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Based on the first‐principles spin‐density functional calculations, we investigate the magnetic interactions between Mn ions in Mn‐doped GaN. We find that the ferromagnetic coupling has a short‐range nature, effective for Mn‐Mn distances up to about 7 A. For Mn concentrations of about 6%, the ferromagnetic solution is more stable than the antiferromagnetic state, while the ferromagnetic stability is weakened by electron doping. Using the calculated exchange coupling and the percolation approach, we estimate the Curie temperature lying above room temperature. Analyzing the Mn d levels, we suggest that the d‐d hybridization between Mn ions is the main reason for stabilizing the ferromagnetic state.

Based on first‐principles pseudopotential calculations, we study the energetics of various donor‐pair defects which have been proposed as electrically deactivating donors in highly n‐type Si. We find that a class of nearest‐neighbor donor pairs such as d1 and DP(1) is energetically most favorable. Among donor‐pair‐vacancy‐interstitial complexes, the DP(2)V‐I defect forms a metastable configuration, with very low energy barriers of 0.03–0.06 eV for transforming into d2, which behaves as an electrically active donor. Thus, the formation of DP(2)V‐I is more probable for samples grown at low temperatures, in good agreement with recent experiments. On the other hand, the DP(4)V‐I defect is unstable against the formation of d4, with no energy barriers.

We report a computational study of coupled magnetic quantum dots, which are the magnetic analogue of the conventional electrically coupled quantum dots. In order to study that problem, we calculated the conductance of the narrow 2DEG in which two magnetic quantum dots are located. The calculated conductance shows several double anti‐resonance‐dips, which are due to the formation of the molecular like states which are extended over those two magnetic quantum dots. This is also shown by the local density of states which is given explicitly by the form of images.

We perform first-principles pseudopotential calculations to investigate the dielectric response for ${\mathrm{MgB}}_{2}$, and calculate the Coulomb repulsion parameter ${\ensuremath{\mu}}^{*}$ using the full dielectric matrix approach. The calculated value for ${\ensuremath{\mu}}^{*}$ is 0.118 when the Debye energy ${\ensuremath{\omega}}_{D}$ of ${\mathrm{MgB}}_{2}$ is used for the phononic cutoff energy ${\ensuremath{\omega}}_{c}$, while ${\ensuremath{\mu}}^{*}=0.151$ for ${\ensuremath{\omega}}_{c}=0.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We find that the local-field effect is significant due to the covalent nature of chemical bonds, enhancing the dielectric screening and thereby reducing the Coulomb repulsion parameter. In addition, due to the small density of states at the Fermi level, the value of ${\ensuremath{\mu}}^{*}$ is smaller than those obtained from first principles for $\mathrm{Nb}$ and $\mathrm{Li}$. We also examine the anisotropy of the Coulomb repulsion $\ensuremath{\mu}(\mathbf{k},{\mathbf{k}}^{\ensuremath{'}})$ on the Fermi surface, and find that the repulsions between the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands are much smaller than those within the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ bands.

Based on first-principles calculations, we suggest a method for fabricating $p$-type ZnO with group-I elements such as Li and Na. With group-I dopants alone, substitutional acceptors are mostly self-compensated by interstitial donors. In ZnO codoped with H impurities, the formation of compensating interstitials is severely suppressed, and the acceptor solubility is greatly enhanced by forming H-acceptor complexes. The H atoms can be easily dissociated from these defect complexes at relatively low annealing temperatures, and thus low-resistivity $p$-type ZnO is achievable with dopants different from group-V elements.

We perform first-principles pseudopotential calculations to investigate the atomic structure and energetics of various defects consisting of B and P impurities and the effect of P on B diffusion in Si. In the equilibrium case, we find that a B-P pair is energetically most stable when P is positioned at a second-neighbor site of B. When excess Si interstitials are generated by implantation, B and P impurities tend to form I s -B-P complexes with self-interstitials (I s ), where P still prefers to a second-neighbor site of B, particularly for high donor concentrations. Using the nudged elastic band method, we examine the diffusion of B and its energy barrier in the presence of P. We find that the diffusion pathways of B are similar to those reported for the I s -B pair without P; however, the migration energy for B diffusion from the I s -B-P complex increases by about 0.2 eV. The increase of the activation energy and the formation of the B-P pair acting as a trap for B diffusion provide a clue for understanding the suppression of B diffusion observed in Si predoped heavily with donor impurities.

