Angle-resolved X-ray Photoelectron Spectroscopy Measurements Research Articles

Electrical characterization of the p-Hg1−xZnxTe interface after anodic sulfidization treatments

We have investigated the p-Hg1−xZnxTe interface, with x in the range of 0.19–0.23, after electrochemical treatments with Na2S in ethylene glycol using admittance spectroscopy of metal–insulator–semiconductor structures made on these passivated surfaces. Native sulfide layers are characterized from angle-resolved x-ray photoelectron spectroscopy measurements and are constituted mainly by ZnS. The surface roughness is ascertained by atomic force microscopy. Free carriers densities and the degree of compensation for the semiconductor are deduced from capacitance measurements: the method is based on a numerical analysis of the normalized capacitance measured at high frequency as a function of the bias voltage. The positive fixed charge density in the native sulfide layer is found to be in the range of 1010–1011 cm−2 as deduced by the shift in the C–V characteristics. C–V plots show a very small hysteresis effect at 77 K, resulting in a low density of slow surface states (∼1×1010 cm−2). The energy distribution of fast interface states is calculated in two ways from capacitance and conductance measurements. The conductance method shows a higher sensitivity. A broad U-shape distribution is obtained for the different samples studied, with a reproducible minimum density of about a few 1010 eV−1 cm−2. The best passivation conditions are obtained with a constant current density of about 100 μA cm−2 and a sulfidization duration of ∼15 min. The time constant of the interface traps is deduced from the energy loss peak and leads to an estimation of their capture cross section. All these results show that a very high interface quality can be obtained on p-Hg1−xZnxTe in a reproducible way by anodic sulfidization.

Electrochemical and XPS studies on the passivation behavior of sputter-deposited W-Cr Alloys in 12 M HCl solution

Nanocrystalline, single bcc solid solutions of W-Cr alloys have been prepared by DC magnetron sputtering over a wide composition range. The passivation behavior of these alloys was studied by electrochemical measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The W-Cr alloys are passivated spontaneously and showed significantly high corrosion resistance in 12 M HCl solution at 30 °C. Their corrosion rates are about one order of magnitude lower than that of sputter-deposited tungsten and about five orders of magnitude lower than that of chromium metal even after prolonged immersion. XPS analysis showed that tungsten is enriched in the spontaneously passivated films formed on the alloys after long immersion time, and the passive films on the W-Cr alloys are found to be composed of double oxyhydroxide of tungsten and chromium ions. Angle-resolved XPS measurements reveal that tungsten and chromium ions are homogeneously distributed in the spontaneously passivated films. The synergistic effect of tungsten and chromium in forming the double oxyhydroxide is responsible for the higher corrosion resistance of the W-Cr alloys than of the alloy component metals.

Electrochemical and XPS studies of the corrosion behavior of sputter-deposited W-Nb alloys in concentrated hydrochloric acid solutions

Amorphous W-Nb alloys containing 52–78 at% niobium have been prepared successfully by DC magnetron sputtering. The W-Nb alloys are passivated spontaneously and show high corrosion resistance in both 6 and 12 M HCl solutions at 30 °C. In particular, their corrosion rates in 12 M HCl solution are about one order of magnitude lower than those of tungsten and niobium. Analysis by X-ray photoelectron spectroscopy (XPS) shows that tungsten is slightly enriched in the spontaneously passive films formed on the alloys after immersion for a long time in HCl solutions at 30 °C, and the passive films are composed of double oxyhydroxides of tungsten and niobium ions. Angle-resolved XPS measurements reveal that tungsten and niobium ions are homogeneously distributed throughout the passive films. The formation of the spontaneous passive film composed of the homogeneous double oxyhydroxide of tungsten and niobium ions is responsible for the high corrosion resistance of the W-Nb alloys.

A surface analytical investigation of the electrochemical double layer on silver electrodes in chloride solutions

Abstract The electrochemical double layer on Ag in NaCl solutions was examined with X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The potential dependent surface concentration of the adsorbed species (Na + , Cl − , H 2 O) as well as the cationic excess charge was determined. The specific adsorption of Cl − is accompanied by coadsorption of cations and water. Angle-resolved XPS measurements and ISS experiments gave more detailed information about the structure of the double layer perpendicular to the silver electrode surface. A layered structure of the specifically adsorbed chloride anions and the coadsorbed cations was determined, in agreement with recently published studies.

