Synthesis, structure and optical properties of high-quality VO2 thin films grown on silicon, quartz and sapphire substrates by high temperature magnetron sputtering: Properties through the transition temperature

Abstract

Vanadium dioxide (VO2) thin films were deposited on silicon (100), quartz, and r-cut sapphire substrates by DC magnetron sputtering at 650 °C. The thin films were characterized by Raman spectroscopy and imaging, scanning electron microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDS), and Transmission Electron Microscopy (TEM). The optical transmittance and hence the insulator-to-metal transition (IMT) and metal-to-insulator transition (MIT) were characterized as a function of temperature. The effects of substrate material on VO2 thin film’s growth, microstructure, stoichiometry and optical properties were investigated. The results show that the as-deposited VO2 thin films on Si, quartz, and sapphire all show a near-zero IR transmission in switched metallic state; especially VO2 thin film grown on sapphire shows superior IMT characteristics compared with previously published works on thin VO2 films, and very close to its bulk single crystal form, which is probably due to the significantly reduced imperfections in the film and the interface, and especially the highly preferred growth with similar paralleled grain orientation of VO2 on sapphire arising from both the high deposition temperature and epitaxial growth. The spectral transmittance of the films on quartz and sapphire substrates was analyzed to extract the optical constants n and k as a function of wavelength. No significant difference between optical properties could be observed for films deposited on quartz and sapphire substrates indicating that the differences in microstructure of these films play a minor role in their optical properties. n and k spectra are, in general, in agreement with previously reported works. The absorption coefficient versus photon energy characteristics indicate that VO2 in the insulating phase possesses a direct bandgap at 2.6 eV and an indirect bandgap of 0.52 eV. Raman imaging was used to map the phase mixture of various regions of the film at micron scale (1.2 μm × 1.2 μm) during heating and cooling scans through the IMT and MIT. The dependence of the integrated Raman intensities at 220 cm−1 and 600 cm−1 on the temperature shows an excellent correlation with the optical transmittance through the IMT and MIT, both during heating and cooling scans. It is shown that the transformation occurs by the growth of one phase at the expense of the other and, in the IMT/MIT region, both phases coexist.