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ABSTRACT: We present theoretical investigations that clarify elemental nitridation processes of corundum Al_{2}O_{3}(0001) and (11[over ¯]02) surfaces. The calculations within the density functional theory framework reveal that the structures with substitutional N atoms beneath the surface are stabilized under nitridation conditions. We also find that the desorption of O atoms at the topmost layer induces outward diffusion of O atoms as well as inward diffusion of N atoms, leading to the transformation into AlN films. The kinetic Monte Carlo simulations in conjunction with density functional theory results indeed observe a dependence of these chemical and structural changes on temperature and pressure.
Physical Review Letters 01/2013; 110(2):026101. · 7.37 Impact Factor
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ABSTRACT: Epitaixial relationship between AlN thin films and Si(001) substrates is systematically investigated using the empirical interatomic potential on the basis of the experimental findings for AlN on Si(001) grown by plasma-assisted molecular beam epitaxy and reactive magnetron sputtering. The calculated results for the structural stability of AlN on Si(001) reveal that (0001)-oriented 2H-AlN becomes stable even on 3C-Si(001) beyond the film thickness of six monolayers with epitaxial relationship of AlN〈01-10〉‖Si[110]. This is consistent with the experimental results found in plasma-assisted molecular beam epitaxy. We also estimate the relative appearance ratio among various rotational domains with AlN〈11-20〉‖Si[110] (Type A) and AlN〈01-10〉‖Si[100] (Type B) in addition to the AlN〈01-10〉‖Si[110] (Type C). The calculated appearance ratio such as 23.7% (Type A), 30.7% (Type B), and 14.8% (Type C) are favourably compared with respective experimental results such as 24.7%, 26.5%, and 22.8% obtained by reactive magnetron sputtering. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2011; 8(5):1569 - 1572.
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ABSTRACT: The band alignments of twin-plane superlattices in semiconductor nanowires are systematically investigated on the basis of density functional calculations. Our calculations demonstrate that for nanowires with small diameters the quantum-confinement effect is prominent within wurtzite structure regions and the energy gap in wurtzite-structured nanowires is remarkably larger than that including zinc-blende structure. This results in the straddling band alignment, in which both electrons and holes are confined in zinc-blende structure region. The analysis using a simple tight-binding methods also clarifies that the straddling band alignments can be realized when the diameters of nanowires are less than 4-8 nm, leading to full control of band alignments by varying the nanowire diameter. Our results provide the ability of band-alignment tuning and open new possibilities for band engineering.
Nano Letters 10/2010; 10(11):4614-8. · 13.20 Impact Factor
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ABSTRACT: In a search for promising ferromagnets exhibiting a giant magnetocrystalline anisotropy (MCA) change by an external electric field, the MCA in the Fe-Co(001) alloy monolayers is determined by means of full-potential linearized augmented plane-wave method. A large MCA change by the electric field is found to appear in the Fe0.75Co0.25 monolayer, and a reasonable origin from a band structural change in the minority-spin d states is obtained, where a position of Fermi level relative to the d band level is a key factor for designing the giant MCA change.
Journal of Physics Conference Series 02/2010; 200(3):032051.
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ABSTRACT: Controlling and designing quantum magnetic properties by an external electric field is a key challenge in modern magnetic physics. Here, from first principles, the effects of an external electric field on the magnetocrystalline anisotropy (MCA) in ferromagnetic transition-metal monolayers are demonstrated which show that the MCA in an Fe(001) monolayer [but not in Co(001) and Ni(001) monolayers] can be controlled by the electric field through a change in band structure, in which small components of the p orbitals near the Fermi level, which are coupled to the d states by the electric field, play a key role. This prediction obtained opens a way to control the MCA by the electric field and invites experiments.
