D.F. Mitchell

A kinetic study of the initial oxidation of the Ni(001) surface by RHEED and x-ray emission

Nickel (001) surfaces were prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction. The oxidation of this surface in pure C2 to form NiO was studied by reflection high energy electron diffraction (RHEED) and X-ray emission. On exposure to oxygen the “clean” surface was found to chemisorb oxygen to produce a coverage of 0.014 microgcm2 in an ordered c(2 × 2) structure. Within this film there appears to exist a number of nucleation sites dependent on temperature and step density. Growth is by oxygen capture at the periphery of these sites to produce oxide islands approximately three oxygen planes thick which spread to cover the surface. At room temperature this film does not thicken with additional oxygen exposure.

Early stages of oxidation of Fe(001) observed by electron diffraction and x-ray emission

Abstract Chemically clean (001) surfaces on iron were prepared by a combination of high temperature oxidations, argon ion bombardments and hydrogen reductions. During such treatments carbon was observed to form a c(2 × 2)-C structure only for surface concentrations below about two monolayers. This pattern exhibited an order-disorder transformation at a temperature which was a function of the carbon surface concentration. On exposure to oxygen the “clean” iron surface was found to react with a near unit sticking coefficient. This value steadily decreased as the oxide film increased. The kinetics of oxide growth was independent of specimen temperature from 25 to 150°C, being dependent only on the total number of molecules/cm 2 striking the surface. Structural information during surface cleaning and the initial stages of growth of oxide films was obtained using reflection high energy electron diffraction (RHEED). For the first monolayer coverage of oxygen only broadening of the iron diffraction-maxima was observed indicating a p(1 × 1)-O structure. Further exposure led to the formation of a fcc type lattice. With longer exposures a spinel type pattern developed. The mean interplanar spacing of the oxide increased with time of exposure from 8.18 ± 0.04 A to 8.34 ± 0.06 A. These results suggest the formation of a cubic oxide in which the iron to oxygen ratio increases with time up to a limiting value between that of γ-Fe 2 O 3 and Fe 3 O 4 .

A kinetic study of the initial oxidation of Ni (111) and (211) surfaces by rheed and X-ray emission

Abstract Ni (111) and (211) surfaces, prepared by a combination of high temperature oxidation, argon ion bombardment and hydrogen reduction, were oxidized between 40 and 300°C in low oxygen pressures. The oxidation processes were studied by reflection high-energy electron diffraction (RHEED). electron excited X-ray emission spectroscopy (XRE) and work function (Δφ) measurements. The oxidation up to ~0.3 ∼g of oxygen per cm 2 was observed to proceed in three stages. Stage 1 : Initial chemisorption associated with the formation of two-dimensional structures (oxygen uptake between 0 and ~0.5 monolayers); on (111) Ni the sequence observed was a >p(2 × 2) followed by a ( 3 × 3 ) R 30° , whereas only a p(2 × 2) structure was produced on (211) Ni. Stage 2: The formation of NiO nuclei and spreading of these nuclei to cover the surface (oxygen uptake between 0.5 and 2.5 monolayers); on (111) Ni whether anti-parallel (111) NiO or polycrystalline NiO nuclei formed depended upon the oxidation pressure and temperature. Stage 3: Logarithmic thickening of the oxide film produced from Stage 2 (oxygen uptake > 2.5 monolayers).

Oxidation mechanisms of β-NiAl + Zr determined by SIMS

Abstract The oxidation mechanisms of single crystal Zr-doped β-NiAl from 800 to 1200 °C were determined using primarily secondary ion mass spectrometry (SIMS) imaging and depth profiling. High spatial resolution SIMS imaging provides a means for critically assessing the effects of diffusion through the boundaries of the various morphologies of α-Al 2 O 3 scales that form on β-NiAl. Transient oxidation occurs by outward Al diffusion through transition Al 2 O 3 scales, θ-Al 2 O 3 in this case, for temperatures from 800 to 1100 °C. Steady-state oxidation occurs by counter-diffusion of oxygen inward and both Al and Zr outward through boundaries in the α-Al 2 O 3 ridge morphology.

