G.N. van Wyk

Sn bulk-to-surface diffusion in a Cu(111)(Sn) single crystal

Abstract The kinetics of the bulk-to-surface Sn diffusion in a dilute Cu(Sn)(111) single crystal had been investigated in the temperature range 678 to 794 K using Auger electron spectroscopy and low energy electron diffraction. It is shown that a sputter time correction is essential to obtain an accurate estimate of the diffusion coefficient D . After deriving the segregation equation for limited surface site accessibility, the modified Darken equations are used, with sputter time corrections, to determine the diffusion coefficient D and the segregation energy ΔG from the kinetic data. The data and fits are characterised by a high degree of accuracy and the fit parameters have the following values: D 0 = (7 ± 6) × 10 −6 m 2 /s, E = 168 ± 5 kJ/mol and ΔG = 76 ± 3 kJ/mol.

SURFACE STRUCTURE AND COMPOSITION OF NIAL(100) BY LOW-ENERGY ION SCATTERING

The atomic structure of the (100) surface of NiAl was investigated by low‐energy ion scattering spectrometry (LEISS) using 1000 eV He+ ions as well as alkali ion impact collision ion scattering spectrometry (ALICISS), using 1450 eV Na+ ions. The ALICISS results were analyzed with a computer program based on a model using hitting probability concepts. The LEISS results showed that the top layer consisted mainly of Al with an Al/Ni ratio of 76/24 and the second layer entirely Ni. The ALICISS results could be best explained by a vertical relaxation of 25% (Δd12=−0.36 A) between the first and second layers and a relaxation of 15% (Δd24=−0.43 A) between the second and fourth layers. These results are discussed in terms of published data from the literature.

Kinetics near the discontinuous surface transition in the Cu(Ag)(111) binary segregating system

Abstract The kinetics of the segregating element silver in the system Cu(111)(Ag) was measured at temperatures close to where the discontinuous transition in the silver surface concentration versus temperature occurs. The seemingly complex kinetic behaviour of two-step diffusion and anomalous segregation processes could be fitted using only the surface segregation energy Δ G , the surface interaction parameter Ω S and the bulk diffusion coefficient D parameters.

Sputtering and radiation-enhanced segregation in amorphous Cu-Ti alloys

Binary amorphous copper-titanium alloys of compositions 44, 53, 65 and 69 at% copper were sputtered with 2 keV Ar+ ions. The surface concentrations were then determined by Auger electron spectroscopy and ion scattering spectrometry. It was found that Cu is sputtered preferentially as it segregates to the surface. A recently developed model for radiation-enhanced surface segregation was used to fit the experimental data. The segregation energy decreases with increasing Cu concentration from 2500 Jmol−1 to zero for the highest concentration, in qualitative agreement with several predictions on the segregating species in the alloy. The radiation-enhanced diffusion coefficients have values in the range (2–3) × 10−20m2s. From the concentration profiles the depth of the altered layer was estimated to about 4 nm which coincides well with the range of the defect distribution caused by the argon ions according to TRIM calculations. A CuTi sputtering ratio of 6 was needed for the best possible fit of the results.

A transmission electron microscope study of electrodeposited nickel films on (001) copper substrates

Abstract Nickel was electrodeposited on to (001) single-crystal copper films. Deposit thickness were in the range 10 A to 500 A. Bicrystals and stripped deposits were examined by transmission electron microscopy and high-resolution electron diffraction. Misfit, as a function of thickness, was determined from moire fringe spacings, splitting of diffraction spots and the spacing of misfit dislocations. The misfit was found to be less than that of vapour-deposited films at corresponding thicknesses. The misfit as calculated from the spacing of misfit dislocations was not in agreement with the misfit as determined from moire fringes or electron diffraction. These discrepancies might be due to foreign material incorporated in the deposit.

Silicon diffusion in an Fe-based amorphous alloy

Abstract The segregation rate of Si in amorphous as well as precrystallized Fe 81 B 13.5 Si 3.5 C 2 was determined as a function of time at different temperatures below the crystallization temperature. Analysis of the segregation kinetics yielded diffusion parameters of E = 72±9 kJ/mol, D 0 = 1.7 × 10 −15 m 2 / s for the amorphous specimen and E = 154±5 kJ/mol, D 0 = 3.6 × 10 −10 m 2 / s for the crystalline specimen. The difference is explained by the initial presence of an oversaturated concentration of structural defects. The correct interpretation of segregation kinetics results for amorphous alloys is discussed.

Diffusion of silicon in Fe-based amorphous and crystalline alloys

The segregation rate of silicon was measured in three different Fe-based amorphous and crystallized alloys with different silicon contents: 3.5, 5 and 9 at%. Analysis of the segregation kinetics yielded the diffusion activation energies E, as well as the frequency factors D0. A linear dependence was found between In D0 and E. In general, the D0 and E values were lower for the amorphous specimens than for the crystalline ones, were independent of silicon content and are explained in terms of an oversaturated concentration of structural defects. In the crystalline specimens, diffusion behaviour was influenced by silicon content near the solubility limit of silicon in iron.

The effect of substrate surface contamination on the electrolytic growth of epitaxial nickel films

Abstract Thin (001) single-crystal copper substrates were contaminated by exposure to the atmosphere. Electrodeposition of nickel took place upon the contaminated surface. Duplex films as well as stripped deposits were examined by transmission electron microscopy and electron diffraction. In contrast to deposition onto clean substrates, small three-dimensional (3-D) nuclei formed at the start of deposition. Deposits became continuous at an average thickness of ≈50A. Apart from the small initial nuclei, larger block-like growths were observed. Contrast phenomena as well as the results of misfit measurements suggested that the lattices were initially strained to give a coherent nickel-copper interface.

The composition and bonding characteristics of the alloying elements in electrodeposited NiCr alloys

Electrodeposited NiCr alloys of different compositions were studied by Auger Electron Spectroscopy (AES) and Electron Spectroscopy for Chemical Analysis (ESCA). It was found that all the chromium in the alloy was present as Cr2O3, while nickel was present in the metallic form. The influence of plating parameters on the compositions of the alloy and the efficiency of the deposition process was investigated in detail. The nature of inclusions in the deposit was also studied and quantitatively compared with the impurities found in thermally prepared NiCr alloys.

Surface Segregation and Preferential Sputtering of Binary Alloys

The surface composition of compounds and alloys very generally differs from their bulk concentration. Minimization of the total energy (Gibbs free energy) leads to surface segregation in alloys with an equilibrium composition at a given temperature, characterized by the segregation energy. Ion bombardment of a solid multicomponent material can cause an altered surface concentration whose steady state value depends on the different sputtering yields of the constituents. Bombardment can further support segregation due to radiation enhanced diffusion, expressed by a diffusion coefficient. We report on experimental results, model calculations and numerical simulations concerning preferential sputtering, segregation and their combined effects for various binary systems, i. e. Ta2Os, PdPt, CuTi, and FeAl. Detailed analysis requires surface analytical methods with different information depths, as provided by ion scattering spectroscopy (ISS) and Auger electron spectroscopy (AES). From such measurements segregation energies and diffusion coefficients can be extracted.