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Electronic Structure of β-Ta Films from X-ray Photoelectron Spectroscopy and First-principles Calculations
Abstract
The electronic structure and chemical bonding of β-Ta synthesized as a thin 0 0 1-oriented film (space group P4¯21m) is investigated by 4f core level and valence band X-ray photoelectron spectroscopy and compared to α-Ta bulk. For the β-phase, the 4f7/2 peak is located at 21.91 eV and with the 4f5/2 at 23.81 eV which is 0.16 eV higher compared to the corresponding 4f peaks of the α-Ta reference. We suggest that this chemical shift originates from electron screening, higher resistivity or strain in the β-Ta film. Furthermore, the 5d–5s states at the bottom of the valence band are shifted by 0.75 eV towards higher binding energy in β-Ta compared to α-Ta. This is a consequence of the lower number of nearest neighbors with four in β-Ta compared to eight in the α-Ta phase. The difference in the electronic structures, spectral line shapes of the valence band and the energy positions of the Ta 4f, 5p core-levels of β-Ta versus α-Ta are discussed in relation to calculated states of β-Ta and α-Ta. In particular, the lower number of states at the Fermi level of β-Ta (0.557 states/eV/atom) versus α-Ta (1.032 states/eV/atom) that according to Mott's law should decrease the conductivity in metals and affect the stability by charge redistribution in the valence band. This is experimentally supported from resistivity measurements of the film yielding a value of ∼170 μΩ cm in comparison to α-Ta bulk with a reported value of ∼13.1 μΩ cm.
... The valence band HAXPES spectra of the 5d TMs show the fine structures in any X-ray polarization. These fine structures cannot be detected in the spectra obtained by conventional SX-PES, [20][21][22][23][24][25][26][27][28][29] since ÁE of HAXPES (150 meV) is much better than that of SX-PES (∼700 meV). In fact, the valence band spectrum of Au for H-pol X-ray shown in Fig. 1(g) is sharper than the previously reported Au HAXPES spectrum with ÁE of ∼400 meV at the excitation energy of 8 keV, 30) whereas the X-ray polarization dependence on the spectral shape is weak in both our experiment and Ref. 30. ...
... The very weak satellites (E B ¼ $26:6 and ∼28.5 eV) at the higher E B side of the Ta 4f core-level spectrum are due to TaO x . 23) ...
... However, significant changes in the Ta 4f core level are seen after RIE treatments, notably a strong increase in the metallic Ta component. The enhancement of the metallic Ta peak is accompanied by a long shoulder at intermediate binding energies due to suboxides, consistent with previous reports [16,54,55]. ...
Superconducting circuits are among the most advanced quantum computing technologies, however their performance is currently limited by losses found in surface oxides and disordered materials. Here, we identify and spatially localize a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy (s-SNOM). Making use of terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle x-ray scattering (GIWAXS) shows that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. By localizing defect centers at the nanoscale, our characterization techniques and results will inform the optimization of fabrication procedures for new low-loss superconducting circuits.
... It is known that the tantalum exists in two allotropic forms, namely alpha and beta, where the β structure is metastable. However, according to the authors of [23], it could exist in the form of thin films and coatings. As mentioned in section "Materials and methods", the Ta element was deposited in the form of a thin film, which is completely in agreement with the results showing the existence of β-Ta within the alloyed zone. ...
The electron-beam cycling additive technique was proposed for the formation of shape memory Ti-Ta coatings on titanium substrate. On a commercially pure Ti plate, Ta film with a thickness of about 4 μm was deposited by direct current (DC) magnetron sputtering. The sample was then subjected to an electron-beam surface alloying by a scanning electron beam. On the already-formed Ti-Ta surface alloy, a Ta coating with the same thickness was further deposited and the specimen was again subjected to electron-beam alloying for the second cycle. The same procedure was repeated for the third cycle. The structure obtained after each cycle Ti-Ta coatings was studied by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-dispersive X-ray spectroscopy (EDX). The Young’s modulus, hardness, and shape memory effect (SME) were studied by nanoindentation experiments. The results showed that the thickness of the Ti-Ta coatings is about 50 μm in all cases, where the Ta content increases after each technological cycle. It was found that the obtained phase composition is in the form of a double-phase structure of α’ martensitic and β phases, where the highest amount of beta is registered in the case of the Ti-Ta coating obtained after the third cycle. The results obtained for the Young’s modulus and hardness showed that both mechanical characteristics decrease significantly after each cycle. Additionally, the elastic depth recovery ratio increases with an increase in the number of cycles.
... The β-phase to α-phase transformation of tantalum produces a unique microstructure with very unusual characteristics, especially for a crystalline material. The β-phase is also metastable and will transform to α when heated, from 750 to 775 °C, as reported in [15]. ...
