Effect of ion bombardment on the structural and optical properties of TiO2 thin films deposited from oxygen/titanium tetraisopropoxide inductively coupled plasma

Abstract

Titanium dioxide films were deposited on silicon substrates from oxygen/titanium tetraisopropoxide inductively coupled radiofrequency plasmas in a helicon reactor operated at low temperature (<150°C) and low pressure (0.4Pa). The effect of the ion energy (Ei), varied in the 15–175eV range, on the morphology, microstructure and optical properties of the films is investigated. Scanning electron microscopy (SEM) shows that at low energy (Ei=15eV), the film exhibits a columnar morphology consisting of a bottom dense layer, an intermediate gradient layer and a top roughness layer. Increasing the ion energy results in more homogeneous films along the growth direction as confirmed by the in-situ kinetic ellipsometry measurements and post deposition spectroscopic ellipsometry data analysis. In addition, the atomic force microscopy (AFM) measurements reveal that the film top surface becomes smoother as Ei is increased. X-ray diffraction (XRD) diagrams show that only anatase is identified in the film deposited at 15eV, whereas the complete phase transformation from anatase to rutile occurs at Ei=75eV. These results are in good agreement with the Fourier transform infrared spectroscopy (FTIR) spectra which also show that the hydroxyl groups absorbed in the films deposited at 15eV, are greatly decreased for Ei≥45eV. Suitable structural models combined with the Tauc–Lorentz dispersion law have been found to accurately fit the spectroscopic ellipsometry experimental data. The results in good agreement with SEM and AFM measurements are also consistent with the structural properties evidenced by XRD and FTIR. The refractive index (n) can be increased significantly by increasing the ion energy from 15eV to 75eV, reaching a value of 2.49 at 1.96eV. Upon increasing the ion energy above 75eV n is shown to decrease due to micropores which are formed in the films.