Sputter cleaning and dry oxidation of CdTe,HgTe, and Hg0.8Cd0.2Te surfaces

The analyses of CdTe, HgTe, and Hg_0.8Cd_0.2Te surfaces by XPS and LEED after Ar⁺ sputtering and after the subsequent onset of a dry oxidation are described, and a quantitative evaluation of the XPS spectra is attempted. The results are: Ar⁺ sputtering yields a perfect unreconstructed CdTe surface of stoichiometric composition, whereas the composition of sputtered HgTe and Hg_0.8Cd_0.2Te surfaces generally deviates from the stoichiometry of the respective compound. This deviation is a function of the energy of the Ar ions (1 to 15 keV) and is characterized by an increasing deficit in Hg as the ion energy is raised. The Hg deficit of sputtered Hg_0.8Cd_0.2Te surfaces is substitutionally compensated by an equivalent increase in Cd, and due to this substitution the resulting surfaces are sufficiently ordered to display a distinct LEED pattern. The oxidation of sputtered CdTe, HgTe, and Hg_0.8Cd_0.2Te surfaces in an O₂ atmosphere is an extremely slow process. Therefore, the surfaces to be oxidized were additionally exposed to UV radiation (low pressure mercury lamp), and due to UV generated ozone as an oxidizing agent ultrathin native oxide layers of up to 15 Å thickness were readily obtained. The predominant constituents of these native oxide layers on Hg_0.8Cd_0.2Te are concluded to be CdTeO₃ and TeO₂.     

Investigation by X-ray photoelectron spectroscopy of the transient oxidation of NiAl

X-ray photoelectron spectroscopy (XPS) has been used to study the oxidation of NiAl in oxygen at atmospheric pressure. Prior to oxidation, the native oxide scale on the specimen was removed by ion sputtering and the specimens were (pre-)heated in vacuum before exposure to oxygen. At low oxidation temperatures (<750 K) scales consisted of Al₂O₃ and NiAl₂O₄, with a thin surface layer of NiO, but at higher temperatures were of Al₂O₃, apart from about 0.5 at % Ni. The Ni content in the latter case was constant throughout the scale and did not increase dramatically near the alloy/oxide interface. In the experimental conditions used in this study, initial formation of NiO and NiAl₂O₄ seems to be avoided at the higher oxidation temperatures ( > 750 K).     

Inhibition of initial surface oxidation by strongly bound hydroxyl species and Cr segregation: H2O and O2 adsorption on Fe-17Cr

The initial stages of surface oxidation of Fe-17Cr (ferritic stainless steel) were investigated at 323 K by X-ray photoelectron spectroscopy (XPS) and inelastic electron background analysis. The results indicated the formation of a mixed iron-chromium oxide layer upon O₂ exposure and the formation of a thin chromium oxyhydroxide layer upon H₂O exposure. The oxidation of Fe did not occur in the latter case. Moreover, it was found that pre-exposing the Fe-17Cr surface to H₂O significantly hinders subsequent oxidation by O₂, thus providing a way to control the formation of nanoscale oxides on stainless steel materials. It was concluded that the formation of strongly bound hydroxyl species together with adsorbate-induced segregation of Cr severely limits the reaction between O₂/H₂O and Fe from the alloy.     

Auger and X-ray photoelectron spectroscopic and electrochemical characterization of titanium thin film electrodes

Auger (AES) and X-ray photoelectron spectroscopic (XPS) characterizations of electrochemically oxidized titanium are described. Surface oxides on thin (200–250 Å) vacuum deposited titanium films were formed under conditions of linear potential scan in 1 N KClO₄, 1 N HClO₄ and 1 N H₂SO₄. Current/voltage, capacitance/voltage and surface conductance/voltage relationships confirmed the irreversible formation of the surface oxide at thickness of 20–30 Å/V, for low applied potentials. Post moretem analysis of the thin films by AES and XPS indicated a mixture of metal and metal oxides (TiO₂, Ti₂O₃, TiO) on each surface, with the higher oxide states predominating on the electrochemically oxidized films. Observation of the L_IIIM_2,3M_4,5, N(E) signal shape in the Auger spectra of the potentially oxidized oxidized films showed a suboxide TiO-like surface rather than an TiO₂ surface state. Deconvolution of the $\text{Ti}\text{(2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext><mtext>,\; </mtext><mtext>3</mtext><mtext>2</mtext>}}\text{)}\text{XPS}$ spectra confirmed the coexistence of multiple oxidation states of Ti during electrochemical or atmospheric oxidation of the films. Ion sputtering of each surface was used to characterize the subsurface metal/metal oxide composition and to correlate the oxygen to metal atomic ratio with electrochemical pretreatment.     

