Optical Properties of TiO₂ Grown by Atomic Layer Deposition Using Various Oxidizing Agents: The Ellipsometry Analysis of Absorption Properties

This study analyzes the optical properties of TiO 2 films grown via atomic layer deposition (ALD) using Tetrakis(dimethylamino)titanium with oxidizing agents such as H 2 O, H 2 O 2 , O 3 , and O 2 ‐plasma. TiO 2 ‐H 2 O exhibited Ti 3+ states and oxygen vacancies characteristic of black TiO 2 , enhancing visible‐NIR light absorption. This effect increased with more ALD cycles due to the surface memory effect, leading to a decreased growth rate. In contrast, films grown with H 2 O 2 , O 3 , and O 2 ‐plasma are more stoichiometric, has fewer defects, and show no visible‐NIR absorption due to the absence of Ti 3+ . TiO 2 films deposited with H 2 O also show increased surface roughness and hydrophobicity, while films grown with other oxidizing agents exhibited higher roughness but decreased hydrophobicity. Ellipsometry and UV–vis spectroscopy confirmed that TiO 2 films grown with H 2 O has an increased refractive index and extinction coefficient in the visible range, along with a bandgap widening due to the Moss‐Burstein effect. Conversely, films grown with other oxidizing agents showed a decreased bandgap with increasing thickness and an increased refractive index but a zero extinction coefficient below the bandgap. Mid‐IR ellipsometry measurements revealed the dielectric response, highlighting the critical role of the oxidizing agent in tailoring the properties of TiO 2 films for energy and environmental applications.