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.     

Electron energy loss spectra from polycrystalline Cr and Cr2O3 before and after surface reduction by Ar bombardment

Electron energy loss spectra (ELS) have been obtained from polycrystalline Cr and Cr₂O₃ before and after surface reduction by 2 keV Ar⁺ bombardment. The primary electron energy used in the ELS measurements was systematically varied from 100 to 1150 eV in order to distinguish surface versus bulk loss processes. Two predominant loss features in the ELS spectra obtained from Cr metal at 9.0 and 23.0 eV are assigned to the surface and bulk plasmon excitations, respectively, and a number of other features arising from single electron transitions from both the bulk and surface Cr 3d bands to higher-lying states in the conduction band are also present. The ELS spectra obtained from Cr₂O₃ exhibit features that originate from both interband transitions and charge-transfer transitions between the Cr and O ions as well as the bulk plasmon at 24.4 eV. The ELS feature at 4.0 eV arises from a charge-transfer transition between the oxygen and chromium ions in the two surface layers beneath the chemisorbed oxygen layer, and the ELS feature at 9.8 eV arises from a similar transition involving the chemisorbed oxygen atoms. The intensity of the ELS peak at 9.8 eV decreases after Ar⁺ sputtering due to the removal of chemisorbed oxygen atoms. Sputtering also increases the number of Cr²⁺ states on the surface, which in turn increases the intensity of the 4.0 eV feature. Furthermore, the ELS spectra obtained from the sputtered Cr₂O₃ surface exhibit features characteristic of both Cr⁰ and Cr₂O₃, indicating that Ar⁺ sputtering reduces Cr₂O₃. The fact that neither the surface- nor the bulk-plasmon features of Cr⁰ can be observed in the ELS spectra obtained from sputtered Cr₂O₃ while the loss features due to Cr⁰ interband transitions are clearly present indicates that Cr⁰ atoms form small clusters lacking a bulk metallic nature during Ar⁺ bombardment of Cr₂O₃.     

Structural and compositional changes in ion-bombarded Ta2O5

A detailed study of the effect of heavy-ion bombardment on Ta₂O₅ has been undertaken using a combination of radioactive tracer techniques, electron microscopy, and Rutherford backscattering. Crystalline Ta₂O₅ is amorphized at ~6 × 10¹³ ions cm^?2, while at a dose of ~5 × 10¹⁶ ions cm^?2 the electron microscopy reveals the development of random grains of a new crystalline phase. By ~1 × 10¹⁷ ions cm^?2 the grains are not yet overlapping but still yield a diffraction pattern consistent with either δ-Ta-O (not to be confused with TaO) or Ta_1?xO₂, thus indicating that Ta₂O₅, like most other transition-metal oxides, is subject to preferential sputtering. Preferential sputtering was confirmed by backscattering analysis of specimens bombarded to high doses, where the average surface composition was found to be Ta_1.8±0.2O₂ or, equivalently, Ta₂O_2.2±0.2. The surface alteration had an average composition independent of the mass and energy of the incident ions.     

Low energy sputtering of neutral Cu2 molecules

The ratios of relative yields of neutral sputtered Cu₂ molecules to neutral sputtered Cu atoms were found to be linearly proportional to the sputtering yield of Cu, from a Cu target under bombardment by Ar⁺ ions (energy 50–90 eV), as determined by secondary neutral mass spectrometry.     

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.     

High-resolution sputter depth profiling with a low pressure hf plasma

An arrangement for sputter-depth profiling by means of a low pressure hf plasma is described. Extremely plane bombarding craters are obtained when the bombarding voltage, the plasma parameters and the geometry of the target assembly are fitted appropriately. A quantitative relation for plane crater conditions is established, and verified by a comparison with crater profiles created in Ta₂O₅ layers by the bombardment with Ar⁺ ions of 100–600 eV. Under optimum conditions the flatness of the crater bottoms being investigated by optical interferometry was in the order of 10Å. The method has been applied for depth profiling of 3500Å thick Ta₂O₅ layers by Sputtered Neutral Mass Spectrometry (SNMS).     

