Metal Exchange and Diffusion during Atomic Layer Deposition of Cobalt and Nickel Sulfides

Atomic layer deposition (ALD) has become an essential tool to fabricate high-quality thin-film materials with atomic-level control. Early ALD studies mostly focused on binary metal oxides, but recently, ALD of multielement compounds (e.g., ternary metal sulfides) has triggered significant interest for numerous emerging applications. However, controllable ALD synthesis of multielement compounds is comparatively much more complicated and challenging, and particularly, the involved mechanisms can be complex and sometimes difficult to interpret. In this work, we show that, as a representative attempt to synthesize ternary cobalt nickel sulfides (CoxNiyS) by ALD, directly depositing nickel sulfide (NiSx) on cobalt sulfide (Co9S8) can be significantly affected by the gas–solid metal exchange reaction on the surface and the metal-ion diffusion inside the film. A series of detailed mechanistic investigation is carried out, with the employment of in situ X-ray photoelectron spectroscopy (XPS) technique and numerical simulation, to understand the crucial role and interplay of the metal exchange and diffusion in the process. The understanding of the mechanisms can provide insights into achieving a high level of controllability in multielement ALD.