Relaxation Of Surface Atoms Research Articles

Atomic surface relaxation

After a recall of the surface relaxation trends, a short review of simple theoretical models is given. The point-ion model which only involves the calculation of the crystal electrostatic energy is shown to give good results near Aluminium and Iron surfaces. The surface atomic relaxation can also be obtained from the crystal energy minimization and is calculated from a simple tight-binding model applied to the transition metal d band. The multilayer oscillatory relaxation is obtained. The asymptotic relaxation value is also calculated and compared to experimental values. This model seems valid close to the surface and can improve the LEED intensity analysis.

Comments on "Lattice relaxation at a metal surface"

In a recent publication, Gupta has attempted a calculation of surface atomic relaxation with the use of the tight-binding (TB) scheme, and has showed that it yields essentially different results from those predicted by the pair-potential method. We point out that the surface atomic relaxation near the TB metal surface depends strongly on the TB scheme employed. We compare Gupta's results of surface atomic relaxation with those obtained with the use of a more rigorous TB approach.

Electronic structure of the Si (111) reconstructed surface in the vacancy model

The self-consistent pseudopotential method is applied to the Si (111) 7 × 7 reconstructed surface in the vacancy model with a simplified 3 × 3 superlattice structure. Numerical results with and without relaxation of surface atoms are presented. It is concluded that the relaxation, if any, is to be much smaller than the atomic distance to explain the photoemission spectrum of the 7 × 7 surface. The importance of the many-body effect is suggested in the photoemission process associated with the dangling bond surface states of Si.

Al on GaAs(110) interface: Possibility of adatom cluster formation

A reexamination of the experimental data and previous electronic-structure calculations on the prototype Schottky system Al/GaAs(110), together with new calculations, indicates that at low coverages and temperatures neither a covalent bond nor a metallic bond is likely to be formed between Al and the substrate. Instead, the predominant species is likely to be Al clusters which interact only weakly and largely nondirectionally with the substrate. In contrast with all previous theoretical models which assume an epitaxially ordered array of chemisorption bonds even at submonolayer coverage, it then appears that the formation of a Schottky barrier as well as other physical and chemical characteristics of the interface (e.g., core level and exciton shifts, valence-band photoemission spectra, gap states, surface atomic relaxation) are not explainable in terms of strong and ordered chemisorption bonds. This weakly interacting cluster model leads to several interesting predictions regarding the atomic structure and spectroscopy of this metal-semiconductor interface at the initial stages of its formation. The properties of the interface at higher temperatures (i.e., after annealing) are discussed in terms of an Al-Ga exchange reaction.

Calculation of structure-sensitive surface peak intensity in mev ion scattering

The surface peak intensity from a silicon crystal has been calculated by the Monte Carlo simulation method assuming a variety of surface structure models and dynamics. Effects of thermal vibration, displacements and relaxation of surface atoms, and also the shadowing effect by adsorbed layers and the effect of multiple scattering of the projectile in penetrating surface layers have been evaluated in the He + ion incident energy range from 0.1 to 2.0 MeV. Since many real surfaces of crystals are covered with adsorbed (deposited) layers or otherwise reconstructed when they are clean, it is of practical usefulness to obtain the calculation results about such surfaces in order to apply the MeV ion scattering technique for surface analyses. The results show the sensitivity and limits of this technique.

Initial steps of interface formation: Surface states and thermodynamics

A general framework about the influence of reactive particles on surface electronic structure and barrier formation at low coverages (ϑ?10−2) is deduced from experimental results obtained under thermodynamic equilibrium conditions on a suitable compound semiconductor ’’model surface.’’ Adsorption isotherms enable a rough classification of weak and strong electronic interactions into physisorption, chemisorption, and surface reaction steps, which depend on temperatures and partial pressures. (1) Covalent bonding of particles on ionic compound semiconductor surfaces, characterized by small partial charges formally attributed to the adsorption complex, may lead to extremely large dipole effects even at very low coverages [e.g.,dipole moments ≳ 15 Debye may formally be attributed to the adsorption complex ZnO (101̄0)/CO2,chem at ϑ<10−2]. Correspondingly, drastic changes are found in surface atom relaxation. Sticking coefficients are close to unity. (2) Ionic bonding, characterized by pronounced band bending, Sticking coefficients are extremely low for ’’acceptor type’’ adsorption.(3) Point defects at compound semiconductor surfaces are, for entropy reasons, thermodynamically stable at high temperatures. We present for the first time quantitative results on equilibrium concentrations of surface point defects [oxygen vacancies at ZnO(101̄0)], which cause surface Fermi level pinning at extremely low coverages (ϑ<10−3).