The aim of the present work is to verify the microstructural behavior of a B 521 tantalum alloy UNS Grade R05200 after welding, in relation to the welding thermal cycle. The joint design was a 1.5 mm thickness circumferential butt welding, on a 32 mm outside diameter pipe, welded in 1 G position (horizontal, flat, and rotating). The chosen welding process was gas tungsten arc welding (GTAW). The microstructural analysis showed the presence of coarse, dendritic-columnar structures, as well as a hexagonal cell, with no cracks noted. Hardness tests showed an increase in hardness, from 120 HV to 425 HV, in the heat-affected zone. Through finite element methods, the behavior of the temperature field was estimated and compared.
... Tantalum exists in two crystalline forms, namely alpha and beta phases, and the identified Ta peak belongs to the beta structure. Although the beta phase is metastable and the bulk Ta exists in the form of almost entirely alpha phase, the authors of [29] found that the β phase can exist in a stable state in the form of thin films. This again correlates with our results showing a presence of undissolved β-Ta, which was deposited on the top of the Ti substrate as an alloying element and is consistent with SEM/EDX investigations. ...
In this study, we present the results of Young’s modulus and coefficient of friction (COF) of Ti–Ta surface alloys formed by electron-beam surface alloying by a scanning electron beam. Ta films were deposited on the top of Ti substrates, and the specimens were then electron-beam surface alloyed, where the beam power was varied from 750 to 1750 W. The structure of the samples was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Young’s modulus was studied by a nanoindentation test. The coefficient of friction was studied by a micromechanical wear experiment. It was found that at 750 W, the Ta film remained undissolved on the top of the Ti, and no alloyed zone was observed. By an increase in the beam power to 1250 and 1750 W, a distinguished alloyed zone is formed, where it is much thicker in the case of 1750 W. The structure of the obtained surface alloys is in the form of double-phase α’and β. In both surface alloys formed by a beam power of 1250 and 1750 W, respectively, Young’s modulus decreases about two times due to different reasons: in the case of alloying by 1250 W, the observed drop is attributed to the larger amount of the β phase, while at 1750 W is it due to the weaker binding forces between the atoms. The results obtained for the COF show that the formation of the Ti–Ta surface alloy on the top of Ti substrate leads to a decrease in the coefficient of friction, where the effect is more pronounced in the case of the formation of Ti–Ta surface alloys by a beam power of 1250 W.
... The diversity of CQDs can be attributed to different carbon precursors and 49 synthetic strategies [11]. In a previous study, as a part of selecting raw materials for CQDs, wood 50 waste was found to be environmentally safer and less expensive than chemical reagents, thus 51 3 maximizing the use of renewable resources. Macromolecular compounds in wood include cellulose, 52 hemicellulose, and lignin. ...
In this study, CexZr1−xO2 solid-solution oxide (x = 0.7) was prepared using hydrothermal method, and the obtained Ce0.7Zr0.3O2 nanoparticles were modified with wood–based carbon quantum dots (CQDs). Furthermore, we report a composite photocatalyst of vacuum heat-treated CQDs/Ce0.7Zr0.3O2 for degrading methylene blue. The structural properties and photocatalytic activity of CQDs/Ce0.7Zr0.3O2 were systematically studied after vacuum heat treatment. Herein, a part of the functional groups present on CQD surfaces changed through high–temperature heating during vacuum heat treatment. The dominant structures sp2 hybrid carbon atoms and amorphous carbon were transformed to defect states of sp3 hybrid oxygen atoms, thus increasing the surface oxygen vacancy concentration. The high activity and excellent stability of vacuum heat-treated CQDs/Ce0.7Zr0.3O2 system can be attributed to increased surface oxygen vacancy concentration, modification of CQDs, and the protruding structure of Ce0.7Zr0.3O2, contributing to efficient charge transfer. This study provides potential systems of CQD-modified photocatalysts and also reveals the substantial effect of vacuum heat treatment on enhancing the photocatalytic properties of composites.
... Tantalum is highly resistant to corrosion by acids and used in the chemical industry for production of lining, claddings, tanks, valves, heat exchangers and so on [2]. Tantalum can be found in two different crystal structures: bulk α-Ta with body-centered cubic (bcc) structure [3] and thin film β-Ta with tetragonal crystal structure [4]. Tantalum capacitors are attractive for portable telephones, personal computers, automotive electronics, and photo cameras. ...
Superconducting circuits are among the most advanced quantum computing technologies; however, their performance is limited by losses found in surface oxides and disordered materials. In this work, we demonstrate the identification and spatial localization of a near-field signature of loss centers on tantalum films using terahertz scattering-type scanning near-field optical microscopy. By utilizing terahertz nanospectroscopy, we observe a localized excess vibrational mode around 0.5 THz and identify this resonance as the boson peak, a signature of amorphous materials. Grazing-incidence wide-angle X-ray scattering reveals that oxides on freshly solvent-cleaned samples are amorphous, whereas crystalline phases emerge after aging in air. Through nanoscale localization of defect centers, our findings provide valuable insights for the optimization of fabrication procedures for new low-loss superconducting circuits.
