J.L. Sullivan

A study of the core level electrons in iron and its three oxides by means of X-ray photoelectron spectroscopy

The core level electrons in iron and its three oxides FeO, Fe3O4 and alpha -Fe2O3 have been studied by means of X-ray photoelectron spectroscopy. The main features of the spectra have been identified and electron binding energies have been measured. The Fe 2p peaks of the oxide spectra are broadened (FWHM approximately 4.5 eV) due to unresolved multiplet splitting, and shake-up satellites can be observed for both FeII and FeIII species. For all the samples studied the O 1s spectra consist of two overlapping peaks, one due to the oxide and a higher binding energy peak due to an adsorbed species. The higher binding energy peak was frequently dominant indicating a high degree of absorption on the oxide surfaces. The effect of argon ion etching on the surfaces has also been investigated and the results indicate that the analysis of surfaces by means of ion etching in conjunction with XPS can give misleading results.

Reduction of oxides of iron, cobalt, titanium and niobium by low-energy ion bombardment

Ion-beam bombardment of solid surfaces with rare gas ions is used extensively in conjunction with XPS and AES for surface cleaning and depth profiling. It is sometimes not appreciated that the process can give rise to extensive chemical changes in the surface to be examined. The purpose of this investigation is to study this ion-induced chemical damage. As a vehicle for this, changes in the elemental chemical states associated with some transition metal oxides have been examined using XPS, when the oxides were subjected to progressive periods of argon ion bombardment. Chemical reduction of all oxides investigated has been observed and the data have been compared with theoretical predictions from a current model for preferential oxygen removal. Measurements of the core electron energy levels in oxides and some hydroxides of iron, cobalt, titanium and niobium have been made and the results are tabulated.

Surface characterisation of plasma-nitrided titanium: an XPS study

DC plasma nitriding was applied to titanium metal sheets in a commercial cell using Ar + NH3 as the admixture and the nitrided surface investigated by means of XPS. As a comparison, in situ nitriding of a chemically pure Ti surface at room temperature was performed by N2+ ion bombardment (1–5 keV) in the electron spectrometer. Synthesis of the complex Ti2p envelope was accomplished using two sets of loss peaks, separated by 1.6 and 3.0 eV from the major TiN-type Ti2p32 and Ti2p12 components. A doublet at 458.8 and 464.5 eV was also included to account for a TiO2-type oxide. The sum of the main Ti TiN peak and the two loss peaks were taken to be representative of nearly stoichiometric TiNx with x ranging from 0.85 to 1.15. Further components derived from the peak synthesis were assigned to TiNxOy and Ti2O3. The stoichiometric nitride is represented by a Ti2p doublet at binding energies of 454.7 and 460.6 eV and a single sharp N 1s peak at 396.7 ± 0.1 eV. On the superstoichiometric samples, especially after N2+ bombardment, a second peak appears at about 395.8 eV with a positive correlation between this peak concentration and the relative amounts of species (TiNxOy, Ti2O3, TiO2) derived from Ti2p components and the surface O and N content. Consecutive Ar+, N2+ and (N2+ + O2+) bombardment leads to significant changes in composition together with rearrangement of short-range chemical structure which is reflected in peak-shape changes of the Ti2p and N 1s lines.

An X-ray photoelectron spectroscopy study of the oxides of GaAs

Abstract In this paper, by the use of X-ray photoelectron spectroscopy, we unequivocally identify the oxides present on GaAs surfaces and accurately measure the binding energies associated with the 2p 3/2 , 3d, and Auger lines in the X-ray photoemission spectra. These measurements intended to provide reliable reference data for further work. We conducted an extensive analysis of the oxidation states of Ga metal and oxide powder reference samples, air exposed GaAs wafers, and wafers subjected to various surface treatments (argon plasma treatments and boiling). Based on this experimental evidence, an assignment of the photoelectron peaks to various chemical states is proposed.

A comparison of ion and fast atom beam reduction in TiO2

Ion beam bombardment of solid surfaces has been used extensively for surface conditioning, surface cleaning and in various aspects of surface and interface analysis. The process can give rise to extensive physical and chemical changes in the bombarded surface. It has recently been suggested that bombardment by means of last atoms, rather than ions of the same mass and energy, produces less surface damage. This investigation was therefore initiated to study the relative damage produced by ion and fast atom bombardment. To do this, changes in elemental chemical states associated with TiO2 have been examined using XPS, when the oxide was subject to progressive periods of argon and helium ion and fast atom bombardment. Chemical reduction of the oxide, according to the interaction TiO2→Ti2O3→TiO→Ti was obtained for both Ar and He bombardment. The degree of reduction was compared and it was found that in all cases ion produced significantly more reduction than fast atoms. This difference was greatest in the case of He bombardment. The difference between atom and ion surface damage may be explained in terms of a high local surface potential produced in the case of ions leading to electrical breakdown and effective reduction in the mean metal-oxygen binding enrgy.

