Tuning VO2 phase stability by a combined effect of Cr doping and oxygen pressure

This work reports a systematic study on the tuning of the insulator-metal transition temperature (TIMT) of VO2 thin films as well as their metastable phases by doping the films with Cr under different oxygen pressures. At a low oxygen pressure, TIMT decreases from 69 to 34 degrees C when increasing the Cr concentration in VO2 from 0 to 10%. At a higher oxygen pressure, TIMT is enhanced from 72 to 90 degrees C with the same Cr concentration. This combined effect of Cr doping and oxygen pressure yields the structural stabilization of metastable (triclinic, T, monoclinic, M* and M2) phases of VO2 thin film at room temperature, as observed by x-ray diffraction (XRD) and Raman measurements. X-ray photoelectron spectroscopy (XPS) measurements confirm the presence of V3+ in the low oxygen pressure stabilized phases - T and M*, while the atomic force microscopy (AFM) images show changes in film roughness. A comparison of the volume fraction monoclinic phases, determined from resistivity and infrared transmission measurements, evidenced the signature of percolation in VO2. Our experimental approach provides a deeper understanding of the phase stability of VO2 films and a new perspective to tuning the IMT over a large scale as needed for technological applications.