Hydrogen-passivated Si Research Articles

Scanning tunnelling spectroscopy and ab initio calculations of single-walled carbonnanotubes interfaced with highly doped hydrogen-passivated Si(100) substrates

The electronic properties of isolated single-walled carbon nanotubes (SWNTs) adsorbed onton- and p-doped hydrogen-passivated Si(100) surfaces are studied by ultrahigh vacuum scanningtunnelling spectroscopy and ab initio density-functional methods. SWNTs identified assemiconductors (s-SWNTs) have well-defined conduction and valence band edges separated by a≈1 eV gap, with the mid-gap Fermi level implying that the s-SWNTs areundoped. Relative s-SWNT/H-Si(100) band alignments inferred fromdI/dV plots are sensitive to the polarity of the substrate doping. Band structure calculations for a(12,4) s-SWNT corroborate experimental data: n-type (p-type) doping of the substrateleads to a shift of the surface bands lower (higher) in energy relative to those of thes-SWNT. The adsorption energy and charge transfer calculated for the (12,4) s-SWNTphysisorbed onto H-Si(100) are considerably less than values reported for the same tube onunpassivated Si(100) and are registration independent. The atomistic results presented herehave critical implications to hybrid electronic and photonic devices that rely upon a directinterface between a SWNT and a technologically relevant semiconductor such as Si orGaAs.

First-principles study of the Young’s modulus of Si ⟨001⟩ nanowires

We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si ⟨001⟩ anowires. The nanowires are taken to have predominantly {100} surfaces, with small {110} facets. The Young's modulus, the equilibrium length, and the residual stress of a series of prismatic wires are found to have a size dependence that scales like the surface area to volume ratio for all but the smallest wires. We analyze the physical origin of the size dependence and compare the results to two existing models.

Lateral Manipulation of Single‐Walled Carbon Nanotubes on H‐Passivated Si(100) Surfaces with an Ultrahigh‐Vacuum Scanning Tunneling Microscope

Ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM) can be used for the manipulation of individual atoms and molecules into complex arrangements for sensitive electrical and structural characterization. However, the systematic UHV STM manipulation of single-walled carbon nanotubes (SWNTs), high-aspect-ratio molecular wires derived from graphene that exist in both semiconducting and metallic forms, has yet to be reported. In this work, we demonstrate the room-temperature lateral manipulation of approximately 1-nm-diameter SWNTs on UHV-prepared, hydrogen-passivated Si(100) surfaces. We show the reproducible actuation of SWNTs having lengths as small as 13 nm, along with the partial division of a two-tube bundle. Moreover, UHV STM desorption of H at the SWNT/Si interface is introduced as a means of locally strengthening the interaction between the tube and the surface. The UHV STM manipulation scheme described here is potentially extensible to the orientational control of SWNTs interfaced with atomically clean semiconducting surfaces, such as InAs(110), GaAs(110), and unpassivated Si(100), for which first-principles calculations based on density functional theory have been reported recently in the literature.

Structure of tetracene films on hydrogen-passivated Si(001) studied via STM, AFM, and NEXAFS

Scanning tunneling microscopy STM, atomic force microscopy AFM, and near-edge x-ray absorption fine structure NEXAFS have been used to study the structure of tetracene films on hydrogen-passivated Si001. STM imaging of the films with nominal thickness of three monolayers 3M L exhibits the characteristic “herringbone” molecular packing known from the bulk crystalline tetracene, showing standing molecules on the ab plane. The dimensions and orientation of the herringbone lattice indicate a commensurate structural relationship between the lattice and the crystalline substrate. The corresponding AFM images illustrate that at and above the third layer of the films, the islands are anisotropic, in contrast with the submonolayer fractals, with two preferred growth directions appearing orthogonal to each other. The polarization dependent NEXAFS measurements indicate that the average molecular tilting angle with respect to the surface first increases with the film thickness up to 3 ML, then stabilizes at a value close to the bulk tetracene case afterwards. The combined results indicate a distinct growth morphological change that occurs around a few monolayers of thickness.