Based on first-principles spin-density functional calculations, we find that Co-doped ZnO energetically favors a spin-glass-like state due to antiferromagnetic interactions between transition metal atoms, while ferromagnetic ordering is stabilized by electron doping. We find a short range nature in both antiferromagnetic and ferromagnetic interactions, and suggest that a very high doping level of Co ions is required to achieve ferromagnetism, together with a sufficient supply of electron carriers. Our results explain experimental features such as the low reproducibility of ferromagnetic samples and the very low saturation magnetization per Co ion.

Using ab initio spin-density-functional calculations, we investigate the electronic and magnetic structures of a ${\mathrm{C}}_{60}$ fullerene during a structural transition to a nanotube segment by a series of Stone-Wales transformations. We find that partly opened intermediate cage structures may acquire a magnetic moment of several Bohr magnetons. Our results offer a possible explanation for the ferromagnetic behavior observed in polymerized ${\mathrm{C}}_{60}$ following exposure to high temperatures and pressures, and suggest the use of carbon nanostructures for magnetic measurements.

Based on first-principles density-functional calculations, we propose a class of nearest-neighbor donor pairs that are energetically favorable in highly n-type Si. These donor pairs comprise dopant atoms either fourfold coordinated at the nearest-neighbor distance or threefold coordinated through bond-breaking relaxations. For P and As dopants, the two defect states are very close in energy, less than 0.1 eV, while the threefold coordinated state is more stable by 0.24 eV for Sb dopants. The former state has a very deep donor level close to the valence band maximum, while the defect level lies deep inside the valence band for the latter. Thus, both the donor pairs are electrically inactive at very high doping levels, and they are suggested to be responsible for the observed saturation of carriers.

When monatomic contacts are stretched, their conductance behaves in qualitatively different ways depending on their constituent atomic elements. Under a single assumption of resonance formation, we show that various conductance behavior can be understood in a unified way in terms of the response of the resonance to stretching. This analysis clarifies the crucial roles played by the number of valence electrons, charge neutrality, and orbital shapes., 2 figures

We investigate the phase transformation of ${\mathrm{HfO}}_{2}$ under hydrostatic pressure through first-principles pseudopotential calculations within the local-density-functional approximation (LDA) and the generalized gradient approximation (GGA). We find that with increasing of pressure, ${\mathrm{HfO}}_{2}$ undergoes a series of structural transformations from monoclinic to orthorhombic I and then to orthorhombic II, consistent with experiments. The calculated transition pressures within the GGA are in good agreement with the measured values, while they are severely underestimated by the LDA. Analyzing the distribution of electron densities for the high-pressure phases, we find that the electron densities of the orthorhombic-II phase are more homogeneous than for the orthorhombic-I phase. Due to this distinct difference in the homogeneity of electron densities, the energy difference between the orthorhombic-I and orthorhombic-II phases is enhanced in the GGA; thus, the transition pressure between the two phases increases significantly.

Using ab initio local spin-density-functional formalism, we study the occurrence of spin polarization in quasi-one-dimensional heterostructured C/BN nanotubes. At the zigzag boundary connecting carbon and boron nitride segments of tubes, we find atomiclike states that acquire magnetization when partly filled. Whereas individual C/BN heterojunctions can be used to spin-polarize electrons during transport, periodic arrangements of heterojunctions in doped systems can lead to the formation of a one-dimensional itinerant ferromagnetic state.

