Table of contents

Volume 6

Number 45, 7 November 1994

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SURFACE AND INTERFACE SCIENCE LETTERS TO THE EDITOR

L687

and

The dissociative chemisorption for hydrogen molecules on the Si(111) (1*1) surface has been studied using a first-principles method. A direct dissociative chemisorption process was found. The direct dissociation happens at the saddle point of the potential energy surface for hydrogen atoms moving on the Si(111) (1*1) surface. A two-dimensional potential energy surface was presented to show the direct dissociation. Different orientations of H2 interacting with the surface were also investigated. The calculations showed that H2 always favours having the molecular axis parallel to the surface.

L693

and

A simple crystal growth model for molecular beam epitaxy (MBE) is proposed. Based on the common features of various MBE material systems, growth modes are classified as either high-temperature growth or low-temperature growth. The transition temperature between the two growth modes is found to be a characteristic value: half the melting temperature (Tm) of each material. The incorporation of growing atoms on the crystal surface is discussed based on a crystal surface model composed of kinks, ledges, and terraces. At temperatures higher than 1/2 Tm, kinks act as incorporation and decomposition sites for growing atoms, and the shape of the growth front step is simple and smooth. At temperatures lower than 1/2 Tm, kinks act only as pinning sites with a zigzag step.

L699

A mechanism of the core rearrangement of excited atomic states near a metal surface is presented. A numerical model is developed and applied to the description of the rearrangement of 1D core states of Ne** and Ne+* to 3P core states. This is ascribed to resonant ionization of 1Dnl states into the continuum: Ne++(3P)+metal, followed by resonant electron capture. This ionization is rendered possible by level shifts near a metal surface. Effects of adsorbates are investigated and it is shown that a lowering of the workfunction induces a strong decrease of the rearrangement.

REVIEW ARTICLES

9495

The energetics of some surface defects for the (100), (110), and (111) surfaces of Cu, Ag, Au, Ni, Pd, and Pt are studied using effective medium theory. Among the calculated data are the surface energy, the energetics of some reconstructions, the formation energy for steps and islands, and the activation energy for diffusion of adatoms.

9519

It is well known that thin films develop large intrinsic stress during their preparation. The intrinsic stress either originates from strained regions within the films (grain boundaries, dislocations, voids, impurities, etc.) or at the film/substrate (lattice mismatch, different thermal expansion, etc) and film/vacuum interfaces (surface stress, adsorption, etc.) or is due to dynamic processes (recrystallization, interdiffusion, etc). Since the magnitude of most of these stress contributions is directly related to film morphology, important structural information can be extracted from measurements of the intrinsic stress. This article presents a thorough discussion of today's understanding of the growth of thin films and reviews the related atomistic mechanisms responsible for intrinsic stress. On the basis of these ideas recent experimental results on the intrinsic stress of UHV deposited polycrystalline and epitaxial thin metal films are discussed. Depending on the respective growth mode of the films-Volmer-Weber, Stranski-Krastanov and Frank-Van der Merwe modes-characteristic stress behaviours are observed. In situ intrinsic stress measurements are therefore a promising new technique to gain additional insight into film growth.

SURFACE AND INTERFACE SCIENCE PAPERS

9551

, and

We have employed a parametrized tight-binding molecular-dynamics scheme in the study of the phenomenon of vacancies on the Si(001) c(4*2) surface. Simulated annealing is performed with a 'fictitious-Lagrangian' procedure to determine the optimal structures of a single and a dimer vacancy on this surface. A monovacancy is found to be less stable than a dimer vacancy, which agrees with experimental observations. We also show that there is a possible anisotropy in the surface migration of a dimer vacancy on the surface. The calculated activation energy for dimer-vacancy diffusion is 0.6 eV higher than that estimated experimentally at high temperatures.

