F.H.P.M. Habraken

A study of the kinetics of the interactions of O2 and N2O with a Cu(111) surface and of the reaction of CO with adsorbed oxygen using aes, LEED and ellipsometry

The interactions of O2 and N2O in the low pressure range with a Cu(111) surface and of CO with adsorbed oxygen have been studied with ellipsometry, Auger electron spectroscopy and LEED. The adsorption of O2 was investigated in the 10−6–10−4 Torr range and at crystal temperatures ranging from 23 to 400°C. O2 chemisorbs dissociatively with an initial reaction probability of about 10−3 and an apparent activation energy of 2–4 kcal/mol, which depends on the substrate temperature, up to a saturation coverage of 0.45. The probability of decomposition of N{ib2}O is 10−5 at 300°C, and the activation energy is 10.4 kcal/mol for 250 < T < 400°C. The oxygen coverage saturates at θ = 0.45 as well. For both oxidation reactions the kinetics can be described with a precursor state model. With LEED no superstructures were observed. The probability of the reaction of CO with adsorbed oxygen is 4 × 10−5 at 250°C and is initially independent of the oxygen coverage. The reaction is assumed to proceed via a Langmuir-Hinshelwood mechanism. The activation energy for the reaction COad + Oad → CO2 is 18–20 kcal/mol.

Elastic recoil detection

In elastic recoil detection (ERD) one determines the yield and energy of particles ejected out of the surface region of samples under MeV ion bombardment. By application of this surface and thin film analysis technique one can obtain quantitative information concerning the depth distribution of light elements in a sample to be analysed. The quantitativity and the depth resolving power are based on knowledge of the recoil cross section and the stopping power of high-energy ions in matter. This paper reviews the fundamentals of this technique and the various experimental methods for recoil identification. Furthermore, important features for material analysis, such as detection limits, depth resolution and elemental range are discussed. Some emphasis is put on the conversion of the spectral contribution of the elements to atomic concentrations in the films for several representative cases. Throughout the review numerous examples are given to illustrate the features of ERD and to demonstrate empirically the accuracy of the quantification method.

The kinetics of the interactions of O2 and N20 with a Cu(110) surface and of the reaction of CO with adsorbed oxygen studied by means of ellipsometry, AES and LEED

Ellipsometry, LEED and Auger electron spectroscopy have been used to study the interactions of O2 and N2O with a clean annealed Cu(110) surface and the reaction of CO with adsorbed oxygen in the monolayer range. Gas pressures were in the range 10−8–10−4 Torr and crystal temperatures varied between 23–400°C. The changes in the ellipsometric angles Δ and ψ per oxygen atom upon adsorption and removal of oxygen depend on the coverage θ, the temperature and on the azimuth of the plane of incidence of the light beam. The kinetics of the chemisorption of oxygen is independent of the crystal temperature, initial sticking probability ≈ 0.2. The LEED data and the adsorption kinetics indicate an attractive interaction in the adsorbed layer in the [001] direction. The initial decomposition probability of N2O at room temperature is 0.15 and decreases with increasing temperature; the maximum coverage is 0.5 monolayer. The LEED patterns observed were the same as those with O2. The reaction probability of CO with adsorbed oxygen increases with decreasing oxygen coverage (order of magnitude ∼10−5, apparent activation energy ∼6 kcal/mol). This increase has been attributed to the operation of the Langmuir-Hinshelwood mechanism.

Plasma‐enhanced growth and composition of silicon oxynitride films

Silicon oxynitride films with varying oxygen/nitrogen ratio were grown from SiH4, N2O, and NH3 by means of a plasma‐enchanced chemical vapor deposition process. The elemental composition of the deposited films was measured by a variety of high‐energy ion beam techniques. To determine the chemical structure we used Fourier transform infrared absorption spectroscopy and electron‐spin resonance. Ellipsometric data and values for mechanical stress are also reported. We show that the entire range of compositions from silicon oxide to silicon nitride can be covered by applying two different processes and by adjusting the N2O/NH3 gas flow ratio of the respective processes. It is suggested that the N2O/SiH4 gas flow ratio is the major deposition characterization parameter, which also controls the chemical structure as far as the hydrogen bonding configuration is concerned. We found that the films contain significant amounts of excess silicon and that the mechanical stress in the oxynitrides is lower than in plasm...

The adsorption and incorporation of oxygen on Cu(110) and its reaction with carbon monoxide

The initial interaction of oxygen with a Cu(110) surface occurs in two stages, characterised by (2 × 1) and c(6 × 2) overlayer structures and then a third stage where the c(6 × 2) structure remains but further oxygen uptake is registered only with ellipsometry. The first stage is nonactivated and is accompanied by a work function increase of 370–420 meV, depending on sample temperature. The subsequent appearance of the extra features associated with the c(6 × 2) structure in the second stage is accompanied by a decrease in work function of ∼100 meV and is characterised by an apparent activation energy of ∼18 kJ mol−1. The adsorbed oxygen can be removed at all coverages by gas-phase CO. The reaction appears to follow Langmuir-Hinshelwood kinetics with an apparent activation energy of ∼25 kJ mol−1.

