Optical properties of HfOx (x < 2) films grown by ion beam sputtering-deposition method

Abstract

The optical properties of the HfOx films of different chemical composition (x ≤ 2) deposited by ion beam sputtering-deposition (IBSD) method were studied. Spectral dependencies of refractive index n(λ) and extinction coefficient k(λ) were determined with ellipsometry in λ = 250–1100 nm wavelength region. The x values (i.e. [O]/[Hf] ratio) for the films were derived from x-ray photoelectron spectroscopy (XPS) data. The spectral dependences of optical constants n(λ) and k(λ) were found to undergo radical changes with x = 1.78–1.82. The films with x < 1.78 demonstrated high extinction coefficient k > 1 with the metallic-like behavior of optical constants spectral dependences. The films with x > 1.82 were found to be transparent, with k = 0 and n(λ) being well approximated by a Cauchy polynomial dependence for dielectrics. Using a sample with a gradient of x, it was established that the transition from the metallic to the dielectric-like behavior of the optical constants occurs not smoothly, but is discontinuous. A sharp jump in the optical constants is observed at x ≈ 1.8. According to XPS data, the transparent films were found to consist of two components only: HfO2 and Hf4O7 suboxide. Cauchy polynomial coefficients for Hf4O7 suboxide and HfO2 were found by using the Bruggeman effective medium approximation.