Effect of hydration of the TiO2 anatase (101) substrate on the atomic layer deposition of alumina films

We have studied the initial processes of atomic layer deposition (ALD) of Al2O3 on TiO2 anatase (101) with density functional theory (DFT) calculations. Our ab initio thermodynamics results predict that, at typical ALD conditions, the initial substrate is almost completely dehydrated, with water being adsorbed only near defects. We show that the first ALD half-reaction, i.e. the adsorption of trimethylaluminium (Al(CH3)(3), TMA), presents a positive Gibbs free energy. The proximity of water at defect sites can considerably stabilise the interaction by elimination of one or two methyl groups. Water molecules thus serve as nucleation seeds for the growth of the overcoating. The island growth mode found in experiments is then attributed to an incomplete hydration of the substrate.