Fullerene coalescence experimentally found in fullerene-embedded single-wall nanotubes under electron-beam irradiation or heat treatment is simulated by minimizing the classical action for many atom systems. The dynamical trajectory for forming a (5,5) C$_{120}$ nanocapsule from two C$_{60}$ fullerene molecules consists of thermal motions around potential basins and ten successive Stone-Wales-type bond rotations after the initial cage-opening process for which energy cost is about 8 eV. Dynamical paths for forming large-diameter nanocapsules with (10,0), (6,6), and (12,0) chiral indexes have more bond rotations than 25 with the transition barriers in a range of 10--12 eV., 4 pages, 2 figures, 1 supplementary movie at http://dielc.kaist.ac.kr/yonghyun/coal.mpeg. To be published in Physical Review Letters

We investigate the energy spectra of clean incommensurate double-walled carbon nanotubes, and find that the overall spectral properties are described by the so-called critical statistics of Anderson metal-insulator transition. In the energy spectra, there exist three different regimes characterized by Wigner-Dyson, Poisson, and semi-Poisson distributions. This feature implies that the electron transport in incommensurate multi-walled nanotubes can be either diffusive, ballistic, or intermediate between them, depending on the position of the Fermi energy., Comment: final version to appear in Phys. Rev. Lett

Based on first-principles theoretical calculations, we find a very stable nitrogen complex in oxynitrides, which consists of two N atoms at O sites and one O vacancy. This N complex is electrically inactive without bonding with hydrogen, removing the electrical activity of O vacancies, and the stability of this complex is greatly enhanced as going from pure oxide to oxynitride films. We suggest that charge traps involving a single N atom, such as a bridging N center, can be deactivated by reactions with O or NO interstitials, and resulting N interstitials are easily depleted into the interface, in good agreement with experiments.

We investigate the transport properties of telescoping carbon nanotubes (TCNT's), where one nanotube slides into the other tube with a larger diameter, through multiband tight-binding theoretical calculations. In armchair TCNT's which consist of armchair nanotubes, since the two ${\ensuremath{\pi}}^{*}$ states of the inner and outer tubes are weakly coupled, the transmission through the ${\ensuremath{\pi}}^{*}$ channel is severely suppressed, while the $\ensuremath{\pi}$ channel is almost transmitted with antiresonance dips. In zigzag TCNT's, rotational symmetries derive selection rules in the coupling of the inner and outer tube states with nonzero angular momentum, and the total transmission depends on composite nanotubes. We find that the (9,0)/(18,0) TCNT exhibits metallic conduction, with antiresonance dips, while the (18,0)/(27,0) TCNT is semiconducting near the Fermi level because the state mixing is forbidden by the selection rule for angular momentum. Although the incommensurate (9,0)/(10,0) TCNT has no selection rule, we find extremely low transmissions near the Fermi level.

Based on first-principles theoretical calculations, we find that on pristine single-wall carbon nanotubes (SWNT's), the dissociative adsorption of ${\mathrm{H}}_{2}$ molecules is severely suppressed due to very high energy barriers of about 3 eV, while H atoms have low-energy barriers less than 0.3 eV. On Li-doped SWNT's, the energy barriers for the dissociative adsorption of ${\mathrm{H}}_{2}$ are lowered by about 0.3--0.5 eV due to the charge transfer to the tube, as compared to the pristine tube. However, these energy barriers are still too high for ${\mathrm{H}}_{2}$ to be directly adsorbed, indicating that Li doping itself does not enhance significantly the H adsorption. On the other hand, LiH molecules are easily adsorbed and dissociated with no energy barrier.

We perform first-principles pseudopotential calculations to investigate the oxidation process by oxygen gas exposure in single-wall carbon nanotubes with a closed tip. Oxygen molecules can be initially adsorbed either on the tube cap or wall, keeping their molecular form, then, a successive transformation occurs into the stable geometry with broken C-C and O-O bonds. The broken-bond configuration of oxidated nanotubes is suggested to be a precursor for etching away the tube cap and wall, particularly, in nanotubes with small diameters, while the oxidative etching is suppressed on the wall of nanotubes with large diameters. Despite a charge transfer from the tube to ${\mathrm{O}}_{2}$ molecule, metallic nanotubes still exhibit the metallic conduction, in good agreement with experiments.

