Role of Nonplasma Oxidizing Agent on the Reaction Step in Atomic Layer Deposition of Titanium Oxide Thin Films

Atomic layer deposition (ALD) is a key technique for fabricating high-quality metal oxide thin films via sequential surface reactions between precursors and oxidizing agents. In the thermal (nonplasma) ALD process for TiO₂ thin films, Titanium-(IV) chloride (TiCl₄), Titanium-(IV) isopropoxide (TTIP), and Tetrakis-(dimethylamido)-titanium-(IV) (TDMAT) are among the most widely used precursors, while H₂O and H₂O₂ serve as the primary oxidizing agents. This study carried out a theoretical investigation of the oxidation step in ALD-TiO₂, focusing on the reaction mechanisms and energetics using density functional theory (DFT). The simulations revealed that H₂O oxidizes the surface via O-H bond dissociation, whereas H₂O₂ proceeds through either O-H or O-O bond cleavage, each with distinct activation barriers and resulting surface structures. The activation barriers for H₂O oxidation on TDMAT, TTIP, and TiCl₄, were approximately 24, 50, and 92 kJ/mol, respectively, while for H₂O₂, the O-H bond cleavage pathway required barriers of about 36, 64, and 87 kJ/mol, respectively, and the O-O cleavage pathway required 77, 190, and 170 kJ/mol, respectively. Despite its higher energy requirement, the O-O pathway of H₂O₂ enables the oxidation of two precursor species with a single molecule, effectively halving the oxidant consumptionan important advantage for industrial-scale applications. Furthermore, this pathway generates twice as many reactive -OH adsorption sites compared to H₂O, directly impacting film growth kinetics and quality. This study provided valuable insight for optimizing ALD process parameters and selecting synergistic precursor-oxidant pairs, thereby advancing the fundamental knowledge and practical capabilities of ALD technology.