Hydrogen-terminated Si Research Articles

Polycrystalline domain structure of pentacene thin films epitaxially grown on a hydrogen-terminated Si(111) surface

Single-monolayer high pentacene (Pn) dendrites grown on a hydrogen-terminated Si(111) surface [H-Si(111)] under ultrahigh vacuum were observed by low-energy electron microscopy and microbeam low-energy electron diffraction analyses. We determined the epitaxial structure (type I) inside a unique polycrystalline domain structure of such dendrites, each of which has six equivalent epitaxial orientations of Pn two-dimensional (2D) unit cells. There are three sets of these cells, which are rotated $\ifmmode\pm\else\textpm\fi{}120\ifmmode^\circ\else\textdegree\fi{}$ relative to each other. Domain boundaries inside each dendrite were successfully observed by scanning tunneling microscopy. In addition, we found another epitaxial relation (type II): the polycrystalline domain structure and lattice parameters are similar to those of the type-I dendrite; however, the 2D unit cells of the type-II dendrite are rotated approximately 90\ifmmode^\circ\else\textdegree\fi{} relative to those of the type-I dendrite. These results suggest that the crystal structure of the dendrites on H-Si(111) is determined mainly by the interaction between Pn molecules. Each dendrite is composed of domains that are exclusively of type I or II. The so-called point-on-line coincidences are found between the Pn 2D lattices of types I and II, and H-Si(111). The higher commensurability of the type-I dendrites than the type-II dendrites results in a higher probability of type-I dendrite formation. Moreover, for both the type-I and type-II dendrites, we found supercell structures. We estimated the minimum interface energy between the dendrite and H-Si(111) from an island's free energy, which is necessary to reproduce the growth of a single-monolayer high dendrite.

Identification of Allylamine and Allylalcohol Adsorbate on Hydrogen-terminated Si(111)

The adsorbates formed by ultraviolet photo-assisted adsorption of allylamine (CH2=CHCH2-NH2) and allylalcohol (CH2=CHCH2- OH) on hydrogen-terminated Si(111) surfaces were examined by vibrational analysis using high-resolution electron energy loss spectroscopy (HREELS). The spectra exhibit the stretching signals for N-H and O-H, indicating the survival of functional groups -NH2 and -OH within the precursor molecules on the adsorbed layers. For the allylamine adsorbate, delivery of gaseous ketene (CH2=C=O) resulted in formation of amide (CH3CONH-) adsorbate. The allylamine adsorbate was evidently functional for peptide-bond formation. These monolayers are prospective materials for usage in nanometer-scale fabrication, chemical functionalization and biochemical analysis.

UHV characterization of ambient-dosed hydrogen-terminated Si(0 0 1)

We report ultra-high vacuum characterization of a high-molecular weight pentiptycene-based copolymer deposited from solution onto H-terminated Si(0 0 1) surfaces under ambient conditions. We found that NMR-grade deuterated chloroform was the best choice of solvent to preserve the cleanliness of the sample, resulting in the lowest sample C and O contamination as determined by AES and STM. The H-terminated Si(0 0 1) – 2 × 1 surface proved to be an excellent metrology tool for measuring the molecular features of the polymer with nanometer resolution. Atomically resolved STM images of single polymer molecules were obtained. The average periodicity along the polymer chain was 1.26 nm, with the width of the polymer varying between 0.98 nm and 1.5 nm. Kinks observed along the polymer chain are consistent with a reduction of the polymer backbone from alkyne to alkene.

Morphology, structure, and magnetism of FeCo thin films electrodeposited on hydrogen-terminated Si(111) surfaces

In this work we describe the fabrication of FeCo alloy (less than 10 at% Co) thin films from aqueous ammonium sulfate solutions onto n-type Si(111) substrates using potentiostatic electrodeposition at room temperature. The incorporation of Co into the deposits tends to inhibit Fe silicide formation and to protect deposits against oxidation under air exposure. As the incorporation of Co was progressively increased, the sizes of nuclei consisting of FeCo alloy increased, leading to films with a highly oriented body-centered cubic structure with crystalline texture, where (110) planes remain preferentially oriented parallel to the film surface.

Selected area deposition of multiwalled carbon nanotubes from solution

A simple method is described for the area-selective deposition of multiwalled carbon nanotubes (MWCNTs) from solution. Selectivity is achieved by functionalizing the MWCNTs with groups that adsorbs preferentially on specific areas of a patterned substrate. Well-characterized octadecylester-functionalized MWCNTs are adsorbed directly from solution on to the substrate. The non-polar hydrocarbon chains interact more strongly with non-polar hydrogen-terminated Si surfaces than do polar oxide surfaces and this gives rise to area-selective adsorption of the MWCNTs on a patterned Si/SiO 2 substrate. Potential applications of this method and its application for other types of patterned substrates are discussed.