XPS study of vanadium surface oxidation by oxygen ion bombardment

Oxidation of vanadium metal surfaces at room temperature by low-energy oxygen ion beams is investigated by X-ray photoelectron spectroscopy (XPS). It is observed that ion-beam irradiation of clean V results in formation of thin oxide layer containing vanadium in oxidation states corresponding to VO, V₂O₃, VO₂ and V₂O₅ oxides. The composition of the products of ion-beam oxidation depends markedly on oxygen ion fluence. The results of angle-resolved XPS measurements are consistent with a structure of oxide film with the outermost part enriched in V₂O₅ and VO₂ oxides and with V₂O₃ and VO oxides located in the inner region of the oxide layer.     

A study of the oxidation of tantalum nitride by ellipsometry and auger electron spectroscopy

Ellipsometry and Auger electron spectroscopy in conjunction with in-situ ion sputtering have been used to study the thermal and anodic oxidation of thin tantalum nitride (Ta₂N) resistor films which are used in microelectronic circuits. In addition, the effects of thermal oxidation on the resistor-conductor contact resistance and conductor adhesion have been investigated.Thermal oxide thickness and stoichometry were estimated by Auger electron spectroscopy. A more accurate determination of oxide thickness was obtained by using multiple angle-of incidence ellipsometry. The thermal oxide was found to demonstrate a parabolic growth rate, in agreement with other workers. It was determined that the contact resistance should not be adversely affected by oxidation of the tantalum nitride at room temperature for storage times of several months; no significant decrease in the conductor adhesion was observed for oxide thicknesses up to 8 nm.A significant difference in the stoichiometry and refractive index of the thermally and anodically oxidized films was found, however. Depletion of the nitrogen from the surface of the anodically oxidized Ta₂N films resulted in an increase in the refractive index of the anodic oxide layer to a value close to that of pure Ta₂O₅.     

Combined SIMS,AES, and XPS investigations of tantalum oxide layers

Thick layers of tantalum oxide prepared by thermal and anodic oxidation have been studied by combined SIMS, AES, and XPS during depth profiling by 3keV Ar⁺ ion sputtering. The chemical composition of these films is revealed by the OKLL and O 1s signals and by the “lattice valence” parameter determined from the TaO _n ^± intensities. Thus the anodic film consists of a contamination layer, an oxygen-rich reactive interface and a thick homogeneous oxide layer followed by an interface to the Ta metal. The thermal oxide shows an oxygen concentration decreasing with depth and a broad oxide-metal interface. In both cases, carbon contamination (carbide) prevents the application of the valence model to the clean Ta substrate. The sputtering yield of the oxides was found to be 0.6 Ta₂O₅/ion.     

Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.     

A study of the 42CrMo4 steel surface by quantitative XPS electron spectroscopy

Quantitative X-ray photoelectron spectroscopy was used to characterize the native oxide film formed on 42CrMo4 steel surface by air exposure in normal conditions. In order to determine the thickness and composition of the oxide layer we have used a stacking layer model together with experimental XPS sputtering depth profiling. At a nanoscale study, to obtain quantitative results one must take into account fundamental parameters like the attenuation depth of photoelectrons. We have found that both lepidocrocit (γ-FeOOH) and magnetite (Fe₃O₄) were present and the total thickness of the oxide layer was 16 monolayers.     

Oxidation and reduction of thin Ru films by gas plasma

The oxidation and reduction of Ru thin films grown on a Si(1 0 0) surface were studied by X-ray photoemission spectroscopy (XPS). Ru thin films were oxidized with O₂ plasma generated by an rf discharge, and their XPS spectra were measured. The spectra were decomposed into several components for Ru suboxides attributable to different stages of oxidation. After sufficient exposure to oxygen, a stoichiometric rutile RuO₂ layer was found to have formed near the surface. Thermal annealing at 500 K resulted in a thicker RuO₂ layer. Experiments demonstrated that the Ru oxide layer can be removed by H(D) atoms via the desorption of water molecules.     

