Surface composition modification of tungsten higher oxide under He<Superscript>+</Superscript> ion bombardment

The surface reduction of higher oxide WO₃ under irradiation by He⁺ ions with the energies 1 and 3 keV in a high vacuum is investigated by X-ray photoelectron spectroscopy. It is found that lower WO₂ and intermediate WO _x (2 < x < 3) oxides form first in WO₃ surface layers under He⁺ ion bombardment, and with an increase in the irradiation dose metallic tungsten forms. It is shown that the degree of irradiated oxide surface metallization increases with an increase in the energy of the bombarding He⁺ ions. A comparison of WO₃ oxide surface composition modification under He⁺ and Ar⁺ ion irradiation is presented.     

Ion-beam reduction of the surface of higher oxides of molybdenum and tungsten

The process of reduction of the surface of oxides MoO₃ and WO₃ under irradiation by Ar⁺ and O ₂ ⁺ ions with an energy of 3 keV in high vacuum is investigated by X-ray photoelectron spectroscopy. It is shown that upon irradiation by Ar⁺ ions, lower and intermediate oxides and unoxidized metals are formed in the surface layers of higher oxides. Irradiation by O ₂ ⁺ ions mainly leads to formation of intermediate oxides with an insignificant content of lower oxides. It is found that the process of ion-beam reduction of the surface of oxides MoO₃ and WO₃ substantially depends on the ion type, irradiation dose, and difference in energy of the metal-oxygen bond in oxides.     

Ion-beam reduction of the surface of tantalum higher oxide

The process of reduction of the surface of higher oxide Ta₂O₅ under irradiation by inert gas (Ar⁺) and chemically active gas (O₂⁺) ions with an energy of 3 keV in high vacuum is investigated by X-ray photoelectron spectroscopy at room temperature. It is found that intermediate oxide TaO₂, lower oxide TaO, and metallic Ta form in the surface layers of Ta₂O₅ under Ar⁺ ion bombardment. An insignificant amount of intermediate oxide TaO₂ forms in the surface layers of Ta₂O₅ under O₂⁺ ion bombardment. Ion-beam-induced reduction of the Ta₂O₅ surface is shown to depend on the type of ion and irradiation dose.     

Ion-beam reduction of the surface of higher niobium oxide

X-ray photoelectron spectroscopy is used to study the process of reduction of the surface of the higher oxide Nb₂O₅ upon bombardment with inert gas ions (Ar⁺) and reactive gas ions (O₂⁺) with an energy of 1 and 3 keV in high vacuum at room temperature. It is found that, upon bombardment with Ar⁺ ions, the lower oxide NbO and the intermediate oxide NbO₂ are formed in the surface layers of the oxide Nb₂O₅. Bombardment with O₂⁺ ions leads to the formation of an extremely insignificant amount of the intermediate oxide NbO₂ in the surface layers of the oxide Nb₂O₅. It is revealed that the process of ion-beam reduction of the surface of the oxide Nb₂O₅ depends on the ion type, dose, and energy of exposure.     

Preferential sputtering of TIC under keV Ar+ ion bombardment: Simultaneous sputtering-ISS measurement with He+ and Ar+ ions

The composition change of the outermost atom layer of TiC(110) under ion bombardment with 1.5–3 keV He⁺ and He⁺ + Ar⁺ ions has been measured by ion scattering spectroscopy with He⁺ ions at different sample temperatures. It has been found that the preferential sputtering of C atoms takes place for both the He⁺ and Ar⁺ ion bombardment, however the preferred sputtering is more pronounced for Ar⁺ ions than for He⁺ ions. The ion bombardment with He⁺ ions at elevated sample temperatures hardly results in any change in surface composition below ～800°C, while Ar⁺ ion bombardment results in C enrichment for elevated temperatures as reported so far.     

Ion-enhanced gas-surface chemistry: The influence of the mass of the incident ion

There are many examples of situations in which a gas-surface reaction rate is increased when the surface is simultaneously subjected to energetic particle bombardment. There are several possible mechanisms which could be involved in this radiation-enhanced gas-surface chemistry. In this study, the reaction rate of silicon, as determined from the etch yield, is measured during irradiation of the Si surface with 1 keV He⁺, Ne⁺, and Ar⁺ ions while the surface is simultaneously subjected to fluxes of XeF₂ or Cl₂ molecules. Etch yields as high as 25 Si atoms/ion are observed for XeF₂ and Ar⁺ on Si. A discussion is presented of the extent to which these results clarify the mechanisms responsible for ion-enhanced gas-surface chemistry.     

