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 = CH3, C2H5, C4H9, C6H5, or CH2C6H5) 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, SiH2, and SiH3 species, whereas Cl-Si(100) surfaces were predominantly terminated by monochloro (SiCl and SiHCl) and dichloro (SiCl2 and SiHCl2) Si species. Methylation produced signals consistent with termination by Si-alkyl bonding arising from SiH(CH3)-, SiH2(CH3)-, and Si(CH3)2-type species.     

High-resolution soft X-ray photoelectron spectroscopic studies and scanning auger microscopy studies of the air oxidation of alkylated silicon(111) surfaces

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 monoleyers (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 approximately 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 degrees vs < or =0.5 degrees 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.     

Highly stable organic modification of Si(111) surfaces: towards reacting Si with further functionalities while preserving the desirable chemical properties of full Si-C atop site terminations

Si(111) surfaces modified by covalent Si-CH=CH-CH3 functionality, with no intervening oxide, show coverage of all atop Si sites and superior chemical stability in ambient conditions and water, as compared with molecules that form nearly full coverage (e.g., Si-C C-CH3 and Si-CH3 surfaces).     

Chemical and electrical passivation of silicon (111) surfaces through functionalization with sterically hindered alkyl groups

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using sterically bulky alkyl groups such as (CH3)2CH- (iso-propyl), (CH3)3C- (tert-butyl), and C6H5- (phenyl) moieties. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.9 binding eV, consistent with carbon bonded to Si. The C 1s XPS data indicated that the alkyls were present at lower coverages than methyl groups on CH(3)-terminated Si(111) surfaces. Despite the lower alkyl group coverage, no Cl was detected after alkylation. Functionalization with the bulky alkyl groups effectively inhibited the oxidation of Si(111) surfaces in air and produced low (<100 cm s(-1)) surface recombination velocities. Transmission infrared spectroscopy indicated that the surfaces were partially H-terminated after the functionalization reaction. Application of a reducing potential, -2.5 V vs Ag+/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon-carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si-H bonds on the surface.     

Low-temperature STM images of methyl-terminated Si(111) surfaces

Low-temperature scanning tunneling microscopy (STM) has been used to image CH(3)-terminated Si(111) surfaces that were prepared through a chlorination/alkylation procedure. The STM data revealed a well-ordered structure commensurate with the atop sites of an unreconstructed 1 x 1 overlayer on the silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 +/- 0.01 nm, which are assigned to adjacent H atoms on the same methyl group. The C-H bonds in each methyl group were observed to be rotated by 7 +/- 3 degrees away from the center of an adjacent methyl group and toward an underlying Si atom. Hence, the predominant interaction that determines the surface structure arises from repulsions between hydrogen atoms on neighboring methyl groups, and secondary interactions unique to the surface are also evident.     

Scanning tunneling microscopy of ethylated Si(111) surfaces prepared by a chlorination/alkylation process

Scanning tunneling microscopy (STM) and computational modeling have been used to study the structure of ethyl-terminated Si(111) surfaces. The ethyl-terminated surface was prepared by treating the H-terminated Si(111) surface with PCl5 to form a Cl-terminated Si(111) surface with subsequent exposure to C(2)H(5)MgCl in tetrahydrofuran to produce an alkylated Si(111) surface. The STM data at 77 K revealed local, close-packed, and relatively ordered regions with a nearest-neighbor spacing of 0.38 nm as well as disordered regions. The average spot density corresponded to approximately 85% of the density of Si atop sites on an unreconstructed Si(111) surface. Molecular dynamics simulations of a Si(111) surface randomly populated with ethyl groups to a total coverage of approximately 80% confirmed that the ethyl-terminated Si(111) surface, in theory, can assume reasonable packing arrangements to accommodate such a high surface coverage, which could be produced by an exoergic surface functionalization route such as the two-step chlorination/alkylation process. Hence, it is possible to consistently interpret the STM data within a model suggested by recent X-ray photoelectron spectroscopic data and infrared absorption data, which indicate that the two-step halogenation/alkylation method can provide a relatively high coverage of ethyl groups on Si(111) surfaces.     

