Thermal decomposition of a silicon-fluoride adlayer: Evidence for spatially inhomogeneous removal of a single monolayer of the silicon substrate

The thermal decomposition of silicon-fluoride adlayers, produced by exposing a clean Si(100) surface to a molecular beam of fluorine, have been investigated under ultrahigh-vacuum conditions with x-ray photoelectron spectroscopy and mass spectrometry. The only gas phase reaction products detected from temperature-programmed desorption are ${\mathrm{SiF}}_2$ and ${\mathrm{SiF}}_4$, the relative yield of each product depending strongly on the coverage of fluorine adatoms. For fluorine coverages below 3–4 monolayers, the major reaction product is ${\mathrm{SiF}}_2$, whereas above 3–4 monolayers the relative yield of ${\mathrm{SiF}}_4$ increases continuously, while that of ${\mathrm{SiF}}_2$ remains constant. Independent of the initial coverage of fluorine, the thermal decomposition is terminated near 800 K by the removal of one monolayer of the Si substrate in the form of ${\mathrm{SiF}}_2$. Results are presented that suggest the removal of this ‘‘final’’ monolayer proceeds inhomogeneously, leaving separate domains in which the local coverage of fluorine is either zero or near saturation.