An angle-resolved xps study of the in-depth structure of passivated amorphous aluminum alloys

Angle-resolved X-ray photo-electron spectroscopy (ARXPS) measurements were performed in order to characterize the in-depth structures of the passive film and underlying alloy below the passive film for sputter-deposited corrosion-resistant amorphous aluminum alloys. Titanium cation is enriched throughout the passive film formed on an amorphous Al36Ti7Mg alloy, while chromium cation is enriched in the exterior of the passive film formed on an amorphous Al36Cr9Mo alloy and is deficient in the interior. The analyses of the in-depth distribution of constituents in the passive films indicate that the field-assisted outward migration of chromium cation in the passive film is faster than that of aluminum cations though the corrosion resistance of chromium is apparently higher than that of aluminum in 1 M HCl. The protective ability of the passive film is attributed to the stability of the chromium-enriched surface layer of the passive film. The outward migration of titanium cation is slower than that of aluminum. The investigation of the structure of the underlying alloy below the passive film shows that chromium and titanium are enriched in the underlying alloy surface and the composition gradually approaches the alloy composition with depth. The thickness of the chromium- or titanium-enriched region estimated is less than 1 nm.

Corrosion-resistant amorphous aluminum alloys and structure of passive films

The corrosion behavior of amorphous Al-Ti-Ni, Al-Cr-Ti and Al-Cr-Mo alloys in 1 M HCl and that of amorphous Al-Ti-Mg alloys in chloride containing neutral solution is investigated. The amorphous Al-Ti-Ni, Al-Ti-Cr and Al-Cr-Mo alloys are passivated spontaneously even in 1 M HCl and show lower corrosion rates and higher pitting potentials in comparison with Al-Ti and Al-Cr binary alloys. Al-Ti-Mg alloys containing relatively low concentration of magnesium possess a better resistance against pitting corrosion. X-ray photoelectron spectroscopy measurement reveals that titanium and chromium are enriched in the passive films formed on Al-Ti-Ni, Al-Ti-Mg and Al-Cr-Mo alloys, respectively. Angle-resolved XPS measurements were performed in order to characterize the in-depth distribution of constituents in the passive films formed on amorphous Al-Ti-Ni, Al-Ti-Mg and Al-Cr-Mo alloys in the passive films of which, passivating elements are enriched.

Spontaneously passivated films on sputter-deposited Cr-Ti alloys in 6 M HCl solution

Among sputter-deposited Cr-Ti alloys, 30–65 at% Ti alloys are amorphized. The Cr-Ti alloys with 22at% or less Ti are bcc single phase alloys. Regardless of the crystallinity, the open circuit potentials of sputter-deposited Cr-Ti alloys are located in the passive regions of both titanium and chromium in 6 M HCl solution open to air at 30 °C, and all Cr-Ti alloys are spontaneously passivated. X-ray photo-electron spectroscopy (XPS) analysis reveals that the spontaneously formed passive film as well as air-formed film is slightly rich in titanium ions mainly because of preferential oxidation of titanium. According to angle-resolved XPS measurement, no concentration gradient of Cr 3+ and Ti 4+ ions has been detected in depth of the passive film. Analysis of binding energies of core electrons of Cr 3+ and Ti 4+ cations shows that Cr 3+ and Ti 4+ cations are located very closely in the film so as to show the electronic interaction, and the film is not composed of a heterogeneous mixture of chromium and titanium oxyhydroxides but of homogeneous double oxyhydroxide consisting of Cr 3+ and Ti 4+ cations. The formation of the homogeneous double oxyhydroxide film containing both Cr 3+ and Ti 4+ cations is responsible for extremely higher corrosion resistance of the homogeneous single phase Cr-Ti alloys in 6 M HC1 open to air at 30 °C in comparison with the corrosion resistance of chromium and titanium metals.

Characterization of Ferritin Adsorption onto Gold

The adsorption of ferritin from phosphate-buffered saline (PBS) onto gold has been examined using a quartz crystal microbalance (QCM), surface plasmon resonance (SPR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Application of these techniques allows elucidation of the surface coverage and layer thickness of ferritin on gold. The kinetics of ferritin adsorption onto gold were investigated by monitoring the resonant frequency shift of a QCM. Adsorption isotherms for ferritin in PBS and in air have been measured by QCM. These isotherms suggest less than monolayer coverage of ferritin on gold. A layer thickness of 4.8 ± 0.2 nm was calculated for the dry ferritin film from QCM data. Measurements of the shift in the reflectivity of light at a fixed angle close to the SPR plasmon resonance were also used to follow the kinetics of ferritin adsorption. Full SPR curves were measured in PBS solution and air and were used to determine the effective thickness of the ferritin layer in both environments. The ferritin layer on gold from SPR data was found to be twice as thick when measured in PBS as it was for a dry film. This difference in thickness is attributed to shrinkage of ferritin with drying. Angle-resolved XPS measurements on a dry ferritin film (preadsorbed on gold) yield a ferritin thickness of 4.7 ± 0.5 nm, a value in good agreement with those determined from QCM and SPR. QCM, SPR, and XPS all yield a surface coverage of 6.3 ± 0.7 mg m−2for dry ferritin layers on gold. AFM enabled examination of the topography of ferritin adsorbed on gold on the nanometer scale and confirmed that ferritin forms an incomplete monolayer. In all cases, ferritin was found to be irreversibly adsorbed to gold and to form a stable protein layer, thus making it well suited as a biological receptor layer for immunosensing applications.