The growth and stability of passive films

This paper will consider the growth and breakdown of passive oxide films on metals and alloys. Emphasis is placed on the use of surface-analytical techniques, particularly secondary ion mass spectrometry (SIMS) to characterize oxides formed on Fe and Fe-Cr alloys in18O-containing solutions, and to determine the oxides stability to subsequent air exposure. It is found that the stability of passive films towards air exposure decreases with increasing Cr content of the alloy. Films formed on Fe in the passive potential range in borate buffer solution are stable towardsex situ exposure, whereas films formed on Fe-26Cr alloys are not stable to air exposure after passivation at any potential in the passive region. This result has implications for examining the thickness and composition of passive films on high-Cr alloys usingex situ techniques. Passive films play a critical role in the initiation of pitting and the processes which influence film breakdown on Fe in Cl−-containing solutions are discussed. The results indicate that pitting is associated with a critical stage in the development of passive oxide films.

Nickel-chromium interface resolution in Auger depth profiles

Abstract Auger depth resolutions for fine-grained crystalline Ni/Cr multilayers have been determined. Resolution is shown to be a function of ion species, ion energy, ion incident angle, and the presence of reactive species. Deterioration of interface resolution with depth is shown to be the result of cumulative surface roughness induced by the ion beam to an extent proportional to the ion velocity normal to the surface. The presence of a reactive species to create an amorphous surface layer is shown to inhibit the development of surface roughness. The best resolution was obtained using low energy Xe ions and large ion beam angles. The ultimate resolution is limited by the fabrication perfection of the standard, and not by the sputtering process or by the Auger mean free path length.

High-Energy Electron-Diffraction and X-Ray Emission Analysis of Surfaces and Their Reaction Products

A technique which can provide both structural and chemical information from surface films at the monolayer level has been developed. For structure studies the grazing incidence, high-energy electron-diffraction method has been applied in ultra-high vaccum. The portion of the specimen surface irradiated during electron-diffraction observations acts as a line source of x-rays located at the focus of a low-atomic-number x-ray spectrometer. The ultra-high-vacuum environment in the diffraction chamber is separated from that of the conventional vacuum spectrometer by an x-ray-transparent window. Characteristic Ka radiation from monolayer quantities of both carbon and oxygen on a low-index surface of iron has been observed. Diffraction patterns and x-ray information may be obtained from such structures simultaneously. Attempts to calibrate the apparatus for carbon and oxygen layers on iron are discussed in some detail. The method appears well suited to the study of kinetic phenomena in surface films of mean thickness less than 100 A.

GaInAs/GaAs strained‐layer superlattices grown by low pressure metalorganic vapor phase epitaxy

Strained‐layer superlattices (SLS’s) of GaxIn1−xAs/GaAs (x≂0.8–0.9) have been grown by low pressure metalorganic vapor phase epitaxy. The reactants were trimethylgallium, trimethylindium, and arsine. The structures were characterized by Auger profiling and low‐temperature photoluminescence. These measurements show that high quality SLS’s with interface widths no larger than 25 A and very narrow luminescence peaks (6.5 meV) similar to the best structures grown by molecular beam epitaxy, can be easily grown by low pressure metalorganic vapor phase epitaxy.

Transport in α-Al2O3 scales on FeAl and NiAl alloys at 1100°C

Abstract The nature of transport processes occurring during the growth of oxides at 1100°C on Fe-25 wt% Al and Ni-32 wt% Al has been investigated. Both materials develop protective α-Al2O3 scales which have been characterized by SEM, RHEED and Auger electron spectroscopy. Two-stage oxidation experiments have been conducted using 16O2 and 18O2 gases; imaging SIMS (secondary ion mass spectrometry) has been used to locate the different isotopic species. The SIMS data (profiles from areas as small as 3.5 μm×3.5 μm, and images with 0.1 μm lateral resolution) illustrate the complexity of the growth processes, and indicate areas where scale growth has taken place predominantly by cation or oxygen ion diffusion.

Effects of annealing on TlBaCaCuO thin films

We have studied the effects of high‐temperature anneals on sequentially deposited TlBaCaCu films in an environment containing O2 and TlOx . From Auger electron spectroscopy depth profiles, it was found that Tl was quite volatile and can be easily eliminated from the thin films after a short treatment at 830 °C. However, the Tl can be incorporated in the films after appropriate treatment.