Structural and electrical transitions in VO2/c-Al2O3 films investigated using simultaneous X-ray reciprocal space mapping and electrical resistance measurements

Abstract

We carried out simultaneous in-situ three-dimensional X-ray reciprocal space mapping and electrical resistance measurements of VO2 films grown on c-cut sapphire to investigate the relationship between the structural phase transition (SPT) and the metal-insulator transition (MIT). The decoupling of the MIT and SPT behaviors was more pronounced in the thinnest film (37 nm), with a difference in transition temperatures (triangle Tc) of approximately 8.3 degrees C. Despite a decrease of over 50% in the M1 fraction, the electrical resistance remained in the insulating phase, indicating a delay in the MIT. This behavior is attributed to the disconnected island morphology, which restricts the formation of continuous conduction pathways. As the film thickness increased, triangle Tcgradually decreased, and eventually both thermal-hysteresis characteristics became similar at a thickness of 360 nm. Furthermore, pronounced six-fold diffuse X-ray scattering was observed around the VO2 M1 (020) Bragg peak, revealing the presence of structural defects and small crystalline domains. This hexagonal pattern originates from three in-plane variants that are rotated by 120 degrees about the film-normal b-axis. Each variant is paired with a 21 screw-axis twin domain, corresponding to a 180 degrees rotation about the same axis. As a result of this combined rotational and twinning symmetry, the diffuse scattering appears along the a* and c* directions of the M1 phase. The correlation length (xi x) of the monoclinic order steadily decreased even in the pre-transition region, exhibiting a trend similar to that of the electrical resistance. When xi x reached approximately 6 nm, the electrical resistance began to decrease rapidly. This correlated behavior suggests a possible interplay between structural disorder, such as defect generation between coherent domains and limited grain connectivity, and charge transport in VO2 films.