H-terminated Si Research Articles

Conduction modulation of π-stacked ethylbenzene wires on Si(100) with substituent groups

For the realization of molecular electronics, one essential goal is the ability to systematically fabricate molecular functional components in a well-controlled manner. Experimental techniques have been developed such that π-stacked ethylbenzene molecules can now be routinely induced to self-assemble on an H-terminated Si(100) surface at precise locations and along precise directions. Electron transport calculations predict that such molecular wires could indeed carry an electrical current, but the Si substrate may play a considerable role as a competing pathway for conducting electrons. In this work, we investigate the effect of placing substituent groups of varying electron donating or withdrawing strengths on the ethylbenzene molecules to determine how they would affect the transport properties of such molecular wires. The systems consist of a line of π-stacked ethylbenzene molecules covalently bonded to a Si substrate. The ethylbenzene line is bridging two Al electrodes to model current through the molecular stack. For our transport calculations, we employ a first-principles technique where density functional theory (DFT) is used within the non-equilibrium Green’s function formalism (NEGF). The calculated density of states suggest that substituent groups are an effective way to shift molecular states relative to the electronic states associated with the Si substrate. The electron transmission spectra obtained from the NEGF–DFT calculations reveal that the transport properties could also be extensively modulated by changing substituent groups. For certain molecules, it is possible to have a transmission peak at the Fermi level of the electrodes, corresponding to high conduction through the molecular wire with essentially no leakage into the Si substrate.

水素終端Si(111)及びSi(110)表面の初期酸化過程

水素終端Si(111)及びSi(110)表面の初期酸化過程

Substrate and drying effect in shape and ordering of micelles inside CTAB–silica mesostructured films

Deviation from a perfect 2D-hexagonal (p6m) structure, for CTAB–silica mesostructured films prepared by adding different amounts of excess ethanol to a solution of CTAB and TEOS just before spin coating on OH- and H-terminated Si substrates, is observed from combined X-ray reflectivity and grazing incidence small angle X-ray scattering measurements. Such a deviation can be well understood in terms of the shape and ordering of the micelles, with or without the silica coating layer's contribution, inside the film. For example, cylindrical shaped micelles, which are initially circular on a hydrophilic OH-terminated Si substrate in order to form a perfect 2D-hexagonal structure, become elliptical (extended along the in-plane) on a hydrophobic H-terminated Si substrate to form a slightly compressed 2D-hexagonal structure due to a different attachment of the film to the substrate. With time, due to the drying of the silica materials and its restricted movement along the in-plane direction, the films on both the substrates are compressed along the out-of-plane direction only, to form observed centered rectangular (c2mm) structures. Also, due to the asymmetric shrinkage, stress is developed, which deteriorates the ordering in the film. The final shape of the micelles, including the silica coating layer's contribution, shows maximum and minimum deviations from the circular shape inside the thick film on a OH–Si substrate and the thin film on a H–Si substrate, respectively. The deviation in the shape of the micelles itself, which is of actual importance, seems to be maximum and minimum inside the thick film on a H–Si substrate and the thin film on a OH–Si substrate, respectively, and is essentially determined by the substrate nature and initial silica wall thickness.

Self-assembly of molecular wires on H-terminated Si(100) surfaces driven by London dispersion forces

First-principles calculations combined with kinetic Monte Carlo simulations are carried out to unambiguously demonstrate the vital role of van der Waals (vdW) interactions in the self-assembly of styrene nanowires on H-terminated Si(100) surfaces. We find that, only with the inclusion of London dispersion forces, accounting for the attractive parts of vdW interactions, are the effective intermolecular interactions reversed from repulsive to attractive. Such attractive interactions, in turn, ensure the preferred growth of long wires under physically realistic conditions as observed experimentally. We further propose a cooperative scheme, invoking the application of an electric field and the selective creation of Si dangling bonds, to drastically improve the ordered arrangement of the molecular nanowires. The present paper represents a significant step forward in the fundamental understanding and precise control of molecular self-assembly guided by London dispersion forces.

