Atomistic calculation of the SiH₃ surface reactivity during plasma deposition of amorphous silicon thin films

We report a direct, statistically significant calculation of the surface reactivity of the SiH₃ radical on hydrogenated amorphous silicon (a-Si:H) using molecular-dynamics simulations of repeated impingement of SiH₃ radicals on growth surfaces of smooth a-Si:H films over the temperature range 475-800 K. SiH₃ can either incorporate into the film by adsorbing onto a surface Si dangling bond or inserting into Si-Si bonds (sticking), or abstract surface H through Eley-Rideal (ER) or Langmuir-Hinshelwood (LH) pathways to produce SiH₄ gas, or react with another surface SiH₃ to desorb as Si₂H₆ (recombination), or leave the film by reflection or desorption. The overall surface reaction probability, β, includes both radical sticking and recombination. In agreement with experimental measurements, β is almost constant over the temperature range studied, as are the probabilities for sticking and recombination, s and γ, respectively; the calculated mean value of β is 0.47 ± 0.03. Energetic analysis of the various surface reactions shows that radical adsorption, radical insertion, and ER abstraction are barrierless processes, which explains the measured temperature independence of β. LH abstraction is activated, but competes with disilane formation, yielding a temperature-independent γ. Also, LH abstraction leads to H elimination from a-Si:H during growth and can partly explain the experimentally measured temperature dependence of the H content in the a-Si:H film.