Atomic Layer Deposition of HfS2 on Oxide Interfaces: A Model Study on the Initial Nucleation Processes

In this work, we investigated the initial nucleation processes during atomic layer deposition (ALD) of HfS2 on an atomically defined Co3O4(111) surface under ultrahigh-vacuum conditions. The nucleation and growth steps were monitored in situ by infrared reflection absorption spectroscopy in combination with density-functional theory. HfS2 was grown by sequential dosing of tetrakis(dimethylamido)hafnium (TDMAH) and D2S onto the surface exposing well-defined OD groups and partially dissociated OD/D2O aggregates. We find that the initial half cycle of the ALD process composes of several regimes. Initially, TDMAH loses all ligands due to a reaction with mobile OD/D2O species on the surface. With increasing exposure to TDMAH, the stoichiometry of the growth nuclei changes. We observe the formation of partially hydrolyzed Hf(NMe2)n(O)x(OD)m species and the consumption of OD/D2O aggregates. In the final growth step, the partially hydrolyzed Hf(NMe2)n(O)x(OD)m species react with TDMAH until all OD groups are consumed. Furthermore, we show that the density and dispersion of the Hf(NMe2)n(O)x(OD)m aggregates formed in the initial ALD half cycle depend dramatically on the growth temperature. Our findings demonstrate that the initial nucleation step of the ALD procedure is a very complex process in which it is essential to control not only the classical growth parameters such as temperature and flux but also the nature and the mobility of the OD/D2O aggregates on the surface.