Temporal and spatial TaO emission generated from UV laser ablation of Ta and Ta2O5 in oxygen ambient

2 O₅ targets in oxygen ambient are presented. Line assignments indicate the presence of the excited Ta(I), Ta(II), and TaO in the plume. At higher oxygen pressure, a single peak appears in the TaO emission spectrum from the laser ablation of Ta while two peaks corresponding to a fast and a slow component of TaO emission are observed from the laser ablation of the Ta₂O₅ target by time-resolved emission spectroscopy. The delay times after laser pulse corresponding to two components of TaO emission from the laser ablation of Ta₂O₅ have been investigated as a function of oxygen pressure, laser fluence, and observation distance from the target surface. The two components of TaO emission could be attributed to different pathways for the generation of excited TaO molecules. A blast wave model is proposed to describe the behavior of the excited TaO in the plume of laser ablation of Ta₂O₅. Received: 1 February 1997/Accepted: 12 March 1997     

The formation of Ta+ secondary ions at oxygen-covered Ta surfaces

Absolute ionization probabilities R for Ta⁺ secondary ions are determined both experimentally from comparative in-situ SNMS and SIMS measurements and theoretically on the basis of recent microscopic ionization theories as a function of the oxygen surface concentration c_O. The theoretically predicted c_O dependence of R for sputtered Ta particles is found to agree well with the experimental results. The comparison between experiment and theory gives quantitative estimates of factors which influence the oxygen induced ion yield enhancement.     

Low energy Ar-ion bombardment effects on the CeO2 surface

The structure and electronic properties of epitaxial grown CeO₂(1 1 1) thin films before and after Ar⁺ bombardment have been comprehensively studied with synchrotron radiation photoemission spectroscopy (SRPES). Ar⁺ bombardment of the surface causes a new emission appearing at 1.6 eV above the Fermi edge which is related to the localized Ce 4f¹ orbital in the reduced oxidation state Ce³⁺. Under the condition of the energy of Ar ions being 1 keV and a constant current density of 0.5 μA/cm², the intensity of the reduced state Ce³⁺ increases with increasing time of sputtering and reaches a constant value after 15 min sputtering, which corresponds to the surface being exposed to 2.8 × 10¹⁵ ions/cm². The reduction of CeO₂ is attributed to a preferential sputtering of oxygen from the surface. As a result, Ar⁺ bombardment leads to a gradual buildup of an, approximately 0.69 nm thick, sputtering altered layer. Our studies have demonstrated that Ar⁺ bombardment is an effective method for reducing CeO₂ to CeO_2−x and the degree of the reduction is related to the energy and amount of Ar ions been exposed to the CeO₂ surface.     

Electronic structures and ferroelectric properties of ABi2Ta2O9 (A=Ca, Sr, and Ba)

The electronic structures of ABi₂Ta₂O₉ (A=Ca, Sr, and Ba) were calculated by using first-principles under optimized structure. As the size of A-site cation decreases from that of Ba²⁺ to Ca²⁺, the band-gap between O 2p and Ta 5d increases from 2.0 to 2.9 eV, which responses to the stronger orbital hybridizations between Ta 5d and O 2p orbits favoring improvement of the ferroelectric property, decrease in leakage current, and increase in both spontaneous polarization and Curie temperature by the structural distortion. In contrast to CaBi₂Ta₂O₉ and SrBi₂Ta₂O₉, the hybridization between Ba 5p orbits and O 2p orbits in BaBi₂Ta₂O₉ has better structural stability.     

Effect of oxide surface metallization under ion-beam irradiation

Changes in the surface chemical composition of WO₃, Ta₂O₅, MoO₃, and Nb₂O₅ oxides after Ar⁺ ion irradiation and those of the WO₃ surface after He⁺ ion irradiation under high vacuum were investigated by X-ray photoelectron spectroscopy. Upon Ar⁺ ion irradiation with an energy of 3 keV, the pronounced effect of ion-beam metallization was observed on the WO₃ oxide surface; a moderate effect was found for the Ta₂O₅ oxide surface; a weak one for the MoO₃ oxide surface; and no effect was discovered for the Nb₂O₅ oxide surface. At the saturation dose, 44 at % W, 12 at % Ta, and 2 at % Mo form on the oxide surfaces. Irradiation by light He⁺ ions with energies of 1 and 3 keV results in WO₃ surface metallization. At the saturation dose, 2 and 10 at % W (at 1 and 3 keV, respectively) forms on the oxide surface. The nature, mechanisms, and features of the oxide surface metallization effect induced by ion-beam irradiation are discussed.     