Reorganization of Au nanoparticle Langmuir-Blodgett films on wet chemically passivated Si(001) surfaces

Growth of dodecanethiol-encapsulated Au nanoparticles on differently terminated (OH-, H-, or Br-terminated) Si(001) substrates by Langmuir-Blodgett method at a constant monolayer surface pressure and their out-of-plane structural modification with time have been investigated. As the substrates have different gradations in the hydrophilic/hydrophobic nature, three different out-of-plane structures have been formed. On H-terminated Si (hydrophobic surface), a fluctuating monolayer of Au nanoparticles has been formed, whereas on Br- (coexistence of hydrophilic and hydrophobic surfaces) and OH-terminated Si (hydrophilic surfaces), trilayer of Au nanoparicles have been formed, but the top layer coverage is more for the OH-terminated Si. The growth of Au nanoparticles on H-terminated Si is similar to the Frank-van der Marwe mode, whereas on Br- and OH-terminated Si, the growth is similar to the Stranski-Krastanov mode. These three different structures modify with time and finally become a thicker monolayer of high density, and positions of naoparticles within the monolayer become random. AFM images of the films also show that positions of the Au nanoparticles are random. Density of the final layer becomes maximum on OH-terminated Si and minimum on H-terminated Si, whereas it becomes intermediate on Br-terminated Si. Reorganization thus helps to obtain nanostructures of tunable nanoparticle density.

Si2H6 Dissociative Chemisorption and Dissociation on Si(100)-(2×1) and Ge(100)-(2×1)

Atomically controlled epitaxy of semiconductor-on-semiconductor is important for the construction of nanometer-scale devices such as quantum dots, qubits for quantum computing, nanoelectromechanical systems (NEMS) oscillators, and nanobiomedical devices. For instance, patterned atomic layer epitaxy (ALE) using a scanning tunneling microscopy tip to depassivate an area of a H-terminated Si(001) surface, creating a pattern for subsequent Si growth, should lead to a new generation of atomically precise structures. Vapor phase epitaxy is well-suited to achieve controlled deposition, as the precursors are not reactive with the H-terminated background, unlike Si atoms from solid-source evaporation. Disilane (Si2H6) is arguably the best precursor for Si ALE on Si or Ge surfaces at moderate temperatures; yet, its adsorption configuration and subsequent decomposition pathways are not well understood. Combining experimental data from in situ infrared absorption spectroscopy (IRAS), scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS) after saturation by disilane of both Si(100)-(2×1) and Ge(100)-(2×1) surfaces, with first-principles calculations of candidate surface structures, we show that Si2H6 chemisorbs through a β-hydride elimination pathway as Si2H5 and H, instead of the previously proposed SiH3, and subsequently decomposes into an ad-monohydride dimer. The initial chemisorption process takes place on a single dimer and produces a monohydride ad-dimer oriented perpendicular to the substrate dimer rows. The ad-dimer can be located either in between two adjacent, initially clean dimers from the same dimer row, or in a bridging position over the trench between two adjacent dimer rows. These findings provide clear guidance for the formation of atomic size structures defined by local removal of hydrogen on H-terminated Si surfaces.

Self-assembled line growth of allyl alcohol on the H-terminated Si(100)-(2 × 1) surface

Using first-principles density-functional calculations, we investigate the growth mechanism of allyl alcohol (ALA) line on the H-terminated Si(100)-(2×1) surface. Unlike the allyl mercaptan (CH2=CH−CH2−SH) line, which was observed to grow across the Si dimer rows, we find that ALA (CH2=CH−CH2−OH) has the line growth along the Si dimer row. The self-assembled growth of ALA line occurs via the radical chain reaction mechanism, similar to the case of a typical alkene molecule, styrene. Our calculated energy profile along the reaction pathway shows that the different growth direction of ALA line compared with that of allyl mercaptan line is ascribed to the great instability of the oxygen radical intermediate, which prevents the line growth across the dimer rows.

