High-Resolution X-ray Photoelectron Spectroscopic Studies of Alkylated Silicon(111) Surfaces

Abstract

Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.

High-Resolution Soft X-ray Photoelectron Spectroscopic Studies and Scanning Auger Microscopy Studies of the Air Oxidation of Alkylated Silicon(111) Surfaces

“…22 Consistently, <15% of a monolayer of silicon oxide is observable by transmission infrared spectroscopy on the freshly prepared Clterminated Si(111) surface. 23 The O 1s XPS signal could arise from a number of sources, including adsorbed solvent from the wet chemical preparation techniques, adsorbed pump oil vapor introduced in the quick-entry load lock, or contaminating dust particles covered with oxygen-containing organic molecules that were not possible to avoid when working in standard laboratory conditions.…”

“…surface Si atoms bonded to the methyl carbon atom, 22 did not shift substantially in binding energy or intensity. A broad feature, centered on the Si 3+ oxidation state at +3.10 eV above the bulk Si 2p 3/2 peak, was evident and had an equivalent coverage of 0.20 ML.…”

“…After 12 h in air (Figure 1b), the monolayer coverage of the peak approximately 0.8 eV higher in binding energy than the bulk peak, assigned to surface Si bonded to Cl atoms, dropped from 0.99 ML 22 to 0.87 ML, the 1.37 eV signal also decreased in intensity, and a broad signal of oxide, centered at +3.13 eV above the bulk Si 2p 3/2 peak, appeared. This oxide feature was accompanied by smaller suboxides 1.85 and 3.74 eV, respectively, above the bulk Si 2p 3/2 peak.…”

Transmission Infrared Spectroscopy of Methyl- and Ethyl-Terminated Silicon(111) Surfaces

“…This behavior indicates that all of the TIRS-observable Si-Cl bonds initially present on the surface have been removed during the alkylation reaction, in agreement with previous XPS observations of this surface. 7 On the CH 3 -terminated Si surface shown in Figure 3, a small, broad peak centered near 2070 cm -1 was observed when TIRS was collected at an incident IR beam angle of 74°off surface normal. The integrated area of this peak was <10% of the surface coverage of the freshly etched H-terminated Si(111) surface reported in Table 1.…”

“…This observation supports previous XPS observations that all Cl is removed from the Si surface during exposure to the alkylmagnesium halide solution, even for functional groups in which not all atop Si(111) atoms can be bonded to the alkyl moiety because of the steric bulk of the alkyl group. 7 The data from the Si-Cl stretching mode (583 cm -1 ) were not as conclusive, although measurement of the area under this peak could have been hampered by interference from the Si-Si phonon region directly above 600 cm -1 .…”