Characterization of water exposed plasma sprayed oxide coating materials using XPS

The surface compositions and oxidation states of non-exposed and water exposed plasma sprayed oxide coatings were studied using X-ray photoelectron spectroscopy (XPS). Coating materials were TiO₂, Al₂O₃ and Cr₂O₃ and their mixtures. Water exposures were performed for free standing coating disks at mild electrolyte (1 mmol NaCl solution) at pH 4, 7 and 9. The exposure time was two weeks.It was observed that pure plasma sprayed TiO₂ material was chemically stable over whole experiment pH range and only slight surface hydroxylation was observed for this material.In case of plasma sprayed Al₂O₃ materials the surface O/Al ratio increased considerably during water exposure especially at exposure pH 7. This was probably result of surface conversion to hydrous form. No surface oxidation state changes were observed for this material.The non-exposed Cr₂O₃ materials contained both Cr(III) and Cr(VI) oxides. The water exposures increased the surface oxygen and Cr(VI) contents at the expense of Cr(III). The most probable reason for that was the dissolution of surface Cr(VI) oxide phase during water exposures and the (re)adsorption of dissolved Cr(VI) species back to the surface.     

The effect of chloride ions on the corroded surface layer of 00Cr22Ni5Mo3N duplex stainless steel under cavitation

The effects of Cl⁻ on the corroded surface layer of 00Cr22Ni5Mo3N duplex stainless steel under cavitation in chloride solutions were investigated using nanoindentation in conjunction with XRD and XPS. The results demonstrate that Cl⁻ had a strong effect on the nano-mechanical properties of the corroded surface layer under cavitation, and there was a threshold Cl⁻ concentration. Furthermore, a close relationship between the nano-mechanical properties and the cavitation corrosion resistance of 00Cr22Ni5Mo3N duplex stainless steel was observed. The degradation of the nano-mechanical properties of the corroded surface layer was accelerated by the synergistic effect between cavitation erosion and corrosion. A key factor was the adsorption of Cl⁻, which caused a preferential dissolution of the ferrous oxides in the passive film layer on the corroded surface layer. Cavitation further promoted the preferential dissolution of the ferrous oxides in the passive film layer. Simultaneously, cavitation accelerated the erosion of the ferrite in the corroded surface layer, resulting in the degradation of the nano-mechanical properties of the corroded surface layer on 00Cr22Ni5Mo3N duplex stainless steel under cavitation.     

Initial oxidation stages on FeCr(100) and FeCr(110) surfaces

Simultaneous LEED and AES are used to follow early stages of oxidation of monocrystalline FeCr(100) and (110) between 700 and 900 K in the oxygen pressure range 10^?9–10^?6 Torr. A chromium-rich oxide region at the alloy/oxide interface is observed, which exhibits different surface structures on oxidized FeCr(100) and FeCr(110). The chromium concentration in this initially formed oxide film is found to be enhanced by low oxygen pressures or high temperatures. During further oxidation different behaviours are observed on FeCr(100) and FeCr(110), which are explained by assuming different ion permeabilities through the initial chromium rich oxide regions on the two surface planes. On FeCr(110) surfaces oxidation is initiated on chromium enriched (100) facets at 800 K or below. At 900 K a film consisting of rhombohedral Cr₂O₃ or (Fe, Cr)₂O₃ is epitaxially growing with its (001) plane parallel to the alloy (110) face. On FeCr(100) surfaces the chromium rich oxide region next to the substrate is of fcc type. As soon as the diffusion of iron from the alloy to the gas/oxide interface is observable, a spinel type oxide is formed and connected with the location of iron in tetrahedral lattice sites. Closer to the fcc lattice the spinel oxide consists of FeCr₂O₄ or a solid solution of FeCr₂O₄ and Fe₃O₄ whereas next to the gas phase the oxide is pure Fe₃O₄.     

