Structural and electrical characterization of polycrystalline NbO₂ thin film vertical devices grown on TiN-coated SiO₂/Si substrates

We report on the electrical properties of polycrystalline NbO₂ thin film vertical devices grown on TiN coated SiO₂/Si substrates using pulsed laser deposition. First, we analyzed the thickness and contact size dependences of threshold switching of NbO₂ films grown in 10 mTorr Ar/O₂ mixed growth pressure, where 25.1%/74.9% of NbO₂/Nb₂O₅ surface composition content was estimated by ex-situ x-ray photoelectron spectroscopy. Then the threshold switching and self-sustained current oscillatory behavior of films with different NbO₂/Nb₂O₅ composition ratios was measured and analyzed. The current-voltage measurement revealed that the leakage current property in the insulating state was dominated by the trap-charge assisted Poole-Frankel conduction mechanism. All films showed threshold switching behavior in agreement with the previously proposed Joule heating mechanism. The second film was grown in lower (1 mTorr) growth pressure, which resulted in a higher (34.2%/65.8%) NbO₂/Nb₂O₅ film surface composition. The film grown in higher growth pressure demonstrated lower off-state leakage current, faster switching, and self-sustained oscillations with higher frequency than the film grown in lower growth pressure.