Spectroscopic studies of the modification of crystalline Si(111) surfaces with covalently-attached alkyl chains using a chlorination/alkylation method

A two-step procedure, involving radical-initiated chlorination of the Si su rface with PCl5 followed by reaction of the chlorinated surface with alkyl- Grignard or alkyl-lithium reagents, has been developed to functionalize cry stalline (1 1 1)-oriented H-terminated Si surfaces. The surface chemistry t hat accompanies these reaction steps has been investigated using X-ray phot oelectron spectroscopy (XPS), Auger electron spectroscopy (AES), temperatur e programmed desorption spectroscopy (TPDS), high-resolution electron energ y loss spectroscopy (HREELS), infrared (IR) spectroscopy in both glancing t ransmission (TIR) and attenuated total multiple internal reflection (ATR) m odes, ellipsometry, and contact angle goniometry. The XPS data show the app earance of the Cl signal after exposure to PCl5 and show its removal, and c oncomitant appearance of a C Is signal, after the alkylation step. Auger el ectron spectra, in combination with TPD spectroscopy, demonstrate the prese nce of Cl after the chlorination process and its subsequent loss after ther mal desorption of Si-Cl fragments due to heating the Si surface to 1200 K. High-resolution XP spectra of the Si 2p region show a peak corresponding to Si-Cl bond formation after the chlorination step, and show the subsequent disappearance of this peak after the alkylation step. IR spectra show the l oss of the perpendicularly polarized silicon monohydride (Si-H) vibration a t 2083 cm(-1) after the chlorination step, whereas HREELS data show the app earance of vibrations due to Si-Cl stretches upon chlorination of the Si su rface. The HREELS data furthermore show the disappearance of the Si-Cl stre tch and the appearance of a Si-C vibration at 650 cm(-1) after alkylation o f the Si surface. Ellipsometric measurements indicate that the thickness of the alkyl overlayer varies monotonically with the length of the alkyl grou p used in the reactant. Contact angle and IR measurements indicate that the packing of alkyl groups in the monolayers produced by this method is less dense than that found in alkylthiol monolayers on Au. As determined by XPS, the alkylated surfaces show enhanced resistance to oxidation by various we t chemical treatments, compared to the H-terminated Si surface. The two-ste p reaction sequence thus provides a simple approach to functionalization of (1 1 1)-oriented, H-terminated silicon surfaces using wet chemical methods .