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.     

Radiation damage of material surfaces under prolonged irradiation with He+ and Ar+ ions with broad energy spectra

The results of studying the redistribution of Be, Al, Ti, Fe, Cu, Zr, Mo, and W atoms incorporated in polycrystalline metal samples under irradiation with He⁺, (He⁺ + Ar⁺), and Ar⁺ ion beams with a broad energy spectrum and an average energy of 10 keV at irradiation doses of 1 × 10²¹ ion/cm² are studied. It is discovered that irradiation at doses exceeding 1 × 10¹⁹ ion/cm² results in local small-crystal formations being produced in a near-surface substrate layer. Their typical dimensions are less than 1–5 μm, and their the density is up to 1–100. They contain incorporated atoms and impurity atoms with a concentration of 0.1–10 at %. Subsequent irradiation at a dose of 1 × 10²⁰ ions/cm² or more leads to disappearance of these formations, mainly because of sputtering processes.     

Planarization of inhomogeneous structures by means of physical ion sputtering

The dynamics of ion-beam etching of test microstructures, simulating fragments of the surface structure of very large-scale modern integrated circuits, has been studied. Aluminum strip microstructures, 0.5 μm high and 1 μm wide deposited on the surface or embedded into SiO₂, were used as test samples. 1 keV Ar⁺ ion beam 1 with a current density 0.5 mA/cm₂, incident on the surface of a sample, rotating at a speed of 60 r/min, at an angle of 87°, has been used in the experiments. The surface morphology evolution was studied using atomic-force microscopy. The experiments demonstrate that physical ion sputtering at glancing incident angles can be used for the planarization of originally inhomogeneous structures. The achieved planarization degree allows one to use this method for defect detection in the metallization multilevel layers of very large-scale modern integrated circuits.     

ISS measurement of surface composition of AuCu alloys by simultaneous ion bombardments with Ar+ and He + ions

Surface composition of Au-Cu(43 at%) alloy under 1.5–5 keV argon ion bombardment has been investigated by ion scattering spectroscopy (ISS). In this experiment, we adopted a specific technique to use mixed He⁺ and Ar⁺ ions as primary beam in order to perform sputtering (Ar⁺) and ISS measurement (He⁺) simultaneously. The outermost atom layer of Au-Cu alloys under Ar⁺ ion bombardment is Au-rich leading to the conclusion that Ar⁺ ion bombardment of AuCu alloys causes the preferential sputtering of Cu atoms, resulting in a Au-rich outermost atom layer and a depletion layer of Au atoms beneath the outermost atom layer due to ion-beam-enhanced surface segregation. This result explains the experimental results obtained by AES as well.     

ESCA studies of metal-oxygen surfaces using argon and oxygen ion-bombardment

ESCA has been used to monitor alterations of catalytically and electrochemically important metal-oxygen surfaces following exposure to Ar⁺ and O₂⁺ ion bombardment. This treatment resulted not only in sputtering, but also, in many cases, in reduction to the corresponding metal or lower oxide. A model based on bulk thermodynamic free energy considerations Is proposed to explain this phenomenon. We have also exploited this approach to obtain an in-depth concentration profile of various oxidation states of an element, to selectively prepare desired surface oxide compositio and to aid in interpreting complex O ls spectra. Results obtained from metal-oxygen surfaces for Ni, Ru and Mo are presented. Ni₂O₃ and RuO₃, which are gross defect structures of the bulk species, are present on NiO and RuO₂ respectively, with the former being confined to the surface layers. The MoO₂, on the other hand, is covered with a surface layer of MoO₃ present as a regular crystal structure.     

The influence of ion bombardment on emission properties of small dust grains

Dust grains that are present in many plasma and vacuum systems and in the space usually carry a non-negligible charge. Their charging significantly depends on surface properties of the grain material. In cold plasma, charging is mainly given by electron attachment, nevertheless, when plasma becomes hot, other processes (secondary electron emission, field emission, etc.) take place. Emission properties of the grain surface could be modified by grain baking or by ion bombardment. Our study is carried out at the dust charging experiment dealing with a single dust grain electro-dynamically levitated in a 3D quadrupole trap. The grain can be exposed to the ion beam in the energy range of 100 eV–5 keV and to the electron beam in the energy range of 100 eV–10 keV. We have chosen He⁺ and Ar⁺ ions for the surface treatment and the observed influence on the surface properties is discussed in terms of secondary emission. A non-negligible shift of the secondary electron emission yield, as well as a change of energy distribution of secondary electrons, were measured after Ar⁺ bombardment. A preliminary study suggests that the effects of He⁺ and Ar⁺ are comparable.     