In this study, a method for forming a highly adhesive and ductile α-phase tantalum film on 304 stainless steel using the bipolar high-power impulse magnetron sputtering (bipolar HiPIMS) technique was investigated. The growth of brittle β-phase tantalum, which is commonly formed when using conventional direct current magnetron sputtering methods (DCMS) at room temperature, was prevented by the high-energy ions directed toward the growing films. The results showed the formation of an extremely thin (<70 nm) α-phase tantalum film without any additional processes, such as substrate biasing or heating. In addition, a high adhesion strength exceeding 45 MPa, broadened intermixed layer, and nanocrystalline microstructure were achieved by the effect of positive voltage. These results indicate that the bipolar HiPIMS technique is a facile deposition method for modifying the substrate surface and forming high-quality crystalline films. Our tantalum films are optimized to protect substrates against corrosion in severely harsh mechanical, chemical, and thermal environments. To confirm the superiority of the bipolar HiPIMS technique over HiPIMS and DCMS, the properties of the films formed using the three methods were analyzed via X-ray diffraction, Auger electron spectroscopy, atomic force microscopy, scanning electron microscopy, and pull-off adhesion testing.
Although understanding filament formation in oxide‐based memristive devices by theory has emerged, there are still fundamental unanswered questions. Importantly, for practical application of thin films the material in its amorphous state is to be considered, but mostly lacking so far, and details on sub‐stoichiometry are also scarce. To gain insight into the optical and electronic properties of sub‐stoichiometric amorphous tantalum oxide (TaOx), the electron energy loss spectrum (EELS) of model systems is characterized theoretically and electron transport characteristics are analyzed in detail. Calculated blue‐shifts by increasing sub‐stoichiometry explained the measurements, potentially suggesting estimation of oxygen vacancy concentrations through EEL spectra. Electron transport results based on TaOx material models validated by EELS measurements show that oxygen vacancy filamentary paths are initiated at low bias upon increasing sub‐stoichiometry yet noting an interplay with the local amorphous structure. Contact resistances at interfaces of the TaOx switching layer and a tantalum scavenging layer or titanium nitride electrode are quantified, indicating the possibility for either oxygen vacancy‐ or metal cluster‐based conduction mechanisms at the interface. The computational work, combined with experimental characterization for validation, provides a basis for investigating effects of sub‐stoichiometry on filament formation in TaOx thin film memristive devices. To explain filament formation in sub‐stoichiometric tantalum oxide (TaOx) thin film memristive devices, theoretical characterization of electron energy loss spectra (EELS) is validated by measurements. Predicted EELS blue‐shifts when increasing sub‐stoichiometry estimate the oxygen vacancy concentration. Electron transport calculations for amorphous TaOx and metal–TaOx–metal stacks of varying sub‐stoichiometry elucidate oxygen vacancy assisted transmission paths of the conduction filaments.
A simple first-principles approach is used to estimate the core level shifts observed in X-ray photoelectron spectroscopy for the 4f electrons of Hf, Ta, W, and Re; these elements were selected because their 4f levels are relatively close to the Fermi energy. The approach is first tested by modeling the surface core level shifts of low-index surfaces of the four elemental metals, followed by its application to the well-studied material TaSe2 in the commensurate charge density wave (CDW) phase, where agreement with experimental data is found to be good, showing that this approach can yield insights into modifications of the CDW. Finally, unterminated surface core level shifts in the hypothetical MXene Ta3C2 are modeled, and the potential of XPS for the investigation of the surface termination of MXenes is demonstrated.
Tantalum (Ta) gradient transitional layer, which was prepared by double-glow plasma surface alloying technology, was designed to improve the tribological properties of AISI 304 stainless steel. The microstructure, morphology, mechanical and tribological properties of surface modified stainless steel were characterized and analyzed. The results showed that the modified Ta coating can improve the surface roughness and tribological properties of the AISI 304 stainless steel greatly. Furthermore, β-Ta is in the majority in gradient transitional layer and it show high critical scratch load. The thickness of Ta coating, surface roughness, load bearing capacity, friction coefficient and wear rate of Ta/AISI 304 increased first and then decreased. Especially, the sample with Ta alloyed time of 30 min show excellent tribological properties, which were closely related with high critical load, low surface roughness, and the phase transformation from metastable β-Ta to stable cubic α-Ta in less oxygen conditions at the role of frictional heat. Furthermore, there are α-Ta nanoparticles with assembled into nanoribbons on the counterface decreasing the friction and wear further. The present study provides useful guidance for the modification of AISI304 and a new prospect on the transition from β-Ta to α-Ta during the friction.
