Electron properties and surface effects of rare-earth systems

3 . Received: 21 March 1997/Accepted: 12 August 1997     

Electronic properties of the single-domain Li/Si(001) surface

Received: 14 October 1996/Accepted: 17 December 1996     

Spin excitations in Kondo insulator YbB12

Cluster based analysis show that the observable three-level YbB₁₂ spin excitations character can be reproduced in the framework of the asymmetric variant of periodic Anderson model with a singlet ground state and two electrons per site. For the macroscopic system the effective Hamiltonian with the direct f-f exchange is justified and the dynamic spin susceptibility of f-electrons is found. It is shown that the lowest spin excitation dispersion has minimum at the antiferromagnetic vector as observed in the experiment. The distinctive feature of analysis is the using of singlet and triplet basis operators.     

External Electric Field Effect on Hydrogenic Donor Impurity in Zinc-Blende InGaN Quantum Dot

The binding energy of a hydrogenic donor impurity in zinc-blende (ZB) InGaN quantum dot (QD) is calculated in the framework of effective-mass envelope-function theory using the plane wave basis. It is shown that the donor binding energy is highly dependent on the impurity position, QD size and the external electric field. The symmetry of the electron probability distribution is broken and the maximum of the donor binding energy is shifted from the centre of QD in the presence of the external electric field. The degenerating energy levels for symmetrical positions with respect to the centre of QD are split. The splitting increases with the increase of QD height while the splitting increases up to a maximum value and then decreases with the increase of QD radius.     

On the low-temperature thermal properties and low-energy Fermi excitations in CeNiSn

The thermal expansion of a single crystal of the intermetallic compound CeNiSn has been measured at low temperatures 0.3 K<T<12 K and in a magnetic field up to 8 T. A large anisotropy of the linear expansion is observed which is strongly influenced by the magnetic field. These data are interpreted within the theory explaining the origin of the quasigap in the heavy fermion spectrum of CeNiSn by the interplay between the heavy fermions and low-energy excitations in non-cubic Kondo lattices.     

Theory of adsorption: Ordered monolayers from Na to Cl on Si(001) and Ge(001)

First principles calculations of clean and adsorbate-covered surfaces of Si(001) and Ge(001) are reported. Chemical trends in the adsorption of ordered Na, K, Ge, As, Sb, S, Se and Cl overlayers are discussed. The calculations are based on the local-density approximation and employ non-local, norm-conserving pseudopotentials together with Gaussian orbital basis sets. The semi-infinite geometry of the substrate is properly taken into account by employing our scattering theoretical method. From total-energy minimization calculations we obtain optimal surface reconstructions which show asymmetric dimers for Si(001), Ge(001) and Ge:Si(001). For As:Si(001), Sb:Si(001) and Sb:Ge(001), we find symmetric adatom dimers in the equilibrium geometries. S or Se adlayers are found to be adsorbed in bridge positions forming a (1×1) unit cell with a geometry very close to the configuration of a terminated bulk lattice. Cl atoms adsorb on top of the dangling bonds of symmetric Si dimers residing in the first substrate-surface layer. Our calculations for Na:Si(001) and K:Si(001) confirm valley-bridge site adsorption for half monolayer coverage. For full monolayer alkali-metal coverage, adsorption in pedestal and valley-bridge positions is found to be energetically most favourable. The calculated optimal adsorption configurations are in excellent agreement with a whole body of recent experimental data on surface-structure determination. For these structural models, we obtain electronic surface band structures which agree very good with a wealth of data from angle-resolved photoemission spectroscopy investigations.     

Electronic structure and optical absorption spectra of CdSe covered with ZnSe and ZnS epilayers

Using the first-principles methods we compute the electronic structure and the absorption spectra for a wurtzite CdSe (0001) slab covered with zincblende ZnSe and ZnS epilayers. For each structure we compute the DOS and the imaginary part of the dielectric function. We find that the semiconductor passivation shifts the ‘near Fermi-level’ states of the bare CdSe slab down to lower energy levels. The migration suggests the decrease of surface effects and energy loss. We observe the substantial reduction of the abnormal peaks in the absorption spectra of the bare CdSe slab, which seems to be a consequence of the DOS migration. This is consistent with the experimental results that a proper passivation enhance the luminescence efficiency. We also study the case that the epilayer surface is terminated with PH₃ and find the PH₃ passivation also reduces the surface state to some extent.     

Ground state magnetic properties of ultrathin fcc Fe and Co films on Cu(001) surfaces

The oxidation characteristics of silicon implanted with a low dose of nitrogen (1–3×10¹⁵cm^–2) have been studied for dry oxidation conditions at 1020°C. The wafers were subjected to a pre-oxidation annealing. Complete inhibition of the oxide growth occurs in the initial stage of oxidation, while the oxidation rate for prolonged oxidation is identical to that for pure silicon. The oxidation resistance increases with the implantation dose. The resistance is attributed to the formation of a nitrogen-rich surface film during annealing. This layer, which consists of only a few monolayers, is presumably composed of oxynitride. The electrical characteristics of MOS capacitors formed on implanted wafers show that the interface state density is not significantly increased by the low-dose N implantation.     

