In Situ Infrared Spectroscopic Study of Atomic Layer-Deposited TiO₂ Thin Films by Nonaqueous Routes

The mechanisms of growth of TiO 2 thin films by atomic layer deposition (ALD) using either acetic acid or ozone as the oxygen source and titanium isopropoxide as the metal source are investigated by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy. The FTIR study of the acetic acid-based process clearly shows a ligand exchange leading to the formation of surface acetate species (vibrational bands at 1527 and 1440 cm –1 ) during the acetic acid pulse. Their removal during the metal alkoxide pulse takes place via the elimination of an ester and the formation of Ti–O–Ti bonds. These findings confirm the expected ester elimination condensation mechanism and demonstrate that the reaction proceeds without intermediate surface hydroxyl species. The in situ FTIR study of the O 3 -based ALD process demonstrates similarities with the process described above, with formation of surface formate and/or carbonate species upon exposure of the surface titanium alkoxide species to ozone. These surface species are removed by the subsequent titanium isopropoxide pulse, leading to the formation of Ti–O–Ti bonds.