Optical properties of HfO2 nanoceramic films as a function of N–Bi co-doping

N-doped HfO2, Bi-doped HfO2 and N–Bi co-doped HfO2 films have similar structures to that of the nanocrystalline monoclinic HfO2 film. After doping with N and/or Bi, an HfO2 film became more stoichiometric and exhibited better nanocrystallinity. The N–Bi co-doped HfO2 film had a lower surface roughness, a higher optical transmission in the VIS–NIR region, and a higher linear refractive index than the HfO2 film. Increasing the substrate temperature resulted in the coexistence of monoclinic HfO2 and tetragonal Bi2O2.3 in the N–Bi co-doped HfO2 film. At a higher substrate temperature, the transparent N–Bi co-doped HfO2 film showed lower transmission in the ultraviolet region, favoring optical applications.