Hydrogen-terminated Silicon Surfaces Research Articles

Electrical Evaluation of Defects at the Si ( 100 ) / HfO2 Interface

This work examines the electrical activity of defects at the interface with Si(100) for thin films (10-20 nm) deposited by injection metal oxide chemical vapor deposition. Based on an analysis of the capacitance-voltage response of structures, two clear peaks are detected in the interface state density profiles, at specific energies in the lower eV) and upper eV) bandgap. The densities of defects responsible for these peaks have been calculated as and respectively. These defects are present when the films were deposited at sufficiently low temperatures to prevent hydrogen passivation. The density of interface defects was not significantly different for films deposited on native oxide or hydrogen-terminated silicon surfaces. The position of these defects in the silicon bandgap corresponds to the location of dangling bond defects in the thermally oxidized system. films deposited at 450°C did not exhibit the prominent interface defects observed on films deposited at indicating hydrogen passivation during deposition. © 2004 The Electrochemical Society. All rights reserved.

Electron-beam lithography with aromatic self-assembled monolayers on silicon surfaces

Aromatic self-assembled monolayers are formed via the coupling of hydroxy head groups to hydrogen-terminated silicon surfaces. We first investigate the application of 4-hydroxy-1,1′-biphenyl as an ultrathin negative tone electron-beam (e-beam) resist using conventional e-beam lithography with a beam energy of 3 keV. We demonstrate the fabrication of nanometer silicon patterns that are transferred using the modified monolayer as a resist mask for a wet chemical etching process in potassium hydroxide. The necessary dose for complete cross linking was determined to be 20 mC/cm2. Using this approach, isolated silicon structures with lateral dimensions down to ∼10 nm and periodic structures with a resolution of ∼20 nm were fabricated. On the other hand, 4′-nitro-4-hydroxy-1,1′-biphenyl has been found not to form monolayers suitable for chemical lithography on hydrogenated silicon surfaces. Upon adsorption, the nitro groups are partially reduced to amino groups by the hydrogenated surface and some of the molecules bind to the surface via the nitrogen terminus.

Wet-chemical passivation and characterization of silicon interfaces for solar cell applications

Abstract The interface properties of silicon solar cell structures were characterized by the two non-destructive and highly surface-sensitive spectroscopic techniques: surface photovoltage and spectroscopic ellipsometry. The resulting charge and density of interface states as well as the microscopic surface roughness and oxide coverage were investigated during silicon wafer preparation and during sample storage in air. The surface state density of hydrogen-terminated silicon surfaces as well as the long-time stability of the hydrogen termination were found to primarily depend on the surface morphology resulting from the wet-chemical oxidation procedures applied before. The smallest interface state densities were obtained by NH 4 F treatment subsequent to oxidation in ultra-pure water at 80°C. Surfaces prepared using this procedure are found to be much more stable upon exposition to clean-room air than those prepared by conventionally prepared H-terminated surfaces. The successful application of the new passivation procedures in photovoltaics is shown for selected examples of different solar cell concepts.

Fluorination-induced back-bond weakening and hydrogen passivation on HF-etched Si surfaces

The mechanism of hydrogen termination of silicon surface after HF etching has been elucidated using a model of weakened Si--Si back bond which complements the previous model of charge polarization. By calculating the bond energy and Mulliken bonding population of Si--Si bonds on silicon (111) surfaces at the level of density functional theory with substantiation using Hartree--Fock theory, and by analyzing net charges on select atoms, we show that as the surface silicon is bonded with fluorine, the bonds between surface silicon atoms and the silicon atoms underneath are significantly weakened due to the high electronegativity of fluorine and the extraordinary strength of Si--F bonds. The weakened Si--Si back bonds would facilitate the reaction between the HF molecule and the Si surface, peeling off the surface layer, thereby leaving behind a hydrogen-terminated Si surface. The reaction may be initiated by a dipole--dipole interaction on the trifluorine-terminated surface or an insertion-type interaction on the monofluorine-terminated surface due to a fluorination-induced bond polarization. The argument is further supported by studies on reaction pathways using various cluster models of fluorine- and hydrogen-terminated silicon surfaces.

Immobilization of Gold Nanoparticles onto Silicon Surfaces by Si−C Covalent Bonds

A series of ω-alkene-1-thiol-stabilized gold nanoparticles were prepared by a wet process and then covalently linked to a hydrogen-terminated silicon(111) surface with Si−C bonds via a thermal hydrosilylation reaction. The modified silicon surfaces were observed mainly by high-resolution scanning electron microscopy (HR-SEM). The HR-SEM images revealed that the gold nanoparticles protected with 2-propene-1-thiol (C3) were covalently bound to the hydrogen-terminated silicon surface, after which the nanoparticles self-fused; on the other hand, gold nanoparticles protected with 5-hexene-1-thiol (C6) or 10-undecene-1-thiol (C11) were linked to the hydrogen-terminated silicon surface independently. These surfaces were stable in air and can be stored for several months without detectable decomposition.