Quantum mechanics calculations of the thermodynamically controlled coverage and structure of alkyl monolayers on Si(111) surfaces

The heat of formation, Delta E, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH3- (methyl), C2H5- (ethyl), (CH3)2CH- (isopropyl), (CH3)3C- (tert-butyl), CH3(CH2)5- (hexyl), CH3(CH2)7- (octyl), and C6H5- (phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, Delta G, for the formation of both Si-C and Si-H bonds from Si-Cl model compounds was also calculated using QM, with four separate Si-H formation mechanisms proposed, to give overall Delta G(S) values for the formation of alkylated Si(111) surfaces through a two step chlorination/alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si-H free energies of formation, Delta G(H), corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C=C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.     

Photochemical-controlled switching based on azobenzene monolayer modified silicon (111) surface

Azobenzene-containing compounds were covalently attached onto Si(111) surfaces via Si-O linkages using a two-step procedure. The modified Si(111) surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy measurements. The monolayer surface showed preferably chemical stability. Switchable photoisomerizability of azobenzene molecules on these modified surfaces was observed in response to alternating UV and visible light exposure. The measured conductivity showed distinct difference with trans and cis forms of azobenzene compounds on as-modified Si(111) surfaces.     

Transmission infrared spectroscopy of methyl- and ethyl-terminated silicon(111) surfaces

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.     

-NH- termination of the Si(111) surface by wet chemistry

For over a quarter of a century the hydrogen-terminated Si(111) single-crystalline surface has been the gold standard as a starting point for silicon surface modification chemistry. However, creating a well-defined and stable interface based on Si-N bonds has remained elusive. Despite the fact that azides, nitro compounds, and amines do lead to the formation of surface Si-N, each of these modification schemes produces additional carbon- or oxygen-containing functional groups that in turn react with the surface itself, leaving contaminants that affect the interface properties for any further modification protocols. We describe the preparation of a Si(111) surface functionalized predominantly with Si-NH-Si species based on chlorination followed by the room temperature ammonia treatment utilizing NH(3)-saturated tetrahydrofuran (THF). The obtained surface has been characterized by infrared spectroscopy and X-ray photoelectron spectroscopy. This analysis was supplemented with DFT calculations. This newly characterized surface will join the previously established H-Si(111) and Cl-Si(111) surfaces as a general starting point for the preparation of oxygen- and carbon-free interfaces, with numerous potential applications.     

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 PCl5 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 CnH(2n+1)- (n = 1 or 2) or C6H5CH2- 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 C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 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.     

Theoretical investigation of the structure and coverage of the Si(111)-OCH3 surface

The surface structure, strain energy, and charge profile of the methoxylated Si(111) surface, Si(111)-OCH3, has been studied using quantum mechanics, and the results are compared to those obtained previously for Si(111)-CH3 and Si(111)-C2H5. The calculations indicate that 100% coverage is feasible for Si(111)-OCH3 (similar to the methylated surface), as compared to only approximately 80% coverage for the ethylated surface. These differences can be understood in terms of nearest-neighbor steric and electrostatic interactions. Enthalpy and free energy calculations indicate that the formation of the Si(111)-OCH3 surface from Si(111)-H and methanol is favorable at 300 K. The calculations have also indicated the conditions under which stacking faults can emerge on Si(111)-OCH3, and such conditions are contrasted with the behavior of Si(111)-CH3 and Si(111)-CH2CH3 surfaces, for which stacking faults are calculated to be energetically feasible when etch pits with sufficiently long edges are present on the surface.     

Chemical force titrations of functionalized Si(111) surfaces

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.     

A non-oxidative approach toward chemically and electrochemically functionalizing Si(111)

A general method for the non-oxidative functionalization of single-crystal silicon(111) surfaces is described. The silicon surface is fully acetylenylated using two-step chlorination/alkylation chemistry. A benzoquinone-masked primary amine is attached to this surface via Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition ("click" chemistry). The benzoquinone is electrochemically reduced, resulting in quantitative cleavage of the molecule and exposing the amine terminus. Molecules presenting a carboxylic acid have been immobilized to the exposed amine sites. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), and contact angle goniometry were utilized to characterize and quantitate each step in the functionalization process. This work represents a strategy for providing a general platform that can incorporate organic and biological molecules on Si(111) with minimal oxidation of the silicon surface.     

Control of the stability, electron-transfer kinetics, and pH-dependent energetics of Si/H2O interfaces through methyl termination of Si(111) surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.     

