X-Ray absorption, photoemission spectroscopy, and Raman scattering analysis of amorphous tantalum oxide with a large extent of oxygen nonstoichiometry

Abstract

The electronic structure and modification of the local interatomic structure of a reactive sputtered amorphous tantalum oxide (a-TaO_x) thin film with the variation of oxygen nonstoichiometry, x in a-TaO_x have been investigated by X-ray absorption spectroscopy (XAS), X-ray photoemission spectroscopy (XPS), Raman scattering spectroscopy, and Rutherford back scattering spectroscopy. A parallel chemical shift of Ta4f_7/2 and O1s core levels observed with the variation of x indicates the Fermi level shift by reduction and oxidation in the framework of the rigid band model. Extended X-ray absorption fine structure (EXAFS) suggests both the increase of average coordination number of the first Ta–O shell in polyhedra and a considerable reduction of the average Ta–O bond length with the increase of x. The relative intensity of Raman shift peaks at 670 cm⁻¹ and 815 cm⁻¹, corresponding to Ta–O stretching of TaO₆ octahedra and TaO₅ probably with a pyramidal form, respectively, drastically changes between x = 2.47 to 1.86, suggesting the change in the predominant polyhedron from TaO₆ to TaO₅ with a modification in multiplicity of oxygen by the reorganization of the polyhedral network.