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

Abstract

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si + , Si 2+ , Si 3+ , and Si 4+ were detected on the Cl-terminated surface, with the highest oxidation state (Si 4+ ) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p 3/2 peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monolyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH 3 -, C 2 H 5 -, or C 6 H 5 CH 2 -terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed ≈0.20 ML of oxide. This oxide was principally composed of Si + and Si 3+ species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p 3/2 peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7°vs e0.5°off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.

Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si–C bonds: surface preparation, passivation and functionalization

“…As the authors concluded, 150 their observations strongly support an oxidative protective mechanism largely due to Si-C bonds disrupting the native oxide growth mechanism 80 and not exclusively resulting from limited access of oxygenated species to the substrate. 32,115,152 Despite the vast literature now available on the tandem chlorination/alkyl Grignard route, 74,102,109,114,124,145,148,150 and the general agreement over the high quality of the reaction product, the most obvious limitation of this methodology appears to be the complicated and inefficient (see Section 3.1) derivatization of films having a C n H 2n+1 structure. Further, the need for stringent conditions (such as the exclusion of water and oxygen) for both the chlorination (generally phosphorus(V) chloride with radical initiators) and the Grignard reaction might discourage the routine use of this two-step strategy.…”

“…31 As such, regardless of the chemical strategy used, complete passivation of the Si (111) surface cannot be achieved due to van der Waals interactions between methylene units of adjacent alkyl chains. 31,150 Second, alkylated and methylated Si(111) surfaces prepared via this route have a satisfactory stability in air 150 and show remarkable electrical properties, 114,145 with the lowest surface recombination velocity (o25 cm s À1 ) reported to date for a passivated (non-hydride-terminated 72 ) silicon surface. 151 The low recombination velocity of charge carriers is supporting evidence that there is a negligible number of surface defect sites (trap density o3 Â 10 À9 cm À2 ).…”

“…Most importantly, this method is claimed to allow for the complete passivation of Si surface sites of the unreconstructed Si(111)-H surface. 74,144,145,150 As schematically depicted in Fig. 6 the distance between Si surface sites on the Si(111) surfaces is 3.8 Å , 95 while the van der Waals radius of a methyl group is ca.…”

“…This method generally results in ordered, 144 electrically and chemically well-passivated alkyl Si (111) 82,109,114,124,[144][145][146][147][148][149][150] and Si(100) 102 surfaces (Fig. 6).…”

“…6), and fully-passivated Si(111)-CH 3 surfaces have been prepared. 109,144,150 Somewhat larger (ca. 4.5-5.0 Å ) is the van der Waals radius of the methylene units in longer alkyl chains (i.e.…”