Marcel A. J. Somers

Simultaneous determination of composition and thickness of thin iron-oxide films from XPS Fe 2p spectra

Abstract The composition and thickness of thin iron-oxide films on pure iron were determined from XPS Fe 2p spectra. To this end the spectra were reconstructed from reference spectra for Fe0, Fe2+ and Fe3+. The appropriate background due to inelastically scattered electrons was calculated for each reference spectrum, using a recent generalization of Tougaards method. For the reconstruction the film thickness and the relative amounts of Fe2+ and Fe3+ in the oxide film were used as fit parameters. A good agreement was obtained between experimental and reconstructed spectra.

Layer-growth kinetics on gaseous nitriding of pure iron: Evaluation of diffusion coefficients for nitrogen in iron nitrides

Models were derived for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example. In the models, the composition dependence of the solute diffusivity in the phases constituting the layers was accounted for by appropriate definition of an effective diffusion coefficient for a (sub)layer. This effective diffusion coefficient is the intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth kinetics of a γ′-Fe4N1-x monolayer on an α-Fe substrate and the growth kinetics of an ε-Fe2N1-z/γ′-Fe4N1-x bilayer on an α-Fe substrate, as observed by gaseous nitriding in an NH3/H2-gas mixture at 843 K. The kinetics of layer development and the evolution of the microstructure were investigated by means of thermogravimetry, layer-thickness measurements, light microscopy, and electron probe X-ray microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition dependence of the intrinsic (and effective) diffusion coefficients was established. Re-evaluating literature data for diffusion in γ′-Fe4N1-x on the basis of the present model, it followed that the previous and present data are consistent. The activation energy for diffusion of nitrogen in γ′-Fe4N1-x was determined from the temperature dependence of the self-diffusion coefficient. The self-diffusion coefficient for nitrogen in ε-Fe2N1-z was significantly larger than that for γ′-Fe4N1-x. This was explained qualitatively, considering the possible mechanisms for interstitial diffusion of nitrogen atoms in the close-packed iron lattices of the ε and γ′ iron nitrides.

Stress, strain, and microstructure of sputter‐deposited Mo thin films

Mo thin films were deposited on glass substrates using direct‐current (dc) planar magnetron sputtering. Mechanical determination of the internal stresses, using the bending‐beam technique, yielded typical compressive‐to‐tensile stress transition curves with increasing working‐gas pressure. The microstructure of the compressively stressed films consists of tightly packed columns, whereas in the tensily stressed films the development of a void network structure surrounding the columnar grains is observed. At elevated working‐gas pressures the onset of microcolumns is observed in the initial stage of film growth. Determination of lattice strains by x‐ray diffraction (XRD), utilizing the sin2 ψ method, encounters more difficulties than the more straightforward stress determination by the bending‐beam method. Here special attention is focused on deviations from linear dependence of dψ with sin2 ψ along with asymmetry of XRD line profiles that results from stress‐depth profiles as well as lateral stress distrib...

Stress, strain, and microstructure in thin tungsten films deposited by dc magnetron sputtering

Tungsten thin films were deposited on glass substrates by direct‐current planar magnetron sputtering. The induced thickness‐averaged film stress within the plane of the film was determined with the bending‐beam technique and changed from compressive to tensile on increasing working‐gas pressure. The microstructure of these films was investigated by cross‐sectional transmission electron microscopy. Compressively stressed films consisted of tightly packed columnar grains, whereas in films with a maximum value for the tensile stress the onset of a void network surrounding the columnar grains was observed. High‐pressure conditions resulted in dendritic‐like film growth, which brought about complete relaxation of internal stresses. The α phase was predominantly found in films under compression, while an increasing amount of β‐W coincided with the transition to the tensile stress regime. Special attention was focused on stress‐depth dependence and the development of two overlapping line profiles in x‐ray diffra...

On the Oxidation of α-Fe and ε-Fe2N1-z: I. Oxidation Kinetics and Microstructural Evolution of the Oxide and Nitride Layers

The oxidation of α-Fe and ɛ-Fe2N1−z at 573 K and 673 K in O2 at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. Upon oxidation at 573 K and 673 K, on α-Fe initially α-Fe2O3 develops, whereas on ɛ-Fe2N1−z initially Fe3O4 develops. In an early stage of oxidation the oxidation rate of ɛ-Fe2N1−z appears to be much larger than of α-Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for ɛ-Fe2N1−z than for α-Fe due to the porous structure of ɛ-Fe2N1−z as prepared by gaseous nitriding of iron. The development of a magnetite layer in-between the hematite layer and the α-Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for α-Fe and ɛ-Fe2N1−z become about equal, indicating that on both substrates the oxide growth is controlled by the same rate limiting step which is attributed to short-circuit diffusion of iron cations. Oxidizing ɛ-Fe2N1−z increases the nitrogen concentration in the remaining ɛ-iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind.

