Bulk Stoichiometry Research Articles

Surface and bulk cation distribution of the spinel system ZnxMg1−xFe2O4

ZnxMg1-xFe2O4 mixed oxide spinels (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by conventional solid state reaction method. Bulk and surface characterization of these ferrites were carried out by different techniques. The ratios Zn/Fe and Mg/Fe determined by PIXE and AAS were nearly as expected from the synthesis mixture, i.e. following the substitution model. XRD shows only a spinel structure (fcc) with increasing lattice parameter as x increases from 0 to 1. The bulk ratios of tetrahedral to octahedral sites occupied by Fe cations measured by NGR are in good agreement with the theoretical bulk stoichiometry. LEIS results indicate a Mg substitution by Fe cations at the surface. Only octahedral sites are proposed at the surface. The transition from inverse to normal spinel was follow through NGR-spectra occurring at x=0.4.

Antimony capping and decapping of InAlSb(100)

A method to protect clean and ordered InAlSb InSb(100) surfaces from atmospheric exposure by the deposition of an amorphous Sb layer is discussed. This layer may be subsequently removed by heat treatment at 490°C to generate a c(8 × 2) reconstructed surface that is predominantly In-rich and to a lesser extent Al-rich, with respect to the bulk stoichiometry.

Crystal chemical controls on the dissolution kinetics of the isostructural sulfates: Celestite, anglesite, and barite

The isostructural sulfates, barite (BaSO 4), celestite (SrSO 4), and anglesite (PbSO 4), share the crystallographic space group Pnma. With their crystal structure and bulk stoichiometry invariant, we investigate controls on the kinetics of sulfate mineral dissolution. Dissolution rates were measured in hydrothermal mixed flow reactors with solutions of pH 2 to 10 and at 25–140°C. Data were analyzed using a first order rate law. In general, dissolution rates follow the order celestite > anglesite > barite. Although the chemistry of lead differs from strontium and barium, anglesite has intermediate reactivity. All minerals exhibit a similar pH dependence of dissolution where rates decline with increasing pH for the range 2 to 5 and are approximately independent of pH over the range of 5 to 9. Above pH 9, anglesite dissolution rate increases sharply with increasing pH. An analysis of the temperature dependence of dissolution at near-neutral pH shows that the reactivity trend is controlled by differences in the preexponential component of the rate constant. Thus, reaction frequency, not energetics, primarily determines relative reactivity and suggests steric and/or solvation controls on reaction rate. Dissolution rates at near-neutral pH correlate inversely with both ionic radius and average bond length of the structural divalent atom. This leads us to consider the nature of the aqueous metal complexes that are released to solution upon hydrolysis. We find a positive correlation between solvation number and dissolution rate. This correlation extends to describe reported rates of anhydrite (CaSO 4) dissolution. Combining evidence, we suggest that rates of isostructural sulfate mineral dissolution are limited by the relative solvation affinity of the divalent metal atoms for near-surface water. Hydrolysis of the sulfate group is probably not rate-limiting since anionic hydration is extremely fast and approximates rates of aqueous diffusion. Dissolution rates are limited by the mineral component having the lowest solvation affinity. This model predicts dissolution rates of sulfate minerals within a solid-solution compositional series and may describe other minerals with strong anisodesmic character. Our study reiterates the role of near-surface solvent properties in controlling mineral reactivity in aqueous solutions.

Bulk and surface stoichiometry of vapor grown mercuric iodide crystals

The stoichiometry of HgI2 is reviewed as a function of the various stages of preparation including synthesis, purification, crystal growth and ending with device fabrication. Original data are presented on surface roughness of free surfaces of HgI2 determined by variable angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) and their relation to stoichiometry. The spread of the “c” lattice constant variation in bad HgI2 nuclear detectors was determined by triaxial X-ray diffraction (TAXRD), and the results are discussed in relation with deviation from stoichiometry. It is suggested that crystals purified by open tube sublimation are stoichiometric. However, high temperature purification treatments, such as melting and even growth, may induce a certain dissociation of HgI2 into Hg2I2 and free iodine. The latter diffuses, through a complex process, out of the crystal leaving a very rough outer surface and inhomogeneous stoichiometry in the bulk, which may be detrimental to device performance as a nuclear radiation detector.

