Initial Growth and Agglomeration during Atomic Layer Deposition of Nickel Sulfide

Atomic layer deposition (ALD) is a highly useful technique to grow thin film materials, and the initial growth of ALD is of particular importance for achieving high-quality materials and interfaces. In this work, we investigate the initial ALD growth of nickel sulfide (NiS) on a SiOx surface, from bis(N,N′-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) and H2S, by using the in situ techniques of X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS). The mechanism of the initial ALD growth is found to be rather different from that in the later steady film growth. In the initial ALD cycles, the XPS results show a drastic cyclic variation of the signals for the Ni–O bonds, with prominently observable Ni–O signals after each Ni(amd)2 dose but almost negligible after the subsequent H2S dose. These results suggest that the Ni–O bonds are first formed on the surface in the Ni(amd)2 half-cycles and then mostly converted to NiS in the following H2S half-cycles. To describe this initial ALD growth process, a reaction-agglomeration mechanistic scheme is proposed. In this scheme, the conversion from Ni–O to NiS in the H2S half-cycles is suggested to be accompanied by the spontaneous agglomeration of the ligand-stripped Ni-containing species to afford NiS clusters. This agglomeration can re-expose the surface of SiOx and therefore allow the surface to further react with Ni(amd)2 in the subsequent ALD cycles. With the further use of LEIS, the transition from the initial reaction-agglomeration growth to the continuous steady film growth is found to be between 100 and 300 ALD cycles. Ex situ atomic force microscopy and transmission electron microscopy are further employed to corroborate the presence of the agglomeration during the initial growth. The results reported herein and the implied growth mechanism should be of great representativeness for the ALD of metal sulfides, and therefore, the findings should be highly valuable for future engineering of functional metal sulfide nanomaterials and interfaces by ALD.