Oxide Growth Kinetics Research Articles

Ultrathin films of lead oxide on gold: Dependence of stoichiometry, stability and thickness on O 2 pressure and annealing temperature

Ultrathin oxide films grown in vacuum are important in many industrial areas, including microelectronics and heterogeneous catalysis. In this paper, the dependence of oxide stoichiometry, growth kinetics, thickness and stability on O 2 pressure and annealing temperature are explored using a high-stability quartz-crystal microbalance and Auger spectroscopy, for the oxidation of lead on gold as a model system. The oxide thickness increases abruptly at specific values of the O 2 pressure, as explained previously using Gibbs free energies. A qualitative difference is found between lead-oxide films which are 1 monolayer thick and those which are 2 or more monolayers thick; the former apparently involve exclusively chemisorbed oxygen and can be oxidized and reduced reversibly using thermal oxidation/annealing cycles, whereas the latter involve an extended lead oxide, are more thermally stable, and have a smaller electron inelastic mean free path. Accurate values of the O 2 sticking probability are obtained.

Non uniformities of silicon oxide films grown in peroxide mixtures

The growth kinetics of silicon chemical oxides in H 2O 2-containing solutions at various pH values and temperatures was studied by electrochemical impedance spectroscopy (EIS), ellipsometry and X-ray photoelectron spectroscopy (XPS). Infrared (IR) spectroscopy was also used to investigate the evolution of the surface chemistry from the initial Si–H hydrogen coverage to the subsequent oxidation states by analysing the Si–O–Si stretching vibration modes. Successive EIS diagrams obtained in SC1 solutions (NH 3 + H 2O 2 + H 2O) as a function of time constituted a series of semicircles indicating that the semiconductor/oxide/electrolyte (SOE) junction can be modelled as an RC circuit, in which the R term increased to almost 1 MΩ cm 2 after 3 h. It is generally known that, in alkaline solutions such as SC1, the oxidation rate rapidly reaches a steady regime controlled by the interfacial charge transfer reaction and the subsequent dissolution of the generated chemical oxide. Accordingly, we propose a mechanism involving a diffusion process of reactants through the oxide barrier followed by a simultaneous dissolution of the layer. Moreover, in acidic media such as SC2 (HCl + H 2O 2 + H 2O), even though the solubility is extremely low, we have extended our model based on the competition between the oxidation rate at the surface and the dissolution on the nanoscopic scale of the built-up oxide. Both EIS and IR spectroscopy of the residual Si–H x bonds lead to a non-uniformity of the surface oxide growth proceeding from island nuclei. In addition, thickness measurements by ellipsometry together with the observed gradual change in the IR spectrum of the Si–O–Si vibration mode were interesting parameters revealing that the oxide growth proceeds simultaneously with an evolution of the structure leading to a more compact and insulating dielectric layer.

Growth mechanism and characterization of chemical oxide films produced in peroxide mixtures on Si(100) surfaces

The growth kinetics of silicon chemical oxides in H 2O 2-containing solutions at various pH values and temperatures was studied by electrochemical impedance spectroscopy (EIS), ellipsometry and X-ray photoelectron spectroscopy. Infrared (IR) spectroscopy was also used to investigate the evolution of the surface chemistry from the initial hydrogen coverage as Si–H bonds to the subsequent oxidation states by analysing the Si–O–Si stretching vibration modes. Successive EIS diagrams obtained as a function of time constituted a series of semicircles indicating that the semiconductor/oxide/electrolyte junction can be modelled as a resistance-capacity (RC) circuit. It was then observed that the resistance term increased with time to almost 1 MΩ cm 2 after 3 h in SC1 solution. It is generally known that, in alkaline solutions such as SC1, the oxidation rate reaches rapidly a steady regime controlled by the interfacial charge transfer reaction and the subsequent dissolution of the generated oxide. Accordingly, a mechanism involving a diffusion process of reactants through the oxide barrier followed by a simultaneous dissolution of the layer is proposed. Likewise, in acidic media such as SC2, even though the solubility is extremely low, we have extended our model based on the competition between the oxidation rate at the surface and the dissolution of the built-up oxide, and thus it was possible to evidence oxide solubility at the nanoscopic scale. In addition, thickness measurements by ellipsometry together with the observed gradual change in the IR spectrum of the Si–O–Si vibration mode were interesting parameters revealing that the oxide growth proceeds simultaneously with an evolution of the structure leading to a more compact and insulating dielectric layer. The whole of the results lead to a model for the oxidation process accounting for the observed structure and complex impedance properties under various conditions of chemical treatment.

