The effect of post-process annealing on optical and electrical properties of mixed HfO2-TiO2 thin film coatings

In this paper a comparison of electrical and optical properties of mixed hafnium-titanium oxides is described. Thin films were deposited with the use of the magnetron co-sputtering method. For further analysis (Hf0.52Ti0.48)Ox and (Hf0.29Ti0.71)Ox coatings, which were amorphous directly after the deposition process, were chosen,. Moreover, post-process annealing was also performed in order to compare the electrical and optical properties of amorphous and nanocrystalline thin films with the same material composition. It was found that the phase transition from amorphous to orthorhombic HfTiO4 occurred in the case of (Hf0.52Ti0.48)Ox coating at 650 degrees C. In turn, the phase transition to TiO2-anatase was observed at the temperature of 600 degrees C in the case of (Hf0.29Ti0.71)Ox thin film. The leakage current for both amorphous coatings was in the range of 10(-7)-10(-8)A/cm(2). After additional annealing and phase transition, the leakage current slightly decreased for (Hf0.29Ti0.71)Ox thin film, while in the case of (Hf0.52Ti0.48)Ox sample the resistance switching effect was observed. The dielectric constant was equal to 24 and 25 for amorphous (Hf0.52Ti0.48)Ox and (Hf0.29Ti0.71)Ox films, respectively. However, after the phase transition it decreased to 15 for (Hf0.52Ti0.48)Ox and increased to 51 for (Hf0.29Ti0.71)Ox film. The results of optical studies showed that amorphous thin films were well transparent in a visible light range with an average transparency of ca. 85%. After the phase transition to HfTiO4-orthorhombic and TiO2-anatase, a slight decrease in the transparency level by 3% and a redshift of the cut-off wavelength was observed. Moreover, additional annealing caused small changes of the optical band gap energy, refractive index and extinction coefficient.