Influence of surface relaxation on the surface d-band width

Abstract We show that the surface d-band width strongly depends on surface atomic relaxation. A compression of the first interplanar spacing equal to a few per cent is sufficient to cancel the surface effect on the d-band width.

Relaxation of (111) silicon surface atoms from studies of Si 4H 9 clusters

Self-consistent Hartree-Fock and generalized valence bond calculations have been performed on clusters modeling the (111) silicon surface. We find that the surface state is accurately described as a dangling bond surface orbital with 93% p character. We determined the optimum relaxation of the surface layer to be 0.08 Å toward the second layer. In the positive ion, the surface atom relaxes toward the second layer by an additional 0.30 Å and for the negative ion the surface atom moves toward the vacuum 0.25 Å. The vertical ionization potential was found to be 5.78 eV (experimental values are 5.6 – 5.9 eV) while the calculated adiabatic electron affinity is 3.02 eV.

Relaxation of (111) silicon surface atoms from studies of Si 4H 9 clusters : A. Redondo, W. A. Goddard III, T. C. McGill and G. T. Surratt. Solid St. Commun.21, 991 (1977)

Relaxation of (111) silicon surface atoms from studies of Si 4H 9 clusters : A. Redondo, W. A. Goddard III, T. C. McGill and G. T. Surratt. Solid St. Commun.21, 991 (1977)

Relaxation of (111) silicon surface atoms from studies of Si4H9 clusters

Abstract Self-consistent Hartree-Fock and generalized valence bond calculations have been performed on clusters modeling the (111) silicon surface. We find that the surface state is accurately described as a dangling bond surface orbital with 93% p character. We determined the optimum relaxation of the surface layer to be 0.08 A toward the second layer. In the positive ion, the surface atom relaxes toward the second layer by an additional 0.30 A and for the negative ion the surface atom moves toward the vacuum 0.25 A. The vertical ionization potential was found to be 5.78 eV (experimental values are 5.6 – 5.9 eV) while the calculated adiabatic electron affinity is 3.02 eV.

Slow electron scattering from metals: III. The coherently elastically scattered primary electrons

Measurements have been taken of the (20), (10), (00), (1̄0) and (2̄0) beam intensities of low energy electrons diffracted from a (111) surface of a twinned silver crystal. Using the apparatus in I, investigations were made with incident electron energies in the range 30 to 300 eV and angles of incidence between 6° and 60° from the surface normal. Measurements for the (00) beam were made in the {10}, {11} and two other azimuths whilst measurements for the other beams were only taken in the {10} azimuth. The (111) surface was chosen because its geometry allows a clear experimental test to be made of the various predictions of the dynamical and kinematical theories of LEED. Results for the clean (111) surface were obtained by measuring the intensity of a given diffracted beam for a given angle of incidence as a function of incident electron energy (hereafter called an intensity plot). Peaks in the intensity plots of the (00) beam, taken in a high index azimuth, showed fairly intense Bragg peaks associated with an inner potential of 14 eV and secondary Bragg peaks of the type predicted by McRae's dynamical theory of LEED. In the low index azimuths the Bragg peaks were only seen for near normal incidence. Away from normal incidence the peaks in the intensity plots were mainly attributed to secondary Bragg peaks. For incident electron energies above the 3rd order Bragg condition in the {10} azimuth, the secondary Bragg peaks only had appreciable strength near the predicted Bragg peak energy, whereas at lower energies this was not so. The experimental results can be interpreted only on a dynamical theory of LEED and not on a kinematical one. Away from normal incidence there appears to be a dominant effect which permits an extra large flux of electrons to be returned to the surface and so to contribute to the emitted beams. This is thought to occur when the primary electron beam is in a condition to be Bragg reflected back along the incident direction, and thus the incident and reflected electrons can form a non-propagating standing electron wave. The occurrence of the secondary Bragg peaks near the Bragg peak positions explains the random variations of the inner potential that are often found because of the erroneous allocation of Bragg orders to these secondary Bragg peaks in an individual intensity plot. Some simple ideas, based on a band structure approach to the dynamical theory of LEED, are presented to explain certain aspects of the experimental results. The reclassification of the Bragg peaks as secondary Bragg peaks throws considerable doubt on the deductions concerning the amplitude of surface atom vibrations in various directions obtained from the experimental Debye-Waller factors of different beams. Deductions concerning the relaxation of surface atoms along the surface normal are similarly dubious.