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

Abstract

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 \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ additional Si-H bonds form within the top 200 \AA{} 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 \ifmmode^\circ\else\textdegree\fi{}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\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ ${\mathrm{cm}}^{2}$/s and is trapped within the top 200 \AA{} of the film at a rate of \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. Most of this H is trapped irreversibly on the time scale of deposition with emission rates 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}7}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. 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.