Effect of substrate temperature on the surface and interface oxidation of NiTi thin films

NiTi shape memory alloy thin films are deposited on pure Cu substrate at substrate ambient temperatures of 300 °C and 450 °C. The surface and interface oxidation of NiTi thin films are characterized by X-ray photoelectron spectroscopy (XPS). After a subsequent annealing treatment the crystallization behavior of the films deposited on substrate at different temperatures is studied by X-ray diffraction (XRD). The effects of substrate temperature on the surface and interface oxidation of NiTi thin films are investigated. In the film surface this is an oxide layer composed of TiO₂. The Ni atom has not been detected on surface. In the film/substrate interface there is an oxide layer with a mixture Ti₂O₃ and NiO in the films deposited at substrate temperatures 300 °C and 450 °C. In the films deposited at ambient temperature, the interface layer contains Ti suboxides (TiO) and metallic Ni.     

Spectroscopic characterization of Ni films on sub-10-nm silica layers: Thermal metamorphosis and chemical bonding

The thermal evolution in the chemical and physical characteristics of the Ni film of thickness 1-50 nm deposited on silica of thickness less than 10 nm was studied. The chemical composition of silica affected the thermal behavior of the Ni overlayer substantially. Nickel deposited on native oxide may diffuse downward into native oxide during annealing and was oxidized. It mainly produced Ni₃O₂ and silicides below 150 °C. Increasing the temperature to 300 °C caused further oxidation of Ni to yield NiO. The sub-10-nm silicon dioxide layer, on the other hand, can inhibit the diffusion of Ni atoms downward when the Ni-deposited sample was annealed. Instead, these atoms aggregated into small particles on the surface at elevated temperatures, causing the substrate to be exposed. The size of the particles produced can be controlled, as it increased almost linearly with the thickness of the Ni film deposited in the low thickness regime. The thinner Ni films yielded smaller, round nanoparticles with better dispersity. The particles formed were strongly adhered to the silicon dioxide surface. The bulk of the particles formed was mainly metallic. Exposing to the air of the Ni particles formed on silicon dioxide mainly produces Ni₂O₃ on the particles.     

Surface XPS characterization of NiTi shape memory alloy after advanced oxidation processes in UV/H2O2 photocatalytic system

Surface structure of NiTi shape memory alloy (SMA) was modified by advanced oxidation processes (AOP) in an ultraviolet (UV)/H₂O₂ photocatalytic system, and then systematically characterized with x-ray photoelectron spectroscopy (XPS). It is found that the AOP in UV/H₂O₂ photocatalytic system leads to formation of titanium oxides film on NiTi substrate. Depth profiles of O, Ni and Ti show such a film possesses a graded interface structure to NiTi substrate and there is no intermediate Ni-rich layer like that produced in conventional high temperature oxidation. Except TiO₂ phase, some titanium suboxides (TiO, Ti₂O₃) may also exist in the titanium oxides film. Oxygen mainly presents in metal oxides and some chemisorbed water and OH⁻ are found in titanium oxides film. Ni nearly reaches zero on the upper surface and relatively depleted in the whole titanium oxides film. The work indicates the AOP in UV/H₂O₂ photocatalytic system is a promising way to favor the widespread application of biomedical NiTi SMA by improving its biocompatibility.     

Analysis profiles of oxide films on chrome steel by auger emission and x-ray photoelectron spectroscopies

