| Summary: | Reaction mechanisms during the growth of multinary compounds by atomic layer deposition can be complex, especially for sulfide materials. For instance, the deposition of copper indium disulfide (CuInS<sub>2</sub>) shows a non-direct correlation between the cycle ratio, the growth per cycle of each binary growth cycles, i.e., Cu<sub>x</sub>S and In<sub>2</sub>S<sub>3</sub>, and the film composition. This evidences side reactions that compete with the direct Atomic Layer Deposition (ALD) growth reactions and makes the deposition of large films very challenging. To develop a robust upscalable recipe, it is essential to understand the chemical surface reactions. In this study, reaction mechanisms in the Cu-In-S ternary system were investigated in-situ by using a quartz crystal microbalance system to monitor mass variations. Pure binary indium sulfide (In<sub>2</sub>S<sub>3</sub>) and copper sulfide (Cu<sub>x</sub>S) thin film depositions on Al<sub>2</sub>O<sub>3</sub> substrate were first studied. Then, precursors were transported to react on Cu<sub>x</sub>S and In<sub>2</sub>S<sub>3</sub> substrates. In this paper, gas-phase ion exchanges are discussed based on the recorded mass variations. A cation exchange between the copper precursor and the In<sub>2</sub>S<sub>3</sub> is highlighted, and a solution to reduce it by controlling the thickness deposited for each stack of binary materials during the CuInS<sub>2</sub> deposition is finally proposed.
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