Chang-Youn Moon

Defects Responsible for the Hole Gas in Ge/Si Core-Shell Nanowires

The origin of the ballistic hole gas recently observed in Ge/Si core-shell nanowires has not been clearly resolved yet, although it is thought to be the result of the band offset at the radial interface. Here we perform spin-polarized density-functional calculations to investigate the defect levels of surface dangling bonds and Au impurities in the Si shell. Without any doping strategy, we find that Si dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one-component Si nanowires. The defect levels lie to within 10 meV from or below the valence band edge for nanowires with diameters larger than 33 A and the Ge fractions above 30%. As carriers are spatially separated from charge traps, scattering is greatly suppressed, leading to the ballistic conduction, in good agreement with experiments.

Low-velocity anisotropic Dirac fermions on the side surface of topological insulators

We report anisotropic Dirac-cone surface bands on a side-surface geometry of the topological insulator Bi

Formation of dopant-pair defects and doping efficiency in B- and P-doped silicon nanowires.

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Enhanced spin-density wave in LaFeSbO from first principles

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Switching the Charge State of Individual Surface Atoms at Si(111)-√3 × √3:B Surfaces

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First-principles Electronic Structure Calculations of Topological Insulators

revealed by first-principles density-functional calculations. We find that the electron velocity in the side-surface Dirac cone is anisotropically reduced from that in the (111)-surface Dirac cone, and the velocity is not in parallel with the wave vector {\bf k} except for {\bf k} in high-symmetry directions. The size of the electron spin depends on the direction of {\bf k} due to anisotropic variation of the noncollinearity of the electron state. Low-energy effective Hamiltonian is proposed for side-surface Dirac fermions, and its implications are presented including refractive transport phenomena occurring at the edges of tological insulators where different surfaces meet.

Energetics of Various Electrically Deactivating Defects in Heavily n‐type Si

First-principles calculations are performed to investigate the stability of dopant-related defects and the dependence of doping efficiency on wire diameter and orientation in hydrogen-passivated silicon nanowires doped with B and P dopants. As the diameter decreases below a critical value, it is energetically more favorable for donor atoms to form donor-pair defects, which consist of two donors separated at the nearest-neighbor distance. While donor-pair defects are unstable in bulk Si, the stability of these defects is greatly enhanced because of the confinement effect in nanostructures, which leads to the increase of band gap and thereby the shallow level of a substitutional donor. As donor-pair defects are electrically inactive defects, the doping efficiency is expected to be suppressed in small-diameter wires, regardless of the presence of surface or interface dangling-bond defects which were previously proposed to be the compensating defects. In the case of B dopants, the formation of pair defects is unfavorable against shallow acceptor levels, in contrast to n-type dopants, without affecting the doping efficiency.

Electronic and magnetic properties of single-wall carbon nanotubes filled with iron atoms

We predict atomic, electronic, and magnetic structures of a hypothetical compound LaOFeSb by first-principles density-functional calculations. It is shown that LaOFeSb prefers a stripe-type antiferromagnetic phase (i.e., spin density wave (SDW) phase) to the non-magnetic (NM) phase, with a larger Fe spin moment and greater SDW-NM energy difference than those of LaOFeAs. The SDW phase is found to favor the orthorhombic structure while the tetragonal structure is more stable in the NM phase. In the NM-phase LaOFeSb, the electronic bandwidth near the Fermi energy is reduced compared with LaOFeAs, indicating smaller orbital overlap between Fe d states and subsequently enhanced intra-atomic exchange coupling. The calculated Fermi surface in the NM phase consists of three hole and two electron sheets, and shows increased nesting between two hole and two electron sheets compared with LaOFeAs. Monotonous changes found in our calculated material properties of LaOFePn (Pn=P, As, and Sb), along with reported superconducting properties of doped LaOFeP and LaOFeAs, suggest that doped LaOFeSb may have a higher superconducting transition temperature.

Band-gap bowing coefficients in large size-mismatched II-VI alloys : first-principles calculations

We show that each surface atom of heavily boron-doped, (111)-oriented silicon with a √3 × √3 reconstruction has electrically switchable two charge states due to the strong electron-lattice coupling at this surface. The structural and electronic properties of the two charge states as well as their energetics are uncovered by employing scanning tunneling microscopy measurements and density functional theory calculations, which reveals that one of the two is a two-electron bound state or surface bipolaron. We also execute the single-atom bit operations on individual surface atoms by controlling their charge states while demonstrating implementation of the atomic scale memory at a silicon surface with an unprecedented recording density.

Mechanism for oxidative etching in carbon nanotubes

Recently found topological insulators (TIs) have been attracting much attention of scientists in various disciplines. TIs are characterized by insulating bulk states and conducting surface states which are robust against perturbations preserving time-reversal symmetry. First-principles calculational methods for electronic structures are based on the density functional theory (DFT) and they are very efficient and reliable in finding out candidates for topological insulators and predicting their physical properties. This article briefly reviews contributions and achievements of DFT calculations in the investigation of topological insulators. 들어가는 글