9561

, and

We present a novel empirical tight-binding model of the hydrogen-silicon system suitable for realistic molecular-dynamics simulations. The hydrogen-silicon and hydrogen-hydrogen interactions are constructed to reproduce the electronic levels and vibrational frequencies of silane (SiH4) and hydrogen molecules, respectively. The potential functions are rescaled with an exponential factor in addition to the simple inverse square law. This smooths the potential change at some appropriate cut-off distances, which is a prerequisite condition in realistic molecular-dynamics simulations. The application of this model to other molecules and surfaces yields excellent agreement with the experimental results, proving the good transferability of our model.

9571

and

Different possible adsorption sites of potassium atoms on a gallium arsenide (110) surface have been investigated using ab initio self-consistent unrestricted Hartree-Fock total-energy cluster calculations with Hay-Wadt effective core potentials. The effects of electron correlation have been included by invoking the concepts of many-body perturbation theory and are found to be highly significant. We find that the K atom adsorption at a site modelled with a KGa5As4H12 cluster is most favoured energetically, followed by K adsorption at the site modelled with the KGa4As5H12 cluster. For molecular potassium, a parallel approach position modelled by a K2Ga5As4H12 cluster is most favoured energetically followed by a vertical approach position modelled by a K2Ga4As5H14 cluster. The effects of charge transfer from K and K2 to the GaAs surface as well as the possibilities of metallization are also analysed and discussed.

9585

The electrical resistivity rho has been measured at temperatures from 1.5 to 75 K for two series of foils with (111) and (110) surfaces on zone-refined Al, under the same current direction of ((112)). It is found that rho is larger on the (111) foil than the (110) one at 1.5-60 K and their difference peaks at around 35 K. The analysis of the data with Fuchs-Sondheimer theory gives the following results. (i) The phonon-limited part of the bulk-electron mean free path, lb(T), is larger along about ((111)) (lb111(T)) than the orientations around (110) (lb110(T)), and agrees with the value from the electron-phonon scattering time calculated by Leung et al. at 20 K. (ii) lb111(T) varies as T-3.0, but lb110(T) varies as T-3.5 at 10-20 K; this is consistent with Meader and Lawrence's theoretical prediction. The surface resistivity rho s of the single crystals is about half of that of the polycrystal of pure Al. It is concluded that the enhancement of rho s due to anisotropy in lb(T) is the main cause of the discrepancies between the measured and predicted rho s in polycrystals.

9601

, and

Energy spectra of positrons re-emitted from the surface of polycrystalline tungsten foil have been measured as a function of incident positron energy E in the range 100 eV-3 keV. The measurements were made using an electrostatic positron-beam apparatus incorporating a hemispherical energy analyser. At high E values the re-emitted spectra tend to a characteristic shape, dominated by a work-function peak at 2.7 eV with a low-energy shoulder comprising approximately 25% of the total spectrum. As E decreases a high-energy tail attributed to epithermal positrons increases steadily in relative intensity and extent. Below E approximately=500 eV the contribution of the low-energy distribution to the spectra increases significantly, and at 100 eV is dominant. These observations are interpreted in terms of elastic and inelastic processes at the tungsten surface.

9613

The effect of perpendicular surface anisotropy on surface spin-wave excitations in a semi-infinite ferromagnet is studied by solving the coupled equations of motion of spin-wave annihilation and creation operators. The ground-state spin arrangement in our model is either as in-plane ordering or a canted one due to a competition between the perpendicular anisotropy in the surface and the bulk anisotropy parallel to the surface, and surface spin-wave frequencies are calculated in both phases. It is shown that at the transition point between these two distinct phases a low-lying surface spin-wave branch becomes soft, and consequently the ellipticity and the amplitude of spin precessions in the surface layer grow large in the vicinity of the transition point.