We investigate the electronic properties of monolayer and bilayer MoS2 on α-quartz substrate through first-principles density functional calculations. Due to the coupling of the MoS2 with the substrate, the valence band edge state at the Brillouin zone center tends to shift upward, reducing the indirect band gap of the MoS2, whereas the direct gap at the K valleys is less sensitive to substrate conditions. By taking into account the van der Waals interactions between the MoS2 and the substrate, we find that monolayer MoS2 exhibits a transition from direct-gap to indirect-gap semiconductor in the presence of surface O-dangling bonds. Moreover, a charge transfer occurs from MoS2 to SiO2, inducing p-type doping and lifting the Kramers degeneracy by breaking the time reversal symmetry. In bilayer MoS2, O-dangling bonds break the inversion symmetry by inducing dipole fields across the interface and thereby lower the energy band associated with the one layer relative to the other. Although the time reversal symmetry is also broken, its effect on the spin splitting is extremely small in the K valleys so that a strong coupling between the spin and valley degrees of freedom takes place, similar to that found in free-standing monolayer MoS2.

We report the results of action-derived molecular dynamics simulations for the migration and coalescence processes of monovacancies in graphene and carbon nanotubes with different chiralities. In carbon nanotubes, the migration pathways and barriers of a monovacancy depend on the tube chirality, while there is no preferential pathway in graphene due to the lattice symmetry and the absence of the curvature effect. The probable pathway changes from the axial to circumferential direction as the chirality varies from armchair to zigzag. The chirality dependence is attributed to the preferential orientation of the reconstructed bond formed around each vacancy site. It is energetically more favourable for two monovacancies to coalesce into a divacancy via alternative movements rather than simultaneous movements. The energy barriers for coalescence are generally determined by the migration barrier for the monovacancy, although there are some variations due to interactions between two diffusing vacancies. In graphene and armchair nanotubes, two monovacancies prefer to migrate along different zigzag atomic chains rather than a single atomic chain connecting these vacancies. On the other hand, in zigzag tubes, the energy barrier for coalescence increases significantly unless monovacancies lie on the same circumference.

A Friedel-sum-rule-aided first-principles conductance calculation is presented for a sodium monatomic wire suspended between two bulk electrodes with sharp tip geometry. The conductance (G) is found to oscillate with the number (L) of atoms in the wire. For odd L, G has a quantized value of G(0) (=2e(2)/h), which is robust to the variation in wire geometry, while G is smaller than G(0) for even L, sensitive to the wire geometry. This even-odd behavior results from the sharp electrode tip and charge neutrality; the former generates resonant states, and the latter pins one of the resonant states at the Fermi energy for odd L. (C) 2002 Elsevier Science B.V. All rights reserved.

We report low-field magnetoresistance measurements of a two-dimensional electron gas formed in a GaAs quantum well, in which half a monolayer of AlAs has been inserted into the center of the well. A large anisotropy is observed in both the mobility and the low field magnetoresistance in the orthogonal $[1\ifmmode\bar\else\textasciimacron\fi{}10]$ and $[110]$ directions. We describe a method of using the anisotropic low field magnetoresistance to calculate the magnitude of the effective potential of the AlAs submonolayer at the GaAs/AlGaAs heterointerface.

We analyze various potential environments such as electrodes, substrates, gate voltages, and flattening deformations in single-wall C nanotubes in terms of circumferential perturbations on nanotube surfaces. Considering the periodicity of perturbations, we derive selection rules in the subband mixing caused by perturbations. Uniform electric fields perpendicular to the tube axis induce band-gap modification such as opening and closure. Thus, locally applied transverse fields cause significant backscattering of the states near the threshold for transmission.

We investigate the electronic and transport properties of (10,10) carbon nanotubes that contain a finite-sized capped (5,5) tube (capsule) and a long chain of ${\mathrm{C}}_{60}$ fullerenes by using the tight-binding model. We find that the total transmissions through the outer tubes are very sensitive to the symmetry of the hybrid tubes. For the (10,10) tube where the (5,5) capsule is aligned so as to maintain mirror symmetries, the total transmission exhibits antiresonances with symmetric line shapes, while breaking mirror symmetries leads to asymmetric line shapes. For encapsulated fullerenes in the (10,10) tube, we find rich structures in transmission, such as antiresonances, resonances, and transmission gaps, which depend on mirror and rotational symmetries. This feature is attributed to the fact that the coupling of incident channels with the bound states of the inner structures is determined by the symmetry of the hybrid tubes.

