Plasmon energy chemical phase mapping of reactive multilayers

An analytical method of quantitative phase identification and mapping on the nanoscale is presented based on correlative similarity mapping from spectrum images within electron energy loss spectroscopy across the low‐loss plasmon region. The method is applied to map the reaction layer formation for heat treated Cu–Al–Al₂O₃ thin films. Coexistence of residual Al pockets next to polycrystalline but epitaxial CuAl₂ as main reaction product is found as well as a distinctive interfacial plasmon region. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interfacial structures of LaAlO<Subscript>3</Subscript> films on Si(100) substrates

This paper investigates the interfacial characteristics of LaAlO₃ (LAO) and LaAlO_xN_y (LAON) films deposited directly on silicon substrates by the pulsed-laser deposition technique. High-resolution transmission electron microscopy (HRTEM) pictures indicate that an interfacial reaction between LAO and Si often exists. The interfacial layer thickness of LAO films deposited in a nitrogen ambient atmosphere is smaller than that of LAO films deposited in an oxygen ambient atmosphere. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to study the composition of the interfacial layer. The shift of the La 3d photoelectron peak to a higher binding energy compared to LaAlO₃, the shift of the Al 2p peak to a higher binding energy compared to LaAlO₃, the shift of the Si 2p peak to a lower binding energy compared to SiO₂ and the intermediate location of the O 1s peak compared to LaAlO₃ and SiO₂ indicate the existence of a La–Al–Si–O bonding structure, which was also proved by the AES depth profile of LAO films. It can be concluded that the interfacial layer is not simply SiO₂ but a compound of La–Al–Si–O. PACS 77.84.Bw; 77.84.-s; 77.55.+f     

Interface characteristics and mechanical properties of short carbon fibers/Al composites with different coatings

Three kinds of coatings, Ni, Cu and Al₂O₃, were obtained on the surface of short carbon fibers (SCFs). The interface characteristics and mechanical properties of SCFs/Al composites with the various coatings were systematically studied in this paper. The results showed that, compared to non-coating, Ni or Cu coating improved the wettability of SCFs and Al melt. However, the harmful phases Al₃Ni or CuAl₂ generated in interface zone and Al matrix result in the lower mechanical properties. Al₂O₃ coating protected the SCFs and prevented the harmful reaction of Al and SCFs. The interface of Al/Al₂O₃/SCF without any other phase was clean and well bonded, and the Al₂O₃-coated SCFs/Al composite had the highest mechanical properties. The interfacial indentation and fracture mechanism of all the composites were analyzed in detail.     

Observation of iron impurity diffusion in silicon under bending stress by Mössbauer spectroscopy

In the present work, the formation of the Al₇₀Cu₂₀Fe₁₀ icosahedral phase by mechanical alloying the elemental powders in a high-energy planetary mill was investigated by X-ray diffraction and Mössbauer spectroscopy. It was verified that the sample milled for 80 h produces an icosahedral phase besides Al(Cu, Fe) solid solution (β-phase) and Al₂Cu intermetallic phase. The Mössbauer spectrum for this sample was fitted with a distribution of quadrupole splitting, a doublet and a sextet, revealing the presence of the icosahedral phase, β-phase and α-Fe, respectively. This compound is not a good hydrogen storage. The results of the X-ray diffraction and Mössbauer spectroscopy of the sample milled for 40 h and annealed at 623°C for 16 h shows essentially single i-phase and tetragonal Al₇Cu₂ Fe phase.     

Surface sensitive spectroscopic study of the interaction of oxygen with Al(111) - low temperature chemisorption and oxidation

The interaction of an atomically clean Al(111) surface with O₂ has been studied using a combination of electron energy loss spectroscopy (EELS) and Auger spectroscopy (AES). Oxygen dissociatively adsorbs and occupies both surface and subsurface binding sites under all exposure conditions in the temperature range 122–700 K. Surface sites are preferentially occupied at low exposures, while higher exposures increasingly favor population of subsurface sites. Studies of O₂ adsorption at temperatures as low as 131 K have shown that formation of Al₂O₃ occurs at high oxygen exposures. The Al₂O₃ produced exhibits a 54 eV Auger transition and a characteristic vibrational spectrum with loss features at 430, 645, and 880 cm⁻¹. Argon ion bombardment of thin monolayer level Al₂O₃ layers leads to preferential loss of Al₂O₃ and a reduction in the subsurface-to-surface oxygen ratio. Electron bombardment of similar, thin Al₂O₃ layers is ineffective in inducing desorption of surface species, whereas thick Al₂O₃ layers are strongly influenced by electron bombardment, as judged from AES behavior. Qualitative models for O₂ adsorption, oxidative annealing, and damage by ion and electron bombardment are given.     

