Multiple adsorption configurations ofNH3molecules on the Si(001) surface

Initial adsorption configurations of $\mathrm{N}{\mathrm{H}}_{3}$ molecules on the Si(001) surface are studied by using scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. In contrast to the previous consensus of single configuration, we demonstrate that there exist two distinct STM features. One is found to be the existing model of on-dimer configuration, that is characterized by the previously unreported ``U-shape'' empty-state feature. The other is identified to be $\mathrm{N}{\mathrm{H}}_{2}$ and H adsorbed at the same side of two adjacent dimers along the dimer row. The identification of all the adsorption configurations would be useful for the quantitative analysis of Si nitridation.