Zinc-blende Surface Research Articles

Effects of composition, crystal structure, and surface orientation on band alignment of divalent metal oxides: A first-principles study

Using first-principles calculations, this paper systematically investigates the ionization potentials (IPs) and electron affinities (EAs) of nonpolar surfaces of binary divalent metal oxides with formally closed-shell electronic structures, namely, BeO, MgO, CaO, SrO, BaO, ZnO, HgO, SnO, and PbO in relevant crystal structures. An emphasis is put on the understanding of the effects of chemical composition, crystal structure, and surface orientation on the surface band positions. Slab models for nonpolar surfaces are automatically generated using a proposed algorithm that provides a set of unique nonpolar surface orientations. A non-self-consistent dielectric-dependent hybrid functional approach is employed that is shown to provide a significant improvement in the band-gap, IP, and EA evaluation over standard density functional theory calculations using the generalized gradient approximation. The valence band maximum, O $2s$ band center, and O-site local potential versus the vacuum level are examined for prototypical rocksalt, wurtzite, and zincblende surfaces of selected systems without atomic relaxation. All of these quantities are found to qualitatively follow the tendency in the Madelung potential at the O site, where decreasing interatomic distance results in deeper energy levels. The valence band maxima versus the vacuum level or the IPs after atomic relaxation also show a similar trend even when other cation species and crystal structures are included in the discussion, while there is much less correlation for EAs. An overall trend indicates that lattice volume reflecting cation species is a determinant factor to the IPs of the divalent metal oxides investigated in this paper although there are surface-specific dipole contributions that cause the surface orientation dependence of the IPs.

Stoichiometry and surface reconstruction of epitaxial CuInSe2(112) films

CuInSe2(112) films were grown on GaAs(111)A substrates by molecular beam epitaxy. The resulting surface stoichiometry was deduced by consideration of results from various surface analytic techniques. The obtainable Cu/In stoichiometry range in XPS was 0.4–1.2, where 1.2 marks the onset of Cu2−xSe phase segregation at the surface and 0.4 corresponds to the copper-depleted surface with ordered defect compound (ODC) composition. For the stoichiometric CuInSe2(112) surface, a c(4×2) reconstruction of the zinc blende surface periodicity is observed in the LEED pattern, with three rotational domains present on the flat GaAs(111) substrate. With the use of stepped (111) substrates, domain formation could be suppressed. By comparison of the LEED data and concentration depth profiles from angle-resolved XPS, two types of surface reconstructions could be distinguished. According to surface energy calculations in the literature, these correspond to surfaces stabilized by either CuIn or 2VCu defects. The surface of copper-poor CuIn3Se5 shows no reconstruction of the zinc blende order.

Ab initio LMTO calculation of surface localized and resonance states of the SiC(110) zinc-blende surface

The surface localized and resonance states of unrelaxed and relaxed SiC(110) zinc-blende surfaces have been investigated within the local density approximation of density functional theory (DFT), employing a first-principles full-potential self-consistent linear muffin-tin orbital (LMTO) method using a thicker (thirteen layer) slab. Intrinsic surface states appear in the fundamental energy gap for the unrelaxed surface. We have allowed non-bond-length-conserving relaxation of the surface atoms to obtain the minimum energy configuration. The shift in the surface states arising from the relaxation of atoms is small in comparison to other studied heteropolar covalent semiconductors because of the smaller value of the relaxation angle of the SiC(110) surface. New surface states have been predicted in the valence band region. The present atomic geometry is somewhat different from other theoretical results. We observe an inward displacement of the Si cation and an outward displacement of the C anion both parallel and perpendicular to the surface, different from the results of Sabisch et al. No experimental data are available for comparison.

Electronic localized and resonance states of InP(1 1 0) surface An ab-initio LMTO approach

We present results of a comprehensive and systematic study of localized and resonance electron states on a InP(1 1 0) zinc-blende surface by employing the first-principles full-potential self-consistent linear muffin tin orbital (LMTO) method. Intrinsic surface states appear in the fundamental energy gap region which shift towards the bulk valence and the conduction band region when the relaxation of the surface atoms are considered. Orbital characters of the surface states at Γ, X, M, X′ symmetry points have been identified. A detailed analysis of the surface localized and resonance states reveal that the present results obtained for the first time are in excellent agreement with the available experimental data.

Electronic localized and resonance states of relaxed GaAs(110) surface — an ab initio LMTO approach

We present results of a comprehensive and systematic study of localized and resonance electron surface states for an unrelaxed as well as relaxed GaAs (110) zinc-blende surfaces by employing a first-principles full-potential self-consistent linear-muffin-tin orbital method and a supercell approach with the local density approximation of density functional theory. Intrinsic surface states appear in the fundamental energy gap for the unrelaxed surface atoms. These intrinsic surface states shift towards the bulk valence and conduction band region when the relaxation of the atoms in the vicinity of the surface are considered. Localized surface and resonance states are identified and are compared with the results of other calculations and the experimental data. The results are in very good agreement with the available angle-resolved photoemission (ARPE) and inverse photoemission data. Orbital characters of localized and resonance states have been investigated. Some new surface states around the symmetry point Γ have been predicted for the first time which are in reasonable agreement with the ARPE data.

