Electronic properties and bonding in ZrH_x thin films investigated by valence-band x-ray photoelectron spectroscopy

摘要

The electronic structure and chemical bonding in reactively magnetron sputtered ZrH_x (x = 0.15, 0.30, 1.16) thin films with oxygen content as low as 0.2 at.% are investigated by 4d valence band, shallow 4p core-level, and 3d core-level x-ray photoelectron spectroscopy. With increasing hydrogen content, we observe significant reduction of the 4d valence states close to the Fermi level as a result of redistribution of intensity toward the H Is-Zr 4d hybridization region at ~6eV below the Fermi level. For low hydrogen content (x = 0.15, 0.30), the films consist of a superposition of hexagonal closest-packed metal (α phase) and understoichiometric δ-ZrH, (CaF_2-type structure) phases, while for x = 1.16, the films form single-phase ZrH_x that largely resembles that of stoichiometric δ-ZrH_2 phase. We show that the cubic δ-ZrH_x phase is metastable as thin film up to x = 1.16, while for higher H contents the structure is predicted to be tetragonally distorted. For the investigated ZrH_(1.16) film, we find chemical shifts of 0.68 and 0.51 eV toward higher binding energies for the Zr 4p_(3/2) and 3d_(5/2) peak positions, respectively. Compared to the Zr metal binding energies of 27.26 and 178.87 eV, this signifies a charge transfer from Zr to H atoms. The change in the electronic structure, spectral line shapes, and chemical shifts as a function of hydrogen content is discussed in relation to the charge transfer from Zr to H that affects the conductivity by charge redistribution in the valence band.