The adsorption and reaction of fluorine on the Si(100) surface

The adsorption and reaction of both molecular and atomic flourine with the Si(100) surface has been examined under ultraligh vacuum conditions with supersonic molecular beam techniques, X-ray photoelectron spectroscopy (XPS), quadrupole mass spectrometry and low-energy ion scattering spectroscopy. Molecular flourine adsorbs dissociatively on the clean Si(100) surface with an initial (zero-coverage) probability of the adsorption of 0.46±0.02, which is essentially independent of both the incident beam energy (〈Etr=105−19 kcal mol−1) and surface temperature (Ts=120–600 K).The coverage-exposure relationship for F2 is characterized by an initial rapid phase of adsorption, which saturates at a coverage of θF≅1.5 monolayers (ML), followed by a much slower phase of adsorption, which does not saturate even at the exposure of 600 ML. For substrate temperatures of 300 K and above, the rapid phase of adsorption is described well by second-order Langmuir kinetics. However, below 300 K, trapping into a mobile (molecular) extrinsic presursor state on the chemisorbed adlayer becomes important, which results in an adsorption probability that is nearly independent of coverage at 120 K. The adsorption of atomic flourine is qualitatively different from molecular flourine. Although the initial probability of 3–4 ML. Temperature-programmed decomposition of silicon-flouride adlayers, produced by exposing the clean Si(100) surface a 120 K to a beam of flourine, yielded SiF2(g) and SiF4(g) as the only gas phase reaction products. The relative yield to these two gas phase reaction products dependes strongly on the initial coverage of the flourine adatoms-below ∼ ML, SiF2(g) in the major reaction product, whereas above ∼3 ML, the yield of SiF2(g) remains constant while that of SiF2(g) increases continuously. Above initial coverages of 2 ML, the thermal decomposition is terminated near 800 K by the removal of one monolayer of the silicon substrate in the form of SiF2(g). A detailed analysis of the decomposition for coverages of 3 ML revealed complex behavior, the kinetics depending sensitively on the initial coverage of flourin adatome. For example, for initial coverages of 1–1.3 ML, zero-order kinetics were found to apply as the coverage decreases from 1.0 to 0.3 ML. A qualitative assessment of the adlayer configuration following partial decomposition suggests that the thermal decomposition in the zero-order regime proceeds inhomogenously, leaving separate domains where the local coverage of flourine is either near saturation or zero. We suggest that the spatially inhomogenous decomposition is a manifestation of preferential reactivity at surface defects such as atomic steps. Investigation of the steady-state reaction of preferential reactivity at surface defects such as atomic steps. Investigation of the steady-state reaction between F2(g) and the Si(100) substrate for temperatures of 650–1200 K shows conclusively that flourine must be adsorbed dissociatively for the gasification reaction [production of SiF2(g)] to occur, e.i., a Languimuir-Hinshelwood mechanism dominates.