Preparation and Heck Reaction of Multidentate Carbosilane Films Derived from Focally Functionalized and Allyl-Terminated Dendrons on Hydrogen-Terminated Silicon(111) Surfaces

Multidentate carbosilane films were prepared by thermally induced hydrosilylation of allyl-terminated carbosilane dendrons of generations 0, 1, and 2 (G0-G2) on hydrogen-terminated silicon(111) surfaces. The dendron molecules contain three (G0), nine (G1), and twenty-seven (G2) allyl groups at the periphery, and a bromophenyl functional group at the focal point. The dendron films were characterized by contact-angle goniometry, ellipsometry, Fourier transform infrared spectroscopy in the attenuated total reflection mode, and X-ray photoelectron spectroscopy (XPS). Upon hydroboration of the remaining allyl groups in the films, the percentage of the introduced boron atoms in the films were measured by XPS. The results indicate the presence of roughly 20%, 27%, and 46% of unreacted allyl groups in the G0, G1, and G2 films, respectively. The mechanistic aspects of the chemisorption of these dendron molecules on H-Si(111) surfaces are discussed. XPS studies indicate that seven G0 molecules cover approximately the same area on the substrate as three G1 molecules and one G2 molecule. After treatment of the G0, G1, and G2 films with 4-fluorostyrene under the Heck reaction conditions, the XPS studies indicate that about 84%, 71%, and 55% of the Br atoms were consumed, yielding the replacement of ca. 58-70% of the reacted Br atoms by the fluorostyryl groups. The remaining bromophenyl groups were inactive toward the Heck reaction, probably due to their disfavorable position/orientation in the films.

Protein-resistant monolayers prepared by hydrosilylation of α-oligo(ethylene glycol)-ω-alkenes on hydrogen-terminated silicon (111) surfaces

Atomically flat, homogeneous, and protein-resistant monolayers can be readily prepared on H-Si(111) surfaces by photo-induced hydrosilylation of alpha-oligo(ethylene glycol)-omega-alkenes.

Microwave-Assisted Chemical Functionalization of Hydrogen-Terminated Porous Silicon Surfaces

This paper describes the chemical functionalization of hydrogen-terminated porous silicon surfaces with simple and functional 1-alkenes under microwave irradiation to give organic monolayers covalently attached to the surface through Si−C bonds. Using microwaves as a source of energy led to a remarkable increase in the rate of the hydrosilylation reaction and a higher surface coverage. This technique yields highly stable organic monolayers and allows the introduction of different functional groups on the surface (acid and ester) required for immobilization of more complex structures on the surface.

Effect of Organic Contamination on the Electrical Degradation of Hydrogen-Terminated Silicon upon Exposure to Air under Ambient Conditions

The degradation of the electrical characteristics of hydrogen-terminated silicon (both n- and p-doped) under typical laboratory environments was investigated. When a hydrogen-terminated silicon (H-Si) surface was exposed to air under ambient conditions, the current density/bias voltage (J-V) curve of the mercury-silicon junction thus formed changed significantly during the first 50 h. For n-type H-Si surfaces, the J-V curve initially maintained ohmic characteristics for a period of 8-12 h, then evolved to diode behavior; for p-doped samples, the current density showed a rapid increase and reached a maximum in about 20 h. In both cases, the electrical property was recovered to some extent after sonication in organic solvents. In agreement with infrared spectroscopic results, organic contamination upon exposure to ambient air is suggested to play an important role in the change of the electrical performance of hydrogen-terminated silicon. © 2003 The Electrochemical Society. All rights reserved.

Adsorption Behavior of Chemical Contaminants by Molecular Simulation

The adsorption process of chemical contaminants on silicon wafers was investigated by using molecular mechanics, molecular dynamics, Monte Carlo method, and computer graphics. The adsorption energy of organic gaseous contaminants, such as DBP, DOP, and D5, on the hydrogen terminated silicon surface and its oxide surface, were estimated and compared. The adsorption energy of chemical contaminants was larger than that of low molecular weight species, such as benzene, H2O and NH3. The adsorption energy on the hydrogen terminated silicon surface was smaller than that on its oxide surface. The equilibrium adsorption amount on the silicon wafer was also calculated. It was indicated that the configuration of the adsorbed DBP molecules over its gas concentration of 1.9x103 ng/m3 showed multi-layer adsorption. It was indicated that adsorption energy and the equilibrium adsorption amount is a good index to estimate the adsorption properties of organic molecules.

