Effects of the Deposition Mechanisms of Silicon Atoms and Tantalum Nanoparticles on the Morphology of Hybrid Films

Recently fabricated materials with a-Si layers, Ta nanoparticle (NP) scaffolds, and 3.5 atom % Ta/Si ratio have porosity ≈20% larger than that of pure a-Si films and improved performance as anodes of Li-ion batteries. Here, we introduce a model for the growth of these hybrid films that assumes first contact aggregation of NPs deposited with thermal energy and surface diffusion of Si atoms to represent the consequences of their deposition by sputtering. Kinetic Monte Carlo simulations of pure Si films show smooth mounded surfaces with morphology control by Si atom diffusion in convex and flat parts of the surface. Simulations of hybrid films show highly porous Ta NP scaffolds, their partial mixing with Si atoms, and growth of Si clusters separated by nanosized gaps with NPs at their bottom tips. This nontrivial self-organization provides higher porosities and high pore connectivity across the whole film thickness, which is beneficial for ion diffusion. Top views of growing films show widening of Si clusters, in qualitative agreement with scanning electron microscopy images of the previous experimental study. The experimentally observed porosity increase is obtained in simulations with ∼10⁴-10⁵ hop attempts per Si atom and hop probability factors ∼0.03-0.1 for each neighboring atom. Simulated hybrid films are rougher than pure Si films due to fluctuations in the NP heights, which qualitatively agrees with the experiments. Simulations with different thicknesses in each layer but constant Ta/Si ratio lead to lower porosity, which suggests that optimal deposition conditions were chosen in the experiments.