Sputtering of Ta2O5 by Ar+ ions at energies below 1 keV

The sputtering of anodically formed Ta₂O₅ layers of about 3500 Å thickness has been studied by Sputtered Neutral Mass Spectroscopy (SNMS). For perpendicular bombardment with Ar⁺ ions up to 900 eV the flux of ejected neutral particles is found to consist almost exclusively of metal atoms Ta and Oxide specific molecules TaO and TaO₂ with intensity ratios in the order 1 : 1 : 10^?1. From depth profiling measurements with SNMS, and from the intensity ratios in the SNMS spectra the total sputtering yield of Ta₂O₅ and the partial yields of Ta, TaO and of oxygen have been determined for normally incident Ar⁺ ions of 100 to 600 eV. After an initial increase the TaO intensity in the SNMS spectra remains constant during the sputter removal of the whole layer. A simple model is derived by which the preferred emission of TaO molecules, and the initial increase of the TaO intensity is referred to ion induced variations of the surface stoichiometry of Ta₂O₅. For optimum TaO production the model predicts equal atomic surface concentrations of Ta and O.     

On the nature of ion bombardment-induced phase transformations in films of transition metals

Abstract

Electron diffraction studies have been made of polycrystalline Ni films irradiated with well separated beams of ions of different nature, namely ions of inert (He⁺, Ne⁺, Ar⁺, Kr⁺, Xe⁺) and reactive (N⁺ and O⁺) gases. The Ni films were prepared under vacuum conditions (P? 3·10^?6Pa during evaporation) preventing an appreciable contamination of the films with impurities. The samples were irradiated at T? 300 K with ion beams of energies from 10 to 100 keV in the dose range between 5·10¹⁶ cm^?2 and the value leading to sample destruction.

Irradiation with noble gas ions revealed no phase transitions in the Ni films. A similar result was obtained in irradiation of Fe and Cr films with He⁺ ions. The bombardment of Ni films with reactive gas ions does cause changes in the lattice structure of the samples under study, depending on the nature of the bombarding ions. The N⁺ ion bombardment gives rise to the hcp phase with the lattice parameters typical of the Ni₃N compound, and the O⁺ ion bombardment results in the fcc phase with the NiO-type parameter.

The conclusion is drawn on the chemical origin of the phase transformations in the Ni films under ion bombardment. The necessity of revising the concept about the polymorphous nature of phase transformations induced in the films of transition metals by ion bombardment is substantiated.     

Interplay of native defect-related photoluminescence response of ZnO nanosticks subjected to 80 keV Ar ion irradiation

We report on the effect of 80 keV Ar⁺ ion irradiation on the luminescence response of zinc oxide (ZnO) nanosticks synthesized using a simple microemulsion route. The formation of nanoscale rods was confirmed from the transmission electron microscopy, whereas the hexagonal wurtzite phase of the nanorods was detected in an X-ray diffraction pattern. The photoluminescence pattern of the nanorods was dominated by various native defect states of ZnO, which are responsible for the quenching of the typical band edge emission of ZnO. Under Ar⁺ ion irradiation at a fluence of 1×10¹³ ions/cm², the band edge emission was recovered owing to the suppression of oxygen vacancy defects. In addition, the formation of new zinc vacancy and ionized zinc interstitial defects were also evident. Conversely, the band edge emission was found to be quenched as a result of the creation of more oxygen vacancy (V_O) defects due to ion irradiation (fluence: 1×10¹⁵ ions/cm²). The nuclear energy loss of the Ar⁺ ions in ZnO is responsible for the formation of point (vacancy-related) defects, while relatively small amount of electronic energy loss of the Ar⁺ ion results in the ionization of the neutral zinc interstitial (Zn_i) defects. The energy deposition scheme of the energetic ions has been elaborated with the help of theoretical modeling that explains the observed features quite satisfactorily.     

