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

     

Unidirectional Pt silicide nanowires grown on vicinal Si(100)

We investigated the structure and electronic properties of unidirectional Pt(2)Si nanowires (NWs) grown on a Si(100)-2 degrees off surface. We found that Pt(2)Si NWs were formed along the step edges of the Si(100)-2 degrees off surface with c(4x6) reconstructions that occurred on the terraces of Si(100) using scanning tunneling microscopy and the structure of formed NWs was found to be Pt(2)Si by core-level photoemission spectroscopy. Moreover, we confirmed that the electronic band structures of the NWs along the NW direction are different from those perpendicular to the NWs and the surface state induced by the Pt(2)Si NWs was observed with a small density of state using the angle-resolved photoemission spectra.     

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).     

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.     

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.     

Lead underpotential deposition on gold single-crystal surfaces: The (100) face and its vicinal faces

Results concerning underpotential deposition of lead on gold single-crystal faces are presented. The (100) face and its vicinal faces are studied by cyclic voltammograms, potential step measurements and phase shift measurements. The explanation of the complex CV of the (100) face is attempted; adsorption, reconstruction and nucleation processes are discussed. Some details concerning the single-crystal techniques are given in the Appendix.     

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.     

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.     

Characterization of enantiospecific chemisorption on chiral Cu surfaces vicinal to Cu(111) and Cu(100) using density functional theory

Surfaces of simple fcc metals such as Cu with nonzero and unequal Miller indices are intrinsically chiral. Density functional theory (DFT) calculations are a useful way to study the enantiospecific adsorption of small chiral molecules on these chiral metal surfaces. We report DFT calculations of seven chiral molecules on several structurally distinct chiral Cu surfaces. These surfaces include two surfaces with (111)-oriented terraces and one with (100)-oriented terraces. Calculations are also described on a surface that was modified to mimic the surface structures that typically appear on real metal surfaces following thermally driven fluctuations in step edges. Our results provide initial information on how variation in the surface structure of intrinsically chiral metal surfaces can affect the enantiospecific adsorption of small molecules on these surfaces.     

Stepwise formation and characterization of covalently linked multiporphyrin-imide architectures on Si(100)

A major challenge in molecular electronics and related fields entails the fabrication of elaborate molecular architectures on electroactive surfaces to yield hybrid molecular/semiconductor systems. A method has been developed for the stepwise synthesis of oligomers of porphyrins linked covalently via imide units. A triallyl-porphyrin bearing an amino group serves as the base unit on Si(100), and the alternating use of a dianhydride (3,3',4,4'-biphenyltetracarboxylic dianhydride) and a porphyrin-diamine for reaction enables the rapid and simple buildup of oligomers composed of 2-5 porphyrins. The properties of these porphyrin "multad" films on Si(100) were interrogated using a variety of techniques. The charge densities of the redox-active porphyrin oligomers were determined via electrochemical methods. The stepwise growth was evaluated in detail via Fourier transform infrared (FTIR) spectroscopy and by selected X-ray photoelectron spectroscopic (XPS) studies. The morphology was probed via AFM methods. Finally, the thickness was evaluated by using a combination of ellipsometry and AFM height profiling, accompanied by selected XPS studies. Collectively, these studies demonstrate that high charge density, ultrathin, multiporphyrin films of relatively well-controlled thickness can be grown in a stepwise fashion using the imide-forming reaction. The increased charge densities afforded by the porphyrin multads may prove important for the fabrication of molecular-based information-storage devices. This bottom-up process for construction of surface-tethered molecular architectures complements the top-down lithographic approach for construction of functional devices with nanoscale dimensions.     

Functionalization of acetylene-terminated monolayers on Si(100) surfaces: a click chemistry approach

In this article, we report the functionalization of alkyne-terminated alkyl monolayers on Si(100) using "click" chemistry, specifically, the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction of azides with surface-bound alkynes. Covalently immobilized, structurally well-defined acetylene-terminated organic monolayers were prepared from a commercially available terminal diyne species using a one-step hydrosilylation procedure. Subsequent derivatization of the alkyne-terminated monolayers in aqueous environments with representative azide species via a selective, reliable, robust cycloaddition process afforded disubstituted surface-bound [1,2,3]-triazole species. Neither activation procedures nor protection/deprotection steps were required, as is the case with more established grafting approaches for silicon surfaces. Detailed characterization using X-ray photoelectron spectroscopy and X-ray reflectometry demonstrated that the surface acetylenes had reacted in moderate to high yield to give surfaces exposing alkyl chains, oligoether anti-fouling moieties, and functionalized aromatic structures. These results demonstrate that click immobilization offers a versatile, experimentally simple, chemically unambiguous modular approach to producing modified silicon surfaces with organic functionality for applications as diverse as biosensors and molecular electronics.     

