Optical characterization of sol–gel derived Nb2O5 thin films

• Sol–gel Nb2O5 thin films with high refractive index were fabricated.
• A calculation methodology for optical characterization was developed.
• Temperature tuning of refractive index in the range 1.82–2.20 was realized.
• An approach of precise control of film thickness was demonstrated.
• High refractive index (2.11) is attained even at temperatures below 120 °C.

The optical properties of thin Nb2O5 films, obtained by spin coating of Nb sol on glass and silicon substrates are investigated. The Nb sol was prepared by a sonocatalytic method using niobium chloride dissolved in ethanol. A calculation procedure for reliable and unambiguous determination of optical constants (refractive index and extinction coefficient) and thickness of films is developed that uses transmittance and reflectance measurements and non-linear curve fitting. By means of the developed procedure, the impacts of the sol ageing and the post deposition annealing on the optical properties and thickness of the films are investigated. It is demonstrated that films with controlled thicknesses and refractive index values in the range from 1.82 to 2.20 (at wavelength of 600 nm) could be obtained using different concentrations of the Nb sol and post deposition annealing in the range 60–650 °C. The possibility of fabrication of single layer anti-reflective coating for silicon is discussed.