A comparative study of dissociative adsorption of NH3, PH3, and AsH3 on Si(001)-(2x1)

Using a first-principles pseudopotential method we have studied the adsorpt ion and dissociation of NH3, PH3, and AsH3 on the Si(001)-(2x1) surface. Ap art from the existence of a barrier for the adsorption of the precursor sta te for arsine, we observe that the global behavior for the chemisorption of the XH3 molecules considered in this work is as follows: the gas phase XH3 adsorbs molecularly to the electrophilic surface Si atom and then dissocia tes into XH2 and H, bonded to the electrophilic and nucleophilic surface si licon dimer atoms, respectively. The energy barrier, corresponding to a the rmal activation, is much smaller than the usual growth temperature, indicat ing that all three molecules will be observed in their dissociated states a t room temperature. All adsorbed systems are characterized by elongated Si- Si dimers that are (almost) symmetric in the dissociative case but asymmetr ic in the molecular case. According to our first-principles calculations, a ll XH3 and XH2 systems retain the pyramidal geometry observed for the gas m olecules. Our calculated vibrational spectra further support the dissociati ve model for the XH3 molecules considered here. (C) 2001 American Institute of Physics.