Combined depth profile analysis with SNMS, SIMS and XPS: Preferential sputtering and oxygen transport in binary metal oxide multilayer systems

AbstractDepth profile measurements of binary metal oxide multilayer systems have been performed by combined SIMS, SNMS and XPS. The combination of data about the particle fluxes and thus the bulk stoichiometry by SNMS, the ionic fluxes by SIMS, and the surface composition and the oxidation state of the metals by XPS provides a detailed insight into the mechanism of preferential sputtering in these systems. We present results on the systems Al2O3/TiO2 and SiO2/HfO2. The signal behaviour at the interfaces cannot be explained by preferential sputtering alone, but must include an oxygen transport mechanism due to chemical driving forces. By comparison of these systems with four other multilayer systems, we have developed a model of combined preferential sputtering and oxygen transport which accounts for the experimental results. The investigated systems can be classified according to their metal/oxygen sputter yield ratio, the degree of preferential sputtering, and the heat of formation of the oxide.

Evaluation of temperature‐dependent surface chemistry in Zr2Fe and ZrVFe via X‐ray photoemission spectroscopy

AbstractThe near atomic composition and the character of chemical bonds in two non‐evaporable gettering alloys have been investigated via core level (Zr 3d; Fe 2p; V 2p; O 1s; C 1s) XPS (hv = 1253.6 eV). The samples have been measured in UHV after in‐air fracturing and various annealing steps (up to 850 °C). For T > 400 °C a progressive and sizeable decrease of the near surface C content along with an increase in the Zr concentration is observed in both alloys. An annealing‐induced Zr enrichment at the surface is found, the metallic atomic concentrations being permanently far from the nominal bulk stoichiometry. The annealing induces a strong evolution in the chemical bonds, the metallic species being completely oxidized in‐air fractured samples while progressively emerging a metallic character at increasing temperatures. Surface metallization proceeds faster in Zr2Fe than in the V‐containing getter.

Density of thin vapour-deposited films of zinc selenide

The density of thin vapour-deposited films of zinc selenide was determined by spectrophotometry combined with Rutherford backscattering spectrometry. Bulk stoichiometry of the films was measured by Rutherford backscattering spectroscopy. It was found that the films were relatively rich in Se. Depth profile study of the films, by x-ray photoelectron spectroscopy, revealed that surfaces were rich in Se while, away from the surface and inside the film, the Se content decreased with depth until the stoichiometric ratio of the compound ZnSe was reached. The density determined was 0.93+or-3% of the bulk value for ZnSe.

Direct analysis of chemically disordered compounds by inverted tensor LEED

We propose a method to evaluate measured LEED intensities of substitutionally disordered compounds to yield directly the layer-dependent deviations from the bulk stoichiometry. The method is based on tensor LEED and its modification to simulate the substitution of surface atoms. It is tested via comparison with theoretical data as well as via the application to experimental spectra from substitutionally disordered alloys. The new approach promises large savings of computer time, particularly when complicated structures are under consideration.

Hreels Studies of KxC60 Thin Films

ABSTRACTHigh resolution electron energy loss spectroscopy has been used to investigate the vibrational and electronic excitations of single-phase, crystalline AxC60 films. Substantial changes in intramolecular mode frequencies and intensities with alkali concentration arise from charge transfer effects as well as free carrier screening. For x=4, splitting of the t1u band is observed, resulting in an insulating phase with an estimated gap of 0.3-0.4eV. Screening in the x=3 phase results in enhanced surface sensitivity and deviations from bulk stoichiometry at the surface of metallic films are observed.

