Effect of structural distortion on the metal-insulator transition in Ar+-implanted VO2 thin films

Abstract

The structural, optical, morphological, and electrical properties of VO₂ films implanted with Ar⁺ ions were studied using Atomic-force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and electrical resistivity measurements. AFM studies of surface morphology revealed that with an increased dose of Ar⁺ implantation, the roughness of the surface nanorelief nearly doubled, and the homogeneity of the structured grain arrangement improved. XRD studies indicated that a higher dose of Ar⁺ implantation reduces the degree of angular distortion of the VO₂ unit cell, attributable to an increase in the concentration of point defects at the film/substrate heterointerface and relaxation of lattice mismatch strains. Using temperature-dependent micro-Raman spectroscopy, the regularities of changes in the frequencies of characteristic vibrations of the long and short V-V dimers along the c-axis of VO₂(M1), the V-V tilting vibrations nearly perpendicular to the c-axis, and the V-O bond stretching vibrations in the oxygen VO₆ octahedron were determined. The strain ε_а along the V-V chain decreases, accompanied by an increase in the length of V-V bonds. The most significant changes in the lengths of L₂ and L₅ bonds were observed, as evidenced by the rise in the compression strain ε_с along the c_M1 axis. Raman and electrical studies of VO₂ nanocrystalline films demonstrated that the observed coexistence of two structural phases (monoclinic and tetragonal) during the metal-insulator transition (MIT) is due to size dispersion and inhomogeneity in strain distribution. A decrease in the temperature coefficient of resistance and the film resistance at room temperature at high implantation doses is explained by the formation of conductive (metallic) vanadium oxide phases. The changes in strain, crystallite size, and Ar⁺ implantation dose have been shown to lower the phase transition temperature of VO₂ to near room temperature, which is essential for their widespread application in functional devices.