Electronic structure of β-Ta films from X-ray photoelectron spectroscopy and first-principles calculations

The electronic structure and chemical bonding of beta-Ta synthesized as a thin 0 0 1-oriented film (space group P (4) over bar2(1)m) is investigated by 4f core level and valence band X-ray photoelectron spectroscopy and compared to alpha-Ta bulk. For the beta-phase, the 4f(7/2) peak is located at 21.91 eV and with the 4f(5/2) at 23.81 eV which is 0.16 eV higher compared to the corresponding 4f peaks of the alpha-Ta reference. We suggest that this chemical shift originates from electron screening, higher resistivity or strain in the beta-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 beta-Ta compared to alpha-Ta. This is a consequence of the lower number of nearest neighbors with four in beta-Ta compared to eight in the alpha-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 beta-Ta versus alpha-Ta are discussed in relation to calculated states of beta-Ta and alpha-Ta. In particular, the lower number of states at the Fermi level of beta-Ta (0.557 states/eV/atom) versus alpha-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 similar to 170 mu Omega cm in comparison to alpha-Ta bulk with a reported value of similar to 13.1 mu Omega cm.