Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 °C. To grow VO(2 )on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (≤150 °C) growth of ultrathin films (100-1000 Å) of VO(2). For this demonstration, the VO(2 )films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO(2) films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10(-4) Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO(2 )films. The crystallinity, phase, and strain of the VO(2 )were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO(2) in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO(2) agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films' optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO(2) as a tunable refractive index material.