THE ADSORPTION AND INCORPORATION OF OXYGEN ON Cu( 100) AND ITS REACTION WITH CARBON MONOXIDE; COMPARISON WITH Cu( 111) AND Cu( 110)

Abstract Ellipsometry, LEED, Auger electron spectroscopy and monitoring of work function changes have been used to study the interactions of O2 and N2O with a clean annealed Cu(100) surface and of the reaction of CO with sorbed oxygen. Gas pressures were in the range 10−7−10−4 Torr and crystal temperatures varied between 25–400°C. The initial interaction of oxygen with Cu(100) occurs in three stages. Oxygen chemisorbs with an initial sticking coefficient of ~10−7 at room temperature and an apparent activation energy of 1.3−3.5 kcal/mol, depending on the substrate temperature. The first stage is the formation of a (√2 × √2)R45° LEED pattern up to a coverage of 0.5, which is converted with an apparent activation energy of 3.2 kcal/mol to a (√2 × 2√2)R45° structure at a coverage of 0.75 in the second stage. The work function increases inthe first stage in an amount of ~300 meV, but decreases in the second stage to the value of the clean surface. In a third stage after an induction period further oxygen uptake could be registered only with ellipsometry. The apparent activation energy is 4.5 kcal/mol. The initial decomposition probability of N2O at room temperature is 5 × 10−5, its apparent activation energy 3.2 kcal/mol. The LEED patterns observed were the same as with O2. The sorbed oxygen can be removed at all coverages with CO. The reaction appears to follow Langmuir-Hinshelwood kinetics with an activation energy for the reaction COad + Oad → CO2 of 19–20 kcal/mol. A comparison is made with the data obtained for Cu(111) and Cu(110).

Hydrogen in low‐pressure chemical‐vapor‐deposited silicon (oxy)nitride films

Silicon (oxy)nitride films (SiOxNy) have been deposited onto silicon by low‐pressure chemical vapor deposition using SiH2Cl2, N2O and NH3 or ND3. Nuclear reaction analysis, elastic recoil detection, and Rutherford backscattering spectrometry have been used to determine the elemental composition of the films with emphasis on the hydrogen and deuterium content. In the as‐deposited, NH3‐grown films the bulk hydrogen concentration is about 3 at. % for an oxygen/nitrogen atomic ratio (O/N) smaller than 0.4, for O/N>0.4 it is lower. In 900 and 1000 °C vacuum annealed films the bulk hydrogen concentration as a function of O/N goes through a maximum at O/N≊0.4. By comparing this observation with the D content in ND3‐grown films as a function of O/N, a model is deduced which explains this behavior. This model involves an oxygen induced increase of the electronegativity of the atoms to which hydrogen/deuterium is bound. Annealing at 1000 °C in a H2/N2 gas mixture of NH3‐grown films results in bulk hydrogen concentr...

Annealing of plasma silicon oxynitride films

The anneal behavior of plasma‐enhanced chemical vapor deposited silicon oxynitride films has been studied using Fourier transform infrared absorption spectroscopy, nuclear reaction analysis, and electron‐spin resonance. The anneal temperature range was 500–1000 °C. It is observed that the oxynitrides which contain only N–H bonds are thermally stable in the temperature range under study. The layers which also contain Si–H bonds are considerably less thermally stable. Abundant hydrogen effusion from these layers is observed at temperatures as low as 600 °C, accompanied by cracking and shrinkage of the films. It is suggested that the coexistence of both Si–H and N–H bonds offers the possibility for cross linking and that consequently the decomposition temperature of both types of bonds is lowered. Evidence for the occurrence of cross linking is found in the infrared difference spectra. Consistently, the silicon unpaired electron density does not increase upon annealing. The Si–H and N–H bands effectively shi...

Influence of Ar pressure on r.f. magnetron-sputtered Ca5(PO4)3OH layers

Abstract Currently, medical and dental implants are often provided with thin calcium phosphate ceramic coatings (preferably Ca 5 (PO 4 ) 3 OH (HA)) to improve their biological behaviour. Previous studies have demonstrated that radio-frequency (r.f.) magnetron sputtering is a suitable technique for depositing such thin Ca−PO layers. Although X-ray diffraction showed that the deposited films had an HA structure, the Ca/P ratio of the layers was higher than the theoretical value of 1.67 for HA. In the present study the influence of the argon pressure and input power on the structure and chemical composition of the sputtered layers, especially the Ca/P ratio, is investigated. Rutherford backscattering spectrometry (RBS), stylus profilometer (alpha-step), X-ray diffraction spectrometry (XRD), and IR spectrometry (FTIR) were used to characterise the coatings. Although the Ca/P ratio became lower when sputtered at lower input power, it was still higher than the value of 1.67 for HA. The density measured for the films sputtered at 200 and 400 W did not show a simple relation with the argon pressure. For all the films after annealing XRD and FTIR showed an HA-like structure and HA bonds. However, the OH bond appeared to be dependent on the partial pressure of water vapour during sputtering.

Mechanism of MeV ion induced hydrogen depletion from organic layers

In many materials, the loss of hydrogen during elastic recoil detection (ERD) analysis and nuclear reaction analysis (NRA) with MeV ions is observed. This phenomenon, which is especially pronounced in organic materials, limits the accuracy of the quantitative measurements feasible with these techniques. The MeV ion induced depletion was investigated in an ion beam analysis study, using a wide range of projectiles and energies. From the results we conclude that the evolution of hydrogen from electropolymerized porphyrin layers is molecular, which leads to a model incorporating second order kinetics. With this model, the dependence of hydrogen content on the primary ion flux is described by only the initial amount of hydrogen, the probability of ion induced bond breaking and a characteristic distance within two liberated hydrogen atoms will combine to a hydrogen molecule. When incorporating the energy deposited by the primary ion within the ion track into the model, the observed quadratic dependence of ion induced damage on the stopping power is also explained.