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

Abstract

Transmission infrared spectroscopy (TIRS) has been used to investigate the surface-bound species formed in the two-step chlorination/alkylation reaction of crystalline (111)-oriented Si surfaces. Spectra were obtained after hydrogen termination, chlorine termination, and reaction of the Cl-Si(111) surface with CH 3 MgX or C 2 H 5 MgX (X ) Cl, Br) to form methyl (CH 3 )-or ethyl (C 2 H 5 )-terminated Si(111) surfaces, respectively. Freshly etched H-terminated Si(111) surfaces that were subsequently chlorinated by immersion in a saturated solution of PCl 5 in chlorobenzene were characterized by complete loss of the Si-H stretching and bending modes at 2083 and 627 cm -1 , respectively, and the appearance of Si-Cl modes at 583 and 528 cm -1 . TIRS of the CH 3 -terminated Si(111) surface exhibited a peak at 1257 cm -1 polarized perpendicular to the surface assigned to the C-H symmetrical bending, or "umbrella" motion, of the methyl group. A peak observed at 757 cm -1 polarized parallel to the surface was assigned to the C-H rocking motion. Alkyl C-H stretch modes on both the CH 3 -and C 2 H 5 -terminated surfaces were observed near 2900 cm -1 . The C 2 H 5 -terminated Si(111) surface additionally exhibited broad bands at 2068 and 2080 cm -1 , respectively, polarized perpendicular to the surface, as well as peaks at 620 and 627 cm -1 , respectively, polarized parallel to the surface. These modes were assigned to the Si-H stretching and bending motions, respectively, resulting from H-termination of surface atoms that did not form Si-C bonds during the ethylation reaction. I. IntroductionAlkylation of silicon(111) surfaces has attracted significant recent attention due to the potential for obtaining molecular level control over the electrical, electrochemical, and chemical properties of Si surfaces. 1-5 Hydrogen-terminated Si(111) surfaces have a very low surface electron-hole recombination velocity, 6 but oxidize rapidly in ambient air, forming a large number of surface electronic trap states. 2 In contrast, alkylated Si surfaces prepared through a two-step chlorination/alkylation procedure show excellent chemical stability and low surface charge carrier recombination rates for extended time periods, even as they are exposed to ambient air. 1,2 Methyl moieties are the only saturated hydrocarbon group which can sterically form a Si-C bond to every atop site on an unreconstructed Si(111) surface, because van der Waals interactions between neighboring methylene units should prevent C 2 H 5 -and other alkyls from terminating every Si atop site. 7 Furthermore, the sp 3 -hybridized Si-C bond on the Si(111) surface should orient the methyl groups on the CH 3 -terminated surfaces normal to the surface plane, as shown in Chart 1. This orientation should produce diagnostic signatures and polarizations in the vibrational spectrum of such surfaces. Recent scanning tunneling microscopy 8 and low-energy electron diffraction studies 9 have revealed both short-range and long-range order on CH 3 -terminated Si surfaces. However, at pr...

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

“…Although nonmethyl saturated alkyl groups cannot sterically cover every atop site on the Si surface, due to van der Waals interactions between methylene groups on adjacent alkyls, freshly prepared alkylated Si surfaces showed no detectable oxide (i.e., <10% of a monolayer) even with the surface-sensitive SXPS methodology. Infrared spectroscopy has shown that the spectroscopically active Si sites on such surfaces that are not terminated with Si-C bonds are terminated with Si-H bonds, 26 consistent with the small but observable chemical shift of the surface Si atoms in the SXPS data reported herein (Figures 3-5) and reported previously. 22 Because H-Si(111) and CH 3 -Si(111) surfaces both show very low surface recombination velocities, it is reasonable that a mixed Si-H/Si-C terminated surface would also exhibit a low surface recombination velocity.…”

Helium atom diffraction measurements of the surface structure and vibrational dynamics of CH3–Si(111) and CD3–Si(111) surfaces

“…This procedure is known to produce atomically flat, H-terminated Si͑111͒ surfaces. 36,38 After etching, the samples were transferred to an N 2 -purged glove-box that contained less than 10 ppm of O 2 .…”

“…30,33,34 The vibrational spectra of the CH 3 -Si͑111͒ surface have been measured by transmission infrared spectroscopy ͑TIRS͒ and by high-resolution electron energy loss spectroscopy ͑HREELS͒, allowing the assignment of the vibrational modes of this interface. [34][35][36] These spectroscopic techniques have provided functional group analysis and a detailed picture of the Si-C covalent bonding. A photoelectron spectroscopic study has shown that methylation creates a surface dipole, slightly shifting the silicon valence and conduction band edges at the surface.…”

The interaction of organic adsorbate vibrations with substrate lattice waves in methyl-Si(111)-(1 × 1)

“…Relative to the hydrogenterminated Si(111) surface, methyl-terminated Si(111) surfaces exhibit enhanced resistance to oxidation in air. 11,12 The vibrational features of methyl-terminated silicon surfaces impact the thermal properties and the ability of the surface to accommodate energy, so these vibrational properties have recently been studied both experimentally [13][14][15][16][17][18][19] and theoretically. 17,20 A comprehensive analysis of the surfacephonon dispersion relations along the entire surface Brillouin a) Author to whom correspondence should be addressed.…”