9621

and

The critical behaviour of a magnetic superlattice is examined within the framework of the effective-field theory with correlations. For a spin- 1/2 Ising model of a superlattice with arbitrary number of magnetic layers in a unit cell we obtain the general formalism of transition temperature Tc derivation. For the case of an alternating superlattice, the transition temperatures Tc are calculated as a function of the film thickness and of the inter- and intralayer exchange constants. The effects of surface modification on finite superlattices are also studied numerically.

9633

, and

Ferroelectric films described by an Ising model in a transverse field have been studied under the mean-field approximation. We discuss an N-layer film of simple cubic symmetry with nearest-neighbour exchange in which the exchange strength and transverse field are assumed to be different from the bulk values in Ns surface layers, and we derive and illustrate expressions for the phase diagrams, order parameter profiles, specific heat and susceptibility. In such films, the Curie temperature can shift to either lower and higher temperature compared with the corresponding bulk value. If the surface exchange strength is strong enough, there is still a phase transition to ferroelectricity even when the transverse field is larger than the bulk critical value. In surface-enhanced films with Ns>or=2 the maximum in the order parameter profile occurs in the layers next to the outermost surface layer. The bulk peak of the specific heat becomes a discontinuity at the film Curie temperature. There is a rounded peak at the bulk Curie temperature if the surface exchange strength is higher than the critical value. The dim susceptibility still diverges at the film Curie temperature, as does the bulk susceptibility, but its magnitude is reduced. Also there is a rounded peak at the bulk Curie temperature when the surface exchange strength exceeds the critical value. The bulk-related character of the specific heat and susceptibility is less pronounced and the surface-related character is more pronounced when the film thickness is small or the surface-layer number is large.

9647

, and

We present Raman scattering spectra of CaF2 samples containing Ca colloids and of MgO:Li with Li precipitates. The samples have been prepared by different thermal and irradiation treatments in order to achieve a range of colloid radii and density. The sizes of the colloids are determined by their optical absorption bands. Exciting in the surface-plasmon band of the metal colloid we observe surface-enhanced Raman scattering (SERS) of the whole density of states of CaF2 or MgO phonons. The spectra are polarized and show a partial T2g character for CaF2. The intensity is enhanced as the laser energy approaches the colloid band maximum, where surface plasmons are excited. Comparing the results of different samples we find that the spectrum intensity is roughly proportional to the total amount of colloid material in the sample. In order to explain the appearance of the one-phonon density of states and its resonant behaviour we discuss the validity of the classical electromagnetic model and propose a mechanism consisting of surface-plasmon excitation followed by electron transfer from the metal to the host and de-excitation in the metal-host interface. In the host, electron-phonon interaction takes place, as in usual Stokes or antiStokes processes, giving rise to single-phonon modes. The lattice distortion and loss of periodicity in the vicinity of the colloid break the k=0 selection rule and account for the appearance of the whole density of states.

9659

and

Light on both sides of Al-Al2O3-Ag junctions is emitted only by the fast surface plasmon polariton mode. The intensity integrated over the spectral distribution and normalized with respect to the tunnel current is about 30 times higher at positive bias (electrons tunnelling into Ag). The explanation of this difference by Kirtley et al.'s model of excitation of the fast mode by hot electrons is corroborated by 'O quenching' of the emission only at positive bias without a change of the optical reflectivity. We postulate increased hot-electron-photon coupling within the inhomogeneous electron gas at sites of atomic-scale surface roughness.

9677

, , and

The Auger (autoionization) spectra produced by bombarding a magnesium target with argon and neon ions of energy in the range 200 eV-5 keV are reported. The observed spectra of both gases contained considerable fine structure broadly similar to that observed in spectra from aluminium and silicon targets. The Mg spectra contained structure not previously reported, which could be indexed as part of a series of transitions from an excited state in MgO to states at and above the ground state in Mg+. The variation of the heights of the main peaks was examined as a function of incident ion energy. The graph of peak height against incidence energy showed a sharp maximum for Ar while this was not the case for either Ne or Mg. The rare gas Auger peaks increased in energy with increasing incident ion energy and the shift could be explained in terms of a simple Doppler model appropriate to the strongly forward scattered experimental conditions used here. No systematic variation in the energies of the Mg peaks was observed.

