Transition between monoclinic and tetragonal β phases induced by reactive oxygen gas in RF-sputtered V2O5 thin films

Thin films of V2O5 are promising materials for applications in chromogenic devices, such as gas sensors and contactless optical thermal sensors. Therefore, controlling the formation of the various phases of V2O5 is important. The device performance, in relation to satisfactory coloration efficiency and fast response, strongly depends on the characteristics of the phase incorporated in the film structure. To better understand the phase formation in these films, thin films of V2O5 were deposited by RF magnetron sputtering using an O-2-reaction technique from a metallic V target, and the influence of RF power and O-2 levels on the transition between the beta-monoclinic and beta-tetragonal phase structures was investigated by X-ray diffractometer. The films were also evaluated using Auger-electron, Raman-, and UV-vis spectrometers to determine their composition, chemical, and electronic properties to assess the effects of the two sputtering parameters. The mechanism underlying the development of film properties is related to the plasma characteristics and species observed by optical emission spectroscopy. Increasing the RF power resulted in a higher phase content of the beta-monoclinic and a-orthorhombic phase, whereas an increasing the oxygen levels induced a phase transition towards the beta-tetragonal phase of V2O5. Films with different phase contents exhibited different optical energy bandgaps. Plasma diagnostics showed that increasing the RF power increased the thickness of plasma sheaths on the target surface. The thinner sheath on the target surface further increased the beta-tetragonal phase content. The variation between beta-monoclinic and beta-tetragonal phase content was expected because of the bombardment of energetic O- ions that were accelerated from the plasma sheath toward the growth surface. A deeper understanding of the transition between beta phases in V2O5 films can enable better phase control, which can improve film application towards various sensing devices, particularly chromic- or temperature-sensors.

Automated summary

Thin films of V2O5 are promising materials for chromogenic devices and contactless optical thermal sensors. The formation of different phases in these films was investigated, and it was found that the performance of the devices strongly depends on the phase incorporated in the film structure. Understanding the transition between beta phases in V2O5 films can enable better phase control and improve film application towards various sensing devices.