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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.

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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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.}
Article
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.
Article
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.
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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. Coat. Technol. 314 (2017) 97-104.
The structure of sputtered tantalum
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D.J. Mills, The structure of sputtered tantalum, Canad. Ceram. Soc. 35 (1966) 48-52.
Program COHEN Harwell version UKAEA
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Centre for Diffraction Data. Ta, Ref
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JCPDS-International Centre for Diffraction Data. Ta, Ref. ID 04-0788, < http:// www.icdd.com/ >.