Dissociative adsorption of PH₃ on the Si(001) surface

Using a first-principles pseudopotential method we have studied the adsorption and dissociation of the common n-type dopant molecule PH₃ on the Si(001) surface. We have found that for low phosphorus coverages (1/4 monolayer) phosphine adsorbs molecularly on one side of the Si-Si dimer and, at temperatures around 140 K, fully dissociates into PH₂ and H, with each component attached to one side of the dimer. For higher phosphorus coverages (1/2 monolayer) the interaction between adjacent dimers plays a decisive role in the dissociation process. For both coverages, the surface is characterized by an elongated dimer, symmetric for the dissociated case and asymmetric for the molecular case. The H-P-H angles and H-P bond lengths for the dissociative case are very close to those obtained for the PH₃ molecule. However, for the molecular case, while the H–P bond length is close to that observed for the PH₃ molecule, the H-P-H angle is ∼8% bigger. Available experimental scanning tunneling microscopy image results are reinterpreted using theoretical images for the model provided in this work. Our dissociative adsorption model is further supported by our calculated vibrational modes, which are in good agreement with available experimental work.