Photoemission results of intermetallic superconductors: Nb3Al and MgB2

We study the superconducting electronic structures of Nb₃Al and MgB₂ using high-resolution spectroscopy. The obtained spectrum of Nb₃Al measured below T_c shows clear opening of the superconducting gap with a sharp pile up in the density of states and a shift of the leading edge. In addition, the spectrum shows a peak-dip-hump line shape expected from the strong-coupling theory. On the other hand, for MgB₂, the superconducting-state spectrum measured at 5.4 K shows a coherent peak with a shoulder structure, in sharp contrast to that expected from a single isotropic gap. The superconducting spectral shape of MgB₂ can be explained in terms of a multicomponent gap.     

Studies of molecular oxygen adsorbed on Cu surfaces

Molecular oxygen adsorbed on (110) and polycrystalline Cu surfaces has been investigated by UPS, XPS, AES, HREELS and LEED. Molecularly adsorbed O₂ on the (110) surface shows the characteristic three-peak He II spectrum due to π_g, π_u and σ_g orbitals, accompanied by an O-O stretching frequency at 660 cm⁻¹. On the polycrystalline Cu surface, adsorbed O₂ shows the three peak He II spectra with a considerably smaller separation between the π_u and σ_g band and two O-O stretching bands at 610 and 880 cm⁻¹. O₂ adsorbed on the Cu(110) surface gives rise to a (1 × 1) LEED pattern and characteristic K π¹π¹ transition in the Auger spectrum.     

Internal photoemission and plasmon gain in solid and thin films of Al

Weak features in the electron spectrum of Al excited by bremsstrahlung radiation from a Cu anode have been studied using a recently developed multidetector. A feature at ～1410 eV which has been identified as the Al 2p internal photoelectron line was found to have an intensity 4.2 × 10^?3 times that of the Al KL_2,3L_2,3:¹D₂ Auger line, in agreement with a simple theoretical treatment. The identification of this feature is confirmed by the observation of an ～67% decrease in its intensity in spectra obtained from clean Al films in the thickness range 3.1–34.0 nm. The intensity of a plasmon gain peak at ～1404 eV is found to be independent of thickness for films of thickness greater than 7.5 nm.     

Interfacial structure and electrical properties of LaAlO3 gate dielectric films on Si by metalorganic chemical vapor deposition

LaAlO₃ (LAO) gate dielectric films were deposited on Si substrates by low-pressure metalorganic chemical vapor deposition. The interfacial structure and composition distribution were investigated by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectroscopy (SIMS), and Auger-electron spectroscopy (AES). HRTEM confirms that there exists an interfacial layer between LAO and Si in most samples. AES, SIMS, and XPS analyses indicate that the interfacial layer is compositionally graded La–Al silicate and the Al element is severely deficient close to the Si surface. Electrical properties of LAO films were evaluated. No evident difference in electrical properties between samples with and without native SiO₂ layers was observed. The electrical properties are discussed in terms of LAO growth mechanisms, in relation to the interfacial structure. PACS 73.40.Qv; 81.15.Gh; 77.55.+f; 68.35.-p     

Surface studies on polycrystalline YBa2Cu3O7−δ with Auger electron and low-energy electron energy-loss spectroscopy

Chemical and electronic properties of scrapecleaned YBa₂Cu₃O_7–(YBCO) ceramics, which were also annealed in vacuum or exposed to H atoms or O₂ molecules, were studied at room temperature by applying Auger electron (AES) and low-energy electron energy-loss spectroscopy (LEELS). Scanning electron micrographs showed such scraped surfaces to consist mainly of small, fractured YBCO crystallites, i.e. of clean YBCO surfaces. With only one exception, all low-energy AES lines were found to be shifted in energy compared to data recorded with surfaces of metallic Cu and Y. The analysis of the high-energy AES lines indicated the existence of non-stoichiometric material in grain boundaries. At least 25 different LEELS features were recorded. The 24.9-eV energy loss, which is due to the excitation of bulk valence-electron plasmons, indicates an enhanced oxygen deficit _s 0.8 at scrape-cleaned YBCO surfaces. Annealing of scraped YBCO samples at approximately 700 K in vacuum and also their exposure to hydrogen atoms resulted in an additional oxygen depletion within a few atomic layers near to the surfaces. These oxygen deficiencies were found to increase the intensity ratios of low-and high-energy AES lines of Cu, Ba, and Y and to intensify the energy-loss peak at 4.4 eV which has been attributed to O–Cu–O dumbbells.     