An Evaluation of the Fe-N Phase Diagram Considering Long-Range Order of N Atoms in γ'-Fe4N1-x and ε-Fe2N1-z

The chemical potential of nitrogen was described as a function of nitrogen content for the Fe-N phases α-Fe[N], γ′-Fe4N1-x, and ε-Fe2N1-z. For α-Fe[N], an ideal, random distribution of the nitrogen atoms over the octahedral interstices of the bcc iron lattice was assumed; for γ′-Fe4N1-x. and ε-Fe2N1-z, the occurrence of a long-range ordered distribution of the nitrogen atoms over the octahedral interstices of the close packed iron sublattices (fcc and hcp, respectively) was taken into account. The theoretical expressions were fitted to nitrogen-absorption isotherm data for the three Fe-N phases. The α/α+ γ′, α +γ′/γ′, γ′/γ′ + ε, andγ′ + ε/ε phase boundaries in the Fe-N phase diagram were calculated from combining the quantitative descriptions for the absorption isotherms with the known composition of NH3/H2 gas mixtures in equilibrium with coexisting α andγ′ phases and in equilibrium with coexistingγ′ and ε phases. Comparison of the present phase boundaries with experimental data and previously calculated phase boundaries showed a major improvement as compared to the previously calculated Fe-N phase diagrams, where long-range order for the nitrogen atoms in theγ′ and ε phases was not accounted for.

Thermodynamics and Long-Range Order of Interstitials in an h.c.p. Lattice: Nitrogen in ε-Fe2N1-z

Abstract The thermodynamics of nitrogen in the e-Fe 2 N 1 − z phase were evaluated. To this end absorption isotherms for nitrogen in e-Fe 2 N 1 − z , depicting the dependence of the nitrogen content on the applied chemical potential of nitrogen, were determined in the temperature range 673–823 K by equilibrating iron foils in gaseous ammonia/hydrogen mixtures. The absorption isotherms could be described very well by a Gorsky-Bragg-Williams approximation for long-range order (LRO) of nitrogen atoms on the hexagonal nitrogen sublattice, formed by the octahedral interstices of the h.c.p. sublattice of iron atoms. The actual occurrence of LRO was evidenced by diffraction analysis. Mossbauer spectra were interpreted as composed of spectra of iron atoms surrounded by 1–3 nitrogen atoms. The relative amounts of the iron environments as determined from the Mossbauer spectra agreed very well with the corresponding probabilities predicted by the LRO model.

On the initial oxidation of iron: Quantification of growth kinetics by the coupled-currents approach

Polycrystalline iron was oxidized at pO2=10−4 Pa and at temperatures ranging from 300 to 500 K. Ellipsometry was used for monitoring the oxide-film thickness as a function of time. The oxidation kinetics were described quantitatively by application of the model due to Fromhold and Cook by considering coupled currents of cations and electrons. At the lower temperatures tunneling is the dominant electron transport mechanism and an excellent agreement of experimental and calculated oxidation kinetics was obtained by adopting a time-dependent difference of the work functions of the metal-oxide and the oxide-oxygen interfaces. At the higher temperatures the experimental kinetics can be described quantitatively for a film thickness up to about 3 nm. Above this thickness electron transport becomes dominated by thermal emission rather than by tunneling. To investigate the influence of the surface pretreatment on the oxidation kinetics a sample was oxidized at pO2=10−4 Pa at 330 K, both after sputter cleaning and ...

The initial oxidation of ε-Fe2N1−x: an XPS investigation

Abstract The initial oxidation of α-Fe and e-Fe 2 N 1− x , subjected either to a sputter cleaning pretreatment or a sputter cleaning plus additional annealing pretreatment, was investigated with XPS. The samples were oxidised at p O 2 =8·10 −5 Pa and temperatures ranging from 300 to 600 K. From the Fe 2p and O 1s spectra the thickness and composition of the oxide film was determined. The composition of the oxide films formed on α-Fe and on e-Fe 2 N 1− x was only a function of oxidation temperature and film thickness and was independent of the composition or pretreatment of the substrate. Analysis of the N 1s spectra provided information on the electric charge on nitrogen atoms and the depth distribution of nitrogen. A linear relation was found between the N 1s electron binding energy and the nitrogen concentration in the substrate. Upon oxidation of the iron nitride, nitrogen atoms accumulated underneath the oxide film. If the nitrogen concentration at that location exceeded the maximum solubility of nitrogen in e-Fe 2 N 1− x an additional N peak appeared in the N 1s spectrum, which indicated the formation of a nitrogen containing phase other than e-Fe 2 N 1− x at the nitride–oxide interface. For the oxygen exposures applied, the oxide-film thickness decreased with increasing nitrogen concentration in the substrate. The effect of nitrogen in the substrate on the initial oxidation was evaluated from the results.

Thermodynamics and Long-Range Order of Nitrogen in γ'-Fe4N1-x

Models are given for the description of the chemical potential of nitrogen in γ′-Fe4N1-x. In previous work, γ′-Fe4N1-x was treated as a (sub)regular solution, thereby assuming that the N atoms are distributed randomly on the sites of their own sublattice. However, in γ′-Fe4N1-x long-range ordering occurs of the N atoms over the sites of their own sublattice. Then, the expression for the configurational entropy should account for the occurrence of ordering. In the present article, the descriptions adopted and tested for γ′-Fe4N1-x are based on a Langmuir-type approach, the Wagner-Schottky (WS) approach, and the Gorsky-Bragg-Williams (GBW) approach. Application of the various models to data of nitrogen-absorption isotherms for the γ′ iron-nitride phase shows that the subregular solution concept fails to describe the experimental data satisfactorily, whereas a very good agreement between theory and experiment is obtained for the WS and GBW approaches. It is shown that, in particular, accounting for the occupation of disorder (octahedral) sites by N atoms is necessary to obtain an accurate description of the chemical potential of nitrogen in γ′-Fe4N1-x.