Optical Constants and Band Gap Evolution with Phase Transition in Sub-20-nm-Thick TiO(2) Films Prepared by ALD

Titanium dioxide (TiO(2)) 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 TiO(2) ultrathin film increase while the band gap of TiO(2) ultrathin film decreases monotonically with an increase in film thickness. Furthermore, with the purpose of studying the temperature dependence of optical properties of TiO(2) ultrathin film, the samples were annealed at temperature from 400 to 900 °C in N(2) 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 TiO(2) 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.