9689

, and

We report a fully quantum analysis of the impact-site dependence of oriented NO collisions with Ag(111). Our simulations are based upon the recent corrected effective-medium potential-energy surfaces of DePristo and Alexander. The main finding is the huge influence of the impact site on both the NO rotational distributions and the steric effect. The global steric effect is found to vary from -8% (centre site) to +42% (atop site). Yet, calculations with the site-averaged potential surfaces yield a residual steric effect (+8%) in good agreement with the experimental observations of Kleyn, Stolte and co-workers. However, it does not match the site average of the steric results (+19%). This suggests that the detailed topology of the surface should be considered in interpreting the experimental orientation effects and that models based on flat surfaces may be inadequate.

PAPERS

9697

, , , and

The temperature dependences of the elastic constants of the antiferromagnetic Invar alloys Fe60Mn40 and Co46Mn54 have been investigated between 4.2 K and 800 K. Anomalies in the temperature behaviour of the sound velocities have been found in the vicinity of the magnetic phase transition indicating a strong magnetoelastic coupling in these antiferromagnetic Invar systems. The Landau theory has been adapted to the particularities of the systems under study for a phenomenological interpretation of the experimental data. Comparing our results with data obtained on ferromagnetic Invar alloys, we conclude that our phenomenological model is able to describe consistently the elastic anomalies in ferromagnetic and antiferromagnetic Invar systems.

9707

, and

The temperature dependence of the velocity of sound is calculated for heavy fermions within a mean field approach. The basic parameters of the periodic Anderson model, V and EO, are taken to be volume dependent. This leads to a volume dependence of the chemical potential, mu , the renormalized f level energy, epsilon /sup /f, the number of f electrons, nf, and the renormalized hybridization V. These effects are all included in the calculation of the temperature dependence of the sound velocity.

9721

DC conductivity, time of flight (TOF) and transient photoconductivity (TP) measurements were performed in order to characterize samples of amorphous arsenic triselenide prepared by thermal evaporation. Analysis of the hole carrier drift mobility data provides strong evidence for carrier interaction with a structured distribution of defect centres rather than the broad featureless distribution that has in the past been proposed for arsenic triselenide. Deviations from power law behaviour in the TP data also provide evidence for the existence of structure. We applied the 'intuitive' thermalization spectroscopic technique to the data and discovered defect centres situated at an energy position E2 approximately 0.42 eV within the mobility gap. At high temperature the thermalization depth of holes is limited by a set of recombination centres situated at E1 approximately 0.65 eV above the valence band edge.

9729

, and

The electronic and impurity states in the corner of well material surrounded by barrier material are investigated using the variational method. The results show that the binding energies of impurity ground states tend to the value of the third impurity excited states in the bulk when the impurity approaches the corner. The dependence of the ground-state electronic level and impurity binding energy on the dielectric mismatch between the well material and barrier material is also discussed.

9737

Linear as well as non-linear contributions to the Zeebeck and Peltier coefficients of a metallic film in contact with the equilibrium metal are calculated within a simple model. The non-linear part of the thermoelectric response survives down to a very low temperature which in principle permits thermoelectric cooling at these conditions. Thermal equilibrium in a metallic constriction between dissimilar metals is evaluated in the non-linear current-carrying regime.

9745

, and

We consider the dynamics of fluxons in two weakly coupled long Josephson transmission lines with localized impurities. The transmission, the pinning and the destruction of the bifluxon mode are obtained. In the case when there is a collision of a free fluxon belonging to one line with a pinned fluxon of the other line, we also observed three outcomes: capture of both fluxons; elastic collision; transmission of both fluxons at the impurity site.

