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

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Silane (SiH 4 ) 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 (SiH 3 ), 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: SiH 4, disilane (Si 2 H 6 ), SiH 3, and an unassigned short-lived species. Furthermore, 17 absorption features of SiH 3 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 SiH 3 a ← a p P 4 (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 cm 2 cm –1 mol –1 .