Ion beam induced reaction of carbon films on Si(1 0 0)

The irradiation effects of 2 MeV He⁺ and Ar⁺ ions on the film structure of the C–Si system were investigated with RHEED and XPS. The formation of SiC phase and/or the growth of epitaxial SiC were possible by He⁺ irradiation for the carbon films up to 0.7 nm in thickness, which was thinner than that by Ar⁺ irradiation. The He⁺ irradiation could not grow the turbostratic graphite which could be grown by Ar⁺ irradiation. The mechanism of the formation and the epitaxial growth of SiC by ion irradiation was discussed from the view point of the energy transfer from the irradiated ions.     

Surface modification of polycrystalline oxides (V<Subscript>2</Subscript>O<Subscript>5</Subscript>, MoO<Subscript>3</Subscript>, and WO<Subscript>3</Subscript>) irradiated with high-power ion beams

The influence of a high-power ion beam on polycrystalline oxides (V₂O₅, MoO₃, and WO₃) is investigated. Oxide irradiation with ion beams with current densities of greater than ～30 A/cm² is established to initiate changes in the color of irradiated layers and lead to surface-layer particle melting. It is demonstrated that a distinctive feature of the interaction between a high-power ion beam and V₂O₅ is the formation of surface nanosheets and nanowires whose characteristic cross-sectional size and thickness are ～1 μm and up to ～40 nm, respectively. The nanosheets are generated near emerging surface cracks if the beam current density is ～100 A/cm². Possible mechanisms of surface nanostructures formation under the action of pulsed ion beams are discussed.     

An XPS study on the surface reduction of V2O5(0 0 1) induced by Ar ion bombardment

The effect of the irradiation with Al Kα X-rays during an XPS measurement upon the surface vanadium oxidation state of a fresh in vacuum cleaved V₂O₅(0 0 1) crystal was examined. Afterwards, the surface reduction of the V₂O₅(0 0 1) surface under Ar⁺ bombardment was studied. The degree of reduction of the vanadium oxide was determined by means of a combined analysis of the O1s and V2p photoelectron lines. Asymmetric line shapes were needed to fit the V³⁺2p photolines, due to the metallic character of V₂O₃ at ambient temperature. Under Ar⁺ bombardment, the V₂O₅(0 0 1) crystal surface reduces rather fast towards the V₂O₃ stoichiometry, after which a much slower reduction of the vanadium oxide occurs.     

Ion-beam irradiation effect on solid-phase growth of β-FeSi2

Effects of Ar⁺ ion-beam irradiation on solid-phase growth of β-FeSi₂ have been investigated. Fe (10 nm)/Si structures were irradiated with 25 keV Ar⁺ (5.0×10¹⁵ cm⁻²) at a temperature of 25°C (sample A) or 400°C (sample B), and subsequently annealed at 800°C. A reference was obtained after annealing without irradiation (sample C). X-ray diffraction results indicated that β-FeSi₂ was formed after annealing at 800°C for 5 h, and the formation rate was the fastest for sample A and the slowest for sample C, i.e., A>BC. However, Auger electron spectroscopy measurements showed that atomic mixing at Fe/Si interface before annealing was B>AC. These results suggested that amorphization of Si substrate, in addition to atomic mixing, enhanced the solid-phase growth of β-FeSi₂, which was confirmed experimentally. Moreover, a direct band gap of 0.89 eV was observed for the sample with pre-amorphization by the Fourier-transform infrared (FT-IR) spectroscopy measurements. These enhancement effects were attributed to that the phase transition to β-FeSi₂ was accelerated by atomic arrangement induced during annihilation of excess vacancies. These enhancement effects can be utilized for nano-fabrication of β-FeSi₂ by using focused ion-beam irradiation.     

Electrochromic phenomena in transition metal oxide thin films prepared by thermal evaporation

We present an experimental study on the electrochromic properties of MoO₃, WO₃ and mixed WO₃-MoO₃ thin films prepared by thermal evaporation. We have constructed symmetric and quasi-symmetric electrochromic cells incorporating the evaporated oxide films as electrochromic layers. Li⁺ doped V₂O₅ films served as ion storage layers. The symmetric cells were found to exhibit significantly improved optical properties compared to the quasi-symmetric ones, with very low luminous transmittance values in the colored state, which makes them suitable for large-area window applications. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.