Research on Field-Induced Oxidation Processing Technology by Contact-Mode AFM in Ambient Air

The mechanism responsible for scanning probe field-induced oxidation in ambient air is attributed to an electrochemical process, i.e., anodic oxidation or anodization,after the analyses is given of a surface of a sample exposed to air. The effects of biases, tip speeds on morphology of field-induced oxidation, are introduced and deduced in the form of kinetics formula of oxidation growth. The field-induced oxidation of hydrogen-passivated Si (Si:H)using contact-mode AFM in air at room temperature is investigated. The result achieved from the experiment and that drawn from theoretical analysis are identical, which indicates the accuracy of the experimental operation.This experiment suggested that it may be used for further investigation of field-induced oxidation technology.

Electrical conduction of carbon nanotube atomic force microscopy tips: Applications in nanofabrication

This paper reports the electrical transport properties of the interface of a multiwalled carbon nanotube (MWNT) in physical end contact with a hydrogen-passivated Si surface and a Pt surface. The electrical measurement was performed in an atomic force microscope (AFM) with a MWNT attached to a scanning probe in contact mode at approximately 50% relative humidity. AFM force-distance spectroscopy was employed to set the degree of contact between the MWNT tip with the surface. The tip-substrate interface dominates the electrical measurement in this configuration, showing electrical conductivity characteristics indicative of the tip-substrate junction. MWNT tips in contact with a Pt surface exhibit a linear I-V behavior with electrical resistances in the range of 30–50kΩ, demonstrating the metallic nature of the MWNT. Results are presented for the investigation of the current-induced joule heating limitations of MWNT tips under ambient conditions. Thinning of the outer walls through a current-induced thermal oxidation process is observed at a current greater than 5μA, exhibiting a current density of greater than 106A∕cm2. For a MWNT tip in end contact with a highly p-doped silicon surface, a diode-like metal-insulator-semiconductor (MIS) junction is measured. Modeling of the MIS junction is presented and compared to the experiments.

Integer quantum Hall effect on hydrogen-passivated silicon (1 1 1) surfaces

We report magnetoresistance measurements of a high-mobility two-dimensional electron system induced on a hydrogen-passivated Si(1 1 1) surface. At temperatures of T = 150 mK and magnetic fields up to 12 T, signatures of the integer quantum Hall effect with the lowest Landau filling factor of 2 have been observed.

Flux dependence of the morphology of a tetracene film on hydrogen-passivated Si(100)

The initial stage of vacuum evaporated tetracene films on hydrogen-passivated Si(100) substrates has been investigated using ex situ atomic force microscopy. Three-dimensional (3D) islands and dendrites are obtained at low deposition rates near equilibrium growth conditions. By increasing the deposition rate to a proper range away from equilibrium conditions, terraced grains are formed with layers of standing molecules and good film connectivity. 3D grains with a high aspect ratio appear once the deposition rate is beyond the optimized range. These flux-dependent results indicate a kinetic path for the formation of uniform tetracene films. The initial growth patterns are discussed within the frame of the morphological instability theory.

Structure and Morphology of Tetracene Thin Films on Hydrogen-Terminated Si(001)

ABSTRACTScanning tunneling microscopy (STM), atomic force microscopy (AFM) and near-edge x-ray absorption fine structure (NEXAFS) have been used to study the structure of tetracene films on hydrogen-passivated Si(001). A distinct growth morphology change that occurs around a few monolayers of film thickness was characterized. This coverage-dependent film structural phase transition leads to a molecularly ordered film structure commensurate with the crystalline substrate.

Formation of SiCH6-mediated Ge quantum dots with strong field emission properties by ultrahigh vacuum chemical vapor deposition

Pretreatment of silicon surface with SiCH6 was used to modify the Stranski-Krastanow growth mode of Ge on Si(001) at 550°C by ultrahigh vacuum chemical vapor deposition. With the appropriate SiCH6 mediation, the elongated Ge hut clusters can be transformed to highly uniform multifaceted domes with a high Ge composition at the core. These SiCH6-mediated Ge dots have an average diameter and height of 38 and 7 nm, respectively. The modified growth mode for the formation of SiCH6-mediated Ge dots can be attributed to (i) an almost hydrogen-passivated Si surface to limit the nucleation sites for dot formation and (ii) the incorporation of Ge atoms, repelled by the C-rich areas, into the existing Ge dots. The results also demonstrate that SiCH6-mediated dots exhibit the improved field emission characteristics compared to shallow Ge huts.

