The extremely narrow hysteresis width of phase transition in nanocrystalline VO2 thin films with the flake grain structures

• Published in: Applied surface science Vol. 261; pp. 83 - 87
• Main Authors: Xu, Xiaofeng, He, Xinfeng, Wang, Haiyang, Gu, Quanju, Shi, Shuaixu, Xing, Huaizhong, Wang, Chunrui, Zhang, Jing, Chen, Xiaoshuang, Chu, Junhao
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.11.2012; Elsevier
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; DC sputtering powers; Deposition by sputtering; Exact sciences and technology; Flake structure; Hysteresis width; Materials science; Methods of deposition of films and coatings; film growth and epitaxy; Nanocrystalline VO2; Optical and dielectric properties (related to treatment conditions); Phase transitions and critical phenomena; Physics; Solid surfaces and solid-solid interfaces; Strain; Surface roughness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); Treatment of materials and its effects on microstructure and properties; Flake structure; Nanocrystalline VO2; 81.15.C; 68.55.J; 81.40.Tv; Surface roughness; DC sputtering powers; Strain; Hysteresis width; Phase transformations; Roughness; Sputtering; Thin films; Nanocrystalline VO; Strains; Vanadium oxide; Hysteresis; Nanocrystal
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2012.07.098

► It is found that the hysteresis width of MIT is only 0.4°C because of the flake nanocrystallines at 132W sputtering power. ► It shows that the transition of the film is very steep with the sheet resistance changes of 3–4 orders at MIT. ► The optimal hysteresis width and the steepest sheet resistance are determined with small flake nanocrystalline. ► The mechanism of narrowing hysteresis width is mainly the strain imbalance of the surface flake structure VO2 thin films in MIT. The nanocrystalline VO2 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 132W 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 VO2 thin films change from nanoparticle structures to flake structures with the DC sputtering powers increased from 66W to 132W and the surface flake grain sizes reduced gradually to minimum at 132W. Meanwhile, the surface roughness also changes into minimum. However, with the powers further increased from 132W to 176W, the surface flake grain sizes become bigger, and then the surface roughness changes poor. At 176W, 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 VO2 thin film of the flake structures at MIT.