Thin Cu films on manganese: aes and photoemission,by synchrotron radiation,investigations

The initial growth of Cu on polycrystalline manganese at room temperature has been investigated by Auger Spectroscopy and UV-synchrotron radiation photoemission spectroscopy, for copper coverages ranging from 1 to 18 Å. By monitoring the changes of the Cu3d level, a strong intermixing at the interface has been observed. The simultaneous growth of a copper phase which is lying on top of the intermixed region has also been detected. Analysis of the Auger data indicates that copper is most probably growing in a layer-by-layer mode, with a simultaneous diffusion into the substrate. Calculations for a layer-by-layer growth model are compared with the experimental data. They support our analysis.     

AES and photoemission studies of Cu growth on Cr2O3

The initial growth (1–20 Å) of Cu on Cr₂O₃ at room temperature has been investigated by Auger Spectroscopy and UV photoemission spectroscopy by synchrotron radiation. Analysis of the Auger peak intensities suggests that copper is growing in a layer-by-layer mode probably with the simultaneous formation of the second and third layers.The photoemission spectra show that deposited copper grows steadily as a single copper species which does not interact strongly with the substrate.     

Geometric structure and electronic states of copper films on a ruthenium (0001) surface

Auger electron spectroscopy (AES), combined with thermal desorption mass spectroscopy (TDS), work function (Δφ) measurements and energy-dependent angular resolved UV photoemission using synchrotron radiation were used to investigate the geometric and electronic properties of submonolayer and monolayer copper films grown by vapor deposition on a clean Ru(0001) substrate. A pronounced influence of the deposition temperature on the morphology of the Cu films was established in that lower temperatures favor an island growth mechanism (Stranski-Krastanov or Volmer-Weber type), whereas higher deposition temperatures lead to a more uniform spreading and a layer-by-layer growth (Frank-van der Merwe type). For Cu films grown under the latter conditions angular resolved photoemission reveals the existence of two-dimensional Cu bands even before the monolayer has reached completion; the experimentally determined band dispersions agree quite well with recent theoretical calculations.     

Auger spectra of carbon monoxide chemisorbed on Pt(111) and Cu(111)

The Auger spectra of carbon monoxide adsorbed on Pt(111) and Cu(111) are compared. The characteristic features now regarded as a fingerprint of this adsorbed species are observed, even for the weakly adsorbed CO on copper which gives complex X-ray photoelectron spectra. No coverage dependence of the spectra was observed on either substrate. The C lsVV spectrum of CO/Cu(111) is dominated by transitions involving the “screening” electron in the 2π orbital.     

Layer-by-layer growth of metal overlayers

We have used plots of the Auger amplitudes versus deposition time to investigate the growth mode of 3d-transition metals on noble metal (100) surfaces. The systems considered are Fe/Cu(100), Fe/Au(100), Co/Cu(100), and Cr/Ag(100). We find that: 1. The Auger plots consist of a succession of straight lines of constant length with sharp breaks in between, i.e. the growth mode is essentially layer-by-layer. 2. From the experimental data points a slight rounding off in the vicinity of the break points cannot be excluded, although a numerical analysis shows that the deviation from perfect layer-by-layer growth is less than 10% of a monolayer for all systems considered.     

The growth mode of aluminium on silver (111)

The geometric and electronic structures occuring during the growth of Al on a single crystal Ag(111) surface have been studied using a combination of low energy electron diffraction (LEED), Auger electron spectroscopy (AES), energy loss spectroscopy (ELS) and work function measurements. The Auger signal versus deposition time plots, which were used to monitor the growth mode, are shown to behave in an identical fashion to that expected for layer-by-layer (Frank-van der Merwe) growth. LEED was used to determine the lateral periodicity of thin Al films and shows that Al forms, at very small coverages, 2D islands which have the same structure as the Ag(111) substrate and which grow together to form the first monolayer. At substrate temperatures of 150 K a well defined (1 × 1) structure with the same orientation as the underlying Ag(111) can be seen up to at least 12 ML. After completion of the third monolayer the ELS spectrum approached that observed for bulk aluminium. At a coverage of 3 ML the work function decreases by 0.4 eV from the clean silver value.     

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ～ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ_Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H₂ and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ_Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H₂2H⁺ couple at platinum electrodes.     

