Short-range order in amorphous oxygen-deficient TaOx thin films and its relation to electrical conductivity

Thin films of tantalum oxide hold promising functional properties for electronic applications such as resistive random-access memory. For this aim, correlating the structure and charge transport properties of oxygen-deficient derivatives is crucial. Here, using electron scattering measurements from nanoscale volumes in a transmission electron microscope (TEM), we report how oxygen content affects short-range order in amorphous TaOx thin films, where 1.34 <= x <= 2.50. By extracting the bond lengths, we observe that the dominant type of Ta-Ta distances change with decreasing oxygen content from next-nearest-neighbor, similar to 3.8 angstrom, to nearest-neighbor, similar to 3 angstrom. We relate this decrease to the Ta-O polyhedral network within the film, namely decreasing oxygen content increases the presence of TaO5 at the expense of TaO6 polyhedra. The reduction in oxygen content is accompanied by a significant reduction of electrical resistivity of the films from over 4.3 x 10(3) to (4 x 0.05)x10(-3) Omega x cm. In particular, we observe a sharp percolative decrease in resistivity of three orders of magnitude, at x similar to 1.9. Ta oxidation states, measured by x-ray photoelectron spectroscopy, suggest that the main polyhedral building block within the TaO2.5 film is TaO6, while in oxygen-deficient films, the relative fractions of TaO5 polyhedra and metallic Ta increase. At even lower oxygen content, x similar to 1.34, TEM and x-ray diffraction detect crystallites of Ta with cubic and metastable tetragonal structures. We propose that TaO5 polyhedra and Ta crystallites increase conductivity due to direct bonding of Ta atoms, as manifested by nearest-neighbor Ta-Ta bond length, thus enabling conductive paths for charge transport.