Electronic excited states of Si(100) and organic molecules adsorbed on Si(100)

The electronically excited states of the Si(100) surface and acetylene, benzene, and 9,10-phenanthrenequinone adsorbed on Si(100) are studied with time-dependent density functional theory. The computational cost of these calculations can be reduced through truncation of the single excitation space. This allows larger cluster models of the surface in conjunction with large adsorbates to be studied. On clean Si(100), the low-lying excitations correspond to transitions between the pi orbitals of the silicon-silicon dimers. These excitations are predicted to occur in the range 0.4-2 eV. When organic molecules are adsorbed on the surface, surface --> molecule, molecule --> surface, and electronic excitations localized within the adsorbate are also observed at higher energies. For acetylene and benzene, the remaining pipi* excitations are found to lie at lower energies than in the corresponding gas-phase species. Even though the aromaticity of 9,10-phenanthrenequinone is retained, significant shifts in the pipi* excitations of the aromatic rings are predicted. This is in part due to structural changes that occur upon adsorption.     

Nanoporosity of Si (100) bars

Si(100) samples cut from a typical bar (100 mm in diameter) prepared using industrial technology are studied. Measurements of the electron work function (EWF) show that the size effects in these samples (a reduction in thickness along with a sample’s area and the EWF) detected earlier were due to nanostructure porosity that was buried by the technological treatment of a bar’s surface. This hidden nanoporosity is assumed to be a manifestation of the secondary crystal structure.     

Grafting cavitands on the Si(100) surface

Cavitand molecules having double bond terminated alkyl chains and different bridging groups at the upper rim have been grafted on H-terminated Si(100) surface via photochemical hydrosilylation of the double bonds. Pure and mixed monolayers have been obtained from mesitylene solutions of either pure cavitand or cavitand/1-octene mixtures. Angle resolved high-resolution X-ray photoelectron spectroscopy has been used as the main tool for the monolayer characterization. The cavitand decorated surface consists of Si-C bonded layers with the upper rim at the top of the layer. Grafting of pure cavitands leads to not-well-packed layers, which are not able to efficiently passivate the Si(100) surface. By contrast, monolayers obtained from cavitand/1-octene mixtures consist of well-packed layers since they prevent silicon oxidation after aging. AFM measurements showed that these monolayers have a structured topography, with objects protruding from the Si(100) surface with average heights compatible with the expected ones for cavitand molecules.     

Facet-dependent lithium intercalation into Si crystals: Si(100) vs. Si(111)

Fast Li transport in battery electrodes is essential to meeting the demanding requirements for a high-rate capability anode. We studied the intercalation of a Li atom into the surface and subsurface layers of Si(100) and Si(111) using density functional calculations with a slab representation of the surfaces. We suggest that the Li atom migrates on the Si surfaces and is subsequently inserted into the inside for both Si(100) and Si(111). The rate-determining steps are the surface incorporation and subsurface diffusion in Si(100) and Si(111), respectively. Our diffusion rate calculations reveal that, once the Li atom is incorporated into the Si surface, Li diffuses faster by at least two orders of magnitude along the <100> direction than along the <111> direction. The importance of careful treatment of the slab thickness for the study of impurity insertion into subsurface layers is also stressed.     

Adsorption of trisilylamine on the Si(100) surface

Adsorption of trisilylamine (TSA) on the Si(100) surface has been studied using temperature programmed desorption (TPD) and time‐of‐flight electron stimulated desorption (TOFESD). TPD spectra exhibit the presence of three desorption states denoted by β₁, β₂, and β₃ associated with the presence of a mono‐, di‐, and tri‐hydride state respectively. This behavior is identical with previously observed desorption studies resulting from atomic hydrogen adsorption, indicating that the nitrogen species in the adsorbate has minimal impact on the surface structure of the hydride. Preliminary electron irradiation studies are reported and indicate that the formation of a thin silicon nitride layer is induced as a result of the irradiation. Copyright © 2008 John Wiley & Sons, Ltd.     