Growth and characterization of metal(II) alkanebisphosphonate multilayer thin films on gold surfaces

Thin films of divalent metal (Zn and Cu) alkanebisphosphonates grow on gold surfaces modified with (4-mercaptobutyl)phosphonic acid by alternate immersion in 5 mM ethanolic solutions of the metal acetate or perchlorate salt and H 2 O 3 P(CH 2 ) n PO 3 H 2 , n=8, 10, 12,and 14. Growth of each layer is remarkably fast. Well-ordered multilayers can be deposited with 10-min adsorption steps, and films of 100-layer thickness are easily prepared. Quartz crystal microbalance (QCM) measurements of mass changes are in quantitative agreement with adsorption of individual layers corresponding to bulk stoichiometry (M 2 [O 3 P(CH 2 ) n PO 3 ].2H 2 O) and pacing density

Study of ZrB 2 (BMn, V, Cr) intermetallic compounds by laser microprobe mass analysis

The effect of the hydrogen activation process on the concentration of intermetallic compound (IMC) alloy components in near-surface layers and in the bulk was investigated by means of laser microprobe mass analysis (LAMMA 1000 prototype). Nine AB 2-type samples were analysed: ZrV 2, ZrV 0.5Cr 1.5, ZrV 0.2Cr 1.8, ZrCr 2, ZrVMn, ZrCrMn, ZrMn 2, Zr 0.8Mn 0.2Mn 2.0 and Zr 0.7Mn 0.3Mn 2.0. For comparison, the samples were measured both in the activated and in the non-activated form. In each case, mass spectra relating to the surface (depth of 0.1 μm or less) and to the bulk (depth of 0.3 μm or more) were recorded. The quantification of the mass spectra intensities was carried out using relative sensitivity coefficients, as related to Zr, i.e. the element common to all the IMCs investigated in this study. The data obtained show that the surface compositions of non-activated and activated materials are quite similar. However, these compositions deviate from the bulk stoichiometry in that Zr is enriched in the near-surface region. It is found that the Zr segregation is oxygen induced in the case of non-activated materials and is hydrogen induced in the case of activated materials, whereby the extension of the Zr enriched layers is thin and thick, respectively.

AES study of Si LVV bonding states in various substoichiometric oxide environments

AbstractAuger electron spectroscopy has been applied to a study of chemical structure effects in silicon oxides caused by changes of the surface or bulk stoichiometry. Ion‐damaged SiO2, vacuum‐deposited SiOx (1 < × < 2) and Si/SiO2 interfaces were investigated. Silicon enrichment causes not only the growth of the elemental Si peak, but also a broadening of the main oxide peak. Detailed comparison of the Si LVV spectra in the three types of samples suggests that the elemental Si component occurs with the same local structure as in bulk silicon.

Kinetic model of Au-GaAs interfacial reaction

Open-system annealing of Au/GaAs leads to vaporization of As species and migration of Ga atoms into Au. These processes are simultaneous only on a macroscale. A simple kinetic model is developed in which they are treated sequential by introducing specific fluxes F(Ga) and F(As). In view of a high diffusivity of Ga in Au, F(Ga) is calculated assuming linear flow at the phase boundary. F(As) is set constant for a given external pressure of arsenic. Conservation of bulk stoichiometry of GaAs implies that the decomposition rate of GaAs is controlled by the slower process. The model accurately describes the observed reaction rate in both capped and open configuration. It is shown that, upon heating, the instantaneous composition of the GaAs surface in contact to the metal evolves from initially As rich to Ga rich when Au becomes saturated with Ga.

