X-ray photoelectron spectroscopy study of magnetic films

Ta/NiO_x/Ni₈₁Fe₁₉/Ta films were prepared by rf reactive and dc magnetron sputtering. The exchange coupling field H_ex and the coercivity H_c of NiO_x/Ni₈₁Fe₁₉ as a function of the ratio of Ar to O₂ during the deposition process were studied. The composition and the chemical states in the interface region of NiO_x/NiFe were also investigated using X-ray photoelectron spectroscopy (XPS) and the peak decomposition technique. The results show that the ratio of Ar to O₂ has a great effect on the chemical states of nickel in NiO_x films. The exchange coupling field H_ex and the coercivity H_c of Ta/NiO_x/Ni₈₁Fe₁₉/Ta are thus seriously affected. XPS is shown to be a powerful tool for characterizing magnetic films. Received: 18 July 2001 / Accepted: 21 December 2001 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-010/6232-7283, E-mail: guanghua_yu@263.net     

Chemical analysis of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy

We studied the chemical state of semiconducting and metallic SmS thin films by X-ray photoelectron spectroscopy (XPS), which were fabricated using dual-target magnetron sputtering by controlling the power applied to both metal and chalcogenide targets. On the basis of the valence band spectra obtained, it was suggested that semiconducting SmS has the final state corresponding to the Sm²⁺(4f⁶) configuration below the Fermi level, and metallic SmS has mainly the Sm³⁺(4f⁵) final state and a virtual band state in the Sm 5d band, contributing to the delocalization of 4f electrons and the emergence of metallic conductivity (4f⁶d⁰-4f⁵d¹). Thus, the spectra of our fabricated SmS thin films correspond to the band structure obtained from the dielectric property. This is the first work performed on the intrinsically prepared metallic SmS while the former works done for the sample transformed from semiconductor to metal phase by hard polishing.     

X-ray photoelectron spectroscopy study of sputter-annealed Ni2.1Mn0.9Ga surface

The modifications in surface composition of Ni_2.1Mn_0.9Ga ferromagnetic shape memory alloy have been investigated using X-ray photoelectron spectroscopy (XPS) under various sputtering and annealing conditions. XPS core-level spectra show that sputtering makes the surface Ni rich. However, by annealing, the Mn content at the surface increases and at about 390 °C the bulk composition is restored. The valence band spectra show evidence of Ni related extra states for the sputtered surface, which decrease with annealing. This behavior is in agreement with the change in surface composition derived from the core-level spectra.     

Thickness determination of molecularly thin lubricant films by angle-dependent X-ray photoelectron spectroscopy

An angle-dependent X-ray photoelectron spectroscopy (XPS) method used to measure the thickness of molecularly thin lubricants was developed. The method was built based on an island model of patched overlayer on a flat substrate by using the photoemission signal solely from the lubricant film. Typical molecularly thin Zdol films on the CHx overcoat of unused commercial magnetic disks were measured to verify the metrology. The lubricant thickness determined by the metrology was equal to the recent result by thermostatic high vacuum atomic force microscopy. The measured deduction in the thickness of the molecularly thin lubricant films, successively irradiated by the monochromatic source operated at 14 kV/250 W, was as low as 1 ? during the first irradiation hour. XPS spectra showed that no hydrocarbons, water or oxygen were adsorbed over the Zdol outer surfaces in the tested XPS conditions. The inelastic mean free path (IMFP) of C 1s in Zdol or in CHx was found to be independent of take off angle (TOA) when TOA < 40°. The IMFP of C 1s in Zdol was ∼63.5 ? and the lubricant island thickness was ∼35 ?.     

Investigation of the interaction of evaporated aluminum with vapor deposited Teflon AF films via X-ray photoelectron spectroscopy

The interfacial bonding and mixing between evaporated aluminum and a vapor deposited Teflon AF (abbreviated to AF) film have been investigated with X-ray photoelectron spectroscopy. Graphite carbon (C–C), and aluminum carbide (Al–C), oxide (Al–O–C) and fluoride (Al–F) are formed when aluminum atoms are deposited on to the AF film. With increasing deposition of aluminum, the concentrations of these newly formed components increase gradually. Moreover, in situ annealing results in remarkable increases in the C–C, Al–C, Al–O–C and Al–F configurations and a decrease in metallic aluminum. No significant diffusion of aluminum into the AF film was observed during the annealing. The Al compounds form a layer at the Al/AF interface that acts as an adhesion promoter and diffusion barrier. Received: 21 October 2002 / Accepted: 22 October 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-431/880-6229, E-mail: sjding@yahoo.com     