In recent years, the phase control of monometallic and alloy nanomaterials has attracted great attention because of the potential to tune the physical and chemical properties of the monometallic and alloy nanomaterials. In this review, an overview of the latest research progress in the phase‐controlled monometallic and alloy nanomaterials is first given. Then, the phase‐controlled synthesis of monometallic and alloy nanomaterials using a chemical reduction method are discussed. The formation mechanisms of these nanomaterials are specifically highlighted. Lastly, the challenges and future perspectives in this new research field are discussed.
Phase structure of sputtered Ta coating in the negative glow space and LPH effect were explored. The whole coating/substrate system is substrate → physically gas-absorbed Fe surface → oxygen-enriched TaOx layer → amorphous Ta → α and β dual phase → single α phase. After LPH course, microstructure of Ta coating shows intact, only a few cracks emerge after 100 laser pulses, exhibiting thin HAZ but thick Fe/Ta ICZ, without martensitic transformation. For the electrodeposited Cr coating, continuous thermal stresses produce many extra micro-crack, substrate oxidation and martensitic transformation, leading to crack propagations and final bulk delamination, without any ICZ.
We report the enhancement of the spin Hall effect in a (111)-oriented epitaxial α-Ta/CoFeB bilayer system, which was previously believed to show the small spin Hall angle (SHA). The SHA of the epitaxial α-Ta/CoFeB structure estimated by spin Hall magnetoresistance measurement is 0.15, a value that is 1.5 times larger than the conventional amorphous-Ta/CoFeB structure. Effective SHA estimated by harmonic Hall measurement qualitatively supports the SMR result, whereas the strength of the field-like field of epitaxial α-Ta samples is similar to that of amorphous Ta samples. According to the resistivity dependence on the SHA, the origin of the enhanced SHA in epitaxial α-Ta is due to the extrinsic contribution to the SHA.
C 1s signal from ubiquitous carbon contamination on samples forming during air exposure, so called adventitious carbon (AdC) layers, is the most common binding energy (BE) reference in x-ray photoelectron spectroscopy studies. We demonstrate here, by using a series of transition metal nitride films with different AdC coverage, that the BE of C 1s peak E_B^F varies by as much as 1.44 eV. This is a factor of ten more than the typical resolvable difference between two chemical states of the same element, which makes BE referencing against the C 1s peak highly unreliable. Surprisingly, we find that C 1s shifts correlate to changes in sample work function ϕ_SA, such that the sum E_B^F+ϕ_SA is constant at 289.50±0.15 eV, irrespective of materials system and air exposure time, indicating vacuum level alignment. This discovery allows for significantly better accuracy of chemical state determination than offered by the conventional methods. Our findings are not specific to nitrides and likely apply to all systems where charge transfer at the AdC/substrate interface is negligible.
In recent years, gas cluster ion beams (GCIB) have become the cutting edge of ion beam technology to sputter etch organic materials in surface analysis. However, little is currently known on the ability of argon cluster ions (Arn⁺) to etch metal oxides and other technologically important inorganic compounds and no depth profiles have previously been reported. In this work, XPS depth profiles through a certified (European standard BCR-261T) 30nm thick Ta2O5 layer grown on Ta foil using monatomic Ar⁺ and Ar1000⁺ cluster ions have been performed at different incident energies. The preferential sputtering of oxygen induced using 6keV Ar1000⁺ ions is lower relative to 3keV and 500eV Ar⁺ ions. Ar⁺ ions exhibit a steady state O/Ta ratio through the bulk oxide but Ar1000⁺ ions show a gradual decrease in the O/Ta ratio as a function of depth. The depth resolution and etch rate is substantially better for the monatomic beam compared to the cluster beam. Higher O concentrations are observed when the underlying Ta bulk metal is sputtered for the Ar1000⁺ profiles compared to the Ar⁺ profiles.
This paper presents the results of self-consistent first-principle calculations for the crystal structure and electronic structure of pure tantalum, TaNO, and TaZrNO within density functional theory (DFT) for the sake of comparison and shows the influence of allowing elements on the interatomic distance and the Fermi level. The large total densities of states (TDOS) value for TaZrNO implies the highest electronic conductivity. The difference in values is due to the Zr metallic atoms presence in TaZrNO compound. There is a strong interaction between Ta and (N, O) (, ) in TaON compound, and Zr presence increases this interaction (, ) in TaZrON compound.
The single-crystal investigation of the self-hosting σ-structure of β-tantalum (β-Ta) at 120 K (low-temperature, LT, structure) and at 293 K (RT-I before cooling and RT-II after cooling and rewarming; RT represents room temperature) shows that this structure is indeed a specific two-component composite where the components have the same (or an integer multiple) lattice constants but different space groups. The space groups of both host (H) and guest (G) components cause systematic absences, which result from their intersection. The highest symmetry of a σ-structure can be described as [H: P42/mnm; G: P4/mbm (cG = 0.5cH); composite: P42/mnm]. A complete analysis of possible symmetries is presented in the Appendix. In β-Ta, two components modify their symmetry during the thermal process 293 K (RT-I) 120 K (LT) 293 K (RT-II): [H: P21m; G: P21m; composite: P21m] [H: P, G: P4/mbm (cG = 0.5cH), composite: P] [H: P21m, G: P4/mbm (cG = 0.5cH), composite: P21m]. Thus, the phase transition is reversible with respect to H and irreversible with respect to G.
