Controlling the optical properties of hafnium dioxide thin films deposited with electron cyclotron resonance ion beam deposition

•Hafnia thin films fabrication by electron cyclotron resonance ion beam deposition.•Utilized model by O'Leary, Johnson, Lim for optical properties of amorphous films.•Optical properties tuned by fine controlling of reactive and sputtering O2.•Decrease of refractive index from 1.91 to 1.72 with the increase of oxygen content.•Increase of bandgap energy from 5.6 to 6 eV with the increase of oxygen content.