Competitive segregation of Si and P on Fe96.5Si3.5 (100) and (110)

Annealing an Fe(96.5)Si(3.5) (100)/(110) bicrystal, containing 90 ppm P, leads immediately to a strong segregation of silicon. The Si atoms, however, desegregate subsequently and are displaced by P, whose segregation enthalpy is larger than that of silicon. The corresponding surface structures formed on both faces have been studied using complementary methods: Scanning tunneling microscopy (STM) to obtain atomically resolved geometrical information and Auger electron spectroscopy (AES) for the determination of the surface composition. Si substitutes surface Fe atoms on both faces and forms ordered surface alloys, whereas P occupies hollow sites on the surface. Si and P form c(2 x 2) superstructures on the (100) surface, whereby each segregated phosphorus atom blocks in the average one silicon segregation site. The (110) surface, on the other hand, is characterized by a c(1 x 3) Si superstructure. Due to the anisotropy of this surface the P/Si exchange proceeds by the formation of silicon coverage decreasing domain boundaries within the silicon structure, which are simultaneously occupied by P atoms. Furthermore the comparison of the AES and STM derived phosphorus coverages indicates a P multilayer segregation on the (110) surface.     

C59Si on the monohydride Si(100):H-(2 x 1) surface

We propose the use of the Si atom in the experimentally observed C59Si molecule as a possible way to controllably anchor fullerene molecules on a Si surface, due to the formation of a strong bond to one of the Si surface atoms. All our results are based on ab initio total energy density functional theory, and we obtain that the binding energy is on the order of 2.1 eV, approximately 1.4 eV more stable than a C60 bonded in a similar situation. A possible route to obtain such adsorption via a (C59Si)2 dimer is examined, and we find the whole process to be exothermic by approximately 0.2 eV.     

Multiredox tetrathiafulvalene-modified oxide-free hydrogen-terminated Si(100) surfaces

Tetrathiafulvalene (TTF) monolayers covalently bound to oxide-free hydrogen-terminated Si(100) surfaces have been prepared from the hydrosilylation reaction involving a TTF-terminated ethyne derivative. FTIR spectroscopy characterization using similarly modified porous Si(100) substrates revealed the presence of vibration bands assigned to the immobilized TTF rings and the Si-C═C- interfacial bonds. Cyclic voltammetry measurements showed the presence of two reversible one-electron systems ascribed to TTF/TTF(.+) and TTF(.+)/TTF(2+) redox couples at ca. 0.40 and 0.75 V vs SCE, respectively, which compare well with the values determined for the electroactive molecule in solution. The amount of immobilized TTF units could be varied in the range from 1.7 × 10(-10) to 5.2 × 10(-10) mol cm(-2) by diluting the TTF-terminated chains with inert n-decenyl chains. The highest coverage obtained for the single-component monolayer is consistent with a densely packed TTF monolayer.     

Infrared Spectroscopic Analysis of Plasma-Treated Si(100)-Surfaces

 Infrared reflection spectroscopy (specular reflection, attenuated total reflection) has been applied in combination with spectroscopic ellipsometry and electron microscopy to analyze the surface structure of plasma-treated Si(100) surfaces. It is shown that plasma treatments in oxygen and fluorine or chlorine-containing gases cause the formation of a thin surface layer having thicknesses of a few nanometers. The layer was identified to consist of SiO₂ for treatments in an oxygen plasma. Analyses of layers formed by treatments in a fluorine-containing plasma do not confirm the generally assumed model. Different Si-F vibration modes were identified in the surface layer caused by a SF₆ plasma. They correlate, however, with SiF and SiF₂ molecules. There are no indications of the existence of the generally assumed SiF₄. Neither has SiOF₂ been proven in layers produced by etching in a SF₆/O₂ plasma.     

