Dissociative adsorption of NF3 on Si(001)-(2×1)

Using the first-principles pseudopotential–generalized gradient approximation (GGA) approach, we have performed an accurate determination of the atomic and electronic structure of the dissociative adsorption for nitrogen trifluoride on Si(001)-(2×1). The chemisorbed system is energetically favourable and occurs in a very similar way to that found for the ammonia molecule: the NF3 molecule dissociates with the formation of SiNF2 and SiF bonds. We find that the system is characterized by elongated (around 6%) and symmetric silicon dimers (the tilt angle is just 1.5°), and by the fact that the SiNF2 group retains the pyramidal geometry of the nitrogen trifluoride molecule. We also observe that the SiN bond is inclined at almost 8.6° with respect to the surface normal. The NSi bond length is 1.85 Å and the NF bond length is 1.47 Å. Finally, our electronic structure calculations suggest that the adsorption of NF3 molecules almost completely passivates the silicon surface.