1 1 −2 2) surfaces are not varied over the wide range of temperature and Ga BEP. This implies that the hydrogen adsorbed stable structures are expected to emerge on the semipolar surfaces during MOVPE regardless of the growth conditions. Furthermore, we clarify that Mg incorporation on GaN(1 −1 0 1) surfaces is enhanced by hydrogen adsorption consistent with experimental findings.

Theoretical Investigation of Effect of Side Facets on Adsorption–Desorption Behaviors of In and P Atoms at Top Layers in InP Nanowires

We investigate the adsorption and desorption behaviors of In and P atoms on the (111)A top layer of InP nanowires by an ab initio-based approach. Our calculated results reveal that In adatoms occupy the lattice sites of the wurtzite structure as well as the zinc blende structure in the central area of the (111)A top layer. On the other hand, the wurtzite structure is found to be feasible when In adatoms are located near the side facets. The wurtzite structure is preferentially formed for nanowires with small diameters, when most of the top layer consists of side facets. The difference in adsorption behaviors between the surfaces at the central and near the side facet on the top layer of nanowires is one the of factors determining the crystal structure of InP nanowires.

Ab initio-based approach to adsorption–desorption behavior on the InAs(1 1 1)A heteroepitaxially grown on GaAs substrate

The surface reconstructions and adsorption–desorption behavior on InAs(111)A heteroepitaxially grown on GaAs(111) substrate are systematically investigated using our ab initio-based approach incorporating the chemical potentials of In atom and As molecules in the gas phase as functions of temperature and beam equivalent pressure. This approach is also applied to qualitatively clarify the dependence of the surface-related properties on the film thickness of InAs considering various models, such as the InAs fully relaxed and coherently grown without/with GaAs interface. The calculated results imply that the most stable surface structure of the InAs(111)A is the In-vacancy surface at high temperatures including conventional growth conditions, whereas the As-trimer surface appears at low temperatures. In addition to these surface structures, As-adatom structure is newly found as a stable phase only in one atomic double-layer of InAs coherently grown on GaAs. This is because the charge redistribution due to the As adsorption strengthens the interatomic bonds with the surface In atom to stabilize the As-adatom surface. Distinctive features in the In adsorption are shown depending on the film thickness with respect to the computational models. Furthermore, the In adsorption on the In-vacancy surface does not occur without simultaneous As adsorption under conventional growth conditions. This reveals that the self-surfactant effect is crucial for the growth processes of InAs similarly to the GaAs growth.

Ab initio-based diffusion theory and tracer diffusion in Ni–Cr and Ni–Fe alloys

In this paper, ab initio modeling is used to predict diffusion relevant thermodynamic and kinetic information for dilute Ni–Cr and Ni–Fe alloys. The modeling results are then used to determine the phenomenological coefficient matrices and the tracer diffusion coefficients for both vacancy and interstitial mediated diffusion. In addition to predicting diffusion coefficients, this ab initio-based approach provides information typically inaccessible to experiments, including the different contributions to diffusion (e.g., electronic excitation effects), the species dependence of interstitial diffusion, and the deviations from Arrhenius-type relations, which are often used to describe and extrapolate experimental diffusion data. It is found that: (1) Cr is the fastest diffusing species in Ni by both vacancy and interstitial diffusion, followed by Fe and then Ni. The enhanced diffusivity of Cr is primarily due to differences in migration barriers and binding energies, not pre-exponential factors. (2) Fe and Cr solutes in Ni have weak interactions with vacancies but Cr solutes bind strongly to interstitial defects. (3) Cr exhibits non-Arrhenius behavior in both vacancy and interstitial mediated diffusion. (4) Temperature dependent electronic contributions have a significant impact on the diffusion in some cases. (5) The vacancy diffusion mechanism in Ni–Cr changes as a function of temperature resulting in vacancy–solute drag below 460 K.

Modeling of Dislocation Interaction with Solutes, Nano-Precipitates and Interfaces: A Multiscale Challenge

We review here several multiscale methods that we have developed to determine dislocation properties and interactions in metals. The review includes: (1) dislocation core properties in fcc and bcc metals; (2) the effect of solutes or nanoprecipitates on the mobility of a screw dislocation in bcc metals; (3) the interaction between dislocations and precipitates in intermetallic compounds; and (4) the transmission of dislocations through coherent and incoherent interfaces. In the concurrent quantum mechanical (QM) and molecular mechanical (MM) coupling approach, the quantum mechanical treatment is spatially confined to a small region, surrounded by a larger classical atomistic region. This approach is particularly useful for systems where quantum mechanical interactions in a small region, such as lattice defects or chemical impurities, can affect the macroscopic properties of a material. We discuss how the coupling across the different scales can be accomplished efficiently and accurately. We have applied this method to study the core structure and mobility of an edge dislocation in Al and of a screw dislocation in Ta, which are prototypical fcc and bcc metals. We find that the local environment of W solutes in Ta has a dramatic effect both on the dislocation mobility and slip paths. Isolated W solutes enhance the dislocation mobility, W nanoclusters of triangular shape pin the dislocation, while those of hexagonal shape result in spontaneous dislocation glide. The first sequential multiscale approach is a hybrid ab initio-based approach of Suzuki’s atomic-row (AR) model, which allows the study of the dislocation core of a screw dislocation in bcc metals. The second hybrid approach, based on an extension of the Peierls-Nabarro model to study the dislocation-interface and the dislocation-precipitate interactions, integrates the atomistic nature from ab initio calculations of the generalized stacking fault energy surface (GSFS) into the parametric dislocation dynamics method. The ab initio-based calculations reveal that Cu nano-clusters in -Fe dramatically alter the core structure of a screw dislocation from non-polarized in pure Fe to polarized, in agreement with experiments. In contrast, Cr clusters do not change the core polarization and increase the Peierls stress, thus hardening Fe. The hybrid method with four different interaction models was applied to study the interaction of a superdislocation with a spherical -precipitate embedded in the � -matrix of a nickel-based superalloy. The dislocation core structure was found to play an important role in determining the critical resolved shear stress. Based on these simulations, analytical equations for the precipitate strengthening are derived. For the Cu/Ni interface, the dislocation is found to dissociate into partials in both Cu and Ni, and the dislocation core is squeezed near the interface facilitating the spreading process, and leaving an interfacial ledge. It is shown that the strength of the bimaterial can be greatly enhanced by the spreading of the glide dislocation, and also increased by the pre-existence of misfit dislocations.

Ab initio-based approach to the reconstruction on InAs(111) A wetting layer grown on GaAs substrate

The surface reconstructions on InAs(111) A wetting layer grown on GaAs substrate are investigated by our ab initio-based approach incorporating the chemical potentials of In atom and As molecules in the vapor phase as functions of temperature and beam equivalent pressure. The calculated results imply that the most stable surface structure of InAs with/without lattice constraint from the substrate is the In-vacancy surface under conventional growth conditions. The In-vacancy surface is dramatically stabilized on the wetting layer, since the atoms around the In-vacancy are easily displaced to effectively lower the strain energy due to the lattice constraint from the GaAs substrate. Distinctive feature between InAs(111) A surfaces with and without lattice constraint is found in the stable adsorption sites. In adatoms favor the In-vacancy site on the InAs without lattice constraint in contrast to the interstitial sites on the InAs wetting layer. These results suggest that the surface structure and adsorption-desorption behavior on the wetting layer are crucial for investigating the growth processes of nanostructures such as quantum dots and stacking fault tetrahedrons.