Luminescence in collisions of low-energy ions with graphite and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Excitation of H⁺, H₂ ⁺, H₃ ⁺, He⁺, and Ar⁺ ions by impact on graphite and Al₂O₃ was investigated by means of emission spectroscopy in the 50–1000 eV energy range of the projectiles. Emission of Balmer series from excited neutral hydrogen is observed for both targets. In addition, for the Al₂O₃ target a continuum emission is observed. The continuum probably originates from excited M_nO_m molecules produced in the collision cascade, when surface atoms bound by ionic bonds are released after the bond breaking caused by neutralization. The spectra obtained under Ar⁺ -bombardment show Ar II lines emitted by backscattered ions.     

Emission features of Ho ion in Nb2O5, Ta2O5 and La2O3 mixed Li2O-ZrO2-SiO2 glasses

Li₂O-ZrO₂-SiO₂: Ho³⁺ glasses mixed with three interesting d-block elemental oxides, viz., Nb₂O₅, Ta₂O₅ and La₂O₃, were prepared. Optical absorption and photoluminescence spectra of these glasses have been recorded at room temperature. The luminescence spectra of Nb₂O₅ and Ta₂O₅ mixed Li₂O-ZrO₂-SiO₂ glasses (free of Ho³⁺ ions) have also exhibited broad emission band in the blue region. This band is attributed to radiative recombination of self-trapped excitons (STEs) localized on substitutionally positioned octahedral Ta⁵⁺ and Nb⁵⁺ ions in the glass network. The Judd-Ofelt theory was successfully applied to characterize Ho³⁺ spectra of all the three glasses. From this theory various radiative properties, like transition probability A, branching ratio β_r and the radiative lifetime τ_r, for ⁵S₂ emission levels in the spectra of these glasses have been evaluated. The radiative lifetime for ⁵S₂ level of Ho³⁺ ions has also been measured and quantum efficiencies were estimated. Among the three glasses studied the La₂O₃ mixed glass exhibited the highest quantum efficiency. The reasons for such higher value have been discussed based on the relationship between the structural modifications taking place around the Ho³⁺ ions.     

Optical properties of La2O3 doped diamond-like carbon films

A novel kind of La₂O₃ doped diamond-like carbon (DLC) films with thickness of 100-120 nm were deposited by unbalanced magnetron sputtering. Raman spectra and photoluminescence properties were measured by Raman spectrometer operated by 325 nm He-Cd laser and 514 nm Ar⁺ laser, respectively. The intensities of Raman spectra and photoluminescence are higher than those of pure DLC films. The La₂O₃ doped DLC films have the potential promising for the application of solar cell coatings.     

X-ray photoelectron spectroscopic studies of PbO surfaces bombarded with He+, Ne+, Ar+, Xe+ and Kr+

The surface composition of PbO has been studied with X-ray photoelectron spectroscopy after bombardment with several inert gas ions of 400 eV. The results show reduction of PbO to metallic Pb with the degree of damage following the order He⁺ > Ne⁺ >Ar⁺. Both Kr⁺ and Xe⁺ did not reduce the oxide. The depth of damage varied from ≈9 Å for He⁺ to ≈1 Å for Ar⁺ bombardment. The results were compared to a collisional and a thermal model of the sputtering process.     

Electron impact ionization of Ar n , (H2O) n , Ar n (H2O) m clusters

Signals from ions forming in a supersonic molecular beam consisting of an argon-water vapor mixture are measured as functions of the exciting electron energy in the range to 120 eV. The thresholds of electron impact excitation of (H₂O) _{n − 1}H⁺ and Ar _n (H₂O _m ⁺ clusters are determined for the first time. It is found that the proton-hydroxyl group binding energy decreases considerably both in the case of water molecule clustering and when mixed Ar _n (H₂O) _m clusters arise.     