Surface Morphology and Photoluminescence of Alq3 Films Grown by Hot-Wall Method

Tris-(8-hydroxyquinoline) aluminum (Alq3) films were deposited on hydrogen-terminated Si(111) substrates changing the substrate temperature from 80 to 160°C by hot-wall method. The deposited films were characterized by surface morphology and photoluminescence (PL). With increasing substrate temperature, the emission intensity of the Alq3 films increases and the wavelength of the emission peaks shifts to lower wavelength from 528 to 507 nm. In addition, the vibronic structure with distinct five peaks was observed in the PL spectra of the films deposited at substrate temperatures of 100 and 120°C.

Preparation of an Ultraclean and Atomically Controlled Hydrogen-Terminated Si(111)-(1× 1) Surface Revealed by High Resolution Electron Energy Loss Spectroscopy, Atomic Force Microscopy, and Scanning Tunneling Microscopy: Aqueous NH4F Etching Process of Si(111)

We propose an improved wet chemical process for preparing a high-quality hydrogen-terminated Si(111)-(1× 1) surface and show an atomically ordered and ultraclean surface without carbon and oxygen contamination. The vibrational properties and surface morphology are investigated by high-resolution electron energy loss spectroscopy (HREELS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). The HREELS spectra and images of AFM and STM reveal the precise aqueous NH4F etching process of Si(111) and indicate the high controllability of steps and terraces at the atomic scale. The surface cleanliness and morphology strongly depend on the etching time. At the etching time of 10 min, we obtain an ultraclean and atomically ordered surface with wide terraces of 36±7 nm step distance. It is confirmed by AFM and STM that 1.0% ammonium sulfite is useful for removing dissolved oxygen in the 40% NH4F etching solution and for preparing a high-quality H:Si(111)-(1× 1) surface with a low density of etch pits. The onset of tunneling current and the gap of 1.39 eV are measured by scanning tunneling spectroscopy. There is no peak at -1.3 eV in comparison with the previous report [Phys. Rev. Lett. 65 (1990) 1917].

Preparation of reactive surface-modified Ag nanoparticles and their assembly on a Sisurface via covalent bonds

Surface-modified Ag nanoparticles containingω-functionalized reactive groups were prepared via the reduction of aqueousAgNO3 in the presence ofa modifying agent, O,O′-di(10-undecene) dithiophosphinic acid (UDDPA). The resulting surface-modified Agnanoparticles were assembled on the surface of hydrogen-terminated Si(111) by a thermalhydrosilylation reaction. The morphologies and structures of the Ag nanoparticles and theirassembled film on the Si substrate were characterized by means of transmission electronmicroscopy (TEM), UV–visible absorption (UV–vis) spectroscopy, Fourier transformationinfrared (FTIR) spectroscopy, atomic force microscopy (AFM) and x-ray photoelectronspectroscopy (XPS). The friction and wear behavior of the assembled film of thesurface-modified Ag nanoparticles sliding against GCr15 steel (AISI52100) ina ball-on-plate configuration was evaluated using a UMT-2 friction and weartester. It was found that the surface-modified Ag nanoparticles had an averagediameter of about 10 nm. The surface-modified Ag nanoparticles had unsaturatedC = C functional groups sitting at the outermost surface and attached to the Si(111)surface via Si–C bonds. Moreover, the assembled film of the surface-modified Agnanoparticles showed good friction-reducing ability for sliding against the steelcounterpart at a small normal load and sliding velocity, but it had poor antiwearability and load-carrying capacity and was liable to lose lubricity under severesliding conditions, owing to a loss of structural integrity and consequent lubricationfailure.

Gold-Catalyzed Oxide Nanopatterns for the Directed Assembly of Ge Island Arrays on Si

The heteroepitaxial growth of Ge on Au-patterned Si(001) is investigated using in situ spectromicroscopy. Patterning of a hydrogen-terminated Si surface with a square array of Au dots followed by brief exposure to air leads to the spontaneous, local oxidation of Si. The resulting oxide nanopattern limits the surface migration of Au during annealing up to 600 degrees C, resulting in complete preservation of the Au pattern. Subsequent deposition of Ge induces a redistribution of Au across the surface even as the oxide nanopattern persists. As a result, the oxide pattern drives the growth of Ge islands into an ordered assembly, while Au decorates the surfaces of the Ge islands and modifies their shape.