In-situ FTIR studies on self-assembled monolayers of surfactant molecules adsorbed on H-terminated Si(111) surfaces in aqueous solutions

The adsorption of a surfactant, sodium di-2-ethylhexyl sulfosuccinate (SDES), [C4H9CH(C2H5)CH2OCO][C4H9CH(C2H5)CH2OCOCH2]CHSO3- Na+, in an aqueous solution on an atomically flat H-terminated Si(111) [abbreviated as H-Si(111)] surface with a hydrophobic property was investigated by in-situ FTIR measurements. Immersion of the H-Si(111) surface in a solution of 1.0 x 10(-2) M SDES for more than 2 h led to formation of a self-assembled monolayer (SAM) with the alkyl chains having a tendency to be assembled perpendicular to the Si surface. The in-situ FTIR observation revealed that the adsorption was nearly complete about 60 min after the start of the immersion, and after that the adsorbed molecules changed their arrangement into an ordered mode. The Si-H peak in the FTIR spectrum remained unchanged with time in aqueous surfactant solution, in contrast to the case of immersion in pure water, indicating that the adsorbed surfactant protects the H-Si(111) surface from oxidation. No structural change in the SAM was observed when a negative potential of -700 mV vs Ag/AgCl was applied to the Si, whereas the adsorbed molecules changed their arrangement, accompanied by their substantial desorption and oxidation of the Si surface, when a positive potential of +700 mV was applied.     

Silicon surfaces as electron acceptors: dative bonding of amines with Si(001) and Si(111) surfaces.

The bonding of the trimethylamine (TMA) and dimethylamine (DMA) with crystalline silicon surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and density-functional computational methods. XPS spectra show that TMA forms stable dative-bonded adducts on both Si(001) and Si(111) surfaces that are characterized by very high N(1s) binding energies of 402.2 eV on Si(001) and 402.4 eV on Si(111). The highly ionic nature of these adducts is further evidenced by comparison with other charge-transfer complexes and through computational chemistry studies. The ability to form these highly ionic charge-transfer complexes between TMA and silicon surfaces stems from the ability to delocalize the donated electron density between different types of chemically distinct atoms within the surface unit cells. Corresponding studies of DMA on Si(001) show only dissociative adsorption via cleavage of the N-H bond. These results show that the unique geometric structures present on silicon surfaces permit silicon atoms to act as excellent electron acceptors.     

Bonding structure of phenylacetylene on hydrogen-terminated Si(111) and Si(100): surface photoelectron spectroscopy analysis and ab initio calculations

Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s → π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH(2)) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.     

Semicarbazide-functionalized Si(111) surfaces for the site-specific immobilization of peptides

The covalent attachment of semicarbazide-functionalized layers to hydrogen-terminated Si(111) surfaces is reported. The surface modification, based on the photoinduced hydrosilylation of a Si(111) surface with protected semicarbazide-functionalized alkenes, was investigated by means of X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). The removal of the protecting group yielded a semicarbazide-terminated monolayer which was reacted with peptides bearing a glyoxylyl group for site-specific alpha-oxo semicarbazone ligation.     

Highly stable organic monolayers for reacting silicon with further functionalities: the effect of the C-C bond nearest the silicon surface

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using various organic molecules having similar backbones but differing in their C-C bond closest to the silicon surface (i.e., C-C vs C=C vs C[triple bond]C bonds). X-ray photoelectron spectroscopic (XPS) data show that functionalization of silicon surfaces with propenyl magnesium bromide (CH3-CH=CH-MgBr) organic molecules gives nearly full coverage of the silicon atop sites, as on methyl- and propynyl-terminated silicon surfaces. Propenyl-terminated silicon surface shows less surface oxidation and is more robust against solvent attacks when compared to methyl- and propynyl-terminated silicon surfaces. We also show a secondary functionalization process of propenyl-terminated silicon surface with 4'-[3-Trifluoromethyl-3H-diazirin-3-yl]-benzoic acid N-hydroxysuccinimide ester [TDBA-OSu] cross-linker. The Si-CH=CH-CH3 surfaces thus offer a means of attaching a variety of chemical moieties to a silicon surface through a short linking group, enabling applications in molecular electronics, energy conversion, catalysis, and sensing.