Electronic Structure of Beta-Ta Films from X-ray Photoelectron Spectroscopy and First-principles Calculations

The electronic structure and chemical bonding of Beta-Ta synthesized as a thin 001-oriented film (space group P421m) is investigated by 4f core level and valence band X-ray photoelectron spectroscopy and compared to alfa-Ta bulk. For the Beta-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 alfa-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 alfa-Ta. This is a consequence of the lower number of nearest neighbors with four in Beta-Ta compared to eight in the alfa-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 alfa-Ta are discussed in relation to calculated states of Beta-Ta and alfa-Ta. In particular, the lower number of states at the Fermi level of Beta-Ta (0.557 states/eV/atom) versus alfa-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 micro-Ohm-cm in comparison to alfa-Ta bulk with a reported value of ~13.1 micro-Ohm-cm.