Investigation of Nb_xTi_1-xO₂ via EXAFS and Functional Correlation to Electrical Nanoscale Devices

The unique electrical characteristics and crystal structure of NbO₂ has attracted much interest in recent years as this material undergoes an insulator metal transition (IMT) between 750 and 800°C accompanied by a change from a distorted rutile to rutile structure [1]. This transition can be utilized in nanoscale devices such as selectors for high density memory arrays, in conjunction with novel two terminal non-volatile devices, and as temporal synapses in neuromorphic circuit architectures. For those future applications, fundamental understanding of the IMT must be better understood to tailor material switching properties. One way to gain additional insight into the IMT, is to dope our material and observe electrical and structural changes. For NbO₂, Ti is a prime candidate, it has a stable dioxide rutile structure and lacks a +5 oxidation state. In conjunction, these differences should lower the transition temperature as the combined lattice structure should be less favorable to the distorted rutile structure. Secondly, the Ti dopant should stabilize the metastable NbO₂ phase due to the lack of the +5 oxidation state. Nb_xTi_1-xO₂ samples were prepared via reactive sputtering in an oxygen deficient atmosphere resulting in phase pure films after a 750 °C annealing step in an inert N₂ environment. Ti content was adjusted from 0 to 30% to observe gradual changes in the crystal structure and electrical performance. Electrical results obtained from NbO₂ and Nb_0.9Ti_0.1O₂ via nanoscale TiN contacts (50x50 nm²) and an Ir top electrode show bulk resistive switching without the requirement for an electroforming step. This approach yielded reliable devices switching more than 10 million times in an operational range between -1.5 and 1.5 V. A reduction in the switching threshold voltage and threshold variation was confirmed with these two concentrations and further substantiated by Nath et. al [1] in an unrelated effort. Bulk switching was confirmed during a thermal anneal with in-situ XRD at the Canadian Light Source (CLS) showing a crystallographic change at around 750 °C accompanied by a sheet resistance change. Furthermore, structural characterization was performed on the full range of Ti doped NbO₂ samples. XRD results show a gradual shift from the low symmetry space group I41/a:2 of NbO₂ to the higher symmetry space group P42/mnm related to NbTiO₄. Most recently, XAFS data of the K-Nb and K-Ti edge were collected and the NbO₂ structure was confirmed via EXAFS modelling with Artemis and FEFF. The remaining data will be used to extract the Nb-Nb (Nb-Ti) dimer distance which is anticipated to be an indicator for the electrically obtained threshold voltage. Furthermore, additional electrical samples will be prepared matching the film properties of the XAFS samples to yield a more detailed picture of the threshold voltage and dimer distance correlation.<br/><br/>[1] Rana, R., Klopf, J. M., Grenzer, J., Schneider, H., Helm, M., & Pashkin, A. (2019). Nonthermal nature of photoinduced insulator-to-metal transition in NbO2. <i>Physical Review B</i>, <i>99</i>(4). https://doi.org/10.1103/PhysRevB.99.041102<br/>[2] Nath, S. K., Nandi, S. K., Ratcliff, T., & Elliman, R. G. (2021). Engineering the Threshold Switching Response of Nb2O5-Based Memristors by Ti Doping. <i>ACS Applied Materials & Interfaces</i>, <i>13</i>(2), 2845–2852. https://doi.org/10.1021/ACSAMI.0C19544