Quantitative XPS analysis of high Tc superconductor surfaces

The procedure of quantitative XPS analysis involving the relative sensitivity factors is most convenient to apply to high T c superconductor surfaces because this procedure does not require standards. However, a considerable limitation of such an approach is its relatively low accuracy. In the present work, a proposition is made to use for this purpose a modification of the relative sensitivity factor approach accounting for the matrix and the instrumental effects. The accuracy of this modification when applied to the binary metal alloys is 2% or better. A quantitative XPS analysis was made for surfaces of the compounds Bi 2Sr 2CuO 6, Bi 2Sr 2CaCu 2O 8, and YBa 2Cu 3O Y. The surface composition determined for the polycrystalline samples corresponds reasonably well to the bulk stoichiometry. Slight deficiency of oxygen was found for the Bi-based compounds. The surface exposed on cleavage of the Bi 2Sr 2CaCu 2O 8 single crystal was found to be enriched with bismuth, which indicates that the cleavage occurs along the BiO planes. This result is in agreement with the STM studies published in the literature.

Alloy sputtering at high fluence: preferential sputtering and competing effects

This work addresses composition profiles and partial sputter yields of an alloy under ion bombardment. The effects of preferential sputtering, mixing, and Gibbsian segregation have been modelled. The theoretical basis is an integro-differential equation derived previously, into which a feasible expression for a segregation current has been included. A dependence on composition of pertinent input parameters like relocation cross sections and atomic volumes has been allowed for but has not been explored explicitly. In the limit of high fluence, a stationary state may be reached with a composition profile that shows more or less pronounced deviations from bulk stoichiometry. Two methods have been designed for direct determination of such stationary solutions. An iterative scheme for numerical solution of the nonlinear balance equation is very efficient and accurate. An approximate scheme, assuming weakly preferential behavior and thus based on a linearized balance equation, turns out to be quite accurate even in case of strongly preferential sputtering. An explicit analytical solution is given for a particular example. On the basis of simple, schematic input, it is seen that the deviation of a stationary composition profile from bulk stoichiometry may be very strong, even when the sputter cross section is only weakly or moderately preferential. This difference is most pronounced in the absence of substantial feeding by mixing and segregation. In case of a nonzero depth of origin of sputtered atoms, composition profiles depend sensitively on the depth dependence of the sputter cross section, even in cases where consideration of only the partial sputter yields would suggest stoichiometric sputtering. A few examples illustrate explicitly the competition between preferential sputtering on the one hand and mixing or segregation on the other.

Surface core-level shifts of barium observed in photoemission of vacuum-fractured BaTiO3(100).

When clean, vacuum-fractured surfaces of BaTiO 3 are analyzed using ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS), more than one set of barium core levels is observed. Sputtering the surface with ions removes lower-binding-energy intensity within the barium line shapes in UPS spectra while little change is noted in XPS. Sputtering of the surface also causes band bending and the creation of band-gap surface states of Ti 3d character. From a comparison of UPS and XPS Ba 4d line shapes from a vacuum-fractured surface, the lower-binding-energy component is assigned to barium with 12-fold oxygen coordination representative of bulk stoichiometry; the higher-binding-energy intensity originates from undercoordinated barium in the region of the sample surface

Monitoring of the aluminum nitride sputtering deposition by soft x-ray emission spectroscopy

The deposition of aluminum nitride, by dc diode reactive sputtering has been studied by in situ soft x-ray emission spectroscopy. The deposition chamber is coupled with a vacuum x-ray computerized spectrometer. The x-ray emission from the film is excited by high energy secondary electrons from the electric discharge. The Al cathode is sputtered at constant current and voltage mode. Films are deposited at different argon/nitrogen gas mixtures, in the bias mode. For each cycle, x-ray intensities, deposition rate at the quartz microbalance and total pressure are recorded. It has been found that the Al Kα and N Kα x-ray intensity ratio decreases with increasing nitrogen flow rate. At partial nitrogen flow rates of 8.5%, a sharp decrease appears, followed by a plateau, where the film has the bulk stoichiometry. Films have been characterized for ultraviolet-visible absorption. Optical band gap and refractive index have been calculated. The fine structure of the Al Kβ line has been studied. A shift in the maximum intensity position is coupled with a satellite line toward the low energy side, both appear before the transition. These effects are characteristic of the chemical bonding between Al and N. X-ray diffractograms show a preferential orientation depending on the gas mixture composition, but the peaks are small and broad.