Study of nickel silicide formation on Si(1 1 0) substrate

Nickel silicide formation on Si(1 1 0) and Si(1 0 0) substrate was investigated in this paper. It is confirmed that nickel monosilicide (NiSi) starts to form after 450 °C annealing for Si(1 0 0) substrate, but a higher annealing temperature is required for NiSi formation on Si(1 1 0) substrate, which is demonstrated by X-ray diffraction (XRD) and Raman scattering spectroscopy. The higher formation temperature of NiSi is attributed to the larger Ni₂Si grain size formed on Si(1 1 0) substrate. Ni silicided Schottky contacts on both Si(1 0 0) and Si(1 1 0) substrates were also fabricated for electrical characteristics evaluation. It clearly reveals that the rectifying characteristics of NiSi/n-Si(1 1 0) Schottky contacts is inferior to that of NiSi/n-Si(1 0 0) Schottky contacts, which is attributed to a lower Schottky barrier height and a rougher contact interface. The formation kinetics for nickel silicide on Si(1 1 0) substrate is also discussed in this paper.     

The Novaco-McTague rotated Xe monolayer on Pt(111): A high-order commensurate locked phase

The structural analysis of the physisorption system Xe/Pt(111) by means of high-resolution helium diffraction shows that the Xe monolayer exhibits a variety of structural phases: commensurate, incommensurate and incommensurate rotated. The observed buckling of the rotated phase demonstrates that a fraction of the Xe atoms is locked in high symmetry sites in agreement with the theoretical coincident site lattice concept of Fuselier et al., i.e. the rotated phase is a higher-order commensurate phase.     

Method of observation of low density interface states by means of X-ray photoelectron spectroscopy under bias and passivation by cyanide ions

X-ray photoelectron spectroscopy (XPS) measurements under bias can observe low density interface states for metal-oxide-semiconductor (MOS) diodes with low densities. This method can give energy distribution of interface states for ultrathin insulating layers for which electrical measurements cannot be performed due to a high density leakage current. During the XPS measurements, a bias voltage is applied to the rear semiconductor surface with respect to the ∼3 nm-thick front platinum layer connected to the ground, and the bias voltage changes the occupation of interface states. Charges accumulated in the interface states shift semiconductor core levels at the interface, and thus the analysis of the bias-induced shifts of the semiconductor core levels measured as a function of the bias voltage gives energy distribution of interface states. In the case of Si-based MOS diodes, the energy distribution and density of interface states strongly depend on the atomic density of silicon dioxide (SiO₂) layers and the interfacial roughness, respectively. All the observed interface state spectra possess peaked-structures, indicating that they are due to defect states. An interface state peak near the Si midgap is attributable to isolated Si dangling bonds at the interface, while those above and below the midgap to Si dangling bonds interacting weakly with Si or oxygen atoms in the SiO₂ layers. A method of the elimination of interface states and defect states in Si using cyanide solutions has been developed. The cyanide method simply involves the immersion of Si in KCN solutions. Due to the high Si-CN bond energy of ∼4.5 eV, the bonds are not ruptured at 800 °C and upon irradiation. The cyanide treatment results in the improvement of the electrical characteristics of MOS diodes and solar cells.     

Prerequisites for high-quality magnetic tunnel junctions: XPS and NMR study of Co/Al bilayers

Aluminum films with thicknesses ranging from 1 nm to 12 nm have been sputtered on 20 nm thick Co layers. The properties of the Co/Al bilayers were studied by X-ray photoemission spectroscopy (XPS) and spin-echo nuclear magnetic resonance (NMR). Both methods show independently that a 1 nm Al film covers the Co surface completely. XPS and NMR also showed that layers thicker than 1 nm Al are not oxidized completely in ambient air. Similarities to and deviations from niobium with Al overlayers (Nb/Al) are described. Prerequisites for the fabrication of tunneling magnetoresistance devices based on Co or NiFe ferromagnets and an aluminum oxide barrier are discussed. Received: 7 July 1999 / Accepted: 11 November 1999 / Published online: 8 March 2000     

The determination of shadowing critical angles in impact collision ion scattering spectroscopy

Critical angles for shadowing in low-energy ion scattering spectroscopy are calculated in the momentum approximation for the Thomas-Fermi-Molière potential and the Ziegler-Biersack-Littmark potential. In the relevant parameter range the results can be fitted by a formula containing five constants depending on the potential only. For a fixed projectile-target combination at a given energy the distance dependence of the critical angle is described by a simple power law. The comparison of the calculated results with experimental data shows that the inclusion of the effect of thermal vibrations on the critical angles improves the agreement between calculations and experiment significantly.     