Ternary SiGeC alloys: growth and properties of a new semiconducting material

The growth and properties of Si 1− y C y and Si 1− x− y Ge x C y alloys pseudomorphically strained on Si(001) will be critically reviewed. Although the bulk solubility of carbon in silicon is small, epitaxial layers with more than 1 at.% C can be fabricated by molecular beam epitaxy and different chemical vapour deposition techniques. One of the most crucial questions is the relation between substitutional and interstitial carbon incorporation, which has a large impact on the electrical and optical properties of these layers. We will show that the interstitial to substitutional carbon ratio is strongly influenced by the chosen growth conditions, like growth temperature and Si growth rate. In addition, angle-resolved X-ray photoelectron spectroscopy measurements indicate a surface segregation of interstitial carbon-containing complexes (more significant for higher growth temperature). Substitutionally incorporated C atoms allow strain manipulation, including the growth of an inversely strained Si 1− x− y Ge x C y layer. Local ordering effects due to atomic size differences and the growth on reconstructed surfaces, the mechanical and structural properties, and the influence of C atoms on band structure of Si and SiGe layers will be discussed.

New Ge substrate cleaning method for Si 1−x−yGe xC y MOMBE growth

A new method of cleaning a Ge wafer based on a wet chemical treatment and Si 1−x−y Ge x C y /Ge(001) metalorganic molecular beam epitaxial (MOMBE) growth has been performed. It consists of removing the native oxide by ammonia aqueous followed by a diluted sulfuric acid rinse and hydroperoxide treatment to form a passive oxide layer. An extremely uniform surface was obtained by 300 s ammonia etching of the native oxide. Thickness of the oxide layer grown in hydroperoxide is estimated to be ≈1.87 nm using angle-resolved X-ray photoelectron spectroscopy measurement. A well-ordered 2×1 reflection high-energy electron diffraction pattern appeared after 400 °C, 30 min annealing in ultra high vacuum. Si 1−x−y Ge x C y MOMBE growth has been performed on cleaned Ge(001) using disilane (Si 2H 6) and monomethylgermane (CH 6Ge). The 〈001〉 axis of the ternary alloy layers is oriented toward Ge(001). No peak corresponding to SiC was detected in X-ray diffraction data. © 1997 Elsevier Science S.A. All rights reserved.

Determination of Negative Binding Sites on Hair Surfaces UsingXPS and Ba2+ Labeling

The affinity for adsorption of cationic conditioning polymers and organics to hair fiber surfaces is largely determined by the concentration of negative sites on the hair surface. In this study, XPS and Ba 2+ labeling were used to determine the negative site concentration on brown and bleached hair samples. Differences in negative site concentration were determined for each hair type as well as along the length of the hair fibers from root to tip end. The middle and root sections of the bleached hair exhibited a higher negative site concentration than the brown hair. The negative site concentration increased from root to tip for the brown hair, but decreased from root to tip for the bleached hair. Differences in negative site concentration between hair samples were correlated with variations in the RSO 3 - concentrations on the fiber surfaces. Barium ion uptake in addition to that associated with RSO 3 - was attributed to carboxyl site adsorption or to Ba(OH) 2 entrapment within the pores of the hair fiber. Angle-resolved XPS measurements suggested that Ba 2+ ions at the outer surface of the fibers were bonded to two negative surface sites, in agreement with theory. Barium ions in the subsurface region of the fibers may be bonded to only one negative site or may be present as entrapped Ba(OH) 2 .