A X-ray photoelectron spectroscopy (XPS) depth-profile study of the naturally formed native oxide on polycrystalline Ta sample is probed by observing the core level spectra, valence band spectra and workfunction changes. The present paper addresses the issue of the presence of different Ta suboxides along the depth of the oxide layer. Core level spectra, valence band and workfunction measurements all manifest the transformation of insulating Ta2O5 to metallic Ta with a graded distribution of Ta sub-oxides. Effect of ion-beam irradiation and variation in the synthesis method in determining the profile is discussed. By using different ion-beam energies, it has been shown that the ion-beam induced effects are negligible in the study. Differences in the valence states reported in literature with the present study are attributed to the variations in the growth methods.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
Using electrodeposition from a bath of molten fluorides, single crystals of tetragonal beta-tantalum have been obtained for the first time at normal pressure. The unit-cell parameters are a = 10.211 (3), c = 5.3064 (10) A, space group P42(1)m. The beta-Ta structure belongs to the sigma-type Frank-Kasper structures which are typical for binary intermetallic compounds and beta-U. In comparison to the sigma-type, additional intercalated Ta atoms (population factor approximately 0.01) have been detected between the atoms located in the channels of the structure. The shorter interatomic distances observed between the channel atoms in comparison with the atoms of the framework justify the 'self-hosting' characteristic. Beta-Ta exhibits common features with the complex tetragonal structures of the high-pressure phases for the elements Rb, Ba, Sr, Bi and Sb.
This is a critical review of MAX-phase carbides and nitrides from an electronic-structure and chemical bonding perspective. This large group of nanolaminated materials is of great scientific and technological interest and exhibits a combination of metallic and ceramic features. These properties are related to the special crystal structure and bonding characteristics with alternating strong MC bonds in high-density MC slabs, and relatively weak MA bonds between the slabs. Here, we review the trend and relationship between the chemical bonding, conductivity, elastic and magnetic properties of the MAX phases in comparison to the parent binary MX compounds with the underlying electronic structure probed by polarized X-ray spectroscopy. Spectroscopic studies constitute important tests of the results of state-of-the-art electronic structure density functional theory that is extensively discussed and are generally consistent. By replacing the elements on the M, A, or X-sites in the crystal structure, the corresponding changes in the conductivity, elasticity, magnetism and other material properties make it possible to tailor the characteristics of this class of materials by controlling the strengths of their chemical bonds.
We present the first measurements of x-ray photoelectron spectroscopy (XPS) core level binding energies (BE’s) for the widely-applicable group IVb-VIb polycrystalline transition metal nitrides (TMN’s) TiN, VN, CrN, ZrN, NbN, MoN, HfN, TaN, and WN as well as AlN and SiN, which are common components in the TMN-based alloy systems. Nitride thin film samples were grown at 400 °C by reactive dc magnetron sputtering from elemental targets in Ar/N2 atmosphere. For XPS measurements, layers are either (i) Ar⁺ ion-etched to remove surface oxides resulting from the air exposure during sample transfer from the growth chamber into the XPS system, or (ii) in situ capped with a few nm thick Cr or W overlayers in the deposition system prior to air-exposure and loading into the XPS instrument. Film elemental composition and phase content is thoroughly characterized with time-of-flight elastic recoil detection analysis (ToF-E ERDA), Rutherford backscattering spectrometry (RBS), and x-ray diffraction. High energy resolution core level XPS spectra acquired with monochromatic Al Kα radiation on the ISO-calibrated instrument reveal that even mild etching conditions result in the formation of a nitrogen-deficient surface layer that substantially affects the extracted binding energy values. These spectra-modifying effects of Ar⁺ ion bombardment increase with increasing the metal atom mass due to an increasing nitrogen-to-metal sputter yield ratio.
The effect of energetic ion bombardment on the properties of tantalum thin films was investigated. To achieve such energetic ion bombardment during the process the Ta thin films were deposited by deep oscillation magnetron sputtering (DOMS), an ionized physical vapor deposition technique related to high power impulse magnetron sputtering. The peak power was between 49 and 130 kW and the substrate was silicon at room temperature and ground potential. The directionality and the energy of the depositing species was controlled by changing the ionization fraction of the Ta species arriving at the substrate at different peak powers. In this work, the surface morphology (AFM), microstructure (SEM), structure (XRD) and hardness and Young's modulus (nanoindentation) of the films were characterized. The ion energy distributions (IEDs) were measured using an electrostatic quadrupole ion energy and mass spectrometer (HIDEN EQP 300). The IEDs showed that the DOMS process applies a very energetic (up to 120 eV) ion bombardment on the growing tantalum films. Therefore, with such conditions it was possible to deposit pure α-Ta (of 2 μm of thickness) without the use of additional equipment, i.e., without substrate bias or substrate heating. Conditions are therefore significantly different than in previous works, offering a much simpler and cheaper solution to up-scale for industrial operation.