Samples of steel from two different sources were examined. The materials had nominally the same bulk composition but different samples from each batch had been hardened under two slightly different conditions.The surface oxide films were analysed by both Auger emission spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) after a number of ion bombardments, and the variation in composition with depth was established. In general the outermost oxide layers were chromium deficient and XPS results suggested the presence of Fe₂O₃ at the surface, together with atmospheric contamination. After ion bombardment the proportion of chromium oxide increased and both iron metal and oxide (Fe₃O₄) were present. The amount of chromium oxide reached a maximum at the steel oxide interface and on further bombardment was replaced by chromium metal. AES and XPS results were in agreement qualitatively and also quantitatively after measurement with a curve analyser of the areas under the peaks of certain elements.The present investigation has shown that AES and XPS can give very similar analyses provided some simple corrections are applied. However, the use of the higher resolution of XPS can provide additional information which cannot be obtained by AES. Thus, using expanded XPS scans, together with suitable curve analysis techniques, it is relatively simple to separate signals from oxide and from free metal regions of the sample, and to study in detail the changes in composition of the oxide from the oxide-air to the oxide-metal interfaces.The study of the four steel samples in this manner has shown that there is a thin region rich in chromium oxide adjacent to the metal. The high level of chromium falls off fairly rapidly nearer the surface and appears to stabilize at a very low level. This is particularly obvious in the thicker oxides (PH), where there is a thick surface layer which is principally iron oxide. It would appear that under the conditions of the heat treatment given to the present samples the thickening of the oxide is due almost entirely to iron oxide. Other work has shown that the low temperature air-formed oxide on these steels is chromium rich and it is suggested that the thickening is due to diffusion of iron through the original film with little movement of the chromium.In view of the high chromium deficiency in the outer layers of the oxides examined it is apparent that the supposedly protective chromium oxide film on chrome steel is situated at the oxide-metal interface and not on the outer surface.The present investigation has shown the existence of two unreported iron satellites. One would appear to be associated with the mixed valence oxide Fe₃O₄ and another with the purely trivalent oxide Fe₂O₃. The satellites could be associated with a strong plasma loss mechanism, or be due to shake-up phenomena as in the case of copper and nickel oxides¹⁴As might be expected from the normal oxidation behaviour of iron, the present oxide films appear to consist of a thin layer of Fe₂O₃ overlying the main Fe₃O₄ oxide. The removal of this outermost Fe₂O₃ layer is rapid and is accompanied by the changes in the satellites mentioned above, together with changes in the position of the Fe 2p_{$\text{3}\text{2}$} oxide peak.In conclusion, it is clear that a detailed XPS examination can not only provide information on overall compositional changes, as can be obtained by AES, but can also provide a comprehensive picture of changes in oxide composition, including those due to oxides of one metal in different valency states.     

Thermal oxidation behaviors and structures of Fe-9Cr-1Mo alloy

Isothermal oxidation behaviors of Fe-9Cr-1Mo alloy were investigated at 600, 700, 750 and 850 ^°C for 72 h in air atmosphere. The oxidation rates were measured using a thermogravimetric analyzer (TGA). The structure and composition of the oxide scale were characterized by X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and conversion electron Mössbauer spectrometry (CEMS). In this study it was found that the oxide layers form duplex structures consisting of Fe and Cr oxides. CEMS spectra are composed of one doublet due to dispersed Fe ³⁺ in Cr₂O₃ oxidation layers produced at high temperatures and two magnetic components due to Fe-Cr-Mo alloy substrate and hematite (Fe, Cr)₂O₃ with lower hyperfine field than pure hematite (52 T). Fe rich oxides are formed at the surface by oxidation at relatively low temperatures of 600 ^°C and 700 ^°C, while Cr rich (Fe, Cr)-oxides are formed in the top surface layers by oxidation at higher than 750 ^°C.     

Surface termination of BaTiO3 (001) single crystals: A combined electron spectroscopic and theoretical study

The surface termination of oxide surfaces is of crucial importance for the growth of a second material like metals or other oxides. In this study we have investigated the surface of a BaTiO₃ (001) single crystal during sample preparation by various electron spectroscopic methods. It is shown by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) that during sputtering a Ba rich overlayer is formed, in which the Ba²⁺ ions are under coordinated. Below this layer, an oxygen deficient BaTiO₃ layer is found. During annealing, we observe the reformation of the crystalline structure. UP and MIE spectra provide clear evidence of a BaO terminated surface. X-ray photoelectron diffraction studies support this result, comparing recorded polar angle scans with calculated intensity modulations using multiple scattering cluster models.     

In situ X-ray photoelectron spectroscopy study of the oxidation of CuGaSe2

The thermal and native oxidation of CuGaSe₂ thin films was studied by in situ X-ray photoelectron spectroscopy (XPS). The special design of the XPS chamber allowed to measure XP-spectra under oxidizing gas atmospheres at pressures of up to 5 mbar (in situ) or in ultra high vacuum (UHV). During thermal oxidation, the formation of predominantly Ga₂O₃ and some amount of SeO₂ were observed, but no copper oxides could be detected in the near surface region of the thin films. The same oxides were found after native oxidation in air under ambient conditions. Only after long term native oxidation for longer than 4 months Cu(OH)₂ was detected. An additional sodium oxide compound formed at the thin film surface, Na_xO and Na₂CO₃ after thermal and native oxidation, respectively. The amount of these sodium oxide compounds depends on the Na content on the as prepared surface. The formation of SeO₂ under humid conditions at 100 °C was found to depend on the surface composition of the thin film.