Controlling the fixed charge and passivation properties of Si(100)/Al2O3 interfaces using ultrathin SiO2 interlayers synthesized by atomic layer deposition

Al2O3 synthesized by atomic layer deposition (ALD) on H-terminated Si(100) exhibits a very thin (∼1 nm) interfacial SiOx layer. At this interface, a high fixed negative charge density, Qf, is present after annealing which contributes to ultralow surface recombination velocities <2 cm/s. Here, we identify the thickness of the interfacial SiO2 layer as a key parameter determining Qf. The SiO2 thickness was controlled by intentionally growing ultrathin SiO2 interlayers (0.7−30 nm) by ALD. Optical second-harmonic generation spectroscopy revealed a marked decrease in Qf for increasing SiO2 thickness between 0 and 5 nm. This phenomenon is consistent with charge injection across the interfacial layer during annealing. For thicker SiO2 interlayers (>∼5 nm), the polarity of the effective charge density changed from negative to positive. The observed changes in Qf and the associated field-effect passivation had a significant influence on the injection-level-dependent minority carrier lifetime of Si.

3D nanostructures by stacking pre-patterned fluid-supported single-crystal Si membranes

The fabrication of complex three-dimensional (3D) structures at sub-100 nm resolution presents a difficult challenge. 3D photonic crystals that contain waveguides, resonant cavities, filters or other devices, and require deep-sub-100 nm dimensional control, are a particular example of this challenge. Multilayer 3D structures can be formed by stacking and bonding thin membranes that have been patterned in advance. This approach enables the full panoply of 2D planar-fabrication techniques to be employed. Membranes containing patterns that are not perfectly regular will exhibit in-plane distortion unless their intrinsic stress is zero. To minimize the effects of intrinsic stress we float individual membranes on the surface of a liquid. Thin single-crystal Si membranes on an oxide substrate are first patterned and then removed by etching the oxide in hydrofluoric acid. The freed Si membranes readily float on the liquid surface, aided by the hydrophobic nature of H-terminated Si. The authors describe methods for cleaning, patterning, manipulating, bonding and stacking such freely floating membranes.

An empirical pseudopotential approach to surface and line-edge roughness scattering in nanostructures: Application to Si thin films and nanowires and to graphene nanoribbons

We present a method to treat scattering of electrons with atomic roughness at interfaces, surfaces, and edges on nanometer-scale structures based on local empirical pseudopotentials. This approach merges the computational advantages of macroscopic models based on the shift of a phenomenological “barrier potential,” with the physical accuracy of models based on modifications of the atomic configuration at the interface/surface/edge. We illustrate the method by considering the dependence of the scattering matrix element on the confinement (inversion) field in free-standing H-terminated Si inversion layers, on the thickness in similarly H-terminated thin-Si bodies, on the diameter of free-standing [100] cylindrical Si nanowires, and on the width of armchair-edge graphene nanoribbons. For these latter structures, we find extremely large scattering rates, whose magnitude — ultimately due to the chirality dependence of the bandgap — renders perturbation theory invalid and prevents us from drawing quantitative conclusions about transport properties. Yet, they show clearly the dominant role played by line-edge roughness in controlling electronic transport in these structures, in agreement with suggestions that transport in narrow and rough ribbons does not occur via extended Bloch states.

Patterned atomic layer epitaxy of Si/Si(001):H

We aim to develop techniques for the building of atomically precise structures. On the H-terminated Si(001) surface, H atoms can be selectively removed using an STM tip with appropriate lithography conditions, creating arbitrary patterns of reactive dangling bonds with atomic precision. The exposed patterns are used as templates for the growth of Si and Ge by gas-source epitaxy, using disilane and digermane as the precursor gases. The quality of the epitaxy, in terms of island size and defect density of the second and subsequent monolayer (ML), is dependent upon the electron exposure. Good-quality growth of the second and following MLs requires a multiple of the exposure required for good-quality growth of the first ML. This is interpreted in terms of remanent hydrogen in island sites in the first ML.