A study of the structure of a passive film using angle-resolved X-ray photo-electron spectroscopy

Angle-resolved X-ray photo-electron spectroscopy (ARXPS) measurements were performed in order to characterize the in-depth distribution of constituents in the passive film formed on amorphous Al-36Cr-9Mo alloy in 1 M HCl. The result showed that chromium cations were enriched in the exterior of the passive film and aluminum cations were rich in the interior. A model of in-depth distribution of constituents in the passive film considering migration and diffusion of cations was proposed. Experimental data by ARXPS were in agreement with the exponential concentration in-depth distribution of cations of the passive film. The analysis indicated that the migration and diffusion of chromium cations in the passive film were faster than those of aluminum cations. The enrichment of chromium at the outermost surface of the passive film was attributed to the faster dissolution of aluminum cations into the solution in comparison with that of chromium cations.

Electrochemical Oxidation of ZrN Hard (PVD) Coatings Studied by XPS

Zirconium nitride (ZrN) belongs to the group of technologically important materials, namely hard tribological coatings. X-ray photoelectron spectroscopy (XPS) in combination with polarization experiments and electrochemical a.c. impedance spectroscopy has been used to study the composition, structure, thickness and electronic properties of layers formed by electrochemical oxidation of ZrN in phthalate buffer (pH 5.0). Results are compared to those obtained for layers formed by thermal oxidation at elevated temperature, i.e. ZrO2 , as well as by air oxidation at room temperature. Electrochemical oxidation of ZrN to ZrO2 proceeds via the formation of a mixed oxynitride/oxide layer, which then transforms into oxide at sufficiently positive potentials. Angle-resolved XPS measurements contribute to a better understanding of the in-depth layer structure. Exposure of ZrN to air at room temperature results in the formation of a thin oxynitride/oxide layer. The layer exhibits a gradually changing in-depth distribution of various species, i.e. nitride, oxynitride and oxide. Analysis of XPS valence band spectra offers valuable information concerning the electronic properties of investigated materials. The experimental valence band spectrum for ZrN correlates well with a theoretically calculated density-of-states diagram from the literature. In contrast to ZrN, which is a metallic conductor, layers formed by electrochemical oxidation, as well as by air oxidation, exhibit insulating properties. The insulating character of the formed layers is supported also by a decreased current density in subsequent cycles in cyclic voltammograms, as well as by the resistivity value (∽1012 Ω · cm) calculated from a.c. impedance measurements. Therefore, the oxide layer formed on ZrN is stable and prevents further oxidation of the ZrN substrate.

The growth and structure of titanium oxide films on Pt(111) investigated by LEED, XPS, ISS, and STM

Titanium oxide films grown on Pt(111) have been investigated using LEED, XPS, ISS, and STM for coverages ranging from 1.0 to 5.0 ML. The oxide overlayers are prepared by vapor phase deposition of titanium metal followed by oxidation in 10 −6 Torr O 2. Partial dissolution of the overlayer into the Pt substrate occurs upon annealing to temperatures in excess of 500°C. Annealing in 1 × 10 −6 Torr O 2 at temperatures ranging from 500°C to 700°C leads to a three-fold symmetric structure with a unit cell of 18.2 A ̊ × 18.2 A ̊ . This structure is observed at coverages ranging from 1.0 to 5.0 ML. XPS measurements of this phase show the stoichiometry of TiO 2. Heating in vacuum at 650°C to 850°C leads to a new structure with a unit cell of 18.2 A ̊ × 13.9 A ̊ which is observed at coverages of 1.0 to 1.5 ML. This oxide overlayer is shown by XPS to have the stoichiometry of Ti 4O 7. Similarities are observed between the ordered overlayers and reported structures of stoichiometric and reduced rutile TiO 2 surfaces. Angle resolved XPS measurements of disordered titanium oxide films, formed by annealing at temperatures below 500°C, indicate that reduced Ti 3+ is concentrated at the oxide /Pt(111) interface.

Preferential Sputtering and Surface Segregation of Mn in Fe–Mn Alloys

The surface composition in Fe-1.9, 5.8 and 5.6 at% Mn alloys was investigated with X-ray photoelectron spectroscopy (XPS). Attention was focused particularly on changes in the surface composition by argon ion sputtering and by heating in ultra high vacuum. The depth distribution of manganese near the surface was examined with angle resolved XPS measurements. It was found that manganese atoms are preferentially sputtered from the surface by the ions, and that by heating above 600 K they are segregated to the surface cleaned by the ions. The onset temperature of the segregation is lower than that expected from the bulk diffusion of manganese, indicating that the surface segregation of manganese atoms may be enhanced by surface damages produced by the ion sputtering

(Ca, Sr)F 2 surface roughness effect on angle resolved XPS measurements

Nanometric scale surface morphology of layers on substrates can be investigated using angle dependent X-ray photoemission spectroscopy. Experimentally we differentiate the two typical surface forms of (Ca, Sr)F 2 for the (100) and the (111) orientations by means of angular photoemission intensity profiles. The surfaces of these fluorides are known to be terminated by facets in the case of the (100) orientation and to be flat in the case of the (111) orientation. We calculate the influence of an idealized triangular surface roughness of layers on photoemission intensity. The calculation is compatible with the measurements. We show that within certain conditions the intensity ratio ( I layer I substrate ) can be used as a measure of the substrate coverage.

