Determining a Line Strength in the ν₃ Band of the Silyl Radical Using Quantum Cascade Laser Absorption Spectroscopy

Silane (SiH₄) plasmas are widely used for the deposition of hydrogenated amorphous silicon (a-Si:H) films. Nevertheless, the chemical processes governing film deposition are still incompletely understood. Moreover, there is still no general method available to determine the absolute concentration of the silyl radical (SiH₃), which is the accepted chemical precursor of a-Si:H films. In this study, a 10% silane in helium RF plasma was spectroscopically investigated between 2085 and 2175 cm^-1 using an external cavity quantum cascade laser (EC-QCL) based spectrometer. This led to the identification of 4 distinct species from their absorption features: SiH₄, disilane (Si₂H₆), SiH₃, and an unassigned short-lived species. Furthermore, 17 absorption features of SiH₃ were identified and unambiguously assigned. Fast spectral scanning of selected absorption features belonging to the four species in a 10 Hz pulsed RF plasma enabled the measurement and interpretation of their temporal behavior in terms of plausible chemical reactions involving silicon containing species. By quantitatively measuring the decay of the SiH₃ a ← a ^pP₄ (5) transition at 2151.3207 cm^-1 after the discharge was stopped, its line strength (S) was determined to be (7.5 ± 5.5) × 10^-20 cm² cm^-1 mol^-1.