We present a mechanism for the compensation of N acceptors in ZnO through real-space multigrid electronic structure calculations within the local-density-functional approximation. We find that at low N doping levels using a normal ${\mathrm{N}}_{2}$ source, O vacancies are the main compensating donors for N acceptors, while N acceptors are compensated via the formation of defect complexes with Zn antisites at high doping levels. When an active plasma ${\mathrm{N}}_{2}$ gas is used to increase the N solubility, N acceptors are still greatly compensated by ${\mathrm{N}}_{2}$ molecules at oxygen sites and N-acceptor--${\mathrm{N}}_{2}$ complexes, explaining the difficulty in achieving low-resistivity p-type ZnO.

We investigate the electron transport in two-dimensional electron systems, where spatially nonuniform magnetic fields are applied, using the noninteracting-electron approach and composite-fermion edge state theory. The nonuniform magnetic fields divide the conductor into three different regions, such that the upper and the lower regions are incompressible with different filling factors, which are controlled by different uniform magnetic fields, while the magnetic field varies monotonously in the middle region. In both cases of the integer and the fractional quantum Hall regimes, it is found that current-carrying magnetic edge states are formed in the middle region. We show that the current in the middle region is proportional to the filling-factor difference between the upper and the lower incompressible regions when the magnetic field in the middle region is not reversed. However, when the magnetic fields in the upper and the lower regions are antiparallel to each other, the current is not simply proportional to the filling-factor difference and depends on the nonuniform distribution of magnetic fields and electron-electron interactions that can give rise to incompressible strips in the middle region. We also study the composite-fermion edge states for fractional quantum-Hall systems with $\ensuremath{\nu}=p/(mp+1)$ under uniform magnetic fields, where p is a negative integer. For both cases of a constant and a diverging composite-fermion mass, we show that the net current carried by composite fermions in each compressible region is the same as that by electrons although the effective composite-fermion chemical potential decreases as the electron chemical potential increases.

Local mirror-symmetry-breaking deformations such as flattening strongly affect electron coherent transport in single-wall armchair carbon nanotubes. Such a local deformation gives rich structures in electron transmission, such as a transmission barrier, resonances, and antiresonances. When local deformations create barriers, a finite perfect tube sandwiched between two deformed regions behaves as a quantum dot. As gate voltage varies, this nanotube device exhibits periodic resonant peak pairs in transmission, each of which results from nearly degenerate bound states of beat type. This feature of resonances is attributed to the mixing of two channels $|\ensuremath{\pi}〉$ and $|{\ensuremath{\pi}}^{*}〉$ due to the breaking of mirror symmetries.

We study the transport properties of hybrid carbon nanotubes that contain a carbon nanotube capsule and a chain of fullerenes. We find that electron transmissions through the outer tubes are very sensitive to the alignment of inner shells. For a (5,5) capsule in the (10,10) tube, mirror symmetries in cross sections determine the lineshape of antiresonances. For a chain of C60 molecules inside the (10,10) tube, we find resonance peaks and barriers in transmission, depending on the existence of mirror and rotational symmetries. This feature is attributed to the fact that the coupling of incident channels with the bound states of inner shells depends on the symmetries.

We investigate the electron coherent transport in single-wall carbon nanotubes under local flattening deformations that break mirror symmetries. Such a local deformation is found to cause transmission barrier, resonances, and anti-resonances in electron transmission. We also find that a finite perfect tube which is sandwiched between two deformed tubes behaves as a quantum dot. This nanotube device exhibits periodic and nearly degenerate resonant peak pairs in transmission, as a gating voltage applied to the dot varies. This feature of resonances is attributed to the mixing of two channels |π〉 and |π*〉, which is derived by the breaking of mirror symmetries.

With the aid of the Friedel sum rule, we perform first-principles calculations of conductances through monatomic Na wires, taking into account the sharp tip geometry and discrete atomic structure of electrodes. We find that conductances (G) depend on the number (L) of atoms in the wires; G is G_0 (= 2e^2/h) for odd L, independent of the wire geometry, while G is generally smaller than $G_0$ and sensitive to the wire structure for even L. This even-odd behavior is attributed to the charge neutrality and resonant character due to the sharp tip structure. We suggest that similar even-odd behavior may appear in other monovalent atomic wires., Comment: 4 pages, 3 figures

On the basis of first-principles calculations, we present a structural model for the formation of H-induced (111) platelets in Si, which involves a structural transformation from a double-layer-H2(*) configuration of H2(*) aggregates into an H-saturated internal (111) surface structure. This reaction process preferably occurs at high H plasma treatment temperatures and subsequently generates H2 molecules in the platelet voids, consistent with experiments. Our model also reveals the important features observed in (111) platelets, such as high-resolution transmission electron microscopy images, step structures, lattice dilation lengths, and H vibrational frequencies.

