Hydrogen-terminated Si Research Articles

Photodegradation activity and stability of porous silicon wafers with (1 0 0) and (1 1 1) oriented crystal planes

Hydrogen-terminated porous Si wafers with (100) and (111) oriented crystal planes were fabricated through a photo-electrochemical etching. It is found that the porosity of silicon wafers and their etch rates are determined as a function of crystal orientation. Due to the anisotropic etching behavior of single-crystal silicon, the hydrogen-terminated porous Si (100) wafers exhibit not only more excellent photodegradation activity but also stronger stability for methyl orange degradation than hydrogen-terminated porous Si (111) wafers under visible light irradiation. For the unetched Si wafers, however, the photodegradation activities of methyl orange exhibit a contrary conclusion.

Morphology and atomic structure of hydrogen-terminated Si(110)-(1 × 1) surfaces prepared by a wet chemical process

We have measured the macroscopic and microscopic surface morphology of hydrogen-terminated Si(110)-(1×1) [H:Si(110)-(1×1)] surfaces prepared by an improved wet chemical process. The observations were performed by high-resolution electron-energy-loss spectroscopy (HREELS), low-energy-electron diffraction (LEED), and scanning tunneling microscopy (STM). At macroscopic scale, it was found that the surface is a mono-hydride terminated surface with a two-dimensional p2mg space group, thus, being a well-defined H:Si(110)-(1×1) surface. At microscopic scale, elongated terraces were observed along the [11¯0] direction reflecting surface anisotropy. The terraces extend in frames with sizes up to a few micrometers. We discussed the macroscopic and microscopic surface morphology by combining our LEED and STM results.

What a difference a bond makes: the structural, chemical, and physical properties of methyl-terminated Si(111) surfaces.

The chemical, electronic, and structural properties of surfaces are affected by the chemical termination of the surface. Two-step halogenation/alkylation of silicon provides a scalable, wet-chemical method for grafting molecules onto the silicon surface. Unlike other commonly studied wet-chemical methods of surface modification, such as self-assembly of monolayers on metals or hydrosilylation on silicon, the two-step method enables attachment of small alkyl chains, even methyl groups, to a silicon surface with high surface coverage and homogeneity. The methyl-terminated Si(111) surface, by comparison to hydrogen-terminated Si(111), offers a unique opportunity to study the effects of the first surface bond connecting the overlayer to the surface. This Account describes studies of methyl-terminated Si(111), which have shown that the H-Si(111) and CH3-Si(111) surfaces are structurally nearly identical, yet impart significantly different chemical and electronic properties to the resulting Si surface. The structure of methyl-terminated Si(111) formed by a two-step halogenation/methylation process has been studied by a variety of spectroscopic methods. A covalent Si-C bond is oriented normal to the surface, with the methyl group situated directly atop a surface Si atom. Multiple spectroscopic methods have shown that methyl groups achieve essentially complete coverage of the surface atoms while maintaining the atomically flat, terraced structure of the original H-Si(111) surface. Thus, the H-Si(111) and CH3-Si(111) surface share essentially identical structures aside from the replacement of a Si-H bond with a Si-C bond. Despite their structural similarity, hydrogen and methyl termination exhibit markedly different chemical passivation. Specifically, CH3-Si(111) exhibits significantly greater oxidation resistance than H-Si(111) in air and in aqueous electrolyte under photoanodic current flow. Both surfaces exhibit similar thermal stability in vacuum, and the Si-H and Si-C bond strengths are expected to be very similar, so the results suggest that methyl termination presents a greater kinetic barrier to oxidation of the underlying Si surface. Hydrogen termination of Si(111) provides nearly perfect electronic passivation of surface states (i.e., less than 1 electronic defect per 40 million surface atoms), but this electronic passivation is rapidly degraded by oxidation in air or under electrochemical conditions. In contrast, methyl termination provides excellent electronic passivation that resists degradation due to oxidation. Moreover, alkylation modifies the surface electronic structure by creating a surface dipole that effectively changes the electron affinity of the Si surface, facilitating modification of the charge-transfer kinetics across Si/metal or Si/electrolyte junctions. This Account also briefly describes recent studies of mixed monolayers formed by the halogenation/alkylation of silicon. Mixed monolayers allow attachment of bulkier groups that enable secondary chemistry at the surface (e.g., attachment of molecular catalysts or seeding of atomic layer deposition) to be combined with methyl termination of remaining unreacted surface sites. Thus, secondary chemistry can be enabled while maintaining the chemical and electronic passivation provided by complete termination of surface atoms with Si-C bonds. Such studies of mixed monolayers demonstrate the potential use of a wet-chemical surface modification scheme that combines both chemical and electronic passivation.

