Photoelectron spectra of fluorinated amorphous silicon (a-Si: F)

Abstract

The photoelectron spectra of amorphous fluorinated Si ($a$-Si: F) prepared in situ by sputtering Si in an argon-Si${\mathrm{F}}_4$ mixture were measured using Al $K\alpha$ excitation x-ray photoelectron spectroscopy and He I and He II excitation ultraviolet photoemission spectroscopy (UPS). The fluorine-induced chemical shift of the Si core levels was determined as 1.15 eV per attached fluorine. From the relative weight of the shifted and unshifted Si $2p$ levels the contribution of the Si-${\mathrm{F}}_1$, Si-${\mathrm{F}}_2$, Si-${\mathrm{F}}_3$, and Si-${\mathrm{F}}_4$ configurations to the total fluorine concentration could be determined. Argon etching experiments reveal a marked fluorine enrichment at the film surfaces which have been identified mainly as Si-${\mathrm{F}}_3$ entities. The Si-F bonding states show up between 7 and 14 eV in the UPS spectra. Partial F $2p$ and $2s$ densities of states for the Si-${\mathrm{F}}_1$, Si-${\mathrm{F}}_2$, and Si-${\mathrm{F}}_3$ configurations have been extracted from the valence-band spectra. Overall, they agree well with the calculations of Ching provided the theoretical densities of states are shifted rigidly by 3.0 eV towards the top of the valence bands. The top of the valence band is found to recede proportional to the amount of fluorine incorporated. The maximum shift observed is 0.9 eV for a fluorine concentration of about 48 at.%.