Interaction of Si2H6 with a Si(111)-77 surface

Abstract

The kinetics and mechanism of Si2H6 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 s0=0.30.1. No additional adsorption states for hydrogen appear in H2 TDS as compared to Si(111)/H. Adsorption at room temperatures leads to a saturation coverage 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 300600 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, Si2H6 adsorbs into this physisorbed state, which upon annealing either desorbs or dissociates in the temperature range between 80 and 170 K. Dissociation of adsorbed Si2H6 occurs via breaking of the Si-Si bond. An unassigned vibrational feature at 760 cm-1 is observed after the annealing to 170 K which has not been observed in other Si/H systems. Further fragmentation into different SiHn species is observed between 170 and 600 K. Beyond the desorption temperature for the 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 Si2H6 molecule is progressively fragmented. © 1989 The American Physical Society.