Chemical reactions and crystallization behavior in thermal ALD of BaTiO3 thin films from bis-(1,2,4 triisopropylcyclopentadienyl)-barium and titanium isopropoxide precursors

Atomic layer deposition (ALD) of high-dielectric constant perovskite ferroelectric materials is important for advancing next-generation nonvolatile and low-voltage solid-state memory devices for microelectronics. While reports exist for the ALD of these materials, few studies examine the fundamental chemical formation processes. Herein, we study the chemical reaction processes for the formation of individual components and the final product in the thermal ALD of barium titanate (BaTiO3 or BTO) thin films from bis-(1,2,4 triisopropylcyclopentadienyl)-barium and titanium isopropoxide precursors with water as the co-reactant. We find that at the process temperature studied (290 °C), the barium precursor forms a barium hydroxide phase, which is extremely susceptible to reaction with CO2 to form BaCO3. Avoiding this carbonate formation is critical, because the BaCO3 + TiO2 reaction has a significantly higher energy barrier than the Ba(OH)2 + TiO2 reaction toward forming BTO. Additionally, this study shows that a crystalline seed layer of anatase TiO2 below the first barium hydroxide layer helps to lower the formation energy for BTO crystallization. These results provide new guidance to the design of thermal ALD processes for depositing crystalline BTO thin films.