Physical Review Letters 06/2009; 102(18):187201. · 7.37 Impact Factor
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ABSTRACT: The formation of p-type carriers in InN {0001} films by Mg doping is theoretically investigated by means of the highly precise thin film full-potential linearized augmented plane-wave method. The first-principles calculations simultaneously simulating both p-type and n-type carriers in the bulk and surface layers, respectively, demonstrate that the formation energies of a substitutional Mg atom in the surface region are lower than those in the bulk due to the compensation mechanism. The Mg is, however, stabilized in the bulk layers due to a large diffusion-barrier height, suggesting a possible mechanism for the stabilization of Mg in the bulk and the formation of p-type carriers.
Physical Review Letters 11/2008; 101(18):186801. · 7.37 Impact Factor
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ABSTRACT: The stabilization of the dipolar structure and its effects on carrier formation in wurtzite films are investigated through a case study of InN and ZnO {0001} films by means of the highly precise all-electron full-potential linearized augmented plane-wave method. The calculated total energies of both the wurtzite and graphitic structures demonstrate that the structural phase transition from the graphitic to wurtzite occurs when the thickness is beyond six (ten) bilayers in InN (ZnO). Our analysis of the calculated energies reveals that the phase transition is due to the competition between the bulk energy and the extra energy originating from the macroscopic electric field of the dipole structure. Further, an analysis of the electronic structure and charge densities in InN {0001} films reveals the screening role of the electric field. The screening effect causes a broadening of N 2p states in the band structure by approximately 1.5 eV and charge redistribution in each polarizable unit. For InN and ZnO {0001} films, we have also found by using pseudohydrogens to passivate the opposite surfaces that the screening has little effect on the surface states and on the work functions. In contrast, the calculated formation energies of defects in InN {0001} films (N vacancy and substitutional Mg) using both pristine and pseudohydrogen passivated surfaces show that the internal electric field of the dipolar structure in wurtzite films may affect the formation of carriers near the surface. Thus, these results provide some fundamental characteristics for the polar surfaces of wurtzite films.
Phys. Rev. B. 01/2008; 77(3).
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ABSTRACT: Electronic structures and half-metallicity at ferromagnetic∕antiferromagnetic (AFM) interfaces in zincblende transition-metal chalcogenides, CrSe/MnSe and CrTe/MnTe, are investigated by means of the first principles full-potential linearized augmented plane-wave method within the LDA+U, and the effect of correlation in the 3d states on the half-metallic interfaces is discussed. The uncompensated AFM interface with the antiparallel alignment of the Cr and Mn moments at the interfaces shows an excellent half-metallicity, where the correlation effect tends to manifest the half-metallic interfaces. This indicates that these interfaces offer a key ingredient as promising exchange bias candidates in having interfaces with 100% spin polarization at the Fermi level.
Journal of Applied Physics 01/2008; 103(7):07C901-07C901-3. · 2.17 Impact Factor
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ABSTRACT: Spin-spiral structures in a free-standing Fe(110) monolayer are determined by the first-principles film full-potential linearized augmented plane wave method with intra-atomic noncollinear magnetism. The results obtained predict that the spin-spiral structures are energetically favourable over the collinear ferromagnetic state. The interatomic exchange parameters, which are evaluated from the formation energy of the spin-spiral structures, indicate that a competition between the nearest-neighbour ferromagnetic interaction and the long-distant antiferromagnetic interactions leads to the stabilization of the spin-spiral structures. In addition, the spin-orbit coupling is found to play an important role in determining the magnetic ground state.
Journal of Physics Condensed Matter 09/2007; 19(36):365222. · 2.55 Impact Factor
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Toru Akiyama
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ABSTRACT: The hexagonal versus cubic structural preference in vertically grown silicon nanowires on Si(111) substrates is systemized by using first-principles pseudopotential calculations. The calculated formation energy for pristine and H-terminated silicon nanowires with both hexagonal and cubic stacking sequences demonstrates that the stability depends on hydrogen chemical potential and the hexagonal-type nanowires are energetically favorable over a wide range of hydrogen chemical potential. This preference offers a possible origin for little detection of the [111]-oriented silicon nanowires with small diameters, qualitatively consistent with experimental findings.
