Chemical equilibration of plasma-deposited amorphous silicon with thermally generated atomic hydrogen

Hydrogenated amorphous silicon (a-Si:H) thin films prepared by plasma-enhanced chemical vapor deposition (PECVD) from ${\mathrm{SiH}}_4$ have been further hydrogenated in situ by exposure to atomic H generated by a filament heated in ${\mathrm{H}}_2$ gas. Upon equilibration of the network with gas-phase H, as many as ∼2×${10}^{21}$ ${\mathrm{cm}}^{\mathrm{-}3}$ additional Si-H bonds form within the top 200 Å of the film without significant etching, surface roughening, or coordination defect generation. Real-time spectroscopic ellipsometry is applied to study the kinetics of near-surface Si-H bond formation at 250 °C in order to improve our understanding of the effects of excess atomic H in the a-Si:H growth environment. Atomic H entering the film surface exhibits an effective diffusion coefficient >3×${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$/s and is trapped within the top 200 Å of the film at a rate of ∼${10}^{\mathrm{-}3}$ ${\mathrm{s}}^{\mathrm{-}1}$. Most of this H is trapped irreversibly on the time scale of deposition with emission rates ${10}^{\mathrm{-}7}$ ${\mathrm{s}}^{\mathrm{-}1}$. We also find that monolayer levels of surface oxide are an effective diffusion barrier to H, preventing chemical equilibration between the gas and solid phases.