Photochemical desorption from chlorinated Si(100) and Si(111) surfaces — Mechanisms and models

Understanding the mechanisms controlling the anisotropy of microetching is particularly critical as the scale of semiconductor devices shrink. Defining complex, dynamic chemical systems such as halogen etching require microscopic measurements combining kinetics, dynamics, surface layer composition and micromorphology on prototypical surfaces. This study is concerned with two important variables in addition to spontaneous chemical etching, the role of electronic defects induced by high level doping in producing site-specefic reaction and the enhancement of etching by irradiation at low fluences.

Substitutional defects introduced by selective doping significantly influence the rate of chlorine etching by forming shallow electronic states that are ionized at room temperature¹. We have shown that chlorine sticking coeficients as well as laser-assisted etching are significantly affected by doping at very high dopant levels. Enhancement for n-type doping is consistent with the simple assumption that holes at the surface should enhance Si-Si surface bond breaking and in disagreement with the fact that heavily p-doped silicon has a higher chlorine sticking coefficient than n-doped material².

Carrier effects generated by photoirradiation with above bandgap photons are considerably more complex than simple doping. A depletion layer and associated electric field are set up at the surface and minority carriers are preferentially swept to the surface. The type of photocarrier present at the surface is determined by both the doping and the photoirradiation.

Using photoinduced etching of heavily doped Si(100) and Si(111) by chlorine at low laser fluences, we studied the mechanism of photostimulated desorption using core-level photoemission and time-of-flight measurements of the photoproducts². These results will be interpreted in terms of field-modified electron-hole transport together with carrier-modified chlorine adsorption and desorption.     

Cycloaddition of benzene on Si(100) and its surface conversions

A comprehensive ab initio study of the adsorption of benzene on the silicon(100) surface is presented. Five potential candidates ([2+2] adduct, [4+2] adduct, two tetra-sigma-bonded structures, and one radical-like structure) for the reaction product are examined to determine the lowest energy adsorption configuration. A [4+2] butterfly structure is determined to be the global minimum (-29.0 kcal/mol), although one of the two tetra-sigma-bonded structures (-26.7 kcal/mol) is similar in energy to it. Multireference perturbation theory suggests that the [4+2] addition mechanism of benzene on Si(100) is very similar to the usual Diels-Alder reaction (i.e., small or zero activation barrier), even though benzene adsorption entails the loss of benzene aromaticity during the reaction. On the other hand, the [2+2] cycloaddition mechanism is shown to require a relatively high activation barrier (17.8 kcal/mol), in which the initial step is to form a (relatively strongly bound) van der Waals complex (-8.9 kcal/mol). However, the net activation barrier relative to reactants is only 8.9 kcal/mol. Careful examination of the interconversion reactions among the reaction products indicates that the two tetra-sigma-bonded structures (that are energetically comparable to the [4+2] product) can be derived from the [2+2] adduct with activation barriers of 15.5 and 21.4 kcal/mol. However, unlike the previous theoretical predictions, it is found that the conversion of the [4+2] product to the tetra-sigma-bonded structures entails huge barriers (>37.0 kcal/mol) and is unlikely to occur. This suggests that the [4+2] product is not only thermodynamically the most stable configuration (lowest energy product) but also kinetically very stable (large barriers with respect to the isomerization to other products).     

Ultrathin cobalt silicide film formation on Si(100)

The interfacial reaction between ultrathin Co film and substrate Si(100) was studied by in situ XPS using monochromatized Al Kα. When the Co is deposited on Si(100) at room temperature, CoSi₂ is formed during the initial stage of Co deposition and then metallic Co starts to grow sequentially. For 8 ML Co deposition on Si(100) the interfacial reaction layer is relatively thin compared with the pure Co overlayer, which is not involved in the interfacial reaction in depth. The Co layers change rapidly to CoSi layers after annealing at 270°C, and then CoSi₂ layers form after annealing at 540°C for 2 min. The CoSi₂ layers are changed to CoSi₂ islands by post‐annealing at >650°C. Copyright © 2003 John Wiley & Sons, Ltd.     