Self-directed growth approach for acetylacetone lines on an H-terminated Si(001)-(2×1) surface

Self-directed growth approach for acetylacetone lines on an H-terminated Si(001)-(2<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo>×</mml:mo></mml:math>1) surface

Enhanced Stability and Electronic Structure of Phenylacetylene Lines on the Si(100)-(2 × 1):H Surface

A recent scanning tunneling microscopy (STM) study (J. Am. Chem. Soc. 2010, 132, 3013) reported the self-assembled growth of phenylacetylene (PA) molecular lines on the H-terminated Si(100)-(2 × 1) surface. Using first-principles density functional calculations within the generalized-gradient approximation, we investigated the proposed chain reaction pathway, thermodynamic stability, and electronic structure of the PA line. We found that the growth of the PA line with sp2 hybridization at the surface is kinetically more facile than that of the styrene line with sp3 hybridization, indicating that self-assembled growth of molecular lines employing alkynes rather than alkenes will be kinetically facilitated. We also found that the thermodynamic stability of the PA line is significantly enhanced compared to that of the styrene line. Analysis of the local density of states (LDOS) shows that, in the PA line, π electrons originating from the terminal C═C bond and the benzene ring are well delocalized across the ...

Hydrosilylation of Styrene on Water-Saturated Si(001)-2×1 at Room Temperature

The Si(001)-2×1 surface saturated by water is characterized by the passivation of nearly all of the dimerized atoms by H/OH terminations, except isolated dangling bonds, whose areal density is in the range of 1.5 ± 0.2 × 10–2 defects per Si atom. Therefore the water-saturated Si(001)-2×1 surface presents similarities with the defective H-terminated Si(001)-2×1 surface (presence of monohydrides and isolated dangling bonds), on which alkene molecules are known to be grafted via a radical-based hydrosilylation mechanism initiated at silicon dangling bonds. These common features stimulated the present study devoted to the reactivity of the water-saturated surface (n+-doped substrate) with styrene (H2Cα═CβH–C6H5) at room temperature. Using synchrotron radiation X-ray photoemission spectroscopy (XPS), our aim was to estimate the extent of styrene growth and to characterize the chemistry of the adsorbed molecule. XPS showed that styrene does react with the surface: after an exposure of 6.7 L (900 s × 0.75 × 10–8 Torr), we estimate that about 0.2 molecules per Si dimer (∼0.1 molecule per Si atom) are grafted on the surface. The C 1s XPS spectrum is consistent with a hydrosilylation product, Si–CαH2–CβH2–C6H5. Indeed we found that the C 1s spectral shape of styrene mono-σ bonded to the water-covered surface is markedly different from that of styrene di-σ bonded to the clean Si(001)-2×1 surface, confirming the specificity of the reaction product formed on the former surface. Mechanistically, a radical-based hydrosilylation reaction is the most plausible, as theoretical works indicate that the activation barrier of the latter mechanism is much lower than that of a direct, concerted mechanism. The C 1s spectral shape also excludes a reaction of the molecule with surface hydroxyls, leading to the formation of monohydroxyls CβOH(H) (radical-based reaction) or of Si–O–C linkages (Markovnikov or anti-Markovnikov addition).

In situ synchrotron based x-ray fluorescence and scattering measurements during atomic layer deposition: Initial growth of HfO2 on Si and Ge substrates

The initial growth of HfO2 was studied by means of synchrotron based in situ x-ray fluorescence (XRF) and grazing incidence small angle x-ray scattering (GISAXS). HfO2 was deposited by atomic layer deposition (ALD) using tetrakis(ethylmethylamino)hafnium and H2O on both oxidized and H-terminated Si and Ge surfaces. XRF quantifies the amount of deposited material during each ALD cycle and shows an inhibition period on H-terminated substrates. No inhibition period is observed on oxidized substrates. The evolution of film roughness was monitored using GISAXS. A correlation is found between the inhibition period and the onset of surface roughness.

Back-reflection Second-harmonic Generation of (111)Si: Theory and Experiment

We consider second-harmonic generation (SHG) from a (111) surface of a tetrahedrally bonded semiconductor illuminated at normal incidence by a focused pump beam of Gaussian cross section as a model of SHG by focused beams. Calculations are done in the anisotropic bond model (ABM) and the results are applied to Si. The unit-cell configuration is simple enough for the calculations to be done analytically, so the results can be compared directly to similar calculations done for amorphous material. Although the differences in unit-cell symmetry occur on the atomic scale, they lead to large differences in the spatial distribution of the emerging radiation. Lateral focusing, which might be expected to increase the bulk contribution to SHG by increasing the lateral field gradient, has little effect; the spatial-dispersion contribution remains dominated by the phase term. Focusing does not inhibit backscattered SHG from the bulk, although our data on the oxidation of H-terminated (111)Si clearly show that in some cases the interface contribution dominates by a wide margin.