Photoemission study of copper phthalocyanine growth on hydrogen-terminated surface: Si(100)2 × 1–H

Copper phthalocyanine molecules have grown at room temperature under ultra high vacuum on hydrogen-terminated surface Si(100)2×1–H. Chemical and electronic properties of the interface were investigated by ultraviolet and X-ray photoemission spectroscopy (UPS, XPS). ResultsResults indicated the existence of an interfacial dipole of 0.36±0.05eV and a band bending of 0.40±0.05eV. These were evidenced by shifts of XPS core levels and change of the vacuum level position. During the growth, the work function was found to decrease from 4.5eV for the substrate to 3.74eV for the highest coverage (40monolayers). This band bending was also due to the shift of the highest occupied molecular orbital.The interfacial dipole was correlated to a rearrangement of molecules on the surface. An energy level diagram of the interface was deduced from a combination of the XPS and UPS results.

Silicon nanocrystal conjugated polymer hybrid solar cells with improved performance

Hybrid inorganic/organic solar cells based on silicon nanocrystals (NCs) and conjugated polymers are optimized through tuning the surface termination of Si NCs and selecting a narrow bandgap conjugated polymer. Here freestanding Si NCs are blended with either P3HT or PTB7 in a solution phase. The resulting mixture is spin-cast to form a bulk hetrojunction structure, on which a hybrid solar cell is fabricated. It is found that by altering the surface termination of Si NCs from predominately chlorine termination to hydrogen termination the power conversion efficiency (PCE) of a hybrid solar cell is significantly improved. PTB7-based solar cells efficiently respond to a wider spectrum of sunlight than P3HT-based ones. This is due to the lower bandgap of PTB7 compared to P3HT (~1.8eV vs. ~2.0eV). Finally, PCE up to 2.2% has been achieved in solar cell fabricated by using hydrogen-terminated Si NCs and PTB7.

SENSING IMMOBILIZED MOLECULES OF STREPTAVIDIN ON A SILICON SURFACE BY MALDI-TOF MASS SPECTROMETRY AND FLUORESCENCE MICROSCOPY

A hydrogen-terminated Si (111) surface was modified to form an aminoterminated monolayer for immobilization of streptavidin. Cleavage of an N-(ωundecylenyl)-phthalimide covered surface using hidrazine yields an amino group-modified surface, which serves as a substrate for the attachment of biotin and subsequently streptavidin. We used surface analytical techniques to characterize the surface and to control the course of functionalization before the immobilization of streptavidin. To confirm the presence of the streptavidin Texas red on the surface two powerful techniques available in a standard biochemical laboratory are used, Fluorescence Microscopy and MALDI-TOF that allow us to detect and determine the immobilized streptavidin. This work provides an avenue for the development of devices in which the exquisite binding specificity of biomolecular recognition is directly coupled to a biosensor. In addition, we have demonstrated that MALDI-TOF and fluorescence microscopy are useful techniques for the characterization of silicon functionalized surfaces.

Substituent effects on the kinetics of bifunctional styrene SAM formation on H-terminated Si.

Self-assembled monolayers (SAMs) on metal and semiconductor surfaces are of interest in electronic devices, molecular and biosensors, and nanostructured surface preparation. Bifunctionalized molecules, where one functional group attaches to the surface while the other remains free for further modification, allow for the rational design of multilayer chemisorbed thin films. In this study, substituted styrenes acted as a model system for SAM formation through an alkene moiety. Substituents ranging from activating to strongly deactivating for aromatic reactions were used to probe the effect of the electronic properties of functionalizing molecules on the rate of SAM formation. Substituted styrene SAMs were formed on hydrogen-terminated p-type Si(100) and n-type Si(111) via sonochemical functionalization. Monolayers were characterized via ellipsometry, IR spectroscopy, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). Initial rates of reaction for molecules that selectively attached through the alkene were further studied. A linear relationship was observed between the initial rates of surface functionalization and the substituent electron donating/withdrawing ability for the substituted styrenes, as described by their respective Hammett constants. This study provides precedent for applying well quantified homogeneous chemical reaction relationships to reactions at the solid-liquid interface.

