Effect of structural disorder on the modification of V-V and V-O bond lengths at the metal-dielectric phase transition in VO2 thin films

The dynamics of anomalous changes in the microstructure, as well as optical and electrical properties of microcrystalline VO2 thin films on Si substrates during a reversible temperature-induced metal-insulator phase transition (MIT), were studied by high-resolution X-ray diffractometry (XRD), micro-Raman spectroscopy, and resistivity measurements. Temperature-dependent XRD studies of microcrystalline VO2 films revealed the coexistence of monoclinic VO2(M1) and tetragonal VO2(R) structural phases near the MIT, attributable to deformation inhomogeneities and grain size variations. It was determined that inhomogeneous microstructural distortion is the primary factor influencing the MIT in strained VO2 films. Concurrently, significant variations in the resistivity hysteresis were observed in association with the phase transitions. The structural phase transition (SPT) in VO2 films was investigated via temperature-dependent micro-Raman spectroscopy, which elucidated the microstructural influence on the MIT. As the VO2(M1) temperature approached the critical SPT temperature (340 K), the narrow Raman spectrum of the insulating phase transitioned into a broadband spectrum characteristic of the metallic VO2(R) phase, marked by four pronounced peaks at approximately 230 cm-1 (B1g), 400 cm-1 (Eg), 550 cm-1 (A1g), and 635 cm-1 (B2g). The analysis of the V-V and V-O vibrational modes in VO2 indicates that alterations in the V-V and V-O bond lengths are crucial to the temperature-driven SPT from the low-temperature monoclinic VO2(M1) phase to the high-temperature tetragonal VO2(R) phase. The observed hysteretic temperature dependence of the Raman intensities for V-V and V-O modes shows a quantitative correlation with the resistivity measurements of the VO2 films, providing insights into the interplay between the SPT and the MIT. The results obtained are pivotal for comprehending the structural and electronic behaviors of nanoscale films and hold significant implications for their application in functional devices.