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.
Bibliographical noteFunding Information:
This work was supported by the NSF/DoE Partnership for Basic Plasma Science and Engineering (Award Nos. ECS-0317345 and ECS-0317459), an NSF/ITR grant (Award No. CTS-0205584), and Camille Dreyfus Teacher-Scholar Awards to two of the authors (E.S.A. and D.M.). Fruitful discussions with S. Agarwal, M.A. Amat, T. Bakos, and M.R. Gungor also are gratefully acknowledged.
Copyright 2008 Elsevier B.V., All rights reserved.
- Amorphous thin films
- Hydrogen atom
- Molecular dynamics
- Plasma processing
- Surface chemical reaction