Optical properties of nonstoichiometric tantalum oxide TaO$_{x}$ ($x<$ 5/2) according to spectral-ellipsometry and Raman-scattering data

Optical properties of amorphous nonstoichiometric tantalum-oxide films of variable composition (TaO$_{x}$, $x$ = 1.94–2.51) in the spectral range of 1.12–4.96 eV, obtained by ion-beam sputtering-deposition of metallic tantalum at different partial oxygen pressures (0.53–9.09 $\times$ 10$^{-3}$ Pa), have been investigated. It is shown by spectral ellipsometry that the character of dispersion of the absorption coefficient and refractive index in TaO$_x$ of variable composition suggests that light-absorbing films with dispersion similar to that in metals are formed at oxygen pressures in the growth chamber below 2.21 $\times$ 10$^{-3}$ Pa, whereas transparent films with dielectric dispersion are formed at pressures above 2.81 $\times$ 10$^{-3}$ Pa. According to the data of quantumchemical simulation, the absorption peak at a photon energy of 4.6 eV in TaO$_{x}$ observed in the absorptioncoefficient dispersion spectrum is due to oxygen vacancy. The peak in the Raman-scattering spectra of TaO$_{x}$ films with metallic dispersion at frequencies of 200–230 cm$^{-1}$ is presumably related to tantalum nanoclusters.