Deep oxidation of aluminum by a DC oxygen plasma

A novel way of oxidising aluminum using a DC oxygen plasma is described. The oxidation is carried out with a pressure of ∼0.1 bar, an electrical current lower than 3 mA, and a working distance between the electrodes of the order of 1 cm. The pressure is seen to have a stronger influence on the results than the working distance. The process does not damage the surface and only minor differences are detected in the topography due to the expansion of the aluminum during oxidation. It is shown that the region affected by the plasma results in a ∼50-nm-thick amorphous aluminum oxide layer (OL). We find that the kinetics of oxide growth can be described as having two main sources, the main one originating from the plasma and the other from the surrounding ionized gas.

Reactivity of solids studied by in situ XAS and XRD

High-temperature oxidation processes of metals are important solid-state reactions from a fundamental point of view and also concerning technical applications. The kinetics of these solid-state reactions can be studied in situ by X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) at elevated temperatures and defined oxygen partial pressures. Oxidation of thin cobalt foils in air was studied by means of Quick X-ray Absorption Spectroscopy (QEXAFS) at the Co–K edge. By deconvolution of the spectra, the fractions of Co, CoO and Co 3O 4 were determined as a function of time. The overall oxidation reaction can be described by a three-step mechanism in which CoO is formed intermediately. By in situ XRD, the growth kinetics of both oxides and the depth-resolved texture of the growing oxides were determined simultaneously. At the beginning of the oxidation process, a nearly statistical distribution of oxide grain orientations is found, whereas the evolution of a distinct texture is observed during further growth of the oxide scale.

Comment on “Growth kinetics of oxides during furnace oxidation of silicon in N2O ambient” [J. Appl. Phys. 78, 2767 (1995)

Bhat, Jia, and Kwong [J. Appl. Phys. 78, 2767 (1995)] proposed a model for explaining the growth kinetics of oxides obtained during thermal oxidation of silicon in N2O. They claimed that they were able to explain the growth rate for different temperatures and the nitrogen content within the silicon oxynitride. We show that Bhat’s model was erroneous, both in the proposal of the corresponding differential equations, and also in their solution. Therefore, all their analysis should be modified so that values for the diffusivities and reaction rates of oxygen and NO through the Si oxynitride can be determined by means of a correct model.

Competition between Stress Generation and Relaxation in Iron Oxide Films. Experiments and Modelling

In the present work, the calculated growth stresses arising from iron oxide layers are compared to measurements of the macroscopic residual stresses. High temperature x-ray diffraction was used for determining the stress created in thin iron oxide layers formed on phosphated α-Fe, as a function of the oxidation time. Experiments are conducted both in situ during oxidation and after cooling, allowing us to derive respectively, growth and thermal stresses. A numerical model was developed to calculate the growth stress and the relaxation phenomena occurring during isothermal oxidation. This model takes into account the mechanical behaviour of both the substrate and the oxide, and parameters such as oxide growth kinetics, substrate thickness, etc. The elasto-viscoplastic model correlates the experimental results with the various parameters of the study. © 2004 Taylor & Francis Ltd.

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Real-time reflection high energy electron diffraction combined with Auger electron spectroscopy (RHEED-AES) was applied to investigate the oxide growth kinetics during initial oxidation on Si (001) 2 × 1 and Si (111) 7 × 7 surfaces. The correlation between the amount of adsorbed oxygen atoms obtained from O KLL Auger electron intensity and the unoxidized surface area estimated from RHEED spot intensities due to 2 × 1 or 7 × 7 structure revealed that adsorbed oxygen was able to migrate on the Si (111) 7 × 7 surface during Langmuir-type adsorption at 580°C, leading to nucleation and lateral growth of oxide as in the oxidation mode of two-dimensional oxide island growth appearing at higher temperatures, while growth of oxide during Langmuir-type adsorption on the Si (001) 2 × 1 surface at 576°C progressed at the random sites where O2 adsorption took place.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

The early stage oxidation and evaporation of Mg–9%Al–1%Zn alloy

Thermogravimetric technique was used to determine the oxidation and evaporation behaviour of AZ91D magnesium alloys with 5 and 10 ppm of beryllium at temperatures between 200 and 500 °C. Depending on temperature and time, the alloy experienced protective or non-protective oxidation with linear or accelerated oxide growth kinetics. During reaction in an oxidizing atmosphere, the additions of beryllium delayed the transient from the protective to non-protective scale formation. In an inert atmosphere, increased beryllium contents reduced the magnesium evaporation rate.