The energy spectra of secondary ions emitted during ion bombardment

Secondary ion energy spectra have been measured for singly charged ions emitted from targets irradiated with 43 keV A⁺ ions. Targets studied include the 3d transition metals (Sc, Ti, V, Cr, Fe, Ni) Cu and Zn, Zr, Al and Si and the compounds SiO₂, Al₂O₃, NaCl, KCl. Energy spectra were measured in the energy range 1–600 eV. In several cases a peak in the energy spectrum in the region around 200 eV has been found. This is in addition to the usual low energy peaks in the region of 5–10 eV. In many cases the low energy peak was observed to decay steadily with irradiation time or to increase with oxygen pressure. In the case of the cleanest Zn spectrum, only the high energy peak can be detected. The data are discussed in relation to current models of secondary ion emission. We conclude that, in general, elemental metal targets which are clean are characterised by the high energy peak in the secondary ion energy spectrum. The slower ions emitted have been neutralised by electron exchange processes. The low energy peaks in unclean, partially clean, oxide coated or compound targets (NaCl, KCl) arise because the neutralisation of the slower ions is either not as efficient or is not possible. The secondary ion emission model of Blaise and Slodzian could account for the emission of ions from most targets.     

Many-electron effect in the angle resolved L3-M23M23 Auger-photoelectron coincidence spectroscopy (APECS) spectrum of metallic Zn

The many-body effect in the L₃-M₂₃M₂₃ Auger-electron spectroscopy (AES) spectrum of metallic Zn is discussed. The lifetime width and residual relaxation energy shift of the two M₂₃-hole state are governed by the (super) Coster-Kronig (sCK) transitions of two M₂₃-hole state. The residual relaxation energy shift and decay width of the two M₂₃-hole state are calculated in an average configuration by an ab initio atomic many-body theory. The agreement with experiment is good. To elucidate the many-body effect in the two-hole states, it is necessary to be able to discriminate individual components of the multiplet-split AES spectrum. We discuss how to discriminate individual components of the multiplet-split L₃-M₂₃M₂₃ AES spectrum of metallic Zn by angle-resolved Auger-photoelectron coincidence spectroscopy (AR-APECS) in order to determine accurately their line shapes, multiplet splitting energies, and spin states (singlet etc.).     

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.     

A study of the charging and dissociation of SiO2 Surfaces by AES

AES is used to determine the initial spectrum of a vacuum-broken SiO₂ surface and to follow its dissociation under the electron beam probe. Both Auger peaks heights and energies are affected by the irradiation. The change in stoichiometry is accompanied by a decrease of the surface charge by 5–8 V. The relation between stoichiometry and charge is explained by the influence of radiation-induced defects on secondary electron emission. The reduction of SiO₂ is characterized in terms of irradiation dose, dissociation cross-section and electron impact efficiency. Resistance to radiation damage is increased by surface carbon contamination. The chemical contribution to the Auger peak energy can be distinguished from the charging effect leading to a shift between element and compound of 12 eV for the silicon peak.     

Properties of nanocrystalline Al–Cu–O films reactively sputtered by DC pulse dual magnetron

The article reports on the effect of the addition of copper in the Al₂O₃ film on its mechanical and optical properties. The Al–Cu–O films were reactively co-sputtered using DC pulse dual magnetron in a mixture of Ar + O₂. One magnetron was equipped with a pure Al target and the second magnetron with a composed Al/Cu target. The amount of Al and Cu in the Al–Cu–O film was controlled by the length of pulse at the Al/Cu target. The Al–Cu–O films with ≤16 at.% Cu were investigated in detail. The addition of Cu in Al₂O₃ film strongly influences its structure and mechanical properties. It is shown that (1) the structure of Al–Cu–O film gradually varies with increasing Cu content from γ-Al₂O₃ at 0 at.% Cu through (Al_8−2x,Cu_3x)O₁₂ nanocrystalline solid solution to CuAl₂O₄ spinel structure, (2) the Al–Cu–O films with ≥3 at.% Cu exhibit (i) relatively high hardness H increasing from ∼15 GPa to ∼20 GPa, (ii) enhanced elastic recovery W_e increasing from ∼67% to ∼76% with increasing Cu content from ∼5 to ∼16 at.% Cu and (iii) low values of Young's modulus E* satisfying the ratio H/E* > 0.1 at ≥5 at.% Cu, and (3) highly elastic Al–Cu–O films with H/E* > 0.1 exhibit enhanced resistance to cracking during indentation under high load.     

