Tailoring the Optical Properties of Zinc Oxide via Bismuth Doping and Co-Doping Strategies—From Birefringence to Metallicity

We make use of density functional theory to explore the effect of Bi as a high-index dopant on the optical properties of 3 metal-oxide structures: Titanium dioxide (TiO₂), Zirconium dioxide (ZrO₂) and Zinc Oxide (ZnO). It was found that Bi_Metal substitutional doping results in reducing the bandgap (E_g) by creating new defect states in the 3 oxides. In case of ZnO, the in-plane refractive index (n_x,y) is doubled (>3) over a wide band in mid and far IR, and huge birefringence is obtained compared to other materials in literature. The presence of Oxygen vacancies (O_v) in the structure was found to lower the losses, which was elaborated by the decreased absorption cross section of a sphere of 250 nm radius, using FDTD. At 12% Bi doping, metallicity is obtained with losses lower than Silver, Gold, and Nitrides. Co-doping of Bi with interstitial Hydrogen doping (H_i) or substitutional Aluminium (Al_Zn) doping give the same effect, which is increasing both the in-plane and out of plane indices even higher, but on the expense of increasing optical losses.