Surface reaction probabilities and kinetics of H, SiH₃, Si₂H₅, CH₃, and C₂H₅ during deposition of a-Si: H and a-C:H from H₂, SiH₄, and CH₄ discharges

The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H, SiH₃, and Si₂H₅ on a growing α-Si:H film, and CH₃ and C₂H₅ on an α-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. For SiH₃ on α-Si:H, β = 0.28±0.03 in excellent agreement with previous determinations using other experimental approaches, and for Si₂H₅, 0.1<β<0.3. For H on α-Si:H, 0.4<β<1 and mostly consists of surface recombination as H₂, while the etching probability of Si as SiH₄ is only ε ≈003 at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution of SiH₄ in H₂ the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surface recombination at the expense of sticking. For CH₃ or C₂H₅ on α-C:H it is shown that β is not constant during the discharge afterglow, decreasing from about 0.01 down to 0.001. This reveals that chemisorption of these radicals on the H-saturated α-C-H surface is entirely governed by the competition between desorption and creation of active sites by ion bombardment or H atoms. The differences between the surface reaction kinetics of SiH₃ on α-Si:H and CH₃ on α-C:H are discussed within a unified model of precursor-mediated chemisorption.