Room Temperature Atomic Layerlike Deposition of ZnS on Organic Thin Films: Role of Substrate Functional Groups and Precursors

The room temperature atomic layerlike deposition (ALLD) of ZnS on functionalized self-assembled monolayers (SAMs) was investigated, using diethyl zinc (DEZ) and in situ generated H₂S as reactants. Depositions on SAMs with three different terminal groups, -CH₃, -OH, and -COOH, were studied. It was found that the reaction of DEZ with the SAM terminal group is critical in determining the film growth rate. Little or no deposition is observed on -CH₃ terminated SAMs because DEZ does not react with the methyl terminal group. ZnS does deposit on both -OH and -COOH terminated SAMs, but the grow rate on -COOH terminated SAMs is ∼10% lower per cycle than on -OH terminated SAMs. DEZ reacts with the hydroxyl group on -OH terminated SAMs, while on -COOH terminated SAMs it reacts with both the hydroxyl and carbonyl bonds of the terminal groups. The carbonyl reaction is found to lead to the formation of ketones rather than deposition of ZnS, lowering the growth rate on -COOH terminated SAMs. SIMS spectra show that both -OH and -COOH terminated SAMs are covered by the deposited ZnS layer after five ALLD cycles. In contrast to ZnO ALLD where the composition of the film differs for the first few layers on -COOH and -OH terminated SAMs, the deposited film composition is the same for both -COOH and -OH terminated SAMs. The deposited film is found to be Zn-rich, suggesting that the reaction of H₂S with the Zn-surface adduct may be incomplete.