Charge ordering at a dielectric gate in itinerant metallic states with low-field memristor properties in VO2 thin films

Abstract

Vanadium dioxide (VO2) − a non-stoichiometric oxide semiconductor (SC) offers exotic properties at a self-confined structure of correlated 3d1-electrons (spins) useful for non-volatile memory devices, smart switches, and human brain-inspired neuromorphic devices. Poor chemical stability and fragile nature limit its technologies of thin films. In view of resolving some of these issues, we developed polymer stabilized VO2 films (thickness t ≤ 100 nm), using VO2 nanocolloids in poly(vinylpyrrolidone) (PVP) (as a VO2 dispersoid, a molecular template, and a film former) in water, at a (100) Si(p++) substrate. Using a nano SiO2/TiO2 gate (t ≤ 10 nm), VO2 is grown (011) preferentially in a confined shape of nanoplates (nanocrystals) along the films, mostly of 15 to 40 nm widths at 20–30 nm crystallite size. The results are described with X-ray diffraction, surface topologies, lattice images, and X-ray photoelectron spectroscopy (XPS) of films in the variable charges 2V4+ → V3+ + V5+ order at the itinerant metallic states. A significant V5+-3d0, ≤ 33 at%, is shown in the XPS bands, which induces metallic states at conducting ‘V4+ → V5+ + e−’ channels. So, a charge-regulated SC → metal transition incurs via an induced M1 → R-VO2 metallic state near room temperature. A memristor VO2@TiO2/Si so made renders a wide current-voltage (I-V) loop at room temperature, with a leakage current that is well controlled at a high-k TiO2 gate. It exhibits a reversible switching at a duly small threshold field, Vt ≤ 0.2 V. This is the smallest Vt tuned so far beneficial for the low field, ≤ 1.0 V, devices. The charge models corroborate the effect of field-induced charge order at the interfaces of ‘conducting through channels’, regulating a reversible I-V hysteresis in an on-off cycle.