Ammonia photodissociation promoted by Si(100).

Using in situ X-ray photoelectron spectroscopy measurements after reaction, we show that hydrogen-terminated Si(100) perturbs the bonding of physisorbed NH3 enabling a photochemical decomposition pathway at wavelengths different from those characteristic of either the molecule in the gas phase or the semiconductor bandgap. UV illumination only of gas phase NH3 at partial pressures from 0.1 to 100 Torr produced a maximum at 10 Torr in the N surface coverage. This is in good agreement with a model of the radical production rate showing that at this pressure the gas density balances the flux of photons at the surface with energies sufficient to dissociate NH3. UV illumination of both the gas phase and the surface produced a monotonic increase in the N coverage with pressure as well as coverages that were 3-10 times higher than when only the gas phase was illuminated. The amine saturation coverage scaled with the UV fluence at 10 Torr and 75 °C, reaching 6.9 × 10(14) atoms/cm(2) (∼1 N atom per Si surface atom) at 19 mW/cm(2) and 12 × 10(14) atoms/cm(2) (∼1.8 N per Si) at 35 mW/cm(2). Monochromatic illumination showed that the wavelengths driving deposition were not correlated with the Si bandgap, but instead were roughly the same as gas phase photodissociation (λ < 220 nm). The primary driving force to replace the hydrogen termination with amine groups was direct photodissociation of NH3 molecules whose electronic structure was perturbed by interaction with the surface. Amine groups enhanced the surface reaction of water present as a contaminant in the source gas. These results show that molecules in weakly bound surface states can have a dramatic impact on the photochemistry.

Anisotropic surface phonon dispersion of the hydrogen-terminated Si(110)-(1×1) surface: One-dimensional phonons propagating along the glide planes

We have measured the surface phonon dispersion curves on the hydrogen-terminated Si(110)-(1×1) surface with the two-dimensional space group of p2mg along the two highly symmetric and rectangular directions of ΓX and ΓX' using high-resolution electron-energy-loss spectroscopy. All the essential energy-loss peaks on H:Si(110) were assigned to the vibrational phonon modes by using the selection rules of inelastic electron scattering including the glide-plane symmetry. Actually, the surface phonon modes of even-symmetry to the glide plane (along ΓX) were observed in the first Brillouin zone, and those of odd-symmetry to the glide plane were in the second Brillouin zone. The detailed assignment was made by referring to theoretical phonon dispersion curves of Gräschus et al. [Phys. Rev. B 56, 6482 (1997)]. We found that the H-Si stretching and bending modes, which exhibit highly anisotropic dispersion, propagate along ΓX direction as a one-dimensional phonon. Judging from the surface structure as well as our classical and quantum mechanical estimations, the H-Si stretching phonon propagates by a direct repulsive interaction between the nearest neighbor H atoms facing each other along ΓX, whereas the H-Si bending phonon propagates by indirect interaction through the substrate Si atomic linkage.

Surface‐Junction Effects on Interfacial Electron Transfer between Bis(terpyridine)iron(II) and Hydrogen‐Terminated Silicon(111) Electrode

Interfacial electron transfer at bis(tpy)–iron(II) complexes (tpy=2,2’:6’,2’’-terpyridine) on Si(111) electrodes was investigated by using four types of surfaceanchor terpyridine ligands. Despite the greater distance, electron transfer between the bis(tpy)–iron(II) unit and the electrode is accelerated in surface-anchor ligands with an additional phenylene group.

