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

Abstract

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.

Optical properties of amorphous SiO2-TiO2 multi-nanolayered coatings for 1064-nm mirror technology

“…4, bottom). Such trend was assigned by Shi et al to a density variation which is due to the increased percentage of volume occupied by voids that occurs when the lm thickness is reduced [62].…”

“…For the TiO 2 sub-units, the complex refractive index values can be compared with those recently obtained in ref. [62] for lms with thickness in the 2-20 nm range. An overall decrease of the refractive index for decreasing thickness was observed in that work; our data are compatible with this nding as we noticed an analogous trend going from the NL 5 to the NL 19 coating -that is, by reducing the thickness of the TiO 2 sub-units (see Fig.…”

Review—On Atomic Layer Deposition: Current Progress and Future Challenges

“…[208][209][210] Various types of precursors along with a wide range of substrates have been employed for atomic layer deposition of TiO 2 as summarized in Table XI. [211][212][213][214][215][216][217][218][219][220][221] Wang et al demonstrated TiO 2 ALD on Au supported Al 2 O 3 and SiO 2 using titanium isopropoxide (TTIP)-millipore water (precursor-reactant) pair at low temperature (150°C). 211 The TiO 2 was preferentially decorated on low coordinated Au surface generating the Au-TiO 2 interface with a growth rate 0.3 Å/cycle.…”

Band Line-up Investigation of Atomic Layer Deposited TiAlO and GaAlO on GaN

“…In the most recent study of ALD TixAl1-xOy films, the band gap of 4.3 eV and 4.1 eV has been inferred for films with 33% Ti and 40% Ti respectively, from UV-vis absorption spectra plotted as ( h ) 2 vs. E [34]. The reported values for the optical band gap of TiO2 have been found to be dependent on the crystallinity of the film and found to be 3.2 eV [23], 3.66-3.73 eV [34], 3.37 eV [35] to 3.7 eV [36]. The band gap of Al2O3 was extracted from the O 1s energy loss feature from the XPS measurement [37] shown in Fig.…”