Phys. Rev. B. 07/2006; 74(3).
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Toru Akiyama
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ABSTRACT: The structural stability and electronic properties of InP nanowires (NWs) are investigated based on first-principles pseudopotential calculations. In contrast to the bulk phase, zinc-blende (ZB) NWs are found to be less favorable over wurtzite (WZ) NWs, in which the surface dangling bonds (DBs) on the NW facets play a crucial role to stabilize the WZ structure. Our analysis of the NW cohesive energy based on the number of DBs also suggests the bistability forming both ZB and WZ NWs around 120 Å diameter and the formation of rotational twin structures around 400 Å diameter being consistent with experiments. Furthermore, the stable WZ NWs are found to be semiconducting whose characteristics are dependent on the surface DBs as well as the NW size and shape. The estimated oscillator strength also indicates the possibility of efficient light emission originating from the direct gap and geometrically restricted excitonic effects.
Phys. Rev. B. 06/2006; 73(23).
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ABSTRACT: To investigate half-metallic exchange bias interfaces, magnetic structures at ferromagnetic (FM)/antiferromagnetic (AFM) interfaces in the zinc blende transition-metal chalcogenides, and with compensated and uncompensated AFM interfaces, were determined by the full-potential linearized augmented plane-wave method. With the uncompensated AFM interface, an antiparallel alignment of the Cr and Mn moments induces an excellent half-metallicity. More striking still, in the compensated AFM interface the Cr moments in the FM layer lie perpendicular to the Mn moments in the AFM layer but the Mn moments strongly cant to induce a net moment so as to retain the half-metallicity. These findings may offer a key ingredient for exchange biased spintronic devices with 100% spin polarization, having a unidirectional anisotropy to control and manipulate spins at the nanoscale.
Physical Review Letters 03/2006; 96(4):047206. · 7.37 Impact Factor
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ABSTRACT: The relative stability between wurtzite and zinc blende structures in
group III-V semiconductor nanowires is systematically investigated based
on an empirical potential, which incorporates electrostatic energy due
to valence-bond and ionic charges. The energy differences between
wurtzite and zinc blende structures of 12 compound nanowires with
diameter of 1-22 nm show that the wurtzite nanowires are stabilized for
small diameter. This structural trend is found to be due to the
contribution of two- and three-coordinated atoms on the nanowire facets
to the system energy. We also find that the critical diameters, where
the nanowires turn out be bistable forming both wurtzite and zinc blende
structures, exist at the diameter of 12-32 nm depending on the ionicity
of semiconductors. The bistability implies the synthesis of nanowires
exhibiting polytypes, and supports the experimental results in GaP,
GaAs, InP, and InAs nanowires.
Japanese Journal of Applied Physics 02/2006; 45:L275-L278. · 1.06 Impact Factor
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ABSTRACT: Magnetic domain wall structures in an Fe (110) monolayer are determined by the highly precise first principles full-potential linearized augmented plane-wave method including intra-atomic noncollinear magnetism. The self-consistent results demonstrate that the magnetization changes from one orientation to the opposite (180 degrees ) orientation within an 8 A width without any abrupt rotation. This narrow domain wall is found to arise from band effects. Our results are consistent with and support domain walls having a 6 A width recently observed in spin-polarized scanning tunneling microscopy experiments.
Physical Review Letters 08/2004; 93(5):057202. · 7.37 Impact Factor
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ABSTRACT: The initial growth processes on GaAs(111)B-(2×2) surfaces are investigated using our ab initio-based approach, in which desorption behavior of As atoms is described by comparing the calculated desorption energy obtained by total-energy electronic-structure calculations with the chemical potential estimated by quantum statistical mechanics. We find that the As-trimer desorption on the (2×2) surfaces with Ga adatoms occurs beyond 400–700 K while the desorption without Ga adatom does beyond 800–1000 K. The promotion of the As-trimer desorption triggered by Ga adatoms called “self-surfactant effect” is also found to be interpreted in terms of the band-energy stabilization.
