Atomic Layer Deposition of HfS2 on Functionalized Self-Assembled Monolayers on Ordered Oxide Surfaces: A Model Study under UHV Conditions

We have investigated the atomic layer deposition (ALD) of HfS2 on both functionalized and non-functionalized self-assembled monolayers (SAMs) grown on an atomically defined Co3O4(111) surface under ultrahigh-vacuum (UHV) conditions. In particular, we compared the growth on the hydrocarbon-terminated biphenyl-4-carboxylic acid (BP4CA, H-Ph-Ph-COOH) SAM to the OH-terminated 4′-hydroxy-4-biphenylcarboxylic acid (4HBP4CA, HO-Ph-Ph-COOH) SAM. The ALD procedure involved sequential dosing of tetrakis(dimethylamido)hafnium (TDMAH) and D2S separated by evacuation periods in between. The surface species were monitored in-situ by infrared reflection absorption spectroscopy, and the vibrational data was interpreted based on simulated spectra by density-functional theory. We observed that the deposition of HfS2 can be very efficiently controlled by the surface termination of the SAM. At 400 K, we observe no deposition at all on the hydrocarbon-terminated BP4CA SAM, while HfS2 efficiently nucleates and grows on the OH-terminated 4HBP4CA SAM. Nucleation of HfS2 is efficiently suppressed on the BP4CA SAM up to temperatures of 400 K. Above this temperature, we observe the onset of slow deposition of HfS2 also on the BP4CA SAM, suggesting that the surface coverage of the SAM decreases and the blocking effect becomes incomplete. For the functionalized 4HBP4CA SAM, we observed that the initial nucleation process comprises several growth regimes. In specific, we propose that the stoichiometry of the Hf(NMe2)nORm nuclei changes during the first ALD half cycle. Moreover, the IR spectroscopic data indicates that the Hf-containing nuclei have a major effect on the structure of the underlying SAM. Our findings suggest that the density of functional groups and the rigidity of the SAM are critical parameters which will have a major impact on the properties of the materials deposited by ALD.