Clean Si Surface Research Articles

Coadsorption of Ethylene and Nitrobenzene on Si(100)-2 × 1: Toward Surface Patterning at the Molecular Level

This study discusses the possible use of semiconductor surfaces for molecular patterning based on the formation of an organic layer consisting of ethylene and nitrobenzene coadsorbed on a Si(100) surface. Ethylene has been shown to chemisorb on every other surface dimer of the stable 2 × 1 reconstruction of a clean Si(100) surface at room temperature at coverages up to 0.5 ML. This partially covered surface can be used as a template for chemisorption of nitrobenzene that forms a very stable surface adduct. Vibrational study by multiple internal reflection IR confirms the coadsorption of ethylene and nitrobenzene, and heating the surface past the desorption temperature of ethylene leaves only the nitrobenzene adduct while eliminating ethylene. Theoretical calculations were performed to supplement the experimental findings. The stability of a full monolayer of ethylene was compared to that of half-monolayer coverage utilizing a cluster model mimicking five silicon surface dimers within the same dimer row. T...

High resolution photoemission study of SiOx/Si(111) interface disruption following in situ HfO2 deposition

We report on an in situ high resolution core level photoemission study of the early stages of interface formation between an ultrathin SiOx layer (∼0.3 nm) grown on the atomically clean Si(111) surface and a HfO2 dielectric layer. Si 2p core level spectra acquired at 130 eV photon energy reveal evidence of a chemically shifted component on the lower binding energy side of the substrate peak which is attributed to interface defect states resulting from the incorporation of silicon atoms from the substrate into the interfacial oxide at room temperature. This evidence of Si/SiOx interface disruption would be expected to increase charge carrier scattering mechanisms in the silicon and contribute to the generally observed mobility degradation in high-k stacks with ultrathin silicon oxide interface layers.

Palladium Thin-Films on Clean and Hydrogen-Terminated Si(110): The Effect of Hydrogen Termination on Metal Adsorption

A combined scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED) study of the room-temperature deposition of palladium on wet-chemically-prepared H/Si(110) and vacuum-prepared Si(110) “16×2” was carried out to gauge the ability of a hydrogen barrier layer to restrict reactive adsorption of Pd on Si(110) (e.g. silicide formation) and to determine the effect of the hydrogen barrier layer on the growth mode of the deposited films. For deposition quantities less than 1.5 monolayers (ML), Pd atoms adsorbed in a random fashion on the clean Si(110) surface, while islands formed on the H/Si(110) surface. However, after ∼3 ML of deposition the film morphology took on a disordered hillock motif for both clean and hydrogen-terminated substrates. The size distribution of the hillocks was remarkably narrow, and indistinguishable for clean and hydrogen terminated substrates. Further, the hillock morphology was invariant with Pd deposition between 3 and ∼8 ML. XPS analysis revealed the deposited films to be chemically similar to Pd2Si for both Si(110) “16×2”