Branched fluoropolymer-Si hybrids via surface-initiated ATRP of pentafluorostyrene on hydrogen-terminated Si(100) surfaces

Linear, branched, and arborescent fluoropolymer-Si hybrids were prepared via surface-initiated atom transfer radical polymerization (ATRP) from the 4-vinylbenzyl chloride (VBC) inimer and ClSO(3)H-modified VBC that were immobilized on hydrogen-terminated Si(100), or Si-H, surfaces. The simple approach of UV-induced coupling of VBC with the Si-H surface provided a stable, Si-C bonded monolayer of "monofunctional" ATRP initiators (the Si-VBC surface). The aromatic rings of the Si-VBC surface were then sulfonated by ClSO(3)H to introduce sulfonyl chloride (-SO(2)Cl) groups and to give rise to a monolayer of "bifunctional" ATRP initiators. Kinetics study indicated that the chain growth of poly(pentafluorostyrene) from the functionalized silicon surfaces was consistent with a "controlled" or "living" process. The chemical composition and functionality of the silicon surface were tailored by the well-defined linear and branched fluoropolymer brushes. Atomic force microscopy images revealed that the surface-initiated ATRP of pentafluorostyrene (PFS) had proceeded uniformly on the Si-VBC surface to give rise to a dense and molecularly flat surface coverage of the linear brushes. The uniformity of surfaces with branched brushes was controlled by varying the feed ratio of the monomer and inimer (VBC in the present case). The living chain ends on the functionalized silicon surfaces were used as the macroinitiators for the synthesis of diblock copolymer brushes, consisting of the PFS and methyl methacrylate polymer blocks.     

Vibrational characterization and electronic properties of long range-ordered, ideally hydrogen-terminated Si(111)

The use of a well defined, long range ordered surface is of fundamental interest for the determination of surface-specific intrinsic physical parameters. Ideally hydrogen terminated Si(111) surfaces are prepared by wet chemical treatment in basic HF solutions. The mechanism of formation is based on preferential etching of defects, leading to an ideal hydrogen terminaison of the (111) plane, without any reconstruction and with a high degree of perfection. Infrared spectroscopy is used to probe the quality of the surfaces by quantifying the extent of the perfect domains.

High resolution photoemission spectroscopy of such highly homogeneous surfaces shows exceptional narrow features in both the valence band and the core level regions. The valence band levels and their dispersion are well described by first-principles calculations using a quasi particle self-energy approach within the Heidin's GW approximation.

Two surfaces core level states are evidenced, arising from the silicon surface atoms and the backbonds. A crystal field effect splits the surface Si 2_p3/2 component. Their position and relative intensity find a satisfactory agreement with recent calculations using first principles perturbation theory.     

DFT characterization of adsorbed NH(x) species on Pt(100) and Pt(111) surfaces

Periodic density functional theory (DFT) calculations using plane waves have been performed to systematically investigate the adsorption and relative stability of ammonia and its dehydrogenated species on Pt(111) and Pt(100) surfaces. Different adsorption geometries and positions have been studied, and in each case, the equilibrium configuration has been determined by relaxation of the system. The vibrational spectra of the various ammonia fragments have been computed, and band assignments have been compared in detail with available experimental data. The adsorption of NH3 (on top) and NH2 (bridge) is more favorable on Pt(100) than on Pt(111), while similar adsorption energies were computed for NH (hollow) and N (hollow) on both surfaces. The remarkably lower adsorption energy of NH2 over Pt(111) as compared with Pt(100) (the difference being approximately 0.7 eV) can be related to different geometric and electronic factors associated with this particular intermediate. Accordingly, the type of platinum surface determines the most stable NH(x) fragment: Pt(100) has more affinity for NH2 species, whereas NH species are preferred over Pt(111).     

Detection of metallic trace impurities on Si(100) surfaces with total reflection X-ray fluorescence (TXRF)

Summary The physical principles and analytical capabilities of TXRF are discussed and compared to other surface sensitive techniques. Metallic trace impurities on silicon surfaces are readily identified with detection limits down to 10¹¹ atoms/cm² (10^–4 monolayers). Other advantages are simple sample preparation and the possibility of analyzing insulating layers without charging problems. The method has been applied to quantify coverages of Fe, Ni, Cu and Au on Si(100) surfaces, deposited from intentionally doped solutions (NH₃/H₂O₂ and HF/NH₄F). It turns out that certain metal/solution combinations cause large surface coverages on the silicon wafer, even if the metal concentration in the solution is very low (g/kg range).

