Chemical and Electrical Passivation of Single-Crystal Silicon(100) Surfaces through a Two-Step Chlorination/Alkylation Process

Single-crystal Si(100) surfaces have been functionalized by using a two-step radical chlorination−Grignard (R= MgCl, R = CH_3, C_2H_5, C_4H_9, C_6H_5, or CH_2C_6H_5) alkylation method. After alkylation, no chlorine was detectable on the surface by X-ray photoelectron spectroscopy (XPS), and the C 1s region showed a silicon-induced peak shift indicative of a Si−C bond. The relative intensity of this peak decreased, as expected, as the steric bulk of the alkyl increased. Despite the lack of full alkyl termination of the atop sites of the Si(100) surface, functionalization significantly reduced the rate of surface oxidation in air compared to that of the H-terminated Si(100) surface, with alkylated surfaces forming less than half a monolayer of oxide after over one month of exposure to air. Studies of the charge-carrier lifetime with rf photoconductivity decay methods indicated a surface recombination velocity of <30 cm s^(-1) for methylated surfaces, and <60 cm s^(-1) for Si surfaces functionalized with the other alkyl groups evaluated. Soft X-ray photoelectron spectroscopic data indicated that the H−Si(100) surfaces were terminated by SiH, SiH_2, and SiH_3 species, whereas Cl−Si(100) surfaces were predominantly terminated by monochloro (SiCl and SiHCl) and dichloro (SiCl_2 and SiHCl_2) Si species. Methylation produced signals consistent with termination by Si−alkyl bonding arising from SiH(CH_3)-, SiH_2(CH_3)-, and Si(CH_3)_2-type species

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

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 PCl_5 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 C_nH_(2n+1)^− (n = 1 or 2) or C_6H_5CH_2^− 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 C_6H_5CH_2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p_(3/2) peak ∼0.20 eV higher in energy than the bulk Si 2p_(3/2) peak. This feature had an integrated area of ∼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