Charge-transfer hole-lifetime broadening of the Cu Auger-electron spectra of high Tc superconductors

The satellite intensity in the Cu L₂₃-VV Auger-electron spectrum of the high T_c superconductor (YBa₂Cu₃O_7−x, 123) is much more enhanced as compared to that of CuO. This enhancement was previously interpreted by Ramaker et al. [D.E. Ramaker, N.H., Turner, F.L. Hutson, Phys. Rev. B 38 (1988) 11368] as a result of the mixing between the ddp and dpp states. Here, d is a hole in the Cu 3d band and p is a hole in the O 2p band. However, the dramatic Auger electron spectroscopy (AES) spectral lineshape change from CuO to 123 is not only in the charge-transfer (CT) satellite but also in the main-line width. The change arises from the transit of the “pairing” of two bound d holes in the ddp state to that of two bound p holes in the dpp state. As a result, in CuO there is no CT satellite but the dd state becomes a resonant state broadened by the CT hole-lifetime broadening, whereas in 123 the dd state becomes a mixture of a resonant-like state and nonresonant band states. The present many-body theory can explain the overall AES lineshape change from CuO to 123.     

An EELS and TPD study of the adsorption and decomposition of acetic acid on the Al(111) surface

The adsorption and decomposition of acetic acid on Al(111) have been studied using electron energy loss spectroscopy (EELS), temperature programmed desorption (TPD), and Auger electron spectroscopy (AES). Acetic acid reacts with clean Al(111) at 120 K to form a surface acetate species. The adsorbed acetate bonds to the surface in a symmetric configuration with C_s symmetry at 120 K. The adsorption of molecular acetic acid occurs at this temperature only after saturation of the surface acetate layer; this physisorbed multilayer desorbs molecularly at 167 K. Thermal decomposition of the adsorbed acetate leads to a carbon- and oxygen-covered surface; the only detectable thermal decomposition product is H₂. Electron irradiation induces a similar decomposition process of the surface acetate.     

SiO2 surface defect centers studied by AES

A theoretical model is proposed on how a Si dangling bond associated with an oxygen vacancy on a SiO₂ surface (E_s′ center) should be observed by Auger electron spectroscopy (AES). The Auger electron distribution N_A(E) for the L₂₃VV transition band is calculated for a stoichiometric SiO₂ surface, and for a SiO_x surface containing Si-(e^?O₃) coordinations. The latter is characterized by an additional L₂₃VD transition band, where D is the energy level of the unpaired electron e^?. The theoretical N_A(E) spectra are compared with experimental N(E) spectra for a pristine, and for an electron radiation damaged quartz surface. Agreement with the theoretical model is obtained if D is assumed to lie ≈2 eV below the conduction band edge. This result shows that AES is uniquely useful in revealing the absolute energy level of localized, occupied surface defect states. As the L₂₃VD transition band (main peak at 86 eV) cannot unambiguously be distinguished from a SiSi₄ coordination L₂₃VV spectrum (main peak at 88 eV), supporting evidence is presented as to why we exclude a SiSi₄ coordination for our particular experimental example. Application of the Si-(e^?O₃) model to the interpretation of SiO₂Si interface Auger spectra is also discussed.     

Auger electron spectroscopy study on the Cr/Al2O3 interfacial reactions

Interfacial reactions of evaporated chromium with surface has been studied using Auger electron spectroscopy (AES). The results reveal that the interfacial region consists of a mixture, which is a double oxide of Cr and Al or two separated oxides. After annealing, the chromium oxide and the metallic Al produced by reduction of the Al³⁺ ions were easily detected by AES at the interface. We suggest that the interfacial reaction occurs mainly by the charge transfer from the 3d electrons of Cr atoms to O 2p orbitals of the Al₂O₃ substrate. The annealing at higher temperature (973 K) is favourable to promote the interfacial reaction between the surface oxygen and the initial few atomic monolayers of the deposited chromium. The results also showed that the change of the relative Auger peak-to-peak height (APPH(%)) of the Cr LMM group peaks can be used as an index to identify the oxidation states of chromium at the Cr/Al₂O₃ interface.     

Photoabsorption of alloys of Al with transition metals V,Fe, Ni and Cu and Pr near the AlL2,3-edge

The onset of Al 2p transitions of VAl₃, FeAl, NiAl, NiAl₃, CuAl₂, PrAl₂ and the disordered alloys VAl (16 at % Al, 28%, 41%), FeAl (11%) is shifted up to 1.1 eV. New pronounced structure develops close to the onset which for NiAl agrees with a density of states calculation by Connolly and Johnson.