Nanopatterning of Alkynes on Hydrogen-Terminated Silicon Surfaces by Scanning Probe-Induced Cathodic Electrografting

The electrochemical cathodic electrografting reaction, previously demonstrated on bulk silicon surfaces, can be patterned on the nanoscale utilizing conducting probe atomic force microscopy (CP-AFM). Alkyne electrografting is a particularly useful chemical technique since it leads to direct covalent attachment of conjugated alkynes to silicon. In addition, application of a forward bias during the reaction renders the surface less sensitive to oxidation and the resulting monolayers are very stable in air and basic aqueous solution. Alkyne monolayer lines can be drawn down to 40 nm resolution using a Pt-coated AFM tip, and the heights of the monolayers scale with the molecular length of the alkyne. The tip is biased (+) and the surface is biased (-) to drive the cathodic electrografting reaction under ambient conditions. The resistance of the monolayers to fluoride, as well as friction force microscopy, indicate that the alkynes are covalently bonded to the surface, not oxide-based, and hydrophobic. The reaction does not work with alkenes, and therefore hydrosilylation is not the primary mode of reaction. Wider lines (300 nm) can be produced using broadened Pt-coated AFM tips. This reaction could be important for the interfacing of conjugated molecules directly to silicon in a spatially controlled fashion.

Dependence of morphology on miscut angle for Si(111) etched in NH4F

Hydrogen-terminated silicon surfaces are important and commonly used in several nanotechnology applications. A significant obstacle to their widespread use has been the repeatable preparation of large, flat surfaces. Using scanning probe microscopy, we have examined the surfaces of several vicinal Si(111) samples, with miscut angles ranging from 1.1° to 0.01°, produced by etching in a NH4F aqueous solution. Although the miscut angle sets the nominal terrace width, we have found that with wet chemical etch processing, as the vicinal angle decreases, the terrace width increases only to a maximum of ∼200 nm, limited by the etching anisotropy. The result is that for miscut angles below a critical angle, the surface roughness actually increases.

Chemomechanical surface patterning and functionalization of silicon surfaces using an atomic force microscope

Surface modification and patterning at the nanoscale is a frontier in science with significant possible applications in biomedical technology and nanoelectronics. Here we show that an atomic force microscope (AFM) can be employed to simultaneously pattern and functionalize hydrogen-terminated silicon (111) surfaces. The AFM probe was used to break Si–H and Si–Si bonds in the presence of reactive molecules, which covalently bonded to the scribed Si surface. Functionalized patches and patterned lines of molecules were produced. Linewidths down to 30 nm were made by varying the force at the tip.

Photoconductivity and spin-dependent photoconductivity of hydrosilylated (111) silicon surfaces

Organic monolayers were prepared on hydrogen-terminated (111) silicon surfaces by thermally induced hydrosilylation with alkenes. The electronic properties of the modified surfaces were studied by photoconductivity and spin-dependent photoconductivity measurements (electrically detected magnetic resonance) and compared to the oxidized and hydrogen-terminated silicon surfaces. The photoconductivity at low intensity of illumination (monomolecular recombination regime) indicates that the hydrosilylated surface has nearly as few defects as the surfaces treated in HF vapor. The paramagnetic defects detected in the spin-dependent photoconductivity are identified as the silicon dangling bond Pb-center. The density of defects at the hydrosilylated (111) silicon surface is determined by electron spin resonance measurements to be about 1013 cm−2.

Synthesis of tripod-shaped oligo(phenylene)s with multiple ethenyl groups at the bases for chemisorption on hydrogen-terminated silicon surfaces

This communication describes an efficient and convergent synthesis of monodisperse, nanometer-sized, and tripod-shaped oligo(phenylene)s with a triallylsilyl group at the base of each leg and a chlorophenyl group at the focal point of the tripod. These molecules were designed as model compounds for the study of chemisorption of rigid molecules containing multiple ethenyl groups on hydrogen-terminated silicon surfaces. The compounds were synthesized from p-chlorophenyl-tris( p-bromophenyl)silane via selective Pd-catalyzed Ishiyama–Miyaura reaction with bis(pinacolato)diboron, followed by Suzuki coupling with aryl dihalides to elongate the legs. The legs were then end-capped with triallylsilyl groups through Suzuki coupling with 4-triallyphenylboronic acid.