Enhanced initial growth of atomic-layer-deposited metal oxides on hydrogen-terminated silicon

A route is presented for activation of hydrogen-terminated Si(100) prior to atomic layer deposition. It is based on our discovery from in situ infrared spectroscopy that organometallic precursors can effectively initiate oxide growth. Narrow nuclear resonance profiling and Rutherford backscattering spectrometry show that surface functionalization by pre-exposure to 108 Langmuir trimethylaluminum at 300 °C leads to enhanced nucleation and to nearly linear growth kinetics of the high-permittivity gate dielectrics aluminum oxide and hafnium oxide.

Monitoring of oxidation of metals in gas environments for energy production

Monitoring of oxidation in gas environments typical of energy production is essential for determination of lifetime and condition of various components operating at high temperatures. One of the most important phenomena to be monitored is the oxide growth kinetics and its changes during the exposure. Three different monitoring methods have been presented in this paper. First, a conventional method called thermogravimetry based on the weight change measurement during oxidation is reviewed. Second, a novel monitoring technique, HT-CER technique, where contact electric resistance is measured as a function of time and distance of the measuring tip is presented. The third approach described involves a method under development, based on the change of capacitance over two growing oxide surfaces. Simple oxidation in air at a typical temperature for superheater tubes in boilers, 600 °C, has been selected as the reference condition for all of the monitoring methods.

Measuring electrical current during scanning probe oxidation

Electrical current is measured during scanning probe oxidation by performing force versus distance curves under the application of a positive sample voltage. It is shown how the time dependence of the current provides information about the kinetics of oxide growth under conditions in which the tip–surface distance is known unequivocally during current acquisition. Current measurements at finite tip–sample distance, in particular, unveil how the geometry of the meniscus influences its electrical conduction properties as well as the role of space charge at very small tip–sample distances.

High temperature oxidation of high purity nickel: oxide scale morphology and growth kinetics

The oxidation of high purity nickel was studied between 600 and 1200°C for scale thickness between 1 and 30 µm. At or above 1100° C, the scale growth kinetics are strictly parabolic. The scales are then compact with columnar and facetted NiO grains. A more complex behaviour is observed below 1000°C: (i) for test temperatures between 1000 and 800°C, mass gain curves cannot be fitted to a parabola, (ii) different scale morphologies and microstructures are observed depending on scale thickness and temperature, (iii) a duplex scale is formed below 800°C. In addition to the possible effect of grain-boundary diffusion, the departure of growth kinetics from simple pure parabolic kinetics could be also related to the complex scale microstructure and its large evolution during scale growth. In situ oxidation of nickel specimens in an environmental SEM equipped with a hot stage specimen holder permitted to follow the morphological evolution of NiO scales. In situ grown NiO scales show the same microstructural features as observed on Ni specimens oxidised for longer duration in pure oxygen at atmospheric pressure.

Kinetic aspects of the formation of aluminium oxide by use of a microwave-induced plasma.

The oxidation of thin aluminium layers in a microwave plasma has been investigated to determine the kinetics of oxide growth. Thin Al-coatings were oxidized by means of a variety of gas mixtures, characterized by different partial pressures of oxygen, in microwave-induced plasmas of different power. To study the whole kinetic process the Al-metal and the oxide formed were investigated by means of a combination of grazing incidence X-ray reflectometry (GIXR) and grazing incidence X-ray diffractometry (GIXRD). XPS and FTIR spectroscopy confirmed the formation of stoichiometric Al(2)O(3). The alumina formed is X-ray amorphous. Quantitative description of oxide formation was achieved indirectly by determination of the decrease in the integrated intensity of the Al(111)-peak and the total thickness of the whole coating. These values enabled calculation of kinetic data. It was found that oxide growth was a combination of two simultaneous processes - diffusion and sputter processes. The diffusion coefficient D (cm(2) s(-1)) and the sputter rate S (nm s(-1)) were determined. The effect of the composition of the gas mixture, microwave power, and concentration of activated oxygen species on the oxidation process will be discussed. For calculation of the activation energy, E(A), of this plasma-enhanced diffusion process the temperature-dependence of D was investigated.

Kinetics of oxide growth and oxygen evolution on p-Si in neutral aqueous electrolytes

The growth of thin oxide films on wafers of p-Si, (100) orientation, was studied in neutral and acidic aqueous solutions up to 10 V by microelectrochemical (cyclic voltammograms and current transient of potential steps) and microellipsometric measurements. The oxidation occurs according to the high-field law with high-field parameters log(i0/A cm–2)=–13.4±1 and β=10.6±0.1 nm V–1. The oxide films represent an almost ideal dielectric material with e=6.5. Oxide growth is not affected by oxygen evolution, which starts at potentials >3 V. The electron transfer process was analysed in relation to the oxygen evolution reaction. Owing to the presence of oxide thicknesses higher than 2 nm at this point, direct tunneling is less probable and a tunneling via traps should be assumed for the charge transfer. Evidence for coupled electronic and ionic conduction is presented. A contribution of the valence band in SiO2 is stated for the corrosion of oxides at potentials >4.5 V.

