The extremely narrow hysteresis width of phase transition in nanocrystalline VO ₂ thin films with the flake grain structures

The nanocrystalline VO ₂ thin films, which surface has a flake grain structure, are achieved by DC sputtering deposition at different sputtering powers. It is found that the hysteresis loop of metal-insulator phase transition (MIT) is almost superposition, and the hysteresis width is only 0.4 °C for the surface flake grain structure that obtained at 132 W DC sputtering power. Moreover, it is shown that the phase transition is very steep, and the film displays 3-4 orders of the change of sheet resistance at MIT. The characterizations of SEM, AFM and four-point probe methods show that the hysteresis width, the orders of the change of sheet resistance and the phase transition become narrower, higher and steeper at MIT, respectively when the surface shapes of the nanocrystalline VO ₂ thin films change from nanoparticle structures to flake structures with the DC sputtering powers increased from 66 W to 132 W and the surface flake grain sizes reduced gradually to minimum at 132 W. Meanwhile, the surface roughness also changes into minimum. However, with the powers further increased from 132 W to 176 W, the surface flake grain sizes become bigger, and then the surface roughness changes poor. At 176 W, the surface flake structures begin to turn into nanoparticle structures. The hysteresis width, the orders of the change of sheet resistance and the phase transition become wider, lower and poorly steeper at MIT, separately. The results reveal that the nanocrystalline shapes and the surface roughness can affect the hysteresis width and the sheet resistance steepness in MIT. Our analysis shows that the mechanism of the narrowed hysteresis width mainly depends on the strain imbalance of the nanocrystalline VO ₂ thin film of the flake structures at MIT.