Atomic layer deposition of zinc indium sulfide films: Mechanistic studies and evidence of surface exchange reactions and diffusion processes

The authors present the elaboration of zinc indium sulfide (ZnIn _xS_y) thin films in the context of a cadmium-free buffer layer development for copper indium gallium diselenide photovoltaic solar cells. The films were deposited by atomic layer deposition (ALD) from ZnEt₂ (DEZ), In(acac)₃ (acac = acetylacetonate), and H₂S at 200 °C. In situ growth kinetics studies were performed with the quartz crystal microbalance technique to determine the respective mass gain per cycle of ZnS and In₂S₃ layers, allowing determination of the atomic compositions of the ZnIn_xS_y thin films to be expected if the deposition strictly follows the rule of mixtures. As the experimental atomic compositions of the ZnIn_xS_y films differ significantly from this rule, a comprehensive study of the growth mechanism was performed to determine the nature of the side reactions. First, an exchange reaction between In₂S₃ and the Zn precursor was identified, though this process is not sufficient to account for the experimental data, and therefore, a second process which corresponds to the diffusion of species within the film was also found to take place. Ultimately, the atomic compositions of the ZnIn_xS_y films can be explained by a rate-limited exchange reaction at the surface between DEZ and the In₂S₃ layer, combined with diffusion of the species in the whole film. More generally, such side reactions should be considered in ALD of multinary compounds, even at low temperature.