Structural Relaxations of Atomic Wires Fabricated on Hydrogen-Terminated Silicon Surfaces

Structural Relaxations of Atomic Wires Fabricated on Hydrogen-Terminated Silicon Surfaces

Alkoxyl monolayers as anti-stiction coatings in Si-based MEMS devices

Reducing surface energy is key to the success of many microelectromechanical systems (MEMS). In this report we present a strategy for the efficient assembly of alkoxyl monolayers onto a silicon surface to control surface energy. This is achieved by an all-liquid process in which the hydrogen terminated silicon surface resulting from aqueous HF etching is coated with a close-packed alkoxyl monolayer. The adhesion to silicon surface is reduced by a factor of 40 by the monolayer coating and friction coefficient of the coated surface is only 10-2. These coatings are successfully implemented in a model MEMS structure: cantilever beam array (CBA). Release-stiction is eliminated for polycrystalline silicon beams with a thickness of only 2 μm but with lengths up to 2 mm. Electrostatic actuation of coated beams in a controlled environment shows that the monolayer coating prevents in-use stiction at relative humidity as high as 90%.

Impact of organic contamination on the electrical properties of hydrogen-terminated silicon under ambient conditions

The electrical properties of hydrogen-terminated silicon (H–Si) surfaces were investigated by the assembly and testing of reliable and reproducible mercury–silicon junctions. When a H–Si surface is exposed to air under ambient conditions, the current density–bias voltage curve of the thus formed mercury–silicon junction initially maintains ohmic characteristics for a period of 8–12 h and then evolves to diode behavior. The current density substantially decreases, but can be recovered to a certain extent upon sonication in organic solvents. In agreement with infrared spectroscopic results, organic contamination of H–Si is suggested to play an important role in the transition of the electrical properties.

Selective Electroless Plating of Copper on (100)-Oriented Single Crystal Silicon Surface Modified by UV-Induced Coupling of 4-Vinylpyridine with the H-Terminated Silicon

A Sn-free electroless plating process for producing patterned and adherent copper deposits on (100)-oriented single-crystal silicon substrates is described. Pristine and resist-patterned Si(100) substrates were etched, initially, by aqueous HF to produce the hydrogen-terminated silicon surfaces (H−Si(100) surfaces). The H−Si(100) surfaces were further functionalized by the UV-induced reactive coupling of 4-vinylpyridine (4VP). The composition and topography of the modified Si(100) surfaces were characterized by X-ray photoelectron spectroscopy and atomic force microscopy, respectively. The coupled 4VP layer exhibited good stability in boiling chloroform, boiling water, sonicating dichloromethane, aqueous solution of HF and aqueous solution of KOH. Not only did the 4VP layer provide chemisorption sites for the palladium complexes without the need for prior sensitization in SnCl2 solution during the electroless plating process but it also served as the adhesion promotion layer and diffusion barrier for the ...

Hydrogen-bonded macrocluster formation of ethanol on silica surfaces in cyclohexane(1).

Adsorption of ethanol onto silica surfaces from ethanol-cyclohexane binary liquids was investigated by a combination of colloidal probe atomic force microscopy, adsorption excess isotherm measurement, and FTIR spectroscopy using the attenuated total reflection (ATR) mode. An unusually long-range attraction was found between the silica (glass) surfaces in the presence of ethanol in the concentration range of 0.1-1.4 mol % at room temperature. At 0.1 mol % ethanol, the attraction appeared at a distance of 35 +/- 3 nm and turned into a repulsion below 3.5 +/- 1.5 nm upon compression. Half of the attraction range agreed with the adsorption layer thickness estimated from the adsorption excess amount by assuming that the adsorption layer was composed only of ethanol. This indicated that the observed long-range attraction was caused by the contact of opposed adsorption layers of ethanol on the silica surfaces and that the sharp increase of repulsion at shorter distance was caused by the overlap of structured ethanol clusters adjacent to the surface. ATR-FTIR spectra demonstrated that ethanol adsorbed on the silica (silicon oxide) surfaces formed hydrogen-bonded clusters (polymers). Practically no ethanol clusters were formed on the hydrogen-terminated silicon surface. These results indicated that the cluster formation involved hydrogen-bonding interactions between surface silanol groups and ethanol hydroxyl groups in addition to those between ethanol hydroxyl groups. At higher temperatures (30-50 degrees C), the range and the strength of attraction decreased owing to the decrease in the hydrogen-bonded clusters monitored by FTIR spectroscopy, reflecting the nature of hydrogen bonding. The range and the strength of the attraction also changed when the ethanol concentration increased: The long-range attraction started to decrease at 0.6 mol % ethanol at room temperature and disappeared at 1.4 mol % while the adsorption excess amount remained almost constant as did the FTIR peak intensity of the hydrogen-bonded OH group of adsorbed ethanol. In the bulk solution, ethanol clusters appeared at 0.5 mol % ethanol; thus, this change in the attraction could be accounted for in terms of the exchange of ethanol molecules between the surface clusters and bulk clusters. The novel self-assembled structure of alcohol on the surface, found in this study may be called a "surface molecular macrocluster" because the hydrogen-bonded clusters extend to distances of ca. 20 nm longer than the typical sizes of common clusters, 2-4 nm, of alcohol (e.g., ethanol).