Self-organization of ripples on Ti irradiated with focused ion beam

30 keV focused Ga⁺ ions were used to raster the metallographically polished surface of commercially pure Ti (CP Ti) at various FIB incidence angles over a wide range of doses (10¹⁶-10¹⁸ ions/cm²) at room temperature. The sputtered surfaces were observed in situ using FIB imaging and later carefully characterized ex situ under scanning electron microscope (SEM) and atomic force microscope (AFM). Ripples were observed on the irradiated surfaces even at the normal FIB incidence angle. The ripple evolution is analyzed as functions of surface diffusion, surface crystallographic orientation, ion dose and incidence angle. It is found that the ripple orientation was progressively influenced by the ion beam direction with incidence angle increasing and in some cases curved ripples or fragmented rods viewed from different angles occurred at high ion doses. The morphological evolution from the well-developed straight ripples to the curved ones is never observed. The formation of ripples is attributed to the competition between the formation of ripples due to anisotropic surface diffusion and the formation of incidence-angle dependent ripples determined by Bradley-Harper (BH) model.     

Investigation of organic impurities adsorbed on and incorporated into electroplated copper layers

At room temperature electroplated copper layers exhibit changes in resistivity, residual stress, and microstructure. This process, known as self-annealing, is intimately linked to the release of organic impurities, which stem from the incorporation of organic additives into the Cu layer in the course of the electroplating process. The behavior of these impurities during self-annealing, represented by the carbon content, could be detected by analytical radio frequency glow discharge optical emission spectrometry (GD-OES) and carrier gas hot extraction (CGHE). The precondition of a quantitative determination is a surface cleaning procedure to remove adsorbed organics from the copper surface. It was observed that at first almost all impurities have to leave the Cu metallization before an accelerated abnormal grain growth can start. The small amount of remaining organic species after self-annealing could be quantified by both examination techniques, GD-OES and CGHE.     

Surface diffusion of carbon atom and carbon dimer on Si(0 0 1) surface

Carbon (C) atom and carbon dimer (C2) are known to be the main projectiles in the deposition of diamond-like carbon (DLC) films. The adsorption and diffusion of the C adatom and addimer (C2) on the fully relaxed Si(0 0 1)-(2 × 1) surface was studied by a combination of the molecular dynamics (MD) and Monte Carlo (MC) simulation. The adsorption sites of the C and C2 on the surface and the potential barriers between these sites were first determined using the semi-empirical many-body Brenner and Tersoff potential. We then estimated their hopping rates and traced their pathways. It is found that the diffusion of both C and C2 is strongly anisotropic in nature. In addition, the C adatom can diffuse a long distance on the surface while the adsorbed C2 is more likely to be confined in a local region. Thus we can expect that smoother films will be formed on the Si(0 0 1) surface with single C atoms as projectile at moderate temperature, while with C2 the films will grow in two-dimensional islands. In addition, relatively higher kinetic energy of the projectile, say, a few tens of eV, is needed to grow DLC films of higher quality. This is consistent with experimental findings.     

Microstructural effects on the formation and degradation of zinc phosphate coatings on 2024-Al alloy

The formation of zinc phosphate (ZPO) coatings on 2024-T3 aluminum alloy was studied using scanning electron microscopy (SEM), scanning Auger microscopy (SAM) and X-ray photoelectron spectroscopy (XPS), with an emphasis on microstructural effects involving second-phase particles and the alloy matrix. Surface polishing results in an Al-Cu-Mg particle surface that contains metallic Cu as well as an overlayer of aluminum and magnesium oxide, while larger amounts of aluminum oxide are present on the Al-Cu-Fe-Mn particle and matrix. When dipped in an acidic ZPO coating solution, the oxide covering the Al-Cu-Mg particle is etched most easily, and metallic Cu near the surface makes that region most cathodic, allowing more coating deposition compared with the other regions. The oxides on the Al-Cu-Fe-Mn and matrix regions are similar, thereby confirming that the observed differences in ZPO coating characteristics at these two regions arise from their underlying electrochemical characteristics. Immersion of a coated 2024-Al sample in corrosive NaCl solution for extended periods indicates that the ZPO provides better protection to the second-phase particles than to the matrix.     

Sb induced (7×7) to (1×1) surface phase transformation of the Si(111) surface

Adsorption of Sb at a very low flux rate results in an epitaxial layer-by-layer growth on Si(111) surface held at room-temperature. Band-bending is not observed for submonolayer Sb coverages while sharp changes in the photoemission features are observed for 1.0 monolayer (ML) Sb adsorption. Changes in the core level binding energy and width in X-ray photoelectron spectroscopy, surface related feature in Electron energy loss spectroscopy and spot intensity ratios in Low energy electron diffraction studies suggest a surface phase transition upon adsorption of 1.0 monolayer of Sb. A plausible model is proposed to explain the abrupt metal-semiconductor transformation at this critical coverage of 1.0 ML.     