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Nachweis von metallischen Spurenverunreinigungen an Si(100)-Oberflächen mit der Totalreflexions-Röntgenfluorescenzanalyse (TXRF)

     

Dehydrative cyclocondensation reactions on hydrogen-terminated Si(100) and Si(111): an ex situ tool for the modification of semiconductor surfaces

Dehydrative cyclocondensation processes for semiconductor surface modification can be generally suggested on the basis of well-known condensation schemes; however, in practice this approach for organic functionalization of semiconductors has never been investigated. Here we report the modification of hydrogen-terminated silicon surfaces by cyclocondensation. The cyclocondensation reactions of nitrobenzene with hydrogen-terminated Si(100) and Si(111) surfaces are investigated and paralleled with selected cycloaddition reactions of nitro- and nitrosobenzene with Si(100)-2x1. Infrared spectroscopy is used to confirm the reactions and verify an intact phenyl ring and C-N bond in the reaction products as well as the depletion of surface hydrogen. High resolution N 1s X-ray photoelectron spectroscopy (XPS) suggests that the major product for both cyclocondensation reactions investigated is a nitrosobenzene adduct that can only be formed following water elimination. Both IR and XPS are augmented by density functional theory (DFT) calculations that are also used to investigate the feasibility of several surface reaction pathways, which are insightful in understanding the relative distribution of products found experimentally. This novel surface modification approach will be generally applicable for semiconductor functionalization in a highly selective and easily controlled manner.     

Stationary and non-stationary etching of Si(100) surfaces with gas phase and adsorbed hydrogen

Stationary and non-stationary etching of Si(100) surfaces by hydrogen were studied between 200 K and 800 K using direct product detection and thermal desorption spectroscopy. Silane was the only etch product observed. The rates of silane SiD_nH_4−n isotopes measured during etching D-saturated Si(100) surfaces with gaseous H illustrate that the etch reaction proceeds between surface silyl and incoming H in a direct (Eley–Rideal or hot-atom) reaction step: H(g)+SiD₃(ad)→SiD₃H(g). Non-stationary etching via silane desorption occurs through disproportionation between surface dihydride and silyl groups, SiH₂(ad)+SiH₃(ad)→SiH₄(g).     

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.     

Angular distributions of H-induced HD and D2 desorptions from the Si(100) surfaces

We measured angular distributions of HD and D2 molecules desorbed via the reactions H+DSi(100)-->HD [abstraction (ABS)] and H+DSi(100)-->D2 [adsorption-induced-desorption (AID)], respectively. It was found that the angular distribution of HD molecules desorbed along ABS is broader than that of D2 molecules desorbed along AID, i.e., the former could be fit with cos(2.0+/-0.2) theta, while the latter with cos(5.0+/-0.5) theta. This difference of the angular distributions between the two reaction paths suggests that their dynamic mechanisms are different. The observed cos2 theta distribution for the ABS reaction was reproduced by the classical trajectory calculations over the London-Eyring-Polanyi-Sato potential-energy surfaces. The simulation suggests that the HD desorption along the ABS path takes place along the direction of Si-D bonds, but the apparent angular distribution is comprised of multiple components reflecting the different orientations of D-occupied Si dimers in the (2 x 1) and (1 x 2) double domain structures.     

Sequential growth in solution of NiFe Prussian blue coordination network nanolayers on Si(100) surfaces

Controlling the elaboration of Coordination Networks (CoNet) on surfaces at the nanoscale remains a challenge. One suitable technique is the Sequential Growth in Solution (SGS), which has the advantage to be simple, cheap and fast. We addressed two issues in this article: i) the controlled synthesis of ultra thin films of CoNet (thickness lower than 10 nm), and ii) the investigation of the influence of the precursors' concentration on the growth process. Si(100) was used because it is possible to prepare atomically flat Si-H surfaces, which is necessary for the growth of ultrathin films. We used, as a model system, the sequential reactions of K(4)[Fe(II)(CN)(6)] and [Ni(II)(H(2)O)(6)]Cl(2) that occur by the substitution of the water molecules in the coordination sphere of Ni(II) by the nitrogen atoms of ferrocyanide. We demonstrated that the nature of the deposited film depends mainly on the relative concentration of the anchoring sites versus the precursors' solution. Attenuated Total Reflection Fourier Transformed Infra Red (ATR-FTIR), X-ray reflectivity, X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) were used to characterize the steps of the growth process.