A fast method for the simulation of XPS and AES spectra

Results of Monte Carlo simulation are often applied for quantitative XPS or AES analysis. In this work we show that for almost homogeneous samples, translations and rotations of (Monte Carlo) simulated electron trajectories lead to a substantial reduction in computation time. The method is applied to angle-resolved XPS measurements. It is shown that the distribution of a low-concentration element in the outermost 50 Å of a solid can be measured quantitatively.

X-Ray Photoelectron Spectroscopic Analysis of the Oxide of GaAs

Oxides of GaAs grown using various oxidation processes were analyzed with X-ray photoelectron spectroscopy (XPS). Oxides investigated were the native (naturally grown or exposed to air), the chemical (grown in boiling deionized water) and the thermal (at 350°C and 500°C in dry oxygen) ones. With the use of a spectral deconvolution technique, all types of suboxides of both As and Ga including elemental arsenic were observed in addition to well-known As2O3 and Ga2O3. Elemental arsenic is considered to be one of the oxidized forms of GaAs. As2O5 was observed in the thermal oxides. In the chemical oxide and the native oxide grown in short exposure to air, elemental arsenic is the main component of oxide, while As2O3 is the dominant species in more highly oxidized films such as the thermal oxide. XPS data suggest that oxidation of As bonded in GaAs proceeds as GaAs→elemental As (As0)→As2O (As1+)→AsO (As2+)→As2O3 (As3+)→As2O5 (As5+). Oxidation of Ga bonded in GaAs advances as GaAs→Ga2O (Ga1+)→GaO (Ga2+)→Ga2O3 (Ga3+). Angle-resolved XPS measurements and semiquantitative analyses of these data were performed and an effective thickness of each oxide was also derived with simplified assumptions. The native and the chemical oxides were nearly stoichiometric. However, the thermal oxide was substantially Ga-rich due to desorption and evaporation of As2O3 from the surface during oxidation.

Characterization of oxide layers on Mg(0001) and comparison of H 2O adsorption on surface and bulk oxides

The composition, continuity, stoichiometry, and thickness of MgO thin films formed on the Mg(0001) surface were characterized by AES, ISS, UPS and XPS. The layer-by-layer mechanism of film growth was supported by comparison of the AES and ISS profiles for oxygen uptake, and by angle-resolved XPS results. Water was found to dissociate on both the thin film and the MgO(100) surfaces, forming surface hydroxyl groups upon adsorption of 200 K. The hydroxyls were characterized by an O(1s) peak at 533.1 eV in XPS and a valence band emission at 11.8 eV in UPS. The coverage of OH species determined by angle-resolved XPS measurements was ∼ 1 monolayer following adsorption at 210 K on a MgO(100) surface which had been sputtered but not annealed. Hydroxyl formation could be suppressed by annealing the sputtered MgO(100) surface prior to H 2O exposure at room temperature. Thus sites of lower coordination number than those on the ideal MgO(100) surface are implicated in H 2O dissociation.

Surface studies of amorphous W75Si25 oxidation

The surface composition and oxide growth kinetics of amorphous W75Si25 sputter-deposited thin films were examined using x-ray photoelectron spectroscopy (XPS), ellipsometry, and thermogravimetric analysis (TGA). These films have been proposed as metallization layers on compound semiconductors for high-temperature applications. The 1-μm-thick films were oxidized in air at temperatures of 30 to 600 °C for times of 10 min to 30 h. Angle resolved XPS measurements indicated that the surface region consisted of a thin layer (∼16 Å) of primarily SiO2 above a layer of mainly WO3 for oxidation temperatures up to 425 °C. At oxidation temperatures between 150 and 425 °C the WO3 signal from the surface region was enhanced. In this range the SiO2 layer was thinner resulting in a larger detected signal from the underlying WO3 layer. For oxidation temperatures above 450 °C, the two oxides appeared to coexist although the surface region was enhanced in SiO2. Ellipsometry and TGA measurements did not indicate a change in oxide growth kinetics in the temperature range 150–550 °C. The kinetics were parabolic and displayed an Arrhenius-type temperature dependence with rates similar to W oxidation.