Amit Kohn

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, ∼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 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 ∼ 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.

We measure the mean inner potential (MIP) of hematite, α-Fe2O3, using electron holography and transmission electron microscopy. Since the MIP is sensitive to valence electrons, we propose its use as a chemical bonding parameter for solids. Hematite can test the sensitivity of the MIP as a bonding parameter because of the Morin magnetic phase transition. Across this transition temperature, no change in the corundum crystal structure can be distinguished, while a change in hybridized Fe-3d and O-2p states was reported, affecting ionic bonding. For a given crystallographic phase, the change in the MIP with temperature is expected to be minor due to thermal expansion. Indeed, we measure the temperature dependence in corundum α-Al2O3(112¯0) between 95 and 295 K showing a constant MIP value of ∼16.8 V within the measurement accuracy of 0.45 V. Thus, our objectives are as follows: measure the MIP of hematite as a function of temperature and examine the sensitivity of the MIP as a bonding parameter for crystals. Measured MIPs of α-Fe2O3(112¯0) above the Morin transition are equal, 17.85 ± 0.50 V, 17.93 ± 0.50 V, at 295 K, 230 K, respectively. Below the Morin transition, at 95 K, a significant reduction of ∼1.3 V is measured to 16.56 ± 0.46 V. We show that this reduction follows charge redistribution resulting in increased ionic bonding.