Organic functionalization of the Si (100) and Ge (100) surfaces by cycloadditions of carbenes and nitrenes: a theoretical prediction

By means of density functional theory (B3LYP/6-31G*) coupled with effective cluster models, we predict that the well-known cycloaddition reactions of carbenes and nitrenes to alkenes in organic chemistry can be employed as a new type of surface reaction to organically functionalize the Si (100) and Ge (100) surfaces at low temperature. The well-established abundance of carbenes and nitrenes addition chemistry in organic chemistry provides versatile flexibility of functionalizing the surfaces of Si (100) and Ge (100), which can potentially impart new organic functionalities to the semiconductors surface for novel applications in a diversity of fields. Our predictions strongly advance the concept of using organic reactions to modify the solid surface in a controlled manner and quite intriguing chemistry can lie in the material featuring the analogous bonding motif. In further perspective, implications for other theoretical work, regarding disilenes, digermenes, silenes, and germenes that all feature the bonding motif similar to alkenes, are also discussed.     

DNA immobilization onto electrochemically functionalized Si(100) surfaces

N-type Si(1 0 0) surfaces were modified by reduction of 4-nitrobenzenediazonium through cyclic voltammetry. Contact mode AFM was employed to produce holes in the deposited layers and cross-sectional profiles were obtained to determine their thicknesses. Layer thickness was found to increase with the number of cyclic potential scans in both aqueous and non-aqueous media. In acetonitrile, the single scan thickness was determined to be approximately 15 nm, whereas for three scans the layer thickness was found to be approximately 35 nm. These thicknesses were also measured and confirmed by ellipsometry. Both thicknesses are indicative of multilayer formation on the silicon surface. Layers formed in acetonitrile were more uniform and of better quality (without holes), compared to those prepared in water. This type of functionalized surface, after further cyclic voltammetric reduction of the nitro groups and treatment with glutaraldehyde, was then used to immobilize single strand DNA-C₆H₁₂NH₂ probe sequences for hybridization with complementary DNA sequences. Fluorescein-labeled probe and target oligonucleotide sequences were used to validate the immobilization of the probe layer and hybridization with the complementary sequence. No binding was observed when using a non-complementary sequence as probe.     

Grazing incidence X-ray absorption spectra of (Si/W) 100/Si multilayer

Tungsten (W) M_III X-ray absorption spectra of a periodic multilayer, (Si/W)₁₀₀/Si, were measured with the change of X-ray grazing angle using sample current method. Under not total reflection condition, the absorbance changed little except at W M_III absorption edge. While under the total reflection condition, the absorbance increased with the increase of the X-ray energy and the increment changed from low to high at the W M_III absorption edge. This result reflected the variation of the X-ray evanescent wavelength caused by the absorption effect of W.     

Electrochemical "switching" of Si(100) modular assemblies

We report on a modular approach for producing well-defined and electrochemically switchable surfaces on Si(100). The switching of these surfaces is shown to change a Si(100) surface from resistant to cell adsorption to promoting cell adhesion. The electrochemical conversion of the modified electrode surface is demonstrated by X-ray photoelectron spectroscopy, X-ray reflectometry, contact angle and cell adhesion studies.     

Surface analytical characterization of oxide-free Si(100) wafer surfaces

Summary Wet-chemical cleaning procedures of Si(100) wafers are surface analytically characterized and compared. Hydrophobic surfaces show considerably less native oxides in comparison to hydrophilic surfaces.The growth of the oxide is determined as a function of exposure to air by means of XPS measurements. The chemically shifted Si2p XPS signal is utilized for the quantification of the growth kinetics.One hour after cleaning no chemically shifted Si2p XPS peak is discernible on the hydrophobic surfaces. Assuming homogeneous oxide growth, the detection limit of native oxides is estimated to be below 0.05 nm using an emission angle of 18° with respect to the wafer surface. The calculation of the oxide thickness from the chemically shifted and nonchemically shifted Si2p XPS peak intensities is carried out according to Finster and Schulze [1]. For more than a day after cleaning no surface oxides can be identified on the hydrophobic surfaces. The oxide growth kinetics is logarithmic. The very slow oxidation rate cannot be attributed to fluorine residues since no fluorine is seen by XPS. We explain the slow oxidation rate by a homogeneous hydrogen saturated Si(100) wafer surface.