Selectivity of metal oxide atomic layer deposition on hydrogen terminated and oxidized Si(001)-(2×1) surface

In this work, the authors used density-functional theory methods and x-ray photoelectron spectroscopy to study the chemical composition and growth rate of HfO2, Al2O3, and TiO2 thin films grown by in-situ atomic layer deposition on both oxidized and hydrogen-terminated Si(001) surfaces. The growth rate of all films is found to be lower on hydrogen-terminated Si with respect to the oxidized Si surface. However, the degree of selectivity is found to be dependent of the deposition material. TiO2 is found to be highly selective with depositions on the hydrogen terminated silicon having growth rates up to 180 times lower than those on oxidized Si, while similar depositions of HfO2 and Al2O3 resulted in growth rates more than half that on oxidized silicon. By means of density-functional theory methods, the authors elucidate the origin of the different growth rates obtained for the three different precursors, from both energetic and kinetic points of view.

Construction of a metal–organic monolayer–semiconductor junction on a hydrogen-terminated Si(111) surface via Si–C covalent linkage and its electrical properties

A metal-organic monolayer-semiconductor junction, exhibiting a diode behaviour, was constructed on a hydrogen-terminated n-type Si(111) by sequential surface reactions of (1) formation of an organic monolayer with a thiol terminal group, (2) platinum deposition onto the thiol group via adsorption of a platinum complex followed by chemical reduction, and finally (3) continuous Ag layer formation by electroless deposition. Rectifying behaviour was observed at this interface.

&lt;i&gt;In Situ&lt;/i&gt; ATR-FTIR Observation about Surfactant/Hydrogen-TerminatedSi(111) Interface in Solution

Development of novel functional devices has been expected by modification for Si surface. This study investigated immobilization and roles of the Si surface with flowing surfactant by in situ ATR-FTIR method. This result suggested that the surfactant prevented oxidation of the hydrogen-terminated Si surface from the higher concentration in aqueous solution. These would guard the Si surface against H2O molecules.

Improvement of Minority Carrier Lifetime and Si Solar Cell Characteristics by Nitric Acid Oxidation Method

The minority carrier lifetime of single crystalline Si increases by the formation of an ultrathin silicon dioxide (SiO2) layer by use of the nitric acid oxidation of Si (NAOS) method. With the NAOS method using 68 or 98 wt% HNO3 solutions, large increases in the lifetime are observed while the enhancement of the lifetime is low for the 40 wt% HNO3 NAOS method. In the case of the hydrogen-terminated Si surfaces formed by hydrofluoric acid (HF) treatment, the initial minority carrier lifetime is high, but it returns to the value before the HF treatment by keeping in air for 5 days. In the case of the NAOS method, on the other hand, the enhanced lifetime remains unchanged with time kept in air. This stability is attributed to high atomic density of the NAOS SiO2 layer which prevents diffusion of oxidizing species. The atomic density of the NAOS SiO2 layer increases with the HNO3 concentration, which leads to an increase in the valence band discontinuity energy at the SiO2/Si interface. When the NAOS SiO2 layer is inserted between the Si substrate and a silicon nitride anti-reflection layer for n-Si-based pn-junction solar cells, the conversion efficiency is increased from 17.2 to 18.9%. The internal quantum efficiency in the short wavelength region (300∼600 nm) is improved by the NAOS SiO2 layer, indicating the front surface is effectively passivated by the NAOS SiO2 layer.

Copper silicide nanocrystals on hydrogen-terminated Si(001)

In this paper we describe the surface characterization of Cu deposited onto nominally-flat and roughened hydrogen-terminated Si(001) surfaces in ultra-high vacuum using scanning tunneling microscopy. Cu forms Cu3Si 3D-islands with markedly different geometries depending on the surface roughness of the underlying H-terminated silicon surface. Anisotropic islands oriented perpendicular to the dimer-rows are observed on the nominally-flat H-terminated surface, while mostly isotropic islands are observed on the rough-engineered H-terminated surface. These results could have implications with respect to both surface-templated growth of nanostructures and Cu-based microelectronics.

