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 ${\mathrm{cm}}^{\ensuremath{-}1}$): 1010 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-${\mathrm{F}}_{4}$ stretching); 930 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-${\mathrm{F}}_{2}$, Si-${\mathrm{F}}_{3}$ stretching); 828 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si-F stretching); 510 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si TO mode induced by F); 380 ${\mathrm{cm}}^{\ensuremath{-}1}$ (Si${\mathrm{F}}_{4}$ bond bending); 300 ${\mathrm{cm}}^{\ensuremath{-}1}$ (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 \ifmmode^\circ\else\textdegree\fi{}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\ifmmode^\circ\else\textdegree\fi{}C, near the crystallization point. An average cross section $\overline{\ensuremath{\Gamma}}$ 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${\mathrm{F}}_{4}$.