Atomic Layer Deposition of Vanadium Dioxide and a Temperature-Dependent Optical Model
Journal Article - Open Access
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 degrees C. To grow VO2 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 degrees C growth of ultrathin films 100-1000 angstrom of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultrahigh vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity 99.999 percent oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 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 VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 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 VO2 as a tunable refractive index material.
- Inorganic Chemistry