Metal-Insulator Transition in Niobium Dioxide Thin Films

Metal-insulator transition (MIT) in NbO2 has not been studied extensively since its discovery. The phase transition in NbO2 can be triggered by different external excitations, and the transition temperature (1081 K) is much higher than those of other materials with MIT property, which promises extended temperature range in circuit applications. Despite of the potential applications, there was no significant advances in the research on MIT of NbO2, both on understanding of underlying mechanisms and insights to the modulation of MIT, largely attributed to the difficulty in synthesizing phase pure NbO2.
My dissertation has been focused on investigating the MIT on thin film NbO2 synthesized by a reactive bias target ion beam deposition (RBTIBD) technique. Epitaxial phase pure NbO2 films were grown on sapphire (0001) substrates, and effect of deposition conditions (Ar/O2 mixture flow rate and substrate heating) on film quality was investigated. It was found that film quality was sensitive to the Nb/O stoichiometry that could be effectively tuned by the Ar/O2 mixture flow rate, while being much less sensitive to substrate heating within the investigated range. Comprehensive structural and transport characterization was conducted on film deposited under optimized conditions. A thin layer (1-2 nm) of Nb2O5 was discovered at the surface due to spontaneous oxidation in the ambient environment. In addition, I investigated the cation substitution as a path to modify microstructures and transport properties of NbO2 films. In particular, effect of V substitution was studied by depositing substitutionally alloyed VxNb1-xO2 films considering similarities between VO2 and NbO2. Structural analysis suggested that the V substitution broke the Nb-Nb dimers in tetragonal NbO2 lattice. With the increase of V concentration, film lattice transited from distorted rutile structure to regular rutile structure.
The effect of applied electric field on the MIT of NbO2 was studied on metal/NbO2/TiN/Si structures. In-situ Nb capping was employed to prevent the formation of Nb2O5. Unipolar threshold switching characteristics was observed, with repeatability of several hundreds of cycles and thermal stability up to 150 °C. By comparing MIT characteristics of structures with and without Nb2O5, it was found that the presence of Nb2O5 resulted in a significantly larger threshold electric field (~ 250 kV/cm) for transition and more visible hysteresis. With in-situ Nb capping, the threshold electric field was 50-80 kV/cm. Band diagrams for both structures were proposed to explain the drastic differences. This new insight on the interfacial oxide can lead to very low power phase transition switches for electronic applications.