Vibrational properties of amorphous silicon alloys

A theoretical model of the vibrational states of recently reported amorphous silicon-fluorine alloys is presented. The infrared absorption, polarized Raman scattering, and phonon state density spectra are calculated for different local silicon-fluorine bonding arrangements. Changes in the infrared and Raman spectra associated with the incorporation of oxygen and hydrogen in the amorphous alloy is also discussed. Our results suggest that unlike the situation in amorphous silicon-hydrogen alloys, the presence of Si-F bonds in the fluorinated materials significantly disturb the bulk Si vibrational bands. It is found that the signatures of isolated SiF₂ and SiF₃ complexes are stretching modes in the region of 800 to 1000 cm^-1 accompanied by one (SiF₂) or two (SiF₃) sharp resonances in the region of 400 to 500 cm^-1, while the SiF unit is distinguished by a single stretching mode near 800 cm^-1. Finally, our results also indicate that Si-H bonds in close proximity with Si-F bonds alter the fluorine vibrational modes. Si-O bonds, on the other hand, do not affect the Si-F stretching mode near 800 cm^-1.