Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route

Abstract

Despite the fact that H-terminated, HF-etched Si crystals are the starting point for construction of most contemporary electronic devices, 1 little is known about the chemical reactions of H-terminated Si surfaces under ambient temperature and pressure. 2,3 Functionalization of Si without partial oxidation and/or formation of electrical defects is potentially important in fabricating improved electronic devices 4,5 as well as in measurement of charge transfer rate constants at semiconductor/ liquid contacts. 6 One recently described approach involves the reaction of HF-etched Si(111) with olefins and organic diacyl peroxides, in which formation of a self-assembled (near)monolayer of Si-alkyls was hypothesized. 2 We report here an alternative strategy to functionalize HF-etched Si surfaces involving halogenation and subsequent reaction with alkyl Grignard or alkyl lithium reagents. We report vibrational spectroscopic and temperature programmed desorption data which confirm that the alkyl groups are bonded covalently to the Si surface, and we demonstrate that such overlayer formation can impede the undesirable oxidation of Si in a variety of environments while providing surfaces of high electrical quality.The H-terminated Si surface 7 was first exposed to PCl 5 for 20-60 min at 80-100 °C, in chlorobenzene with benzoyl peroxide as the radical initiator. 8,9 Upon chlorination, the XP survey spectra (Figure 1) showed peaks at 270.2 ( 0.4 binding electron volts, BeV, (Cl 2s) and 199.3 ( 0.4 BeV (Cl 2p), indicating that this procedure yielded Cl on the surface. The high-resolution XP spectrum of the Si 2p peak of this surface displayed, in addition to the substrate Si signal, an additional peak located at 0.98 ( 0.12 BeV higher in binding energy (Figure 2) whose position and intensity was consistent with the formation of a surface Si-Cl bond. 10 Auger electron spectra (AES) also confirmed the presence of Cl on the silicon surface. High resolution electron energy loss spectra (HREELS) of this surface exhibited a characteristic peak at 560 cm -1 that was not present on the H-terminated Si surface, confirming the formation of covalent Si-Cl bonds at the surface. 11 Temperature programmed desorption spectra of the chlorinated surface showed dominant signals at 64 (SiCl), 71 (Cl 2 ), and 135 (SiCl 3 ) amu, peaking at 670 and 850 K, which is characteristic of chlorinated silicon surfaces. 10,12 The 560 cm -1 peak in the HREELS and the Cl peak in the AES disappeared following thermal desorption.Exposure of the chlorinated Si surface to alkyl-Li (RLi: R

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

“…A twostep chlorination/alkylation sequence on hydride-terminated silicon surfaces, as initially reported by Bansal et al, 74 has also been demonstrated on numerous occasions. 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.…”

“…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.…”

“…Ensuring that the coordination requirements of surface atoms are satisfied reduces the reactivity of the etched surface and allows for temporary handling of the sample under ambient conditions which are not stringent in terms of the presence of moisture and oxygen. Hydrogen is the most commonly used passivating agent for surface preparation, 72 but strategies relying on halogens such as iodine 73 and chlorine 74 have been also reported. Hydrideterminated silicon surfaces are typically prepared by chemical etching in fluoride-containing solutions, with dilute (o10%) hydrofluoric acid used to passivate the Si(100) orientation and ca.…”

“…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.…”

“…The layers obtained showed only minor degradation upon immersion in both concentrated aqueous potassium hydroxide and 40% aqueous ammonium fluoride solutions, as judged from attenuation of the methylene stretch vibrations in the FTIR spectrum. While this direct approach surely represents a simplified protocol in regards to the two-step procedure of Bansal et al, 74 conclusive evidence on its mechanism is still lacking. This lack of evidence is despite a suggestion of a role played by alkyl halide contaminants 159 from the Grignard solution proposed by Chazalviel and co-workers.…”