Lateral interactions in the thermal desorption of H2 from clean Cu/Ni(110) alloy surfaces

The adsorption of hydrogen on a clean Cu10%/Ni90% (110) alloy single crystal was studied using flash desorption spectroscopy (FDS), Auger electron spectroscopy (AES), and work function measurements. Surface compositions were varied from 100% Ni to 35% Ni. The hydrogen chemisorption on a-surface of 100% nickel revealed strong attractive interactions between the hydrogen atoms in accordance with previous work on Ni(100). Three desorption states (β₁, β₂ and α) appeared in the desorption spectra. The highest temperature (α) state was occupied only after the initial population of the β₂-state. As the amount of copper was increased in the nickel substrate, desorption from the higher energy binding α-state was reduced, indicating a decrease in the attractive interactions among hydrogen atoms. The hydrogen coverage at saturation was not affected by the addition of copper to the nickel substrate until the copper concentration was greater than 25% at which a sharp reduction in saturation coverage occurred. This phenomenon was apparently due to the adsorption of hydrogen on Ni atoms followed by occupation of NiNi and CuNi bridged adsorption sites, while occupation of CuCu sites was restricted due to an energy barrier to migration.     

Leed,auger, and electron energy loss studies of Ni epitaxially grown on Cu(100)

Nickel was epitaxially deposited onto a clean, flat Cu(100) surface. Low energy electron diffraction I(E) curves were recorded for 0.6, 1.1, and 2.7 monolayer (ML) Ni coverage. Multilayer relaxation was considered in theoretical calculations, which were compared with experiment by means of the R|ΔE| factor. The estimated relaxations of the first and second interlayer spacings are estimated to be − 2% and + 1.5% for clean Cu(100), − 2% and − 1.5% for 1 ML Ni coverage, relative to the bulk Cu interlayer spacing of 1.81 Å, and −1% and 0% for 3 ML Ni coverage, relative to the bulk Ni spacing of 1.76 Å. Decreasing the surface Debye temperature of the Ni layer to 268 K from the bulk value of 440 K improves the agreement between theory and experiment. The optimum inner potential values are − 9 and − 10 eV for clean Cu(100) and Ni on Cu(100), respectively. Auger electron spectroscopy was used to determine the thickness of the Ni films, and LEED indicates layer-by-layer growth until about 4 layers, when the LEED spots begin to spread, indicating island formation. Electron energy loss spectra were obtained with primary electron energies of 150 and 300 eV. The 3p core ionization transition was clearly observed after 0.5 ML Ni coverage. Peaks at 3.8 and 7.5 eV for clean Cu are ascribed to interband transitions, and shift to higher energy with Ni coverage. Peaks at 10 and 16 eV for clean Cu (ascribed to an interband transition and a surface plasmon, respectively) disappear with Ni coverage. Bulk plasmon peaks at 19 and 27 eV remain unshifted with Ni coverage. The effect of 0.9 and 1.3 ML Ni coverage of Cu(100) on the chemisorption of Co and oxygen was also studied. The behavior of the surface towards oxygen chemisorption was similar to that of the pure Ni surface. For a large exposure of oxygen (50 L and more) the EEL and Auger spectra are very similar to those observed for NiO. In the case of CO, for submonolayer Ni coverage, the surface shows a more Cu-like behavior, while for larger Ni coverage (a monolayer and more) there is a great similarity with the behavior of the pure Ni(100) surface.     

Iron deposition on the five-fold surface of the icosahedral Al-Pd-Mn quasicrystal

The room temperature adsorption behaviour of Fe on the five-fold surface of i-Al-Pd-Mn has been studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). A complex growth scenario for Fe adsorption on this substrate is observed with STM. At coverages up to about 3 MLE (monolayer equivalent), layer-by-layer growth is observed whereby small clusters and islands are formed which eventually coalesce into almost complete monolayers. No LEED pattern is observed, indicating that the layers are disordered. The AES results rule out intermixing. Above this coverage, there is a transition to a multilayer island growth mode. The islands are rotated by 72° and have the bcc(1 1 0) Fe structure. The results are compared with previous work on Fe adsorption on this substrate and on Al and Fe single crystal substrates.     