Ab initio LMTO calculation of the electronic structure of an ordered monolayer of Sb on a relaxed GaAs(110) surface

We present electronic surface states as obtained by a comprehensive and systematic study of a monolayer of Sb on a relaxed GaAs(110) zinc-blende surface using the first-principles full-potential self-consistent linear muffin tin orbital (LMTO) method. By considering the well accepted epitaxial continued layer structure (ECLS) model we investigate the surface states in the fundamental band gap along high-symmetry directions of the surface Brillouin zone. For the ordered overlayer Sb/GaAs(110), intrinsic surface states appear in the energy gap region which shift towards the bulk valence and the conduction band region when the relaxations of the surface atoms are considered. A detailed analysis of the surface and resonance states reveals that they are in excellent agreement with the available experimental data and the existing pseudopotential calculations.

Ab initio molecular orbital calculations on large lattice cluster models: Use of translational symmetry

Translational symmetry has been shown to be useful in the calculation of electronic structures of large lattice models. The number of unique integrals has been derived for cases of different dimensionality. For the unique integrals zero screening and approximation methods are described. The method has been applied to arrays of hydrogen atoms and to a zincblende surface model. When the size of the system is increased the translationally unique integrals are shown to become either zero or they can be calculated by simple coulombic approximations.

Relaxation of the ZnTe and CuCl (110) surfaces

Ab initio molecular dynamics simulations of the ZnTe and CuCl (110) surfaces are employed to study surface atomic relaxation. We believe that these are the first such computations for heteropolar semiconductors, and for their surfaces in particular. The molecular dynamics follows the Sankey–Niklewski method, and electrostatic interactions are incorporated using Ewald’s scheme for Gaussian atomic charge distributions. Hence the electrons are treated in the local-density approximation, forces are computed using the Hellmann–Feynman method, and atoms move to equilibrium according to Newton’s laws. Using ‘‘dynamical quenching,’’ we allow the ‘‘ideal’’ surfaces to relax according to these laws of physics and then address a controversy concerning whether Coulomb forces can play a significant role in determining the (110) zinc blende surface relaxation: Coulomb effects are not negligible for ZnTe (110) and are as dominant as covalent effects for CuCl (110). They reduce the (almost rigid) bond rotation angle ω1 due to movement of the anions up out of the (110) surface by about 6° for ZnTe, and cause the CuCl anion rotation angle to be nearly zero.

Properties of Fe single-crystal films grown on (100)GaAs by molecular-beam epitaxy

Single-crystal (100)Fe films 90–330 Å thick have been grown on etch-annealed (100)GaAs substrates by molecular-beam-epitaxy techniques. Ferromagnetic resonance data indicate that the two in-plane 〈110〉 directions are inequivalent and, together with magnetometry data, show that the average film magnetization decreases as the thickness decreases. The inequivalence is attributed to the nature of the interface bonding at a (100) zinc-blende surface. The decreased magnetization is attributed to the formation of Fe2As microclusters in the film due to As diffusion which is supported by Auger and electron diffraction studies. In general, the Fe films grown to date on etch-annealed (100)GaAs substrates are significantly inferior to those grown on (110)GaAs.

Polar surfaces of wurtzite and zincblende lattices

Madelung potentials are calculated at lattice sites along the [0001] wurtzite and [111] zincblende axes, both in the bulk and at the polar surfaces. The potentials are then used to compute the electrostatic surface energies of various (0001) wurtzite and (111) zincblende surface arrangements. All arrangements considered satisfy the ionic stability criterion, which requires a net charge at the polar surface. The surface energy results predict reconstructed and faceted surface structures that agree well with experimental low energy electron diffraction work reported in the literature.

Submicroscopic Topography of Oxidized Surface of Stibnite and Etched Surface of Zincblende

Submicroscopic topographies of oxidized stibnite and of etched zincblende were examined by electron microscopes. Crystallites of antimony oxide formed on stibnite have a roof-like shape bounded by two well-developed and four less developed {111} faces. This is in agreement with the conclusion drawn previously from electron diffraction study (Miyake, Sc. Pap. I. P. C. R. 34 (1938), 565). The direct observation by electron microscope gives some further information on the growth of the oxide. Zincblende surfaces etched by hydrochloric acid has a shape which is slightly different from the negative triakis tetrahedron inferred from electron diffraction study (Uyeda et al., Proc. Phys. -Math. Soc. Japan 24 (1941) 1049). In the diffraction patterns from this surface, structures of reflexion spots finer than those previously reported were observed and analysed by the theory of the dynamical refraction effect.