Atomic Layer Deposition of Ruthenium Using the Novel Precursor bis(2,6,6-trimethyl-cyclohexadienyl)ruthenium

A recently reported ruthenium molecule, bis(2,6,6-trimethyl-cyclohexadienyl)ruthenium, has been developed and characterized as a precursor for atomic layer deposition (ALD) of ruthenium. This molecule, which has never been reported as an ALD precursor, was developed to address low growth rates, high nucleation barriers, and undesirable precursor phases commonly associated with other Ru precursors such as RuCp and Ru(EtCp)2. The newly developed precursor has similar vapor pressure to both RuCp and Ru(EtCp)2 but offers significant improvement in stability as evaluated by thermogravimetric analysis and differential scanning calorimetry. In an ALD process, it provides good self-limiting growth, with a 0.5 A/cycle growth rate under saturated dose conditions in a temperature between 250 and 300 °C. Furthermore, the precursor exhibits considerably better nucleation characteristics on SiO2, TiO2, and H-terminated Si surfaces, compared to RuCp2 and Ru(EtCp)2.

Using patterned H-resist for controlled three-dimensional growth of nanostructures

We present a study addressing the effectiveness of a monolayer of hydrogen as the lithographic resist for controlled three-dimensional (3D) growth of nanostructures on the Si(100) surface. Nanoscale regions on the H-terminated Si(100) were defined by H-desorption lithography via the biased tip of a scanning tunneling microscope (STM) to create well-defined regions of surface “dangling bonds,” and the growth of 3D nanostructures within these regions was achieved using a simultaneous disilane deposition and STM H-desorption technique. We demonstrate that 3D growth is strongly confined within STM depassivated regions while unpatterned H:Si(100) regions are robust against adsorption of the precursor molecules.

Roles of 2D Liquid in Reduction of the Glass-Transition Temperature of Thin Molecular Solid Films

To clarify the properties of a liquidlike phase formed on the surface, self-diffusion of molecules in monolayer and multilayer films has been investigated by measuring their uptake behaviors in porous Si substrates using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. It is found that self-diffusion of water, ethanol, and 3-methylpentane in the first monolayer commences at temperatures of ca. 110, 85, and 50 K, respectively, which are considerably lower than the corresponding glass-transition temperatures (Tg) in the bulk. The surface mobility of water appears to be not influenced by hydrogen bonds with substrates: The molecules form droplets on the H-terminated Si substrate at 110 K, whereas they start to diffuse into pores of the OH-terminated Si substrate at the same temperature. The ethanol and 3-methylpentane molecules tend to diffuse into pores of both hydrophilic and hydrophobic Si substrates, although a small number of much slower diffusers remain on the surface. Multilayer films are also incorporated into pores at temperatures well below Tg because of sequential diffusion of the topmost-layer molecules. The origins of nanoconfinement effects on reduction of Tg are discussed based on these findings.

Isolated Silicon Dangling Bonds on a Water-Saturated n+-Doped Si(001)-2 × 1 Surface: An XPS and STM Study

Using Si 2p core-level X-ray photoelectron spectroscopy we have measured the upward band bending at the surface of n+-doped water-saturated Si(001)-2 × 1 and inferred the macroscopic negative surface charge density of the surface. These macroscopic results are in excellent accord with the microscopic view provided by dual-bias scanning tunneling microscopy showing that the isolated silicon dangling bonds (∼1.2 × 10−2 defects per Si atom) bear indeed a negative charge. Noting the structural analogy between isolated dangling bonds on water-saturated Si(001) and H-terminated Si(001), in the final, prospective section of the paper, we raise the question of the possible role that these defects could play in radical chain reactions with π-bonded molecules, in relationship with the hydride/hydroxyl patterns that are resolved in the scanning tunneling images.