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

## TL;DR

This paper studies how silicon atoms and tantalum nanoparticles combine to form hybrid films with higher porosity, which could improve Li-ion battery anodes.

## Contribution

A new model and simulations explain how deposition mechanisms affect hybrid film morphology and porosity.

## Key findings

- Hybrid films with Ta nanoparticles show higher porosity and pore connectivity than pure silicon films.
- Simulations show Si clusters grow around nanoparticles, leading to rougher surfaces and better ion diffusion.
- Optimal deposition conditions in experiments are confirmed by simulation results matching experimental porosity.

## Abstract

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.