Electrical and optical properties of TiO₂ thin films prepared by plasma-enhanced atomic layer deposition

We report on the electrical and optical characterisation of the high-permittivity (high-κ) TiO₂ thin films grown by plasma enhanced atomic layer deposition on Si (100) and glass substrates, respectively. TiO₂ films were incorporated in metal-oxide semiconductor (MOS) capacitor structures with an Al metal gate electrode. The as-deposited films were amorphous; however upon annealing in the temperature range 500-900°C, crystalline TiO₂ in the anatase phase was formed. This was further confirmed by performing Raman measurements where the characteristic features corresponding to the anatase phase were observed. Transmittance and absorption spectra of the as-deposited and annealed films were performed by UV-Vis measurements showing more than 70% of transmittance. The formation of stoichiometric TiO₂ was revealed by X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS) analysis performed on annealed samples (500-900°C). The dielectric constants were calculated from capacitance-voltage (C-V) curves of the MOS structure on the as-deposited film and annealed films revealing a significant improvement of the dielectric constants from 10 to 75 at AC frequencies of 100 kHz for the 700°C annealed TiO₂ thin films. The increase in the dielectric constant for annealed films could be attributed to the transformation of film structure from amorphous to polycrystalline (anatase). However, the transformation of amorphous to crystalline phase, leads to an increase in the leakage current which was also found best fitted with Schottky emission mechanism at moderated electric fields.