Search Results (119 total)

First-principles calculations show that the electronic structure of graphene on SiO₂ strongly depends on the surface polarity and interface geometry. Surface dangling bonds mediate the coupling to graphene and can induce hole or electron doping via charge transfer even in the absence of extrinsic impurities in substrate. In an interface geometry where graphene is weakly bonded to an O-polar surface, graphene is p doped, whereas n doping takes place on a Si-polar surface with active dangling bonds. We suggest that electron and hole doping domains observed on SiO₂ are related to different surface polarities.

We perform first-principles matrix Green’s function calculations to study the coherent charge tunneling through ultrathin SiO₂ layers in metal-oxide-semiconductor devices. The tunneling behavior is analyzed within the atomistic picture based on the overlap of Si-induced gap states in the oxide region. We find that, while interface roughness defects such as suboxide bonds and protruded O atoms only weakly affect the tunneling current, a network of oxygen vacancies composed of Si-Si bonds across the oxide layer drastically increases the gate leakage current due to the defect-assisted tunneling. We show that the formation of such percolation paths is energetically favorable in the nonequilibrium situation, and even the oxygen divacancy is enough to result in the dielectric breakdown for ultrathin oxide layers.

Based on perturbation theory and local-density-functional calculations, we study the effect of atomic-scale defects, whose potentials vary on the scale of interatomic distance, on the electronic structure of graphene in the region of low energies. If defects are identical, for example, vacancies at the same sublattice sites or the Stone-Wales defects with the same orientations, the degeneracy at the Dirac point of graphene is removed with an energy splitting proportional to λ for low disorder densities (λ), which is attributed to the breaking of the intrinsic symmetry of the honeycomb lattice by the presence of atomic-scale defects. However, the degeneracy at the Dirac point is nearly restored if physically equivalent disorders, which are generated by the symmetry operations of graphene, such as reflection and rotation, coexist with similar concentrations.

We perform a comparative study for the quantum transport of telescoping carbon nanotubes, where the (5,5) and (10,10) nanotubes are coaxially aligned, using first-principles local-density-functional and tight-binding calculations. In both calculations, the intertube conductance initially increases as the hybridized length in the contact region increases, and then decreases, exhibiting a maximum conductance. However, the calculated conductances from first principles are generally smaller than those from the single π-orbital tight-binding model. In the first-principles calculations, we obtain the maximum intertube conductance that does not exceed G₀  (=2e²∕h), while individual tubes have two conducting channels, giving the conductance of 2G₀. On the other hand, the single π-orbital tight-binding model gives the maximum conductance close to 2G₀, similar to previous calculations. Using a double-wall nanotube, we examine the effect of interwall interactions on conductance and find that the π^* states of the inner and outer tubes are strongly coupled in the tight-binding model, allowing for an extra conducting channel, while the π^* channel is closed in the first-principles calculations.

We perform first-principles density-functional calculations to study the chemical bonding effect of Ge atoms on the diffusion pathway and migration barrier of a B dopant in Si. The binding energy of a B-Ge pair is extremely small, thus, it is ruled out that the pairing of the B and Ge atoms immobilizes the B atom. When a Ge atom is located in the first neighborhood of a substitutional B, the B impurity is still likely to diffuse via an interstitialcy mechanism by forming a pair with a self-interstitial (I_s) without pair dissociation, similar to that previously suggested in pure Si. We find that the presence of the Ge atom increases the migration barrier by a sizable amount, which can affect the B diffusivity, while the formation energy of the stable I_s-B-Ge complex is little affected. The effect of the Ge atom is most significant in the first neighborhood of the B interstitial, thus, the Ge chemical bonding effect plays a role in the retardation of B diffusion observed in SiGe alloys.

We perform first-principles theoretical calculations to study the bonding and electronic characteristics of AlO₂ nanotube bundles. We find that the nanotubes are tightly linked by O-O peroxy or Al-O-Al bridge bonds, forming crystalline bundles, where tubes are arranged in a triangular or a square lattice in the two-dimensional plane perpendicular to their common axes. Intertube interactions mostly occur between the outer O shells of individual tubes, and the metallicity of bundles is strongly affected by intertube distance and doping. The bundles with only the peroxy bonds are semiconducting, while those having purely the bridge bonds or a mixture of two types of intertube bonds are metallic. A semiconductor-metal transition can occur by applying compressive strains perpendicular to the tube axes or by intercalating the Li ions, accompanied with the change of intertube bonds. We also examine the formation of bundles in the form of Al₂O₃, where tubes are connected by bridge-type bonds. We find that these bundles are energetically more favorable than the AlO₂ nanotube bundles and exhibit the semiconducting behavior.

We perform first-principles pseudopotential calculations to study the influence of Mn doping on the stability of two polytypes, wurtzite and zinc-blende, in GaN. In Mn δ-doped GaN and GaMnN alloys, we find similar critical concentrations of the Mn ions for stabilizing the zinc-blende phase against the wurtzite phase. Using a slab geometry of hexagonal lattices, we find that it is energetically unfavorable to form inversion domains with Mn exposure, in contrast to Mg doping. At the initial stage of epitaxial growth, a stacking fault that leads to the cubic bonds can be generated with the Mn exposure to the Ga-polar surface. However, the influence of the Mn δ-doped layer on the formation of the cubic phase is only effective for GaN layers deposited up to two monolayers. We find that the Mn ions are energetically more stable on the growth front than in the bulk, indicating that these ions act as a surfactant. Thus it is possible to grow cubic GaN if the Mn ions are periodically supplied or diffuse out from the Mn δ-doped layer to the growth front during the growth process.

