We have studied the energetics and bonding geometry of the ammonia adsorbed Si(100) surface by performing density-functional total-energy calculations within generalized gradient approximation. Ammonia molecules are found to adsorb on the down atoms of buckled Si dimers with an adsorption energy of 1.2 eV, but there exists a low-energy (of about 0.6 eV) activation pathway to the more stable dissociative chemisorption where ${\mathrm{NH}}_2$ (H) bonds to the down (up) Si atom. This energy diagram implies that the molecular physisorption may survive at sufficiently low temperature, but thermal activation at higher temperatures possibly carries the system into the dissociated state over the energy barrier. Especially, the calculated adsorption energy (of 2.0 eV) and bonding geometry for the dissociated state is in good agreement with the measurements in recent desorption and photoelectron diffraction experiments.

• Received 9 March 1998

DOI:https://doi.org/10.1103/PhysRevB.58.4903