Investigating individual arsenic dopant atoms in silicon using low-temperature scanning tunnelling microscopy

We study subsurface arsenic dopants in a hydrogen-terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant-related features that fall into two classes, which we call As1 and As2. When imaged in occupied states, the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D0) charge state when imaged in filled states, but become positively charged (D+) through electrostatic ionization when imaged under empty-state conditions, similar to previous observations of acceptors in GaAs. Density functional theory calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction-band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled-state images and an isotropic depression in empty-state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

Controlling the Atomic Layer Deposition of Titanium Dioxide on Silicon: Dependence on Surface Termination

Patterned fabrication depends on selective deposition that can be best achieved with atomic layer deposition (ALD). For the growth of TiO2 by ALD using TiCl4 and H2O, X-ray photoelectron spectroscopy reveals a marked difference in growth on oxidized and hydrogen-terminated silicon surfaces, characterized by typical and predictable deposition rates observed on SiO2 surfaces that can be 185 times greater than the deposition rates on hydrogen-terminated Si(100) and Si(111) surfaces. Large-scale patterning is demonstrated using wet chemistry, and nanometer-scale patterned TiO2 growth is achieved through scanning tunneling microscopy (STM) tip-based lithography and ALD. The initial adsorption mechanisms of TiCl4 on clean, hydrogen-terminated, and OH-terminated Si(100)-(2 × 1) surfaces are investigated in detail through density functional theory calculations. Varying the reactive groups on the substrate is found to strongly affect the probability of precursor nucleation on the surface during the ALD process. Th...

Interaction of C60with clean and hydrogenated SiC-(3×3) probed through the unoccupied electronic states

The effect of hydrogenation on the conduction bands of the Si-rich $6H$-SiC$(0001)$-(3\ifmmode\times\else\texttimes\fi{}3) reconstruction is studied using inverse photoemission spectroscopy in order to distinguish surface from bulk states. These results are exploited for the comparative study of the interaction of C${}_{60}$ adsorbed on (3\ifmmode\times\else\texttimes\fi{}3) and on hydrogen-terminated (3\ifmmode\times\else\texttimes\fi{}3). For the latter, as in the case of hydrogen-terminated Si, C${}_{60}$ is electronically decoupled from the substrate. Upon annealing a C${}_{60}$ thick film deposited on hydrogenated (3\ifmmode\times\else\texttimes\fi{}3) up to 670 K, there is a hint for a possible hydrogen transfer to the C${}_{60}$ molecules. The initially physisorbed molecules then adopt covalent bonding with Si, forming the contact layer. Part of the substrate is already found uncovered at this temperature. By further annealing up to 860 K all H atoms have desorbed. Finally, at 1100 K the remaining covalently bound C${}_{60}$ have desorbed. Unexpectedly, the structural damages caused by H and C${}_{60}$ deposition and by the successive annealing steps do not prevent a final restoration of the initial (3\ifmmode\times\else\texttimes\fi{}3) reconstruction at about 1100 K.

Photochemical grafting of methyl groups on a Si(1 1 1) surface using a Grignard reagent

The photochemical grafting of methyl groups onto an n-type Si(111) substrate was successfully achieved using a Grignard reagent. The preparation involved illuminating a hydrogen-terminated Si(111) that was immersed in a CH3MgBr-THF solution. The success was attributed to the ability of the n-type hydrogenated substrate to produce holes on its surface when illuminated. The rate of grafting methyl groups onto the silicon surface was higher when a larger illumination intensity or when a substrate with lower dopant concentration was used. In addition, the methylated layer has an atomically flat structure, has a hydrophobic surface, and has electron affinity that was lower than the bulk Si.

Size dependence of the optical gap of “small” silicon quantum dots: Ab initio and empirical correlation schemes

We present a comparative study of the energy-gap dependence on diameter d of “small” (d<20Å) hydrogen-terminated Si quantum dots, using density functional theory (DFT) with the hybrid functional of Becke, Lee, Parr and Yang (B3LYP). These accurate real space ab initio calculations [see [1], Garoufalis et al., Phys. Rev. Lett. 87 (2001) 276402] are used to compare the size dependence of the band gap according to quantum confinement theory in relation to the empirical bond-order-length-strength (BOLS) correlation mechanism, usually applied to larger nanocrystals. Our results for the gap variation, in the range of diameters considered here, are in very good agreement with quantum confinement theory and they reproduce by extrapolation the experimental band gap of bulk silicon with high accuracy (error smaller than 1%). On the contrary, extrapolation of fitted band gaps by BOLS scheme, grossly overestimates the band gap of bulk (by almost 80%); whereas, forcing the agreement of bulk band gap results in largely underestimated dot band gaps, in the range of diameters considered here.