Dispersive charge carrier mobility in a surface oxide layer

The electrode impedance spectrum of the oxide produced on a Zr–1%Nb surface in an aqueous solution at 290 °C is analysed as a function of temperature between 20 °C and 290 °C. The portion of the spectrum, relevant to the oxide, is treated in terms of a parallel CPE∥Rox equivalent circuit (CPE denoting constant phase element, Rox ohmic resistor). Considering also the effect of temperature CPE is understood as a dispersive resistance described by the continuous time random walk theory of Scher and Lax. Distinction is made between the kinetics of oxide growth and of non-Faraday charge transport, the time constant of the latter being obtained directly from impedance data.

Oxidation studies of Au‐Al alloys using x‐ray photoelectron spectroscopy (XPS) and x‐ray absorption near‐edge structure (XANES)

AbstractThe oxidation of thin‐film Au‐ Al alloy, as well as pure Al, has been studied using XPS across a wide range of water vapour and air exposures (0.1 L to 1 × 1014 L). The alloys and the pure Al all undergo a similar set of oxide growth kinetics. In addition to the three stages of early growth kinetics identified for Al in previous studies, a fourth stage is identified at air exposure doses of >109 L. During this stage, the average composition of the film changes from Al2O3 to Al2O3·H2O. The Al 2p lineshapes for the oxides grown on pure Al and Au‐Al alloy are different; the origins of this appear to result, in part, from a change in the Fermi level as the oxide thickens. The Al L2,3 X‐ray absorption near‐edge structure (XANES) furthermore provides a complementary probe of the different oxide structures on alloy and Al metal. Thus, their chemical characteristics are affected also by the presence of the gold. This difference in oxide structure is detected at the interface and not through the oxide generally. Copyright © 2001 John Wiley & Sons, Ltd.

AFM induced formation of SiO 2 structures in the electrochemical nanocell

Nanostructures of SiO 2 are formed in the so-called nanocell by AFM tip-induced oxidation of Si. The highly resistive electrolyte is formed by a thin water film in wet gas atmosphere. AFM measurements prove that the water film thickness increases with both p(H 2O) and electric field strength. Water consumption by Si oxidation and water electrolysis is compensated by field enhanced water condensation from the gas phase. Due to the absence of a reference electrode, current dependent potential drops at the tip and in the water film cannot be compensated. At constant cell voltage Δ U, the potential drop within the formed oxide Δ ϕ ox depends not only on Δ U, but on the polarisation time t and the lateral coordinate x as well. Measurements of current transients in the nanocell and a macroscopic cell show that the oxide growth kinetics are similar, but the quantitative result differs due to the 3D structure of the oxide and the variation of Δ ϕ ox( t, x). Investigation of electron transfer reactions in the macro- and nanocell show that anodic oxygen evolution is possible at high electric fields which is explained by hole tunnelling. Direct tunnelling of electrons from the tip to the Si substrate can only take place in the initial stages of oxide formation through few oxide monolayers. Therefore, faradaic processes can be evaluated in the later stage of experiment. For electrochemical nanosystem technology, the oxidation in a one-step process allows the formation of positive Si-structures, while a multistep process followed by etching of the oxide allows the defined formation of pits.

Transport through chromia films

Chromium oxide surface films that form in situ on alloy surfaces are the basis for providing high-temperature corrosion resistance when such alloys are used in high-temperature service. While the slow growth kinetics of chromium oxide is integral to its acting as a corrosion barrier, its periodic growth and spallation finally render alloys unprotective when the chromium concentration in the alloy gradually decreases from about ∼25–30% to about 10–15%. At these latter concentrations the ability of the alloy surface to form a continuous chromium oxide film becomes severely compromised. The ability to decrease the growth kinetics of chromium oxide films can thus prolong the service life of such alloys. Certain rare earth elements such as Ce and Y, whether they are introduced into the alloy as a dispersion of oxides or ion-implanted on the surface, have the ability to significantly reduce the growth rate of chromium oxide. Concomitantly, the major migrating species in the oxide film changes from chromium to oxygen. There is controversy in the literature on the mechanisms leading to these effects. The present study provides further advances in our understanding of this important effect.