Positron trapping at quantum-dot-like particles on metal surfaces

Measurements of the positron annihilation-induced Auger electron (PAES) spectra from the Fe-Cu alloy surfaces with quantum-dot-like Cu nanoparticles embedded in Fe reveal a decrease of the Fe M_2,3VV positron annihilation-induced Auger signal intensity and an enhancement of the Cu one for surfaces created by enriching the Cu content of the Fe-Cu alloy. These experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics at the Fe(1 0 0) surface with quantum-dot-like Cu nanoparticles embedded in the top atomic layers of the host substrate. Estimates of the positron binding energy and annihilation characteristics reveal their strong sensitivity to the nanoparticle coverage. Theoretical core annihilation probabilities are compared with experimental ones estimated from the measured Auger peak intensities. The observed behavior of the Fe and Cu PAES signal intensities is explained by theoretical calculations as being due to trapping of positrons in the regions of Cu nanoparticles embedded in the top atomic layers of Fe.     

An electrochemical determination of oxygen adsorption on copper

The adsorption of oxygen on polycrystalline and thin film copper samples was studied with the solid state electrochemical cell: Cu,Cu₂O¦7.5wt%CaO/ZrO₂¦Cu in ultrahigh vacuum. The oxygen partial pressure in the bulk of the copper sample was controlled electrochemically by applying a voltage across the cell, while the oxygen coverage at the copper free surface was monitored by Auger Electron Spectroscopy (AES). This technique has enabled us to establish much lower oxygen partial pressures at high temperatures than normally attainable in ultrahigh vacuum. In this paper we report the results of reversible oxygen adsorption isotherms on polycrystalline copper at 928, 970 and 1093 K. The results agree reasonably well with the deductions of earlier surface energy measurements and indicate a surprising degree of stability for chemisorbed oxygen on polycrystalline copper. Isosteric heats of adsorption are calculated with and without the inclusion of the earlier surface energy measurements and are compared to previous differential heats of adsorption determined calorimetrically.     

Leed measurements of Fe epitaxially grown on Cu(100)

Iron was epitaxially grown on a Cu(100) surface. Low energy electron diffraction (LEED) intensity versus energy curves were recorded for 1 and 10 layers of iron on Cu(100) at room temperature. A full dynamical analysis was performed using the renormalized forward scattering perturbation method. The surface Debye temperatures were determined to be 233 K for 1 ML Fe and 380 K for 10 layers of Fe. The value obtained for fcc iron was 550 K. A multiple relaxation approach was employed in analyzing the experimental data. The estimated interlayer spacings for the first and second layers were 1.78±0.02 Å (first) and 1.81±0.02 Å (second) for 1 ML Fe, and 1.81±0.02 Å (first) and 1.78±0.02 Å (second) for 10 layers of Fe on Cu(100). Auger electron spectroscopy was used to determine the thickness of the Fe films, and the LEED measurements indicate approximately a layer-by-layer growth until about 17 layers at room temperature. At higher temperatures there is evidence of iron diffusion or copper surface segregation.     

AES studies of palladium films formed on copper and mica

Thin epitaxial films of palladium were grown on epitaxial copper films and cleaved mica in ultra high vacuum. The growth modes of these films were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), transmission electron microscopy (TEM), and TEM replica techniques. Layer by layer growth of Pd on Cu and mica was observed and inelastic mean free paths of Auger electrons for energies of 60 eV (Cu MMM) and 329 eV (Pd MNN) were calculated. These values were 5.7 and 6.9 Å respectively. The thermal stability of monocrystalline and polycrystalline Pd/Cu bilayer films at 483 K was also investigated by AES and TEM. It was found that Pd agglomerates on the Cu at this temperature to form a Stranski-Krastanov growth morphology. The agglomeration is much more rapid on polycrystalline films, suggesting that high surface diffusivity paths (grain boundaries and possibly other defects) enhance the surface diffusion of Pd on Cu.     

Evidence of epitaxial growth of bcc Co on Cr(100)

We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 × 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.     

Auger and other characteristic energies in secondary electron spectra from Al surfaces

The energy spectra of secondary electrons back-scattered from clean, oxygen covered, and Cu covered Al surfaces have been determined. The data support the previous suggestion that Auger electrons can experience both characteristic energy loss and absorption phenomena. From the experimental results it was not possible to determine whether densities of states of electrons in the valence band affected the Al L_2,3 VV Auger spectrum. This portion of the spectrum was greatly changed by oxygen absorption on the Al surface, but little affected by less than a monolayer of Cu. Conversely, characteristic loss spectra were less sensitive to oxygen on the surface, but were highly sensitive to the presence of copper at even less than monolayer coverage. A correlation between characteristic loss and “true” secondary spectra from clean surfaces was established and possible reasons for the correlation are discussed.     