Based on first-principles pseudopotential calculations, we investigated the electronic structure of various P-related defects in ZnO and the p-type doping efficiency for two forms of P dopant sources such as P₂O₅ and Zn₃P₂. As compared to N dopants, a substitutional P at an O site has a higher ionization energy of about 0.62 eV, which makes it difficult to achieve p-type ZnO. Under Zn-rich growth conditions, P_O acceptors are compensated by dominant donors such as P_Zn, leading to n-type conduction. Although a P_Zn-2V_Zn complex, which consists of a substitutional P at a Zn antisite and two Zn vacancies, acts as an acceptor, the formation of Zn vacancies is more probable on going to O-rich conditions for the dopant source using P₂O₅. On the other hand, when Zn₃P₂ is used as the P dopant source, the P_Zn-2V_Zn complex is energetically more favorable and becomes the dominant acceptor under O-rich growth conditions.

Based on first-principles theoretical calculations, we propose a tubular structure of aluminates, which exhibit metallic conduction and are energetically stable in the form of AlO₂, with fewer strain energies compared with MoS₂ nanotubes with similar diameters. The stability of AlO₂ nanotubes is also maintained with Li doping inside the tube cavity. For zigzag nanotubes with small diameters, more electron conduction occurs through the outer O shell with longer Al-O bonds, while the whole tube wall contributes to electron conduction for large diameter tubes or armchair tubes, which have similar inner and outer Al-O bond lengths. We suggest that conducting aluminate nanotubes can be promising materials for nanoscale electronic devices.

We study the trend of structural stability and magnetic moment for MnX (X=N, P, As, and Sb) binary compounds in the NiAs and zinc-blende structures through first-principles spin-density-functional calculations. The exchange splitting and magnetic moment are generally lower in the stable structure, which corresponds to the NiAs structure for MnP, MnAs, and MnSb whereas the zinc-blende structure for MnN. With the exception of MnN, we find the increasing trend of the stability, exchange splitting, and magnetic moment of the ferromanetic state with the anion size along the series MnP, MnAs, and MnSb. Since the N p level is much lower than other anion p levels, the Mn-N bond is more ionic, and thus MnN favors the zinc-blende structure with a lower coordination number. For MnN and MnP with small and light anions, the ground state is an antiferromagnetic state, while a ferromagnetic state is more favorable for MnAs and MnSb, which have larger equilibrium volumes and thereby reduced p-d and d-d couplings for the majority spin channel. Due to the volume and different bonding effects, MnAs and MnSb show a large exchange splitting for the d states and exhibit a nearly half-metallic behavior. The large volume increases the anion p-type character near the Fermi level, and thus the ferromagnetic state is stabilized through double exchange interactions.

We study the electronic and magnetic properties of single-wall carbon nanotubes filled with Fe nanowires through local-spin-density-functional calculations. We find that the magnetic moments of Fe-filled carbon nanotubes for the ferromagnetic state are greatly enhanced due to the reduced coordination number of the Fe atoms on the nanowire surface, compared with bulk Fe. The increase of magnetic moments is more effective for thin nanowires, where the Fe atoms interact very weakly with the nanotube and thus their magnetic properties inside the tube are similar to those for the free-standing nanowires. For thick Fe nanowires, undercoordinated Fe atoms interact more strongly with the carbon nanotube, and thereby the magnetic moments are reduced. The analysis of the densities of states near the Fermi level shows that electron conduction mostly occurs along the Fe wires protected from oxidation by carbon coating. Our calculations suggest that for applications to spin transport devices, it is desirable to form thin Fe wires inside single-wall nanotubes with large diameters.

We perform first-principles pseudopotential calculations to investigate the dielectric response for MgB₂, and calculate the Coulomb repulsion parameter μ^* using the full dielectric matrix approach. The calculated value for μ^* is 0.118 when the Debye energy ω_D of MgB₂ is used for the phononic cutoff energy ω_c, while μ^*=0.151 for ω_c=0.5 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 μ^* is smaller than those obtained from first principles for Nb and Li. We also examine the anisotropy of the Coulomb repulsion μ(k,k^′) on the Fermi surface, and find that the repulsions between the σ and π bands are much smaller than those within the σ and π 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 C₆₀ 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 C₆₀ 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 behaviors 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.

We investigate the phase transformation of HfO₂ 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, HfO₂ 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) C₁₂₀ nanocapsule from two C₆₀ 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.

We investigate the energy spectra of clean incommensurate double-walled carbon nanotubes, and find that the overall spectral properties are described by the critical statistics similar to that known in the 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 multiwalled nanotubes can be either diffusive, ballistic, or intermediate between them, depending on the position of the Fermi energy.

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 π^* states of the inner and outer tubes are weakly coupled, the transmission through the π^* channel is severely suppressed, while the π 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.