Simulations of Scanning Tunneling Microscopy for B-/P-doped Si(111) Surfaces

We have investigated scanning tunneling microscopy (STM) for hydrogen-terminated Si(111) surfaces that have substitutional dopants in the subsurface region, using first-principles calculations within the density functional theory. The STM image for the occupied state indicates protrusion around a subsurface B atom and depression around a subsurface P atom. For the STM image of the unoccupied state, protrusions and depressions are interchanged. These dopant features on the STM image can be explained by the change in electrostatic potential around a dopant ion. The most stable depth of the substitutional B or P atom in H-terminated Si(111) surfaces is determined to be the second atomic layer from the topmost Si layer. The effect of a dangling bond created by the desorption of H atoms on the STM image of the subsurface dopant has also been studied.

Two-Dimensional Island Shape Determined by Detachment

Effect of an anisotropic detachment on a heteroepitaxial island shape is studied by means of a kinetic Monte Carlo simulation of a square lattice gas model. Only with molecular deposition followed by surface diffusion, islands grow in a ramified dendritic shape, similar to diffusion-limited aggregate (DLA). Introduction of molecular detachment from edges makes islands compact. To understand an anisotropic island shape observed in the experiment of pentacene growth on a hydrogen-terminated Si(111) vicinal surface, asymmetry in detachment around the substrate step is assumed. Edge molecules detach more to the higher terrace than to the lower terrace. The island edge from which molecules are easy to detach is smooth and the one hard to detach is dendritic. If islands are close to each other, islands tend to align in a line, since detached molecules from the smooth edge of the right island are fed to the dendritic and fast growing edge of the left island.

Origin of Differing Reactivities of Aliphatic Chains on H−Si(111) and Oxide Surfaces with Metal

The interaction of deposited metals with monolayer films is critical to the understanding of, and ultimate utility of, the emerging arena of molecular electronics. We present the results of a thorough study of the interaction of vapor-deposited Au and Ag on alkane films attached to Si substrates. Two distinct categories of films are studied: C18 films formed from the hydrosilation reaction of octadecyl trichlorosilane with thin SiO2 layers and C18 films formed from the direct attachment of functionalized alkanes with hydrogen-terminated Si. Two direct attachment chemistries were studied: (Si)3−Si−O−C linkages of 1-octadecanol and octadecanal on H-terminated Si(111) and (Si)3−Si−C linkages of 1-octadecene on H-terminated Si(111). The reactivity of the films was studied with p-polarized backside reflection absorption spectroscopy (pb-RAIRS), sputter depth profiling X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and device electrical tests. Independe...

Vertically Oriented Germanium Nanowires Grown from Gold Colloids on Silicon Substrates and Subsequent Gold Removal

A linker-free method to deposit citrate-stabilized Au colloids onto hydrogen-terminated Si by acidifying the Au colloid solution with HF or HCl is presented. This method prevents oxide formation and provides a model system for studying orientation control of nanowires by epitaxy. Conditions are reported that result in vertically oriented Ge nanowires of uniform diameter and length on Si(111). We then present a method to remove Au catalysts from the nanowires with aqueous triiodide and HCl.

Oxidation of Atomically Flat and Hydrogen-Terminated Si ( 111 ) Surfaces by Hydrogen Peroxide

The electrochemical current-voltage curve of a surface measured in an aqueous solution is very sensitive to the surface condition. Using this property, we analyzed the surface after chemical oxidation by . Thermodynamically, back bonds of surface bonds are oxidized more easily than the bonds. However, when the surface was atomically fattened and hydrogen-terminated, the oxidation rate of the back bonds was lowered and the surface bonds and back bonds were oxidized simultaneously. The lowered reactivity of the back bonds was attributed to the hindrance of the approach of molecules to the back bonds by the bonds regularly stretching perpendicular to the surface. Fourier transform infrared (FTIR) spectra showed that the back bonds on step sites were oxidized very easily, and this was attributed to easy access of molecules to the bonds on the step sites. These results indicate that the regular structure of the atomically flattened surface is the reason for its good stability against chemicals.

Controlled growth of aluminum oxide thin films on hydrogen terminated Si(0 0 1) surface

Auger electron spectroscopy, energy electron loss spectroscopy, atomic force microscopy and transmission electron microscopy were used to characterize ultra-thin aluminum oxide films grown on hydrogen-terminated Si(0 0 1)–H substrates via a specific atomic layer deposition and oxidation technique. Oxide thin films grown in such a way are highly stable with temperature at least up to 700 °C. Band gap was estimated to be 6.6±0.2 eV, independent of thickness. Formation of the oxide layer slightly increases the initial roughness of silicon surface. Furthermore, no silicon oxide was found at the aluminum oxide–silicon interface.