New metastable behavior of deep levels is found in hydrogenated GaAs doped with Si. A deep level at 0.60 eV below the conduction‐band minimum (Ec) is generated during hydrogenation and shows metastable for the Ec − 0.42 eV trap. From the defect transformations observed in biased anneals, these defects are found to be metastable defects associated with hydrogen atoms. Especially, the 400 K biased‐anneal experiments indicate that an Ec−0.33 eV trap could be an electric field induced defect, transformed from other intrinsic defects. The Ec − 0.60 eV trap in hydrogenated GaAs could be a hydrogen complex associated with Ec − 0.42 eV trap and the hydrogen atom plays an important role in a metastability of deep level defects in GaAs.

The effects of hydrogen plasma exposure on the characteristics of an n‐channel metal‐oxide‐semiconductor field‐effect transitor are studied. The helium plasma gives almost no changes in maximum transonductance and subthreshold slope whereas the hydrogen plasma degrades maximum transconductance and increases subthreshold slope. These results indicate that the excess interface traps are generated by hydrogenation, which is also confirmed by quasi‐static capacitance‐voltage analysis.

We find an analogy between the single skyrmion state in the quantum Hall system and the BCS superconducting state and address that the quantum mechanical origin of the skyrmion is electronic pairing. The skyrmion phase is found to be unstable for magnetic fields above the critical field $B_{c}(T)$ at temperature $T$, which is well represented by the relation $B_c(T)/B_{c}(0) \approx {[1-(T/T_c)^3]}^{1/2}$., revtex, two figures, to appear in Phys. Rev. B (Rapid Communications)

We study the effect of weak localization on the transition temperatures of superconductors using time-reversed scattered state pairs, and find that the weak localization effect weakens electron-phonon interactions. With solving the BCS $T_{c}$ equation, the calculated values for $T_c$ are in good agreement with experimental data for various two- and three-dimensional disordered superconductors. We also find that the critical sheet resistance for the suppression of superconductivity in thin films does not satisfy the universal behavior but depends on sample, in good agreement with experiments. but depends on sample, in good agreement with experiments., 14 pages, Revtex, 5 ps figures

We use the {ital N}-point Pad{acute e} approximant to make the analytic continuation of the dynamic response function. This continuation scheme reproduces very efficiently the plasmon energy dispersion and the dynamic scattering cross section with double peaks for Al. For Si with covalent bondings, the energy-loss function exhibits the anomalous feature in the long-wavelength limit, which was also found in previous work. At large wave vectors, the local-field effect shifts the plasmon excitation peak to higher energy, however, this shift is compensated by the exchange-correlation effect. {copyright} {ital 1996 The American Physical Society.}

We investigate the effects of electron correlations on the ground state energy and the chemical potential of a droplet confined by a parabolic potential at high magnetic fields. We demonstrate the importance of correlations in estimating the transition field at which the first edge reconstruction of the maximum density droplet occurs in the spin polarized regime., 11 pages (revtex) 3 postscript figures are included at the end of the tex file. To appear in Phys. Rev. B

We perform self-consistent ab initio pseudopotential calculations to study the structural properties of Mg- and Zn-based binary compounds and the stability of MgSe-ZnSe ordered superlattices. For more ionic MgS and MgSe, the rocksalt structure is found to be more favorable over both the zinc-blende and wurtzite phases, while the zinc-blende structure is most stable for ZnS and ZnSe. Because of the imperfect d orbital screening, the Zn-based compounds have smaller lattice constants and larger bulk moduli. For both the bulk and epitaxial MgSe-ZnSe superlattices ordered in the CuAu-I, CuPt, and chalcopyrite structures, the lattice mismatch between binary constituents gives rise to high strain energy, resulting in lattice instability against phase segregation into their binary components at T=0. Similar lattice instabilities are also found for epitaxial common-anion ${\mathrm{Mg}}_{\mathit{x}}$${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$S superlattices ordered in the chalcopyrite or famatinite structure. However, we find that common-cation ${\mathrm{MgS}}_{\mathit{y}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ and ${\mathrm{ZnS}}_{\mathit{y}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$ superlattices grown on (001) GaAs and ordered in the chalcopyrite or famatinite structure are thermodynamically stable and exhibit the charge transfer from less ionic bonds to more ionic bonds, similar to the common-anion superlattices. Since the electronegativity of the S atom is higher than that for the Se atom, the charge transfer to the S atom compensates for the strain energy and stabilizes epitaxial superlattices.

