S. E. Kul’kova

Effect of oxygen vacancies on adhesion at the Nb/Al2O3 and Ni/ZrO2 interfaces

The atomic and electronic structures of the Nb/Al2O3(0001) and Ni/ZrO2(001) interfaces are calculated using density-functional theory. The formation energy of oxygen vacancies is estimated in bulk materials and in surface layers and interfaces for different uppermost atomic layers of oxide surfaces. The work of separation of metal films from oxide surfaces is determined. The effect of oxygen vacancies on the bonding of transition metals to atoms of a substrate determining adhesion at the metal-oxide interfaces is discussed. It is shown that the Nb(Ni)-O interaction at the interfaces weakens in the presence of surface oxygen vacancies.

Vacancies and their complexes in FCC metals

The formation energies of vacancies and their complexes in copper and nickel at zero and finite temperatures are calculated by the embedded-atom method in the quasi-harmonic approximation. The role of temperature effects in the formation of various atomic configurations of intrinsic point defects is studied.

Electronic structure and optical properties of zirconia

The effects of substitutional impurities and oxygen vacancies on the electronic structure and optical properties of cubic zirconia were studied using band-structure calculations. It is shown that oxygen vacancies produce additional states near the Fermi level, whereas impurity atoms make an insignificant contribution to the states in the valence band and at the bottom of the conduction band, and their effect has a predominantly electrostatic character. The mechanisms of the stabilization of the high-temperature ZrO2 polymorphs are elucidated. The calculation results agree well with x-ray photoelectron spectroscopy and optical data

Electronic structure and lattice stability in the dihydrides of titanium, zirconium, and hafnium

A self-consistent linear MT-orbital method in the atomic sphere approximation (LMTO-ASA) is used to calculate the electronic structure of the dihydrides of the group IV metals in their cubic and tetragonal phases. The effect of tetragonal deformation and hydrogen vacancies on the electronic characteristics is studied. Satisfactory agreement is obtained with experimental data of photoelectron spectra. The nature of the instability in the high-temperature cubic phase is discussed.

Atomic and electron structure of the GaAs (001) surface

In the context of the pseudopotential approach, the atomic structure of four structural reconstructions of the Ga-enriched polar surface GaAs (001) is investigated. The geometry parameters of the surface structures of α-, β-, β2-, and ξ-GaAs (001)-(4 × 2) are determined. The electron structure and relative surface energies are calculated. The Cs absorption on the Ga-stabilized ξ structure GaAs (001)-(4 × 2) is considered. The highest absorption energy (2.57 eV) is obtained for the site S5, which is characteristic of the increased coordination of the Cs atom by the As atoms. The analysis of the electron structure enabled the clarification of the mechanism of the Cs bond on the GaAs (001) surface and the variations introduced by this mechanism into the surface electron structure of the substrate.

Investigation of Heusler alloy-semiconductor interfaces

Using the electron density functional theory, the electronic structure and magnetic properties of possible contacts on the (001) interface between XYZ and X2YZ Heusler alloys (NiMnSb, Co2 MnSi) and III–V semiconductors (InP, GaAs) are studied. It is demonstrated that, in both cases, the high degree of spin polarization is achieved in Ni/P(As) or Co/As contacts. The influence of structure defects located on the surface and interfaces on the spin polarization at the Fermi level is studied. The nature of surface states at the Heusler alloy-semiconductor interface and electron factors that favor preservation or loss of the half-metallic behavior in the contacts are analyzed. Calculations of the local magnetic moments show that the magnetic properties of atoms in the contact are not changed significantly at the interface because of the partial compensation of their coordination by atoms of the semiconductor. The spin polarization can be increased by doping of the X element sublattice.

Theoretical study of adhesion at the metal-zirconium dioxide interfaces

The atomic and electronic structure of the interfaces between metals with body-centered cubic (bcc) and face-centered cubic (fcc) structures and zirconium dioxide is studied systematically using the ab initio methods of the electron density functional theory (DFT). It is shown that high adhesion properties can be attained at the nonstoichiometric polar Me(001)/ZrO2(001) interface with bcc metals from the middle of the 4d–5d periods (Mo, Ta, W, and Nb). Charge transfer from the metal to the oxide substrate ensures the strong ionic chemical bond on the metal-ceramic interfaces. The structural and electronic factors responsible for lowering of adhesion at differently oriented interfaces are analyzed. It is shown that a decrease of adhesion at the (110) nonpolar stoichiometric interface is due to an increase in the interfacial spacing as well as a decrease in the number of metal-oxygen bonds. The effect of doping with oxides (CaO, MgO, and Y2O3) stabilizing zirconium dioxide at low temperatures on the adhesion energy at the Me(001)/ZrO2(001) interface is analyzed.

New Ga-enriched reconstructions on the GaAs(001) surface

To prepare structure-ordered GaAs(001) surfaces at low temperatures, GaAs(001) surfaces coated with native oxides were exposed in an atomic hydrogen flow in the temperature range 280–450 °C. The new Ga-enriched GaAs(001) surfaces with the (4 × 4) and (2 × 4)/c(2 × 8) reconstructions were prepared and studied by the methods of X-ray photoelectron spectroscopy, low-energy electron diffraction, and high-resolution characteristic electron energy loss spectroscopy. For the GaAs(001)-(2 × 4) surface, the structure of the Ga-stabilized surface has been proposed and ab initio computed within the (2 × 4) Ga-trimer unit cell model.

Cesium adsorption on the β2-GaAs(001) surface

The results of first-principles calculations of the cesium adsorption energy on the β2-GaAs(001) surface performed within approaches of the density functional theory are presented for two possible terminations of the surface. It is shown that, among the considered high-symmetry positions, the energy-preferred position for cesium is position T3 when the surface layer contains arsenic and position T4 for gallium terminated surface. Cesium introduces insignificant perturbations in the positions of surface-layer atoms, and surface dimers do not break even in the case of adsorption at the dimer bridge and top positions. It is shown that cesium bonding to the GaAs (001) substrate can be explained by sp hybridization of arsenic and gallium orbitals as well as by formation of cesium states mixed with delocalized states of a clean surface. At low coverage, more preferable adsorbate sites are those with nearest neighbor arsenic atoms for both surface terminations.

To the quantum theory of chemical activity of the surface of transition metals

The dissociation of a gas molecule and the formation of a new chemical bond upon adsorption of this molecule on the surface of a transition metal are studied using the method of equations of motion. It is shown that both processes involve the formation of a mixed intermediate state during the adsorbate-substrate interaction. The dissociation is caused by a resonance growth of the vibrational mode, whereby the dissociation barrier is determined by the hybridization energy and by the frequency of electron transitions between molecular levels and the d electron energy levels of the metal in the mixed intermediate state. The resonance conditions for the formation of new surface structures are established.