Non-contact Atomic Force Microscopy Simulations of Hydrogen-terminated Si(100) Surfaces with a Methyl

Toward the development of a simulator of scanning probe microscope for systems with adsorbed organic molecules, we simulate non-contact atomic force microscope images of hydrogen-terminated Si(001)2×1 surfaces with an isolated methyl using a Si tip with and without a hydrogen atom at the apex, using a density-functional based tight-binding method. We examine the constant height, constant frequency and energy dissipation images. In addition to normal images of constant height and constant frequency modes, we obtain the exotic energy dissipation images, where a low dissipation spot is surrounded by a ring of high dissipation region. These images can be understood from the site-dependence of force hysteresis.

Design and Fabrication of Micro-textures for Inducing a Superhydrophobic Behavior on Hydrophilic Materials

Artificial superhydrophobic surfaces are typically fabricated by tuning the surface roughness of intrinsically hydrophobic surfaces. We report here the design and fabrication of micro-textures for inducing a superhydrophobic behavior on hydrogen-terminated Si surfaces with an intrinsic water contact angle of approximately 74 degrees . The micro-textures consist of overhang structures with well-defined geometries fabricated by microfabrication technologies, which provide positions to support the liquid and prevent the liquid from entering into the indents between the micro-textures. As a result, water is in contact with a composite surface of solid and air, which induces the observed macroscopic superhydrophobic behavior.

Simultaneous measurement of potential and dopant atom distributions on wet-prepared Si(111):H surfaces by scanning tunneling microscopy

The authors demonstrate the ability of scanning tunneling microscopy and spectroscopy to simultaneously measure the distributions of both the surface potential and the individual dopant atoms on the atomically flat hydrogen-terminated Si(111) surfaces prepared by an aqueous NH4F etching without disturbing the original dopant distribution. At the p-n junctions, the acceptor and donor atoms were detected distinctly, and the variation in the observed height reflected the surface potential under the biasing condition. Further, a strong correlation between the dopant fluctuations and the surface potential distributions was identified.

Interactions between radical growth precursors on plasma-deposited silicon thin-film surfaces

We present a detailed analysis of the interactions between growth precursors, SiH3 radicals, on surfaces of silicon thin films. The analysis is based on a synergistic combination of density functional theory calculations on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of film growth on surfaces of MD-generated hydrogenated amorphous silicon (a-Si:H) thin films. In particular, the authors find that two interacting growth precursors may either form disilane (Si2H6) and desorb from the surface, or disproportionate, resulting in the formation of a surface dihydride (adsorbed SiH2 species) and gas-phase silane (SiH4). The reaction barrier for disilane formation is found to be strongly dependent on the local chemical environment on the silicon surface and reduces (or vanishes) if one/both of the interacting precursors is/are in a "fast diffusing state," i.e., attached to fivefold coordinated surface Si atoms. Finally, activation energy barriers in excess of 1 eV are obtained for two chemisorbed (i.e., bonded to a fourfold coordinated surface Si atom) SiH3 radicals. Activation energy barriers for disproportionation follow the same tendency, though, in most cases, higher barriers are obtained compared to disilane formation reactions starting from the same initial configuration. MD simulations confirm that disilane formation and disproportionation reactions also occur on a-Si:H growth surfaces, preferentially in configurations where at least one of the SiH3 radicals is in a "diffusive state." Our results are in agreement with experimental observations and results of plasma process simulators showing that the primary source for disilane in low-power plasmas may be the substrate surface.

Thermal Immobilization of Ferrocene Derivatives on (111) Surface of n-Type Silicon: Parallel between Vinylferrocene and Ferrocenecarboxaldehyde

Monolayers attached to a Si(111) surface through Si-C-C or Si-O-C covalent bonds were prepared by the thermally activated reaction (150 degrees C) of vinylferrocene (VFC) or ferrocenecarboxaldehyde (FCA) molecules with hydrogen-terminated Si(111) substrate in order to compare their reactivities. The resulting monolayers gave a couple of redox waves on voltammograms due to ferrocenyl moieties tethered at the surface. The voltammetric quantification revealed that the growth of electrochemically active layers was terminated within 5 h and the final surface coverages of the active ferrocenyl moieties were 58% and 16% for VFC- and FCA-based monolayers, respectively, indicating that the aldehyde molecule is less reactive. X-ray photoelectron spectroscopy and ellipsometry, however, gave an indication that the growth of the VFC layer did not self-terminate and proceeded beyond a monolayer, while this overgrown part of the layer was not electrochemically active.