9755

, , and

Superconductivity and structure of YBCO with added WO3 have been investigated. The result indicates that the addition of up to 5% WO3 does not destroy the superconducting properties of YBCO. It is found that the samples with the WO3 addition directly quenched from 900 degrees C in an oxygen atmosphere have a transition temperature of 88 K. The tetragonal-to-orthorhombic transformation takes place within a short time in the YBCO-WO3 samples. Extra peaks can be observed in the x-ray diffraction pattern, indicating that there is a second phase in the samples.

9759

, and

The phase transitions occurring in the Ising square antiferromagnet with first- (J1) and second- (J2) nearest-neighbour interactions are studied using several mean-field approximations and for a wide range of R=J2/J1. The largest approximation used corresponds to a nine-point cluster approximation of the cluster variation method. In this case, the transition temperatures as a function of R are found to be in excellent agreement with those obtained by other methods. The mean-field approximations predict a first-order transition in the range 0.5<R<or approximately=1.2, where the critical exponents associated with the paramagnetic to superantiferromagnetic transition have been reported to vary continuously with R. In that range of R, the mean-field approximations also predict a crossover between two distinct instability temperatures, or spinodals, taking place immediately below the first-order transition. Mean-field results are also given for the magnetization m, the specific heat Cnu , the magnetic susceptibility X, the staggered susceptibility Xs, and the pair correlation function sigma ij between i and j sites.

9773

, and

A simple analysis of the breaking of the symmetry of the phonon modes by slow distortions is presented. A phenomenological model is put forward for an explanation of the recently discovered 'forbidden mode' in Fe65Ni35 Invar alloy.

9781

, , and

The time-differential perturbed angular correlation technique has been used to study the electric field gradient at 181Ta impurities in the intermetallic compound Hf2Ni. The results of the measurements show the presence of a single nuclear quadrupole interaction. At room temperature the frequency of the interaction is omega Q=92(1) Mrad s-1. The electric field gradient Vzz and asymmetry parameter eta exhibit a pronounced temperature dependence in a range varying from 78 to 1125 K.

9789

and

Effects of compression on the Raman and IR modes of KOH(D), RbOH(D) and CsOH(D) have been measured at pressures up to 20 GPa and temperatures down to 100 K using the diamond anvil technique. For all three hydroxides an ordering transition into the respective hydrogen-bonded low-temperature phases was found below 1 GPa at room temperature. In addition, the spectra of KOH(D) and RbOH(D) show significant changes at pressures above 6.5 GPa, which point to the existence of a new phase. The decrease in frequency of the IR- and Raman-active O-H(D) stretching vibrations with increasing compression indicates that hydrogen bonds exist also in the new high-pressure phase VI. The variation of the 0-0 distance at the phase transition IV implies VI is estimated by using a double-minimum potential.

9807

Two decay components of luminescence of oxygen-vacancy centres were observed in alkaline-earth fluoride and barium fluorohalide crystals at 17-300 K. We denoted them as slow and very slow components. Very slow components are observed in all crystals except for type II oxygen centres in BaFBr. Above 100-150 K the integrated intensity of very slow components decreases in parallel with the decay time with thermal energy 0.1-0.2 eV. The integrated intensity of the slow component is not decreased from 17 to 300 K, while the decay time decreases from several hundred microseconds to several microseconds. This behaviour is explained by thermal interaction between three closed excited levels with different radiative lifetimes. We tentatively assume that these three levels belong to different oxygen-vacancy centre configurations in the lowest excited state.

9817

, , and

The self-consistent full multiple-scattering analysis of the spin-dependent Mn K-edge X-ray absorption near-edge structure (XANES) of MnF2 has been carried out. We succeeded in calculating the spin-dependent XANES over a 40 eV energy range in the continuum above the absorption threshold with the help of a self-consistent spin-polarized method in agreement with experimental data. We show that both the spin-dependent transition matrix element and the Mn p partial density of states contribute to the spin-dependent Mn K-edge XANES of MnF2.