Thin films of β-Ta and α-Cr were deposited on Si(1 0 0) and 1000 Å SiO2/Si(1 0 0), by high power impulse magnetron sputtering (HiPIMS) and direct current magnetron sputtering (dcMS) in hydrogen-containing plasmas. The films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction, scanning electron microscopy, elastic recoil detection analysis, and four-point probe measurements. The results showed that 001-oriented β-Ta films containing up to ~8 at% hydrogen were obtained with HiPIMS, albeit with no chemical shift evident in XPS. The 110 oriented α-Cr films display a hydrogen content less than the detection limit of 1 at%, but H2 favors the growth of high-purity films for both metals. The β-Ta films deposited with dcMS are columnar, which seems independent of H2 presence in the plasma, while the films grown by HIPIMS are more fine-grained. The latter type of microstructure was present for the α-Cr films and found to be independent on choice of technique or hydrogen in the plasma. The β-Ta films show a resistivity of ~140–180 µΩ cm, while α-Cr films exhibit values around 30 µΩ cm; the lowest values obtained for films deposited by HiPIMS and with hydrogen in the plasma for both metals.
An unusual behavior of the surface core-level shifts on the three low-index surfaces of Be metal has been revealed. Three surface-shifted Be 1s components were identified in photoemission studies of both the (0001) and the (10bar {1}0) surfaces while for the (11bar {2}0) surface only one surface-shifted component was resolved. On the close-packed (0001) surface, components shifted by -825, -570, and -265 meV relative to the bulk component were revealed and interpreted to originate from the three outermost surface layers. For the (10bar {1}0) surface, components shifted by -700, -500, and -220 meV were determined and interpreted to originate from the first, second, and third plus fourth atomic layers. For the (11bar {2}0) surface only one component shifted by -410 meV was resolved and interpreted to originate from atoms in the outermost layer. These findings are most unusual since two subsurface core-level shifts have previously not been observed on a close-packed metal surface, and larger surface shifts are determined for the close-packed (0001) surface compared to the more open (10bar {1}0) and (11bar {2}0) surfaces. This latter is contrary to all previous findings on other metal surfaces and also contrary to the predictions of the partial-shift model, which previously has been successfully applied to several metallic systems. The experimental findings are reviewed and reasons for this unusual behavior of the surface shifts are discussed in view of recent theoretical results.
It is pointed out that the spectra of X-ray induced fast photoelectrons from metal should have a characteristic skew line shape resulting from Kondo-like many-electron interactions of the metallic conduction electrons with the accompanying deep hole in the final state. The same line shape should also occur for the discrete line spectra of X-rays emitted from metals. This mechanism could account for the well-known asymmetries observed for K alpha lines.
A 7.5μ thick β-Ta film with very high preferred orientation has been studied with both the Gandolfi and precession cameras. A hexagonal subcell with a = 2.831, c = 5.337 Å has been found. The smallest hexagonal cell that accounts for all the observed spacings contains 144 subcells with a = 34 Å. A structural model is proposed based on analysis of the subcell. It suggests that there are domains in the form of hexagonal prisms about 1000 Å or more in height and about 250 Å in diameter. The domains are divided into 12 coherently diffracting prismatic zones with impurity species, principally Ar and H2, incorporated into the zone boundaries. The structure within a zone is related to the simple h.c.p. structure. The domains are stacked together side by side, normal to the plane of the film, to form layers. Many layers in turn may be added together to form the complete film.
We present a theoretical and experimental study of the surface core level shifts for the three low index faces of Ta. The results are compared to those obtained with similar faces of W. The overall agreement is quite satisfactory.
The singularities of x-ray absorption or emission in metals are studied by a new "one-body" method, which describes the scattering of conduction electrons by the transient potential due to the deep hole. Using the linked-cluster theorem, the net transition rate in the time representation is expressed as the product of two factors: a one-electron transient Green's function L, and the deep-level Green's function G. These factors obey simple Dyson equations, which can be solved asymptotically by using Muskhelishvili's method. The x-ray transition rate is found to behave as 1εalpha, where ε is the frequency measured from the threshold, and alpha an exponent involving the various phase shifts deltal which describe scattering by the deep hole. alpha may be >0 (infinite threshold) or
Ta films with tetragonal crystalline structure (β-phase), deposited by magnetron sputtering on different substrates (steel, silicon, and silicon dioxide), have been studied. In all cases, very highly preferred (001) orientation was observed in X-ray diffraction measurements. All diffraction data revealed two weak reflections corresponding to d-spacing of 0.5272 and 0.1777 nm. The presence of the two peaks, attributed to (001) and (003) reflections, indicates that β-Ta films exhibit a high preference for the space group of P-421m over P42/mnm, previously proposed for β-Ta. Differences in relative intensities of (00l) reflections, calculated for single crystal β-Ta σ-type Frank–Kasper structure and those measured in the films, are attributed to defects in the films. Molecular dynamics simulations performed on tantalum clusters with six different initial configurations using the embedded-atom-method potential revealed the stability of β-Ta, which might explain its growth on many substrates under various deposition conditions.