Trends in the bonding of CO to the surfaces of Pt3M alloys (M=Ti, Co, and Sn)

Studies of the surface composition and CO chemisorption on the [111] and [100] oriented single crystals of the isostructural [face-centered-cubic (fcc)] alloys of bulk stoichiometry Pt3M (M=Ti, Co, and Sn) are reviewed. In principle, the study of these alloys is ideal for separating ensemble effects from electronic effects in adsorbate bonding, since the surfaces of these ordered alloys formed by bulk truncation have the M atoms in fixed positions without any M–M pair sites. Also by varying M from Ti to Co to Sn one can examine the effect of the d and s–p orbital contributions to the intermetallic bond on the binding energy of CO. In practice, one must account for surface segregation, and the formation of surfaces that do not have the ideal structure and composition. The strength of the intermetallic bond is found to play a major role in determining the composition of the surface. In the very exothermic alloys Pt3Ti and Pt3Sn, the thermodynamic tendency for the atom with the lower surface energy to be at the surface determines which of the two bulk planes forms the surface plane in the [100] orientation, either pure Pt(Pt3Ti) or 50% M(Pt3Sn). In CoPt3 where the intermetallic bonding is relatively weak, the lower surface energy of Pt produces pure Pt planes on both the [111] and [100] orientations. Photoemission and thermal desorption studies show that the intermetallic bonding in all three alloys has only a small effect on the binding energy of CO, a decrease in adsorption energy of 20 kJ/mol. The surprising invariance of adsorption energy on Pt3M surfaces having such a large variation in the d-orbital configuration of the M atom is attributed to the predominance of s–p orbital contributions to the intermetallic bond in these alloys.

The temperature dependent variation of bulk and surface composition of In xGa 1− xAs on GaAs grown by chemical beam epitaxy studied by RHEED, X-ray diffraction and XPS

The bulk as well as near-surface composition of In x Ga 1− x As epilayers on GaAs grown by chemical beam epitaxy (CBE) has been investigated as a function of triethylindium (TEIn) flow rate and substrate temperature by reflection high energy electron diffraction (RHEED), X-ray diffraction, and X-ray photo electron spectroscopy (XPS). To clarify whether the bulk stoichiometry of CBE-grown ternaries can be extracted from the growth rate change as determined by the change in the period of RHEED oscillations from binary to ternary compound growth, a systematic study of growth rate change as a function of ternary bulk composition determined by X-ray diffraction has been performed at various temperatures. Our results show that for low growth temperatures there is a linear relationship between the two methods of determination, whereas no correlation is found for higher growth temperatures, in contrast to the MBE case where the two methods of determination give identical results. In the near surface region the epilayer composition is determined in situ by XPS. Using a new method for analysis of XPS spectra, where the inelastically scattered electrons are taken into account, an extraction of quantitative depth information on the upper monolayers is possible. Our results show that In surface segregation is of minor importance at 470°C, but surface segregation of In occurs in In x Ga 1− x As at growth temperatures from 550 to 600°C. For samples grown at 550 and 600°C the surface In enrichment is not restricted to the upper monolayers, but extends to a depth ⩾ 50–75 .

Stoichiometry and local atomic arrangements in crystals

Valence crystals have bulk stoichiometries that are electroneutral with respect to formal atomic valence, and structures in which cation sites are formed entirely by anions and vice versa. This description is shown to be equivalent to a set of stoichiometric and structural rules limiting local atomic configurations in valence crystals. Algebraic equivalents of these rules are used to derive a necessary mathematical link between bulk stoichiometry and local atomic arrangements in valence crystals. In the limit of no charge imbalances on anion sites, this link reduces to the electrostatic valence principle of Pauling. Numerical analysis shows that limits upon the coordination numbers of cations and anions severely restrict the number and type of local configurations that could ever be combined to form an electroneutral bulk stoichiometry, regardless of the valence of the anion or the magnitude of charge imbalances on anion sites.