Effect of Covalent Surface Functionalization of Si on the Activity of Trifluoromethanesulfonic Anhydride for Suppressing Surface Recombination.

An examination of the efficacy of combining physisorbed and chemisorbed passivation strategies on crystalline Si has been performed. This report compares the influence of a linear alkyl adsorbate tethered by either a Si-C or Si-Si linkage, prepared by reaction of Si(111) with organometallic Grignard reagents or organosilanes, respectively. These modified surfaces are first analyzed and compared by IR and X-ray photoelectron spectroscopies. Their behavior toward a known potent physisorbate, trifluoromethanesulfonic anhydride (Tf2O), is then examined. Microwave photoconductivity measurements were obtained which indicate that, while Tf2O shows a beneficial lowering of surface recombination on both surface types initially, only surfaces featuring Si-C linkages exhibit long-lasting suppressed surface recombination. The data for Grignard-treated Si after exposure to Tf2O in fact represent the longest known report of surface recombination suppression by a physisorbate. Conversely, the data for the Si surfaces prepared by dehydrogenative coupling suggest that these passivating groups themselves introduce defect states that cannot be ameliorated by Tf2O physisorption.