  We report on resistive switching (RS) characteristics of W/WO3-x/Pt-based
thin film memristors modulated by precisely controlled oxygen
non-stoichiometry. RS properties of the devices with varied oxygen vacancy (VO)
concentration have been studied by measuring their DC current voltage
properties. Switchability of the resistance states in the memristors have 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 high VOs concentration (~1*1021
cm-3) exhibited lower initial resistance and memory window of only 15, which
has been increased to ~6500 with reducing VOs concentration to ~5.8*1020 cm-3.
Forming-free, stable RS with memory window of ~2000 have been realized for a
memristor possessing VOs concentration of ~6.2*1020 cm-3. Investigation of the
conduction mechanism suggests that tailoring VOs concentration modifies the
formation and dimension of the conducting filaments as well as the Schottky
barrier height at WO3-x/Pt interface which deterministically modulates RS
characteristics of the WO3-x based memristors.