Diffusion-limited submonolayer pentacene thin film growth on hydrogen-passivated Si(1 1 1) substrates

Growth of ultrathin pentacene films is investigated as a function of coverage by atomic force microscopy. Initially, pentacene grows as monolayer fractal islands and evolves into compact islands before coalescence. Stabilization factors, against diffusion-limited-aggregation in terms of interaction between islands and interlayer monomer transport, are proposed to explain the shape transition. Simulations based on a simple model of heterogeneous film growth are found to agree with experimental observations. The role of surface diffusion in island shape transition is revealed by a comparison between pentacene growth on the hydrogen terminated and oxidized Si substrates.

High mobility two-dimensional electron system on hydrogen-passivated silicon(111) surfaces

We have fabricated and characterized a field-effect transistor in which an electric field is applied through an encapsulated vacuum cavity and induces a two-dimensional electron system on a hydrogen-passivated Si(111) surface. This vacuum cavity preserves the ambient sensitive surface and is created via room temperature contact bonding of two Si substrates. Hall measurements are made on the H–Si(111) surface prepared in aqueous ammonium fluoride solution. We obtain electron densities up to 6.5×1011cm−2 and peak mobilities of ∼8000cm2∕Vs at 4.2K.

“Band bending” in copper phthalocyanine on hydrogen-passivated Si(111)

Ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) were employed to study the electronic density of states of copper phthalocyanine (CuPc) layers deposited onto hydrogen passivated Si(1 1 1) substrates. The highest occupied and lowest unoccupied molecular orbital (HOMO respectively LUMO) features are found to shift gradually in the same direction with increasing film thickness. At approximately 15 nm, the shifts saturate with a total amount of about 0.4 eV.

Ab initioabsorption spectra of Ge nanocrystals

Ab initio absorption spectra and optical gaps for hydrogen-passivated Ge nanocrystals are calculated using time-dependent density functional theory within the adiabatic local density approximation. The results are compared to previous effective mass, tight-binding, empirical pseudopotential, and ``$\ensuremath{\Delta}$ self-consistent field'' calculations and shed light on the validity of the various approximations used. By comparing our results with calculations for hydrogen-passivated Si nanocrystals, we predict that the Ge optical gap is smaller than that of Si for any nanocrystal size.

Gas phase chlorination of hydrogen-passivated silicon surfaces

A simple method is described to functionalize hydrogen-passivated Si(111) and Si(100) surfaces with chlorine (Cl2) gas. Infrared-absorption spectroscopy provides a positive identification of chlorination and mechanistic information on the chlorination of H-terminated Si surfaces, and on the structure and stability of chlorine-terminated Si surfaces (Cl∕Si). We find that the chlorination process does not change the surface morphology: H∕Si(111)-(1×1) surfaces and HF-etched Si(100) surfaces remain atomically flat and atomically rough, respectively, upon chlorination. Chlorinated S: surfaces are stable in a nitrogen atmosphere for over 12 hours.

Thin PTCDA films on Si(0 0 1): 2. Electronic structure

We have studied the thin film formation and the electronic structure of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA), on clean and on hydrogen-passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), angle-resolved photoelectron spectroscopy (ARPES), near edge X-ray absorption fine structure (NEXAFS) and low energy electron diffraction (LEED). On the H passivated surface the changes in the electronic structure of the substrate and the molecules with increasing film thickness are very small. The molecular orbitals show a dispersive behavior, indicating that the PTCDA layers are ordered. On the reactive clean surface the anhydride groups of the molecule interact with the substrate as indicated by changes in the core level binding energies. This results in a much lower ordering in the film compared to PTCDA on a passivated silicon surface. There is no sign of decomposition of the molecule because of the more reactive substrate.

