Spectroscopic ellipsometry study of Co-doped TiO2 films

Co-doped TiO2 films were characterized by spectroscopic ellipsometry to determine their thickness, deposition rate and optical properties as function of substrate temperature and background gas composition. To fit the data we used a combination of a single Tauc-Lorentz oscillator with the Drude free electron model to take in account the free electrons present in the film. The Co doping and the addition of H, to the gas phase during film growth cause the formation of a titanium oxide which containsfree electrons that absorb the energy of the red part of the spectrum, causing k to increase. The n of the films at 1.5 eV is about 2.3 eV. The fittings also show that the n of films decreases and k increases at the surface. This can be related to a segregation of Co to the surface, which in some cases, of high substrate temperature and high H-2 flow during deposition, can lead to an even higher concentration of free electrons at the surface.