A method for the experimental determination of the net atomic charge via X-ray photoemission spectroscopy

A method for the experimental determination of the net charge distribution over the atoms in a large class of compounds is described. The method is based on X-ray photoemission spectroscopy and requires the determination of two core-level energies per constituting atomic species. Two special cases of large conceptual and practical interest are discussed: the silicon charge in CoSi₂, Si_1-xGe_x (0≤x≤0.3), SiC, Si₃N₄ and SiO₂, and the aluminum charge in aluminum-doped zeolites. Received: 20 March 2000 / Accepted: 28 March 2000 / Published online: 21 March 2001     

Organisation of carbon and boron nitride layers in mixed nanoparticles and nanotubes synthesised by arc discharge

2 bonds attests for the presence of ordered BN domains and of carbon domains; (ii) the elemental profiles show that BN layers and carbon layers are immiscible with a radial organisation into two to five domains; and (iii) the sets of layers at free surfaces – including the inner surfaces of tubes – are always made of carbon. The origin of this chemical organisation, which is most likely obtained during the growth, is discussed. For the hafnium-boride metallic particles coated by C/BN envelopes, a model based on the solidification from the outside to the inside of isolated liquid-like droplets is proposed: the carbon phase solidifies first according to theoretical phase diagrams, and forms the outer shells. For the tubes, a directional eutectic solidification process is shown to account for the observed C/BN/C sequence, in a vapour–liquid–solid scheme, with an hafnium-rich liquid-like particle at the tip of the tube. Received: 26 November 1998 / Accepted: 14 January 1999     

Enhancement of the secondary ion emission induced by fast clusters

The emission yields of the secondary ions are measured by using a conventional time of flight (TOF) technique under bombardments of Mg and C₂, Ni and Si₂ with different energies, and C_n, Si_n and Ni_n (n = 1-3) with the different charge states and with energy of 1.5 MeV per atom, respectively. For the bombardments of C_n, Si_n and Ni_n, the enhancements of the secondary ion emissions increase with increasing cluster sizes and charge states. For the bombardments of Mg and C₂, Ni and Si₂, although the mass and the nuclear charges of C₂ and Si₂ are the same as or equivalent to Mg or Ni, respectively, the enhancements of the secondary ion emissions induced by the clusters of C₂ and Si₂ in a wide energy range are also clearly indicated. The instantaneous collective interaction of the cluster constituents plays an important role in the secondary ion emissions.     

Characterization of oxide layers on amorphous Zr-based alloys by Auger electron spectroscopy with sputter depth profiling

Amorphous Zr-Cu-Ni-Al-[Ti, Nb] ribbons prepared by melt spinning under argon atmosphere were subjected to electrochemical investigations. Passive films developed at potentiostatic anodic polarization in sulphuric acid solution were investigated by Auger electron spectroscopy (AES) and sputter depth profiling.Changes in the shape of the Auger peaks have been analyzed by factor analysis of the spectra obtained during depth profiling. Pronounced changes in shape and position occur for the Zr, Al, and Ti Auger transitions, but not for Cu and Ni. At least three different peak shapes for O(KVV) were found and attributed to different oxygen binding states. The alloy composition has no significant effect on the thickness and composition of the oxide layer.In multi-element alloys preferential sputtering is a common phenomenon. In the steady state of sputtering, a significant depletion in Cu is found. At the oxide/metal interface, a distinct enrichment of copper is found for all alloys and treatments. The degree of this Cu enrichment depends on the pretreatment. It is higher for the electrochemically-passivated samples than for samples with oxide layers grown during melt spinning.     

XPS study of the surface chemistry of Ag-covered L-CVD SnO2 thin films

In this paper, we present the results of X-ray photoelectron spectroscopy characterization of SnO₂ thin films prepared by laser chemical vapour deposition (L-CVD) and subsequently covered by Ag atoms just after deposition and after long-term exposed to dry air, subsequent annealing in ultra high vacuum at 400 °C and dry air oxidation at 400 °C. Using the standard analytical procedure based on atomic sensitivity factors, the variation of surface chemistry defined in terms of the relative concentration of the main components of the films after the above-mentioned procedures has been determined. It was confirmed that after dry air exposure as well as dry air oxidation, the layers undergo an oxidation reaching almost SnO₂ stoichiometry. Besides, during ultra high vacuum annealing, the films undergo reduction to almost SnO stoichiometry. At the same time, Ag atoms deposited at the top of layers diffuse into the subsurface layers. This was confirmed by X-ray photoelectron spectroscopy depth profiling analysis.