Phys. Rev. B. 77(23).
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ABSTRACT: The stability of spin-spiral and domain wall structures in an Fe monolayer on a W(1 1 0) substrate is theoretically investigated. By analyzing the exchange parameters obtained from first principles total energy calculations, we find that a competition between the nearest-neighbor ferromagnetic and long-distant antiferromagnetic exchange interactions leads to a stabilization of the spin-spiral structures. When the strong magnetocrystalline anisotropy (MCA) arising from the Fe/W(1 1 0) interface is introduced, however, the formation of the spin-spiral structures is suppressed and the ground state appears to be the ferromagnetic state—as observed in experiments. In addition, the strong MCA is found to play a key role in determining the domain wall structures.
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ABSTRACT: Surface phase diagram of recently proposed GaAs(0 0 1)-(2 × 4)γ is systematically investigated by using our ab initio-based approach. We focus on the (4 × 7) domain consisting of c(4 × 4)-like and (2 × 4)-like regions to clarify surface dimer constituents as functions of temperature T and As (As2 and As4) pressure pAs by comparing chemical potentials of surface dimers in the vapor phase with that on the surface. The calculated results under As4 imply that Ga dimers in the c(4 × 4)-like region tend to become stable with increase of temperature and appear at the conventional growth condition such as T ∼ 800 K and pAs ∼ 10−6 Torr, while the (2 × 4)-like region favors As dimers. This is consistent with temperature dependence of change in surface dimer constituents on the c(4 × 4) and (2 × 4)β2 clarified in our previous study. Furthermore, the surface phase transition from the c(4 × 4) to (2 × 4)β2 via (2 × 4)γ is discussed on the basis of the phase diagram obtained in this study.
Applied Surface Science.
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ABSTRACT: Structures and electronic states of Mg incorporated into InN surfaces in various orientations including nonpolar (101̅ 0) and (112̅ 0) as well as polar (0001) and (0001̅ ) surfaces are systematically investigated by performing first-principles pseudopotential calculations. Employing a thermodynamic approach, the calculated surface energies demonstrate characteristic features in the stability of Mg-incorporated surfaces depending on the growth condition. The calculated density of states also predict that regardless of surface orientation, Mg acceptors at the surface are compensated by the extra electrons originating from the surface states of In layers in bare surfaces.
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ABSTRACT: The magnetocrystalline anisotropy (MCA) for an Fe monolayer on MgO substrate (Fe/MgO) and that sandwiched by MgO (MgO/Fe/MgO) is investigated by means of the first principles full-potential linearized augmented plane-wave method, and the effects of an external electric field on the MCA is discussed. In both systems, the MCA is found to be modified through a change in the d band structure around Fermi level by the external electric field.
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ABSTRACT: The atomic arrangements in zinc blende structured GaNxAs1−x thin films coherently grown on V-grooved substrates are theoretically investigated using empirical interatomic potentials and Monte Carlo simulation. The resultant atomic arrangements in GaNxAs1−x strongly depend on concentration x and substrate lattice parameter asub. Surface segregation of As or N is mainly found in GaNxAs1−x with large lattice mismatch to the substrate. On the other hand, the novel atomic arrangements such as layered segregation or ordered structure are found in GaNxAs1−x at the specific region such as (x, asub) = (0.5, 5.3), (0.3, 5.3), and (0.3, 5.1). This specific region corresponds to that with negative excess energy and with sufficient N and As atoms remaining in thin film layers even after their surface segregation. The formation of the novel atomic arrangements is discussed in terms of bond lengths in the surface layers. These results suggest that various novel atomic arrangements in alloy semiconductor thin films appear depending on x and asub which control degree of lattice constraint.
