Simulating the atomic layer deposition of alumina from first principles

Atomic Layer Deposition (ALD) is a chemical vapour deposition technique that is suitable for the controlled growth of thin, conformal oxide films. Insulating layers of alumina (Al2O3) are fabricated by ALD for electroluminescent flat-screen-displays, memory capacitors and read/write heads. Successful precursors for alumina ALD are trimethylaluminium (TMA) and water. Various models for the mechanism of alumina ALD have been proposed but definitive evidence for the surface intermediates is lacking. We therefore use density functional theory to investigate the atomic-scale structure and reactivity of bare and hydroxylated alumina surfaces with respect to TMA. The competition between dissociative chemisorption and molecular desorption is quantified. The calculations show that TMA adsorbs and dissociates at both O and OH sites on the surface, but that these lead to different ALD efficiencies. In this way we clarify how the degree of surface hydroxylation and hence the substrate temperature affects the ALD growth rate.