Bonding of fluorine in amorphous hydrogenated silicon

Abstract

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 ${\mathrm{cm}}^{-1\mathrm{}}$): 1010 ${\mathrm{cm}}^{-1\mathrm{}}$ (Si-${\mathrm{F}}_4$ stretching); 930 ${\mathrm{cm}}^{-1\mathrm{}}$ (Si-${\mathrm{F}}_2$, Si-${\mathrm{F}}_3$ stretching); 828 ${\mathrm{cm}}^{-1\mathrm{}}$ (Si-F stretching); 510 ${\mathrm{cm}}^{-1\mathrm{}}$ (Si TO mode induced by F); 380 ${\mathrm{cm}}^{-1\mathrm{}}$ (Si${\mathrm{F}}_4$ bond bending); 300 ${\mathrm{cm}}^{-1\mathrm{}}$ (Si-F, Si-${\mathrm{F}}_2$ wagging). The intensities of these bands were measured as a function of preparation condition and annealing temperature. In addition to the Si${\mathrm{F}}_4$ molecules trapped in the films during preparation, annealing temperatures above 300 °C produced a transformation of some Si—F groups into Si${\mathrm{F}}_4$ molecules. Gas-evolution experiments show that fluorine-containing molecules (F, HF, Si${\mathrm{F}}_4$) evolve all in one sharp peak around 680°C, near the crystallization point. An average cross section $\overline{\Gamma }$ for Si-F vibrations of 11.2 ${\mathrm{cm}}^2$/mmol and bond was obtained in good agreement with similar values in gaseous and solid Si${\mathrm{F}}_4$.