Effect of ion and neutral sputtering on single crystal TiO2

Abstract Alteration of the structure and composition of solid surfaces under energetic ion or neutral particle impact is a major concern in surface analysis, mainly in connection with depth profiling. It has been established that preferential sputtering occurs in many compounds and systems and that this leads to chemical transformations. Titanium compounds, especially the oxides, possess various stoichiometric and non-stoichiometric states and thus offer a suitable vehicle for investigating these effects. This paper reports studies on the reduction of single crystal TiO 2 by He, Ar and N 2 ion and fast atom bombardment. Attempts have been made in this study to comprehensively characterize the chemical and electronic state of the ion or neutral bombarded single crystalline TiO 2 surfaces by X-ray Photoelectron Spectroscopy (XPS). Differences observed in the extent of bombardment induced reduction due to various incident species (He, He + , Ar, Ar + , N 2 , N 2 + have been discussed and explanations of these differences in terms of a compensating electron leakage current have been proposed.

Chromium oxide coatings applied to magnetic tape heads for improved wear resistance

Abstract This paper reports on an investigation of the wear of chromium oxide based very thin films. Linear data tape Advanced Digital Recording (ADR™) heads coated with 20- and 40-nm thick chromium oxide films have been tested subject to temperature/humidity matrix of 10 to 40°C/10 to 80% in order to assess the wear behaviour of the coating as a function of environment. The tested heads were analysed at various stages of wear, by use of optical microscopy (OM), atomic force microscopy (AFM) and Auger electron spectroscopy (AES). The results show that the most severe damage occurs at the highest relative humidity (80% RH) and for a given humidity, at the lowest temperature (10°C). Inversely, stain transferred from the tape to the head surface predominates at the lowest humidity (10% RH). Stain therefore appears to protect the coating against wear with the degree of protection increasing with the temperature. The wear process differs according to the coating thickness. This is attributed to the location of the maximum subsurface stress with respect to the coating/substrate interface. It is shown that this maximum stress occurs below the interface for 20 nm and at the interface for 40 nm thick coatings. This correlates to different observed wear modes.

Study of low energy high dose nitrogen implantation in aluminium, iron, copper and gold

To advance the understanding of fundamental physical and chemical processes occurring in ion bombardment of metals, ion beam nitridation of aluminium, iron, copper and gold is studied in the energy range 2-5 keV, at current densities of 1 and 5 μA cm -2 , for each ion energy. The concentration profiles of nitrogen-ion-implanted metals were measured by x-ray photoelectron spectroscopy (XPS). The chemical composition and chemical structure of the implanted metals were also investigated.

Boundary lubrication and oxidational wear

Describes the results of studies of the wear of aluminium bronze on steel in kerosene containing an organic acid additive. Based on the experimental results a single dominant mechanism has been isolated and a theoretical model for oxidational wear under boundary lubrication has been proposed. From the model an expression relating wear rate to applied load and involving a number of other surface parameters has been developed. As a test for the validity of the model the expression was used to evaluate contact temperature as a function of load in inserting measured wear rates. Good agreement was found between the calculated temperatures and estimated experimental values.

An investigation into which factors control the nanotribological behaviour of thin sputtered carbon films

Ultra-thin (20–100 nm) films deposited on Si surfaces can improve their mechanical and tribological properties. As a stepping stone towards the optimization of such ultra-thin films, herein we report experimental nanoscratch and nanowear data on a-C films of thickness in the range 200–1000 nm on Si aiming to (1) understand the role of film thickness on the nanoscratch behaviour, (2) determine whether the same factors (substrate bias, H/E ratio, etc) are at play for thick films as for the thin films, (3) determine possible design rules for thinner films enabling their optimization for MEMS applications and (4) evaluate the use of the multi-pass (3-scan) procedure for clarifying the locus of failure. To a first approximation, the critical load for total film failure in the nanoscratch test is proportional to thickness provided the films are not too stressed. a-C films of 1 µm with very high H/E, deposited under high substrate bias, perform well at low load but very poorly in more highly loaded situations. Not only do they exhibit low critical loads but also failure involves extensive delamination outside of the scratch track. This is not observed on thinner films. A suitable strategy for optimizing wear resistance for thin films for MEMS applications is to aim to maximize H/E. For the 200 nm films studied here, the films with the highest H/E showed slightly improved scratch resistance.