Low-dimensional electron gas at semiconductor surfaces

In recent years, it has become possible to create well-ordered semiconductor surfaces with metallic surface states by using self-assembly of metal atoms. Since these states lie in the band gap of the semiconductor, they completely decouple from the substrate. In addition to two-dimensional structures it is possible to obtain arrays of one-dimensional atomic chains, which may be viewed as the ultimate nanowires. The dimensionality can be varied systematically by using vicinal surfaces with variable step spacing. Angle-resolved photoemission and scanning tunnelling spectroscopy reveal surprising features, such as a fractional band filling, nanoscale phase separation into doped and undoped chain segments, and a spin-splitting at a non-magnetic surface. Prospects for one-dimensional electron gas physics in atomic chains are discussed.     

The electric field effect on binding energy of hydrogenic impurity in zinc-blende GaN/AlxGa1−xN spherical quantum dot

Within the framework of effective mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende GaN/Al_xGa_1−xN spherical quantum dot (QD) is investigated using the plane wave basis. The results show that the binding energy is highly dependent on impurity position, QD size, Al content and external field. The binding energy is largest when the donor impurity is located at the centre of the QD and the binding energy of impurity is degenerate for symmetrical positions with respect to the centre of QD without the external electric field. The maximum of the donor binding energy is shifted from the centre of QD and the degenerating energy levels for symmetrical positions with respect to the centre of QD are split in the presence of the external electric field. The binding energy is more sensitive to the external electric field for the larger QD and lower Al content. In addition, the Stark shift of the binding energy is also calculated.     

Electronic structure and magnetism of FeGe2

The electronic band structure of FeGe₂ has been calculated using the self-consistent full potential non-orthogonal local orbital minimum basis scheme based on the density functional theory. In the band structure of FeSn₂, Fe 3d and Sn 5p states play important roles near the Fermi level. Our calculations show that large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.     

Electronic structure of nearly ferromagnetic compound HfZn2

The electronic structure of HfZn₂ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that HfZn₂ shows ferromagnetic instability. Large enhancement of the static susceptibility over its non-interacting value is found due to a peak in the density of states at the Fermi level.     

Quantization of electronic states on metal surfaces

An electron in front of a metal surface experiences an attractive force due to the induced image charge. Band gaps in the band structure can prevent a penetration into the metal along certain directions. The Coulomb-like potential supports bound states in front of the surface which correspond to a hydrogen atom in one dimension. These image states can be measured with high resolution by two-photon photoemission. The adsorption of metals modifies the states. If the electrons can penetrate into the metal, quantum-well states can develop corresponding to standing waves in the overlayer. Image states on small islands show the quantization effects due to the lateral localization. The spectroscopy of image states by two-photon photoemission permits the investigation of growth and morphology of deposited metal layers, a well as the illustration of fundamental quantum-mechanical effects.     

Photoemission and scanning tunneling experiments with small particles on solid surfaces

The fascination of research with nanometersized objects in contact with a macroscopic surface will be illustrated by two examples: mass-selected supported transition-metal clusters and C₆₀ molecules on metallic single-crystal substrates. The preparation, mass-selection and deposition of the small particles will be described in some detail. The main experimental techniques involved in the characterization of their electronic and structural properties are photoelectron spectroscopy and scanning tunneling methods. For the transition-metal clusters the evolution of the valence band with cluster size reveals a trend to metal formation. When the tip of a Scanning Tunneling Microscope (STM) is placed above individual C₆₀ molecules intense light emission is observed. The diameter of this emission spot is approximately 4 Å. This observation indicates the possibility of an optical spectroscopic analysis on the scale of individual molecules.     

Electronic structure and magnetic properties of Y4Co3

The electronic structure of Y₄Co₃ has been studied based on the density functional theory within the local-density approximation. The calculation indicates that Y₄Co₃ is very close to ferromagnetic instability. The Fermi surfaces are composed mainly of 3d electrons of Co and 4d electrons of Y.     

Electronic structure and magnetism of V monolayer on a TM(0 0 1) (TM=Cr,Mo, W) surfaces: First-principles calculations

We investigated the electronic structure and magnetism of V monolayer (ML) on a TM(0 0 1)(TM=Cr, Mo, W) surface, using the full potential linearized augmented plane wave method based on density functional theory. General gradient approximation is used for exchange-correlation potential. The magnetic moments of the V ML's are calculated to be 1.33, 1.35, and 1.30 μ_B on Cr(0 0 1), Mo(0 0 1), and W(0 0 1) surfaces, respectively. All of the TM surfaces are coupled antiferromagnetically to the V monolayer and their magnetic moments are calculated to be −1.01, −0.26 and −0.17 μ_B for Cr, Mo and W substrates, respectively.     

The influence of H2 plasma treatment on the field emission of amorphous GaN film

The influence of H₂ plasma treatment on the field emission properties of amorphous GaN (a-GaN) films is studied. It is found that the treatment makes little change to the surface morphology. The current density of the treated film decreases from 400 to 30 μA/cm² at the applied field of about 30 V/μm. The treatment can reduce the defects in a-GaN films, and therefore the treatment results in the weakening of the tunneling emission of the a-GaN film at the high field region. The treatment also seems to change the conduction mechanism of the a-GaN film.     

Effects of an electric field on the confined hydrogen atom in a parabolic potential well

Using the perturbation method, the confined hydrogen atom by a parabolic potential well is investigated. The binding energy of the confined hydrogen atom in a parabolic potential well is calculated as a function of the confined potential radius and as a function of the intensity of an applied electric field. It is shown that the binding energy of the confined hydrogen atom is highly dependent on the confined potential radius and the intensity of an applied electric field.