Interface properties of metal/cytosine/Si(1 1 1):H heterostructures studied by means of SERS and DFT

In this work the interface formation between cytosine and hydrogen-passivated Si(1 1 1) substrates, the growth of cytosine layers as well as the interface between a metal (In or Ag) and the cytosine layers are studied. Cytosine was thermally evaporated by organic molecular beam deposition (OMBD) onto H-passivated Si(1 1 1) substrates under ultra-high vacuum conditions. Metal deposition on monolayers (∼0.4 nm) of cytosine leads to an enhancement of the Raman signal via the surface-enhanced Raman scattering (SERS) effect. The interaction with metals is found to be very different due to the large difference in the ionisation potential of Ag and In (IP Ag = 7.58 eV, IP In = 5.78 eV). The signal enhancement arises mainly from contributions due to molecule–metal charge transfer. Density functional theory calculations were employed for modelling the interaction of metal atoms with cytosine. Computational approaches were carried out on silver–cytosine and indium–cytosine complexes using the B3LYP density functional with the LANL2DZ basis set.

X-ray studies of self-assembled organic monolayers grown on hydrogen-terminated Si(111).

The structure of self-assembled monolayers (SAMs) of undecylenic acid methyl ester (SAM-1) and undec-10-enoic acid 2-bromo-ethyl ester (SAM-2) grown on hydrogen-passivated Si(111) were studied by X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The two different SAMs were grown by immersion of H-Si(111) substrates into the two different concentrated esters. UV irradiation during immersion was used to create Si dangling bond sites that act as initiators of the surface free-radical addition process that leads to film growth. The XRR structural analysis reveals that the molecules of SAM-1 and SAM-2 respectively have area densities corresponding to 50% and 57% of the density of Si(111) surface dangling bonds and produce films with less than 4 angstroms root-mean-square roughness that have layer thicknesses of 12.2 and 13.2 angstroms. Considering the molecular lengths, these thicknesses correspond to a 38 degrees and 23 degrees tilt angle for the respective molecules. For SAM-2/Si(111) samples, XRF analysis reveals a 0.58 monolayer (ML) Br total coverage. Single-crystal Bragg diffraction XSW analysis reveals (unexpectedly) that 0.48 ML of these Br atoms are at a Si(111) lattice position height that is identical to the T1 site that was previously found by XSW analysis for Br adsorbed onto Si(111) from a methanol solution and from ultrahigh vacuum. From the combined XPS, XRR, XRF, and XSW evidence, it is concluded that Br abstraction by reactive surface dangling bonds competes with olefin addition to the surface.

Ultrahigh-vacuum scanning tunneling microscopy and spectroscopy of single-walled carbon nanotubes on hydrogen-passivated Si(100) surfaces

Single-walled carbon nanotubes (SWNTs) have been studied on a Si(100)-2×1:H surface using an ultrahigh-vacuum (UHV) scanning tunneling microscope (STM). Dry deposition of SWNTs in situ establishes the pristine interface necessary to elucidate fundamental physical and electronic interactions between SWNTs and silicon. We have achieved simultaneous atomic resolution STM images of isolated SWNTs and the local H-passivated Si(100) substrate. Scanning tunneling spectroscopy served to characterize both semiconducting and metallic SWNTs. In each case, electronic features unique to the nanotube can be identified within the substrate band gap. In contrast to previous UHV STM studies of SWNTs on Au(111), our investigation is motivated by the technological relevance of the Si(100) substrate and the potential for nanofabrication of hybrid SWNT-Si electronic devices on the Si(100)-2×1:H platform.

Molecular Electronics on Silicon: An Ultrahigh Vacuum Scanning Tunneling Microscopy Study

An ultrahigh vacuum scanning tunneling microscope (STM) was employed as a tool to characterize two distinct molecular electronic strategies on the Si(100) surface. Initially, the self-directed growth of one-dimensional styrene molecular chains on hydrogen-passivated Si(100) was investigated. High-resolution empty states imaging of these styrene nanostructures confirms alignment of phenyl groups along the chain. However, attempts at STM charge transport measurements were limited by tip induced desorption of styrene molecules. Consequently, an alternative oxygen radical chemistry was also investigated. In particular, the chemical adsorption of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) onto clean Si(100) leads to the formation of an exceptionally stable silicon-oxygen bond that can withstand high bias charge transport measurements up to +/- 5 volts. Direct charge transport measurements through individual TEMPO molecules on degenerately n-type doped Si(100) reveal room temperature negative differential resistance behavior for negative sample biases exceeding - 3 volts.