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.
Highlights
Titanium dioxide (TiO2) became a promising material in different applications for its excellent optical and electrical properties and chemical stability such as large band gap, high refractive index, high dielectric constant, and highly active surface [1–4]
The TiO2 ultrathin films with different thicknesses were obtained by controlling the numbers of atomic layer deposition (ALD) cycles from 50 to 600 cycles
The surface morphologies of all films are smooth with different root mean square (RMS) roughness values, which indicates that TiO2 ultrathin films were well fabricated (Fig. 1h)
Summary
Titanium dioxide (TiO2) became a promising material in different applications for its excellent optical and electrical properties and chemical stability such as large band gap, high refractive index, high dielectric constant, and highly active surface [1–4]. To achieve high-quality TiO2 ultrathin films, the ALD method has been employed to deposit samples on single crystal Si substrates. The optical constants and optical band gap of the TiO2 ultrathin film are obtained, and the influence of nano size and temperature on the optical properties is revealed. These results will be helpful to the applications of ultrathin TiO2 in optoelectronic semiconductor devices such as memory, field-effect transistors, and inverted organic solar cells. A novel method to monitor the phase transition in ultrathin films is proposed by deducing from the band gap evolution
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