• Levi, George
• Azulay, Amram
• Basha, Adham
• Amrani, Tamir

Abstract

<p>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 TaO_x 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, ∼3.8 Å, to nearest-neighbor, ∼3 Å. 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 × 10³ to (4 ± 0.05)×10⁻³ Ω × cm. In particular, we observe a sharp percolative decrease in resistivity of three orders of magnitude, at x ∼ 1.9. Ta oxidation states, measured by x-ray photoelectron spectroscopy, suggest that the main polyhedral building block within the TaO_2.5 film is TaO₆, while in oxygen-deficient films, the relative fractions of TaO₅ polyhedra and metallic Ta increase. At even lower oxygen content, x ∼ 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.</p>

Topics

• impedance spectroscopy
• amorphous
• resistivity
• x-ray diffraction
• thin film
• x-ray photoelectron spectroscopy
• Oxygen
• transmission electron microscopy
• random
• oxygen content
• electrical conductivity
• tantalum