The local structure of beta-tantalum was investigated by comparing experimental extended x-ray absorption fine structure (EXAFS) measurements with calculated spectra of proposed models. Four possible structure candidates were examined: a beta-Uranium based structure, a distorted A15 structure, a bcc-Ta based superlattice structure with N interstitials and a simple hcp structure. The local structural measurements were found to be consistent with the beta-Uranium based model containing 30 atoms per unit cell and having the space group P42/mnm. The thermal effect analysis on x-ray diffraction and EXAFS spectra, which reveals that beta-Ta is highly disordered, agrees with the low symmetry and anisotropic system of the beta-U model.
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.
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The influence of ion bombardment on the composition of surfaces was investigated by means of ESCA. The bombardment of metal
oxides with inert gas ions results, not only in sputtering of the surface, but also in reduction of the oxides. The rate of
reduction is particularly high when the oxide/metal interface is within the range of the bombarding ions. Ion induced reduction
was found in oxide layers, thinner than the escape depth of the photoelectrons, on Mo, W, Nb, Ta, Ti, Zr, Si, and Bi. The
relationship between reduction phenomena, on the one hand, and the ion energy, angle of incidence, mass of the gas used for
bombardment, and ion current density, on the other hand, was investigated in the case of the Mo/Mo-oxide system.
Ion bombardment of surfaces may also result in the formation of new compounds. Two examples of this are the formation of carbides
through the bombardment of contaminated surfaces and the ion induced formation of C-F compounds from a mixture of K2SiF6 and carbon.
An analytical method for precise determination of lattice constants of cubic crystals, from x-ray diffractometric measurements, was formulated. The method minimizes both systematic and random errors and enables the estimation of the uncertainties in the constants. For higher precision, a weighting factor may also be used.
The method was applied to diffractometric data from a Si powder standard reference material and was considered to be highly reliable for precise measurements.
The effects of argon‐ion bombardment on the structure and properties of sputtered tantalum films have been studied. Applied substrate bias voltage was used to control the bombardment energy in a hollow‐cathode‐enhanced low‐pressure magnetron sputtering system. The films were characterized by x‐ray diffraction, electrical measurements, Rutherford backscattering spectrometry, and stress measurements. The findings concerning the effects of negative substrate bias on film resistivity and structure run counter to earlier work. In particular, as opposed to results found in many early studies, which primarily involved higher‐pressure discharges, at zero bias voltage the films have low resistivity and contain the bcc phase. Increasing the bias to -100 V, increases the resistivity dramatically, and induces formation of β‐Ta, with no significant change in film impurity levels. The difference from earlier work is attributed to the lower relative impurity flux, as well as more energetic substrate bombardment in low‐pressure magnetron sputtering. Energetic substrate bombardment is clearly demonstrated by the high level of argon content in films deposited with no applied bias (2%). At very high bias voltage, argon incorporation increases dramatically and resistivity increases in the β phase. Additionally, a decrease in the compressive stress and change in preferred orientation occur. The results suggest that the formation of the β phase is not controlled by impurity effects, but by Ta forward scattering and related stress changes.
Electron microscopy at 100, 200 and 650 kV and X-ray diffraction analyses have been performed on sputtered tantalum films on thermally oxidized, silicon single-crystal substrates. The structure of both r.f. and d.c. sputtered films with no bias is body-centered cubic (b.c.c.) with a lattice parameter of 3.33 Å. However, the structure of the negatively biased d.c. sputtered films has been interpreted as a b.c.c. based superlattice structure with slight tetragonal distortion. The experimental values of the lattice parameters are and , with a ratio of 0.894. These results will be interpreted in terms of interstitial ordering and biaxial stresses in b.c.c. thin films. This interpretation of ordering of impurities in a b.c.c. lattice can resolve some of the inconsistencies which exist in the literature regarding the structure of sputtered tantalum.
