THE ADSORPTION OF DISILANE ON SI(111)-7X7 AS STUDIED BY MULTIPLE INTERNAL-REFLECTION SPECTROSCOPY

The adsorption of Si2H6 on Si(111)-7 x 7 has been studied between 120 and 500 K using multiple internal reflection infrared spectroscopy. Si2H6 adsorbs dissociatively on the Si(111)-7 x 7 surface at temperatures as low as 120 K. The adsorption occurs by Si-Si bond scission resulting in the formation of SiH3 species. Evidence for molecular multilayer formation at 120 K is indicated by the linear increase in integrated intensity of the absorption features attributed to molecular Si2H6 with increasing exposure and by significant lineshape differences as compared with chemisorbed SiH3. By use of the polarization selection rules for infrared spectroscopy, we can identify the SiH3 symmetric stretch at 2154 cm-1 and the asymmetric stretch at 2130 cm-1 at 150 K. Ab initio cluster calculations suggest that a highly strained SiH3 surface molecule with an expanded Si-Si-H bond angle will result in the observed splitting that we observe for the two modes. At or above 250 K the infrared spectra suggest that Si-H bond scission occurs in conjunction with Si-Si bond breaking. Exposures of disilane at 400 K result in a surface that is dominated by surface monohydrides with some higher hydrides possibly present. At an adsorption temperature of 500 K the infrared spectra show only one strong absorption feature at 2090 cm-1 with significant intensity parallel to the surface indicating the presence of only tilted silicon monohydride.