Microstructure of VO2 Thin Films Synthesized by Pulsed Laser Deposition

Abstract

In this paper, pulsed laser deposition (PLD) at low partial oxygen pressure (~10 mTorr) was used to obtain VO2 thin films. During the PLD, the deposition temperature and number of pulses were varied in order to obtain a good sample crystallinity. It was showed by atomic force microscopy (AFM) micrographs that the mean grain size increased from ~ 40 nm to ~ 90 nm at a variation of the deposition temperature from 400 ˚C to 500 ˚C. Further, by increasing of both substrate temperature and number of pulses, the mean grain size increases to 220 nm. According to the Rietveld refinement of the experimental X-ray diffraction (XRD) pattern, within the grain size increasing, the mean crystallite size increased from 14 nm to 22 nm, as well as a decreasing of the lattice strain from 0.29% to 0.20%. These dependencies further imply a decreasing of the dislocation density of 2.3 to 0.9 \times 1012 cm-2. At the same time, the optical band gap decreased from 0.72 eV (400 ˚C) to 0.66 eV (500 ˚C), reaching 0.60 eV (600 ˚C). Further investigations performed by X-ray photoelectron spectroscopy (XPS) showed the vanadium oxide presence, by the spin-orbit splitting of approximately 7.5 eV between V 2p3/2 and V 2p1/2 orbitals. Finally, the electrical measurements done in the range of 250 – 370 K reveal a close relationship between the dislocation density and the observed resistance-temperature dependence.