Cooperative nitrogen insertion processes: Thermal transformation ofNH3on a Si(100) surface

Abstract

The thermal behavior of an ammonia-covered Si(100) surface is investigated by infrared spectroscopy and density functional methods. Upon adsorption at room temperature, $(\mathrm{Si})\mathrm{N}{\mathrm{H}}_{2}$ and Si-H species are formed on the surface. Comparison of the vibrational studies with density functional calculations suggests that the $(\mathrm{Si})\mathrm{N}{\mathrm{H}}_{2}$ structures are preferentially located on the same side along the silicon dimer row on a $(2\ifmmode\times\else\texttimes\fi{}1)$ reconstructed Si(100) surface, although a mixture of different long-range configurations is likely formed. Decomposition of these $(\mathrm{Si})\mathrm{N}{\mathrm{H}}_{2}$ species is observed to start at temperatures as low as $500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Theoretical predictions of the vibrational modes indicate that at this point, the spectrum is composed of a combination of ${(\mathrm{Si})}_{2}\mathrm{N}\mathrm{H}$ and ${(\mathrm{Si})}_{3}\mathrm{N}$ vibrational signatures, which result from insertion of N into Si-Si bonds. Our computational study of the formation of ${(\mathrm{Si})}_{2}\mathrm{N}\mathrm{H}$ structures indicates that subsurface insertion is more feasible if the strain imposed during the insertion in a Si dimer is attenuated by a ${(\mathrm{Si})}_{2}\mathrm{N}\mathrm{H}$ structure already inserted in the neighboring dimer along the same silicon dimer row. This cooperative reaction lowers the energetic requirements for subsurface insertion, providing a theoretical explanation for the mechanism of thermal decomposition of $\mathrm{N}{\mathrm{H}}_{3}$ on Si(100) and for other systems where subsurface migration is observed experimentally.