Bonding of fluorine in amorphous hydrogenated silicon

The infrared spectra of amorphous fluorinated silicon samples (a-Si: F,H) have been measured. The following lines were identified (in addition to the hydrogen-induced bands at 2090, 1985, 890, 840, and 630 cm^-1): 1010 cm^-1 (Si-F₄ stretching); 930 cm^-1 (Si-F₂, Si-F₃ stretching); 828 cm^-1 (Si-F stretching); 510 cm^-1 (Si TO mode induced by F); 380 cm^-1 (SiF₄ bond bending); 300 cm^-1 (Si-F, Si-F₂ wagging). The intensities of these bands were measured as a function of preparation condition and annealing temperature. In addition to the SiF₄ molecules trapped in the films during preparation, annealing temperatures above 300 ^∘{}C produced a transformation of some Si---F groups into SiF₄ molecules. Gas-evolution experiments show that fluorine-containing molecules (F, HF, SiF₄) evolve all in one sharp peak around 680^∘{}C, near the crystallization point. An average cross section Γ for Si-F vibrations of 11.2 cm²/mmol and bond was obtained in good agreement with similar values in gaseous and solid SiF₄.