Interaction ofSi2H6with a Si(111)-77 surface

Abstract

The kinetics and mechanism of ${\mathrm{Si}}_2$ ${\mathrm{H}}_6$ chemisorption on a clean Si(111)-7×7 surface have been studied under ultrahigh-vacuum conditions, by means of thermal desorption spectroscopy (TDS), low-energy electron diffraction (LEED), electron-energy-loss spectroscopy (EELS), and ultraviolet photoemission spectroscopy (UPS) measurements. Disilane adsorbs dissociatively at room temperature with an initial sticking coefficient $s_0$=0.3±0.1. No additional adsorption states for hydrogen appear in ${\mathrm{H}}_2$ TDS as compared to Si(111)/H. Adsorption at room temperatures leads to a saturation coverage ${\Theta }_{\mathrm{H}}$=0.3, at which LEED still shows a 7×7 pattern. The initial sticking coefficient exhibits a negative activation energy of -1.7 kcal/mol in the temperature range 300–600 K. A molecular precursor state exists for the dissociation of the molecule whose adsorption energy is lowered by the presence of chemisorbed hydrogen. At 80 K, ${\mathrm{Si}}_2$ ${\mathrm{H}}_6$ adsorbs into this physisorbed state, which upon annealing either desorbs or dissociates in the temperature range between 80 and 170 K. Dissociation of adsorbed ${\mathrm{Si}}_2$ ${\mathrm{H}}_6$ occurs via breaking of the Si-Si bond. An unassigned vibrational feature at 760 ${\mathrm{cm}}^{\mathrm{-}1}$ is observed after the annealing to 170 K which has not been observed in other Si/H systems. Further fragmentation into different ${\mathrm{SiH}}_{\mathrm{n}}$ species is observed between 170 and 600 K. Beyond the desorption temperature for the ${\beta }_2$ state, which is above 700 K, only monohydride SiH is left on the surface. The formation of the SiH species above 700 K is associated with the complete removal of the surface states of the Si(111)-7×7 in UPS, whereas the surface states at a lower annealing temperature just show a reduction in their intensity. The changes seen in UPS can be explained by a stepwise structural rearrangement of the 7×7 structure as the ${\mathrm{Si}}_2$ ${\mathrm{H}}_6$ molecule is progressively fragmented.