The X-ray diffraction patterns obtained from thin films of β-Ta and b.c.c.-Ta are complicated by variations in type and degree of preferred orientation, and by variations in cell parameters. A discussion of the effects of these variations on the modified Debye-Scherrer X-ray diffraction patterns is given. The diffraction patterns from the commonly observed (200) β-Ta and (110) and (111) b.c.c.-Ta preferred orientations are illustrated, as well as those from mixtures of the two phases. For the case in which the film is a mixture of β-Ta and b.c.c.-Ta phases it is shown that the unit cells of the two phases expand in parallel. The difficulty in determining the orientation and relative amount of a phase present in a film containing both phases on the basis of diffractometer traces alone is emphasized.
Using a novel pulsed power supply in combination with a standard circular flat magnetron source, operated with a Cu target, a peak power density of 2800 W cm-2 was achieved. This results in a very intense plasma with peak ion current densities of up to 3.4 A cm−2 at the substrate situated 10 cm from the target. The ionized fraction of the deposited Cu flux was estimated to be approximately 70% from deposition rate measurements. The potential for high-aspect-ratio trench filling applications by high power pulsed magnetron sputtering is demonstrated by deposition in via-structures. The high power pulsed technique also results in a higher degree of target utilization and an improved thickness uniformity of the deposited films compared with conventional d.c. magnetron sputtering.
This work reports the analysis of the prototype triode-sputtered, 150-μm thick tantalum coatings deposited with a 2.5-μm niobium under-layer onto the bore of a large-diameter A723 steel cylinder at Pacific Northwest Lab for wear and erosion protection. Our phase determination was based on X-ray diffraction (XRD), wavelength dispersive X-ray fluorescence (WDXRF), energy-dispersive X-ray (EDX), hardness and electrical resistivity measurements. Both X-ray diffraction and radius-of-curvature methods were used to determine residual stresses. A locally developed high-resolution pole figure technique was used to perform texture analysis. The post-firing, debonded coating showed: α-tantalum with preferred [110] orientation and high tensile surface stresses, surface tantalum oxides, entrapped krypton sputtering gas, interstitial oxygen and other impurities. The surface and sub-surface pole figures revealed broadened poles and bcc α-tantalum crystalline structure.
Tantalum offers a number of attractive properties for gun bore coating applications, including a high melting temperature, high ductility, and an environmentally friendly deposition method. However, vapor-deposited tantalum can appear in both the characteristic bcc phase found in the bulk material, and in a very brittle and less desirable ‘beta’ phase. Presence of the beta phase in bore coatings is considered undesirable because of its brittleness and resulting failure as the coating is stressed. A high-rate triode sputtering system with a cylindrical coating geometry was used to produce thick tantalum coatings on 4340 steel smooth bore cylindrical substrates. A systematic series of tests were performed to evaluate the effects of sputtering gas species (Ar, Kr, Xe) and substrate temperature (100–300°C) during deposition on the phase and microstructure of the coatings. Heavier sputtering gases and higher substrate temperatures were found to promote the formation of bcc-phase tantalum coatings. Use of a movable target assembly was shown to promote the production of dense, single-phase tantalum coatings.
Generalized gradient approximations (GGA{close_quote}s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. {copyright} {ital 1996 The American Physical Society.}
The role of lattice strain on the cluster core-level binding energy (BE) shifts was investigated. It was observed that lattice strain had changed the chemical bonding between the metal atoms and the changer had induced BE shifts. The electronic wave functions (WF) for clusters were also calculated. The Hartree-Fock self-consistent-field (SCF) molecular orbitals for the ground state of the cluster was used to obtain the intial state contribution to BE. It is found that the difference between the energies of the ground state and the SCF WF for the ionized state is the total BE.
A new density functional (DF) of the generalized gradient approximation (GGA) type for general chemistry applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C(6) x R(-6). A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common density functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on standard thermochemical benchmark sets, for 40 noncovalently bound complexes, including large stacked aromatic molecules and group II element clusters, and for the computation of molecular geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for standard functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean absolute deviation of only 3.8 kcal mol(-1). The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the average CCSD(T) accuracy. The basic strategy in the development to restrict the density functional description to shorter electron correlation lengths scales and to describe situations with medium to large interatomic distances by damped C(6) x R(-6) terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chemical method for large systems where dispersion forces are of general importance.
- G Hägg
G. Hägg, Allmän och Oorganisk Kemi, Almqvist & Wiksell, Uppsala, 1989.
Calculated density of states of α and β-Ta. The number of states at E F is 1 032 and 0.5569 states/eV/atom, respectively
Fig. 6. Calculated density of states of α and β-Ta. The number of states at E F is 1
032 and 0.5569 states/eV/atom, respectively.
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The structure of sputtered tantalum
- D J Mills
D.J. Mills, The structure of sputtered tantalum, Canad. Ceram. Soc. 35 (1966)
48-52.
Program COHEN Harwell version UKAEA
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J.A.C. Marples, J.L. Shaw, Program COHEN Harwell version UKAEA, Brit. Rep.
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JCPDS-International Centre for Diffraction Data. Ta, Ref. ID 04-0788, < http://
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