Ion survival probabilities for 3 keV Ar+ scattering from La,Yb, and chemisorbed H2, O2, and H2O on La surfaces

TOF spectra of scattered primary and surface recoiled neutrals and ions for 3 keV Ar⁺ bombardment of clean La and Yb and H₂, O₂, and H₂O saturated La surfaces are presented. The spectra are analyzed in terms of single (SS) and multiple (MS) scattering of the primary ions and surface recoiling (SR) of adsorbate atoms. Measurement of spectra of neutrals + ions and neutrals alone allows determination of scattered ion fractions Y. The Y values for the SS event are high for clean La (37%) and lower for adsorbate covered La (32% for H₂, 13% for O₂, and 8% for H₂O); Yb exhibits a similar behavior, i.e. 16% for clean Yb and 5% for O₂ + H₂O covered Yb. Photon emission accompanying the scattering collision has been observed from clean La and Yb and adsorbate covered La. A preferential inelastic energy loss of 15 ± 3 eV for the SS event has been observed for scattered neutrals as opposed to ions for La and H₂ saturated La at 135°. These results are interpreted within the models for Auger and resonant electronic charge exchange transitions during approach or departure of an ion with a surface and the electron promotions occuring during close atomic encounters where the electron shells are interpenetrating.     

DC discharge experiment in an Ar/N2/CO2 ternary mixture: A laboratory simulation of the Martian ionosphere's plasma environment

A low-pressure DC plasma discharge sustained in a 1.6%Ar–2.7%N₂–95.3%CO₂ ternary mixture is studied. This plasma was generated in a total pressure range from 1.0 to 4.0 Torr, a power of 6.3 W and a 12 l/min flow rate of gases. The electron temperature was found to be 8.41 eV and the ion density, in the order of 10⁹ cm⁻³. The species observed in the plasma mixture were CO₂, CO₂⁺, CN, CO, CO⁺, O₂, O₂⁺, N₂, N₂⁺, NO, C⁺, Ar and Ar⁺. At the pressure range in the present study, the species observed do not change their intensity due to an increase in the pressure and they separate in two groups according to their emission intensity: the band of the first group (CO₂, CO₂⁺ and CN) is approximately a factor of 3 more intense than that of the second group (CO, CO⁺, O₂, O₂⁺, N₂, N₂⁺, NO, C⁺, Ar and Ar⁺). The behavior of the emission intensities may be correlated to the constant ion density and electron temperature measured. Also, we observed the same constant behavior in the ratios of the neutral and positive species intensities to that of the N₂ intensity, as a function of pressure. This may suggest that the different rate coefficients and cross sections of elastic collision, excitation and de-excitation of electronic or vibrational levels, inelastic and superelastic collisions of electrons with the gas phase and products, neutral–neutral interactions, resonant charged transfer processes, recombination, to mention some, to produce these species change in the same proportion, as a function of the pressure to keep the relative ratios of the species almost constant.     

Comparative SNMS and SIMS studies of oxidized Ce and Gd

Polycrystalline Ce and Gd surfaces of various oxidation states have been investigated in situ and alternately by Sputtered Neutral Mass Spectrometry SNMS and by SIMS. For the bombardment with 4 keV Ar⁺ ions the dominating peaks in the mass spectra of postionized sputtered neutrals and positive secondary ions refer to metal atoms Me and monoxide molecules MeO. At oxygen concentrations of several atomic percent the sensitivity of SNMS and SIMS are of the same order. Characteristic differences are found between the behaviour of corresponding SNMS and SIMS signals: For SNMS the intensitiesI(MeO⁰) and I(Me⁰) in general vary complementarily when the oxygen is removed from the surface. The intensity ratios $\text{I(MeO}{}^0\text{)}\text{I(Me}{}^0\text{)}$ decrease monotonously by 1 to 2 orders of magnitude. With SIMS both I(MeO⁺) and I(Me⁺) are found to decrease with the oxygen content. The SIMS ratios $\text{I(MeO}{}^{\mathrm{+}}\text{I(Me}{}^{\mathrm{+}}\text{)}$ display an oscillatory behaviour with only little variation of their absolute values. As an example, quantitative results for the behaviour of the ionisation probability α⁺_Me for the secondary ions Ce⁺ and Gd⁺ are derived for low oxygen concentrations.     

MO2,M2O5和MO3氧化物对锂硼酸盐玻璃离子导电性的影响

本工作测量了45Li₂O·50B₂O₃·5M_mO_n(M=Al,Ti,Zr,P,V,Nb,Ta,Cr,Mo,W)玻璃的总电导率和电子电导率,以Raman光谱研究了玻璃结构,依据作为锂离子定域体的阴离子团的性质讨论了MO₂,M₂O₃和MO₃氧化物对玻璃离子导电性的影响。 关键词：