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Oberflächenanalytische Charakterisierung oxidfreier Si(100)-Waferoberflächen

     

Thermal behavior of perfluoroalkylsiloxane monolayers on the oxidized Si(100) surface

The thermal stability of perfluoralkylsiloxane monolayers in a vacuum is investigated via X-ray photoelectron spectroscopy (XPS) for temperatures up to 600 degrees C. 1H,1H,2H,2H,-perfluorodecyltrichlorosilane (FDTS) monolayers are deposited on oxidized Si(100) surfaces from the vapor phase with various degrees of surface coverage. Significant monolayer desorption is observed to occur at temperatures below 300 degrees C regardless of the initial monolayer coverage. The desorption mechanism follows first-order kinetics and is independent of the initial coverage. Removal of FDTS is found to occur by the loss of the entire molecular chain, as evidenced by the fact that the CF(3)/CF(2) peak area ratios remain unaffected by the annealing process although CF(n)()/Si peak ratio declines with annealing. This is in sharp contrast to the behavior observed for octadecyltrichlorosilane monolayer for which elevated temperature leads to C-C bond breakage and successive shortening of the alkyl chain. It is also shown that the binding energy and the shape of the F 1s line are good indicators of the degree of disorder in the chain, as well as a measure of the interaction of the chain with the silicon surface.     

Density-functional study of the cycloaddition of acrylonitrile on the Si(100) surface

Using a density functional approach, we have explored the cycloaddition of acrylonitrile on the Si(100) surface. The buckling of the surface dimers characteristic for the (2x1) reconstructed surface is shown to favor structures with a dipolar moment such as the resonant form of acrylonitrile with cumulative double bonds. The bond of acrylonitrile via a single C atom is a possible intermediate leading to the nitrile structure of the adsorbed molecule.     

Structural and electron-transfer characteristics of carbon-tethered porphyrin monolayers on Si(100)

Structural and electron-transfer characteristics are reported for two classes of zinc porphyrin monolayers attached to Si(100) surfaces via Si-C bonds. One class, designated ZnP(CH(2))(n)- (n = 2-4), contains an alkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being p-tolyl groups. The other, designated ZnPPh(CH(2))(n)- (n = 0-3), contains a phenyl or phenylalkyl linker appended to the meso-position of the porphyrin, with the nonlinking substituents being mesityl groups. Both classes of zinc porphyrin monolayers on Si(100) were examined using Fourier transform infrared spectroscopy and various electrochemical methods. The studies reveal the following: (1) The structural and electron-transfer characteristics of the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers are generally similar to those of monolayers formed from porphyrins with analogous linkers, but anchored with an O, a S, or a Se atom. (2) The ZnP(CH(2))(n)-, ZnPPh-, and ZnPPhCH(2)- monolayers exhibit lower saturation coverages and have their porphyrin ring more tilted with respect to the surface normal than the ZnPPh(CH(2))(2)- and ZnPPh(CH(2))(3)- monolayers. (3) The electron-transfer rates for both the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- classes of monolayers monotonically decrease as the length of the linker increases. (4) For all the ZnP(CH(2))(n)- and ZnPPh(CH(2))(n)- monolayers, both electron-transfer rates and charge-dissipation rates decrease monotonically as the surface coverage increases. Collectively, the studies reported herein provide a detailed picture of how the linker type influences the structural and electron-transfer characteristics of these general classes of monolayers.