We present a mechanism for the compensation of N acceptors in ZnSe through first-principles pseudopotential calculations. In Se-rich conditions, the formation of ${\mathrm{N}}_{2}$ molecules that are electrically inert neutralizes the acceptor activity, with the maximum acceptor density achievable with N dopants being about ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, in good agreement with experiments. Going to Zn-rich conditions, the hole density is increased by an order of magnitude, suggesting a promising low-resistance p-type doping; however, a self-compensation still occurs due to a [100]-split interstitial N-N complex occupying a Se site, which behaves as a double donor.

We examine the atomic and electronic structure of Na-adsorbed Si(100)-p(2\ifmmode\times\else\texttimes\fi{}2) surfaces for various Na coverages (FTHETA) through first-principles pseudopotential calculations. At FTHETA=1/4, we find that the 4\ifmmode\times\else\texttimes\fi{}1 structure with linear Na chains adsorbed on the valley bridge sites is energetically most stable, while substrate Si dimers are rearranged by buckling towards the Na chains. At FTHETA=1/2, the adsorption site of Na is the valley bridge site; however, the 2\ifmmode\times\else\texttimes\fi{}2 structure is found to be more stable than the 2\ifmmode\times\else\texttimes\fi{}1 structure. From the calculated formation energies, we suggest that the saturation coverage is one monolayer with the Na atoms occupying the pedestal and valley bridge sites, exhibiting a 2\ifmmode\times\else\texttimes\fi{}1 reconstruction. The coverage dependences of surface geometry and work function are discussed.

Using hybrid functional calculations, we study the diffusion and thermal stability of hydrogen in Mg-doped GaN. Compared with the generalized gradient approximation, we obtain a higher activation barrier for dissociating a Mg–H complex, which is attributed to the increase in the binding energy of Mg–H. Kinetic Monte Carlo simulations yield the annealing temperature of around 800 °C for activating Mg acceptors, close to the measured values. The results provide an insight to understanding the annealing effect such that the annealing temperature generally increases with the Mg–H concentration, and the retrapping of H is partly responsible for the low doping efficiencies at high Mg concentrations.

We study the bonding character, electronic structure, and defects of amorphous Si through first-principles molecular-dynamics simulations. We generate the disordered Si network from the quenching of incompletely melted samples and find more dangling bonds than floating bonds, while in previous liquid-quench simulations more floating-bond defects were generated. The structural parameters such as the pair-correlation and bond-angle distribution functions are in good agreement with experimental data. The electronic structures of the dangling and floating bonds in the amorphous network are examined by analyzing the angular momentum decomposition of the local density of states. The localized states in the gap are found to be mostly p-like at a threefold coordinated site, consistent with the picture that paramagnetic active defects in amorphous Si are due to dangling bonds.

We study the structural and electronic properties of various polytypes of SiC through self-consistent ab initio pseudopotential calculations. For the wurtzite (2H), 4H, and 6H structures, the equilibrium lattice constants and bulk moduli are very similar to those for the cubic structure. The energies calculated for the polytypes considered here are very close to within 4.3 meV/atom, which may explain the polytypism of SiC. The 4H structure is found to be lowest in energy because of the attractive interactions between the alternating cubic and hexagonal stacking layers, while the wurtzite structure is most unstable among the polytypes. We find the asymmetric charge distribution for a Si-C bond to be on the boundary separating the zinc-blende and wurtzite phases, which should be related to the polytypism of SiC. In the hexagonal polytypes, the M conduction-band energy increases, while that of the K point decreases as the hexagonal close packing becomes more prominent. Thus, the conduction-band minimum state located at the X point for cubic SiC changes to the M point, and then to the K point for the 2H structure. For the cubic structure, the density of states near the conduction-band edge increases slowly with energy, while it shows very rapidly increasing behavior for the 6H polytype because its conduction-band edge states are flattened due to the band folding and the energy-increasing behavior of the M state when the hexagonal-close-packing nature is enhanced.

We investigate the optical properties and the dielectric response of Al through self-consistent ab initio pseudopotential calculations. The frequency and wave-vector dependent dielectric function of Al is calculated for both the real and imaginary parts within the random-phase approximation. The accuracy of the calculations is tested using Kramers-Kr\"onig relations and sum rules. In the limit q\ensuremath{\rightarrow}0, we find major optical peaks at 0.5 and 1.5 eV, which originate from the electronic structure, in good agreement with experimental data. For the wave vectors along the [100] direction, the electron-energy-loss function is calculated to examine the plasmon mode. Considering the exchange-correlation and core polarization effects, we find good agreement of the calculated plasma frequencies with experiments. The anisotropy of the plasmon dispersion is also investigated.

We present an efficient scheme of diagonalizing large Hamiltonian matrices in a self-consistent manner. In the framework of the preconditioned conjugate gradient minimization of the energy functional, we replace the modified Jacobi relaxation for preconditioning and use for band-by-band minimization the restricted-block Davidson algorithm, in which only the previous wave functions and the relaxation vectors are included additionally for subspace diagonalization. Our scheme is found to be comparable with the preconditioned conjugate gradient method for both large ordered and disordered Si systems, while it is more rapidly converged for systems with transition-metal elements.

Based on density functional calculations, we propose a defect model that can explain flat band voltage shifts, especially in n+ poly-Si/HfO2 gate stacks. For two interface structures, with Si electrodes on top of crystalline and amorphous HfO2, we find the formation of O-vacancies at the interface, which exhibit weak Si–Si dimer bonds and low formation energies, very different from those in the oxide. Due to weak dimer bonds, charge trap levels lie near the Si conduction band edge, leading to the Fermi level pinning and flat band voltage shifts in n+ poly-Si gate electrodes.

We perform density functional calculations to investigate the magnetic coupling properties of Co-doped ZnO nanowires (NWs). The ferromagnetism of NWs is strongly affected by the position of the minority Co ta levels and their population that is controlled by additional electron doping. While the antiferromagnetic state is energetically more favorable than the ferromagnetic state in carrier-free NWs, electron doping greatly enhances the stability of ferromagnetism. Compared with bulk ZnO, the minority ta levels relative to the conduction band edge have a tendency to decrease with decreasing of the wire diameter, indicating that electron concentrations to achieve the ferromagnetism are much reduced. The short-range nature of the magnetic coupling between two Co ions suggests that sufficiently high doping levels of the Co ions are needed to yield ferromagnetic NWs.

We investigate the structural and dynamical properties of the DX center in Si-doped GaAs under pressure through self-consistent ab initio pseudopotential calculations. The negatively charged broken-bond configuration for the DX center is found to be the ground state against the shallow-donor level at pressures above 24 kbar, in good agreement with measured values. This configuration is also found to be more stable than a tetrahedrally symmetric breathing-mode distortion of the donor neighboring atoms, which induces a localized level

For isovalent impurities such as Be, Mg, Ca, Sr, and Cd and group-I element such as Na in ZnO, first-principles local-density-functional calculations show that the interstitial position of H depends on the type of impurities, either occupying a bond center (BC) site or an antibonding (AB) site adjacent to the impurity atom. The AB site is more favorable in the vicinity of Na, Ca, Sr, and Cd, while the stable position is the BC site in the case of Be. We find that both electronegativity and atomic size play a role in switching the H interstitial position between the BC and AB sites. Previous studies have suggested that two infrared lines observed at 3611 and 3326 cm−1 result from hydrogen atoms positioned at BC and AB sites, respectively. The results for the H bonding sites and defect concentrations suggest that Ca is the most probable impurity as the origin of the infrared line at 3326 cm−1. However, for impurities around which H is positioned at the AB site, the calculated local vibrational frequencies a...