Optical Constants and Band Gap Evolution with Phase Transition in Sub-20-nm-Thick TiO₂ Films Prepared by ALD

Titanium dioxide (TiO₂) ultrathin films with different thicknesses below 20 nm were grown by atomic layer deposition (ALD) on silicon substrates at 300 °C. Spectroscopic ellipsometry (SE) measurements were operated to investigate the effect of thickness on the optical properties of ultrathin films in the spectra range from 200 to 1000 nm with Forouhi-Bloomer (F-B) dispersion relation. It has been found that the refractive index and extinction coefficient of the investigated TiO₂ ultrathin film increase while the band gap of TiO₂ ultrathin film decreases monotonically with an increase in film thickness. Furthermore, with the purpose of studying the temperature dependence of optical properties of TiO₂ ultrathin film, the samples were annealed at temperature from 400 to 900 °C in N₂ atmosphere. The crystalline structure of deposited and annealed films was deduced by SE and supported by X-ray diffraction (XRD). It was revealed that the anatase TiO₂ film started to transform into rutile phase when the annealing temperature was up to 800 °C. In this paper, a constructive and effective method of monitoring the phase transition in ultrathin films by SE has been proposed when the phase transition is not so obvious analyzed by XRD.