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The electrical properties and performance characteristics of niobium dioxide (NbO2)-based threshold switching devices are examined at cryogenic temperatures. Substoichiometric Nb2O5 was deposited via magnetron sputtering and patterned in microscale (2×2−15×15 μm2) crossbar Au/Ru/NbOx/Pt devices and electroformed at 3–5 V to make NbO2 filaments. At cryogenic temperatures, the threshold voltage (Vth) increased by more than a factor of 3. The hold voltage (Vh) was significantly lower than the threshold voltage for fast voltage sweeps (200 ms per measurement). If the sample is allowed to cool between voltage measurements, the hold voltage increases, but never reaches the threshold voltage, indicating the presence of nonvolatile Nb2O5 in the filament. The devices have an activation energy of Ea≈1.4 eV, lower than other NbO2 devices reported. Our work shows that even nominally “bad” selector devices can be improved by reducing the leakage current and increasing the sample resistance at cryogenic temperatures. 

Niobium dioxide has a volatile memristive phase change that occurs ∼800 °C that makes it an ideal candidate for future neuromorphic electronics. A straightforward optical system has been developed on a horizontal tube furnace for in situ spectral measurements as an as-grown Nb2O5 film is annealed and ultimately crystallizes as NbO2. The system measures the changing spectral transmissivity of Nb2O5 as it undergoes both reduction and crystallization processes. We were also able to measure the transition from metallic-to-non-metallic NbO2 during the cooldown phase, which is shown to occur about 100 °C lower on a sapphire substrate than fused silica. After annealing, the material properties of the Nb2O5 and NbO2 were assessed via x-ray photoelectron spectroscopy, x-ray diffraction, and 4-point resistivity, confirming that we have made crystalline NbO2. 

The stabilization of the threshold switching characteristics of memristive NbOx is examined as a function of sample growth and device characteristics. Sub-stoichiometric Nb2O5 was deposited via magnetron sputtering and patterned in nanoscale (50×50–170×170nm2) W/Ir/NbOx/TiN devices and microscale (2×2–15×15μm2) crossbar Au/Ru/NbOx/Pt devices. Annealing the nanoscale devices at 700 °C removed the need for electroforming the devices. The smallest nanoscale devices showed a large asymmetry in the IV curves for positive and negative bias that switched to symmetric behavior for the larger and microscale devices. Electroforming the microscale crossbar devices created conducting NbO2 filaments with symmetric IV curves whose behavior did not change as the device area increased. The smallest devices showed the largest threshold voltages and most stable threshold switching. As the nanoscale device area increased, the resistance of the devices scaled with the area as R∝A−1, indicating a crystallized bulk NbO2 device. When the nanoscale device size was comparable to the size of the filaments, the annealed nanoscale devices showed similar electrical responses as the electroformed microscale crossbar devices, indicating filament-like behavior in even annealed devices without electroforming. Finally, the addition of up to 1.8% Ti dopant into the films did not improve or stabilize the threshold switching in the microscale crossbar devices.