Interaction of aluminum with iridium surface: Adsorption, desorption, dissolution, and formation of surface compounds

The interaction of aluminum with an iridium (111) surface was studied in ultrahigh vacuum by Auger electron spectroscopy over the broad temperature range 300–2000 K. At room temperature, layer-by-layer growth of an aluminum film was observed, with a monolayer forming in coherent relation to the substrate. Deposition at 1100–1300 K gives rise to the formation of surface aluminide Ir₄Al with an adatom concentration N _Al = (4.20 ± 0.15) × 10¹⁴ cm^?2. It was shown that aluminum escapes out of the surface aluminide by thermal desorption in the 1300–1700-K temperature interval, with the desorption activation energy changing from ～4.5 to ～5.7 eV as the coverage decreases from the value corresponding to the surface aluminide (taken for unity) down to zero.     

Growth of Cu Films on Si(111)-7×7 Surfaces at Low Temperature: A Scanning Tunnelling Microscopy Study

Morphologies of Cu(111) films on Si(111)-7×7 surfaces prepared at lowtemperature are investigated by scanning tunnelling microscopy (STM) andreflection high-energy electron diffraction (RHEED). At the initial growth stage, Cu films are flat due to the formation of silicide at the interface that decreases the mismatch between Cu films and the Si substrate. Different from the usual multilayer growth of Cu/Cu(111), on the silicide layer a layer-by-layer growth is observed. The two dimensional (2D) growth is explained by the enhanced high island density at low deposition temperature. Increasing deposition rateproduces films with different morphologies, which is the result of Ostwald ripening.     

Segregation effects in noble-metal film growth on Bi(0001): insights from Auger analysis

The growth of reactive adatom-substrate overlayers has been examined during the deposition of Ag and Cu on Bi(0001) using primarily Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). When viewed by the commonly used signal-versus-time plot, the Auger data exhibit a featureless behavior with a temperature dependence that is suggestive of cluster or compound formation. Surprisingly, when the same data are portrayed on a substrate-versus-adatom plot of the signals, the results are quite unlike those previously reported in the literature, in that the curvature of the data is the opposite of that typically found as the adatom signal increases. Heavy adatom deposition is needed to totally obscure the substrate signal. In the absence of factors that are known to affect the Auger yield in such overlayer systems, these data are found to be consistent with a growth process in which a layer of Bi substrate atoms is continuously segregated at the vacuum-solid interface. This interpretation is consistent with predictions from surface free-energy arguments, in that the Bi has a much lower energy than Ag or Cu and, thus, has a strong tendency to stay on top of the growing film.     

The structure of epitaxially grown metal films on single crystal surfaces of other metals: Gold on Pt(100) and platinum on Au(100)

Gold was evaporated onto Pt(100) and platinum was evaporated onto a Au(100) single crystal surface. Deposition of gold onto Pt(100) removed the $\text{(}\text{14}\text{1}\text{1}\text{5}\text{)}$ reconstructed surface structure and at a coverage of about 0.5 monolayer a (1 × 1) pattern fully developed. This pattern remained unchanged up to 2 gold monolayers. Multilayers of gold produced (1 × 5) and (1 × 7) surface structures after annealing. These observations imply variable interatomic distances in the gold layers. The (1 × 5) and (1 × 7) surface structures can be explained by the formation of a hexagonal top atomic layer on a substrate that retains a square lattice. The well known structure of clean Au(100) did not form, even at 32 layers of gold on Pt(100). Platinum deposited onto Au(100) removed its surface reconstruction yielding a fully developed (1 × 1) pattern at about one-half layer. This pattern remained unchanged upon further platinum deposition. The absence of new reconstructions in this case may be linked with the growth mechanism that is inferred from the variation of the Auger signal intensities of the substrate and adsorbate metals with coverage of the adsorbate. It was found that platinum on Au(100) forms microcrystallites (Volmer-Weber type growth), while gold on Pt(100) grows layer-by-layer (Frank-van der Merwe growth mechanism).