Multiscale simulation study on growth characteristics of infrared adaptive optics ZnS thin films under different deposition parameters

Zinc sulfide (ZnS) thin films are crucial for high-density deformable mirrors in infrared adaptive optics systems due to their excellent transmittance in the 0.4–12 μm range and robust thermal stability. However, achieving optimal film microstructure and surface roughness remains challenging, particularly in thermal evaporation processes where sulfur loss leads to zinc-rich films with increased porosity and roughness. This study employs a multiscale computational approach, integrating Monte Carlo (MC) and Molecular Dynamics (MD) simulations within a modified ballistic deposition model, to investigate ZnS thin film growth on single-crystal silicon substrates. The model incorporates angle-dependent deposition, shadowing effects, surface migration, and stoichiometric variations to systematically analyze the impact of deposition angle (20°–60°), temperature (300-450 K), rate (1–1.5 nm/s), and stoichiometry (Zn:S = 1 and 1.5) on film morphology and roughness. The results show that a deposition angle of 20° produces relatively dense films with moderate voids, while a 60° angle increases porosity and thickness due to enhanced shadowing. Higher deposition rates slightly elevate roughness, whereas elevated temperatures (450 K) promote denser structures and smoother surfaces by enhancing surface diffusion. Notably, a stoichiometry of 1.5, reflecting sulfur loss, significantly increases void formation and roughness, reducing film density and refractive index, consistent with experimental observations. These findings highlight the critical role of stoichiometric control in thermal evaporation and suggest minimizing ion source power in ion-assisted deposition to maintain a Zn:S ratio of 1 while reducing film stress. The simulation results provide theoretical guidance for optimizing ZnS thin film fabrication for deformable mirrors in infrared adaptive optics systems, balancing microstructure, optical performance, and mechanical stability.