High density memory storage capacity, in-memory computation and meuromorphic computing utilizing memristors are expected to solve the limitation of von-Neumann computing architecture. Controlling oxygen vacancy (VO) defects in metal oxide thin film based memristors holds the potential of designing resistive switching (RS) properties for memroy storage and neuromorphic applications. Herein, we report on RS characteristics of CMOS compatible WO3-x based memristors modulated by precisely controlled oxygen non-stoichiometry. Switchability of the resistance states has been found to depend strongly on the VOs concentration in the WO3-x layer. Depending on x, the memristors exhibited forming-free bipolar, forming-required bipolar, and non-formable characteristics. Devices with moderate VOs concentration (~5.8×1020 cm-3) exhibited a large Roff/Ron ratio of ~6500, and reset voltage-controlled multi-level resistance states. A forming-free, stable multi-level RS has been realized for a memristor possessing VOs concentration of ~6.2×1020 cm-3. WO3-x-based memristors with higher VOs concentrations (~8.9×1020 cm-3-1×1021 cm-3) exhibited lower initial resistance, low Roff/Ron ratios (~15-63) and forming-free synaptic functions with reasonable conduction modulation linearity. Investigation of the conduction mechanism suggests that tailoring VOs concentration modifies the formation and dimension of the conducting filaments and the Schottky barrier height at the WO3-x/Pt interface, which paves the way for designing WO3-x -based memristors for memory storage and neuromorphic applications.

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