First principles simulation of reaction steps in the atomic layer deposition of titania: dependence of growth on Lewis acidity of titanocene precursor

Abstract

It is a common finding that titanocene-derived precursors do not yield TiO₂ films in atomic layer deposition (ALD) with water. For instance, ALD with Ti(OMe)₄ and water gives 0.5 Å/cycle, while TiCp*(OMe)₃ does not show any growth (Me = CH₃, Cp* = C₅(CH₃)₅). From mass spectrometry we found that Ti(OMe)₄ occurs in the gas phase practically exclusively as a monomer. We then used first principles density functional theory (DFT) to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* ligand of TiCp*(OMe)₃ lowers the Lewis acidity of the Ti centre and prevents its coordination to surface O (‘densification’) during both of the ALD pulses. The effect of Cp* on Ti seems to be both steric (full coordination sphere) and electronic (lower electrophilicity). This crucial step in the sequence of ALD reactions is therefore not possible in the case of TiCp*(OMe)₃ + H₂O, which means that there is no deposition of TiO₂ films.