Electronic structure and charge transport in nonstoichiometric tantalum oxide

The atomic and electronic structure of nonstoichiometric oxygen-deficient tantalum oxide TaO_x<2.5 grown by ion beam sputtering deposition was studied. The TaO_x film content was analyzed by x-ray photoelectron spectroscopy and by quantum-chemistry simulation. TaO_x is composed of Ta₂O₅, metallic tantalum clusters and tantalum suboxides. A method for evaluating the stoichiometry parameter of TaO_x from the comparison of experimental and theoretical photoelectron valence band spectra is proposed. The charge transport properties of TaO_x were experimentally studied and the transport mechanism was quantitatively analyzed with four theoretical dielectric conductivity models. It was found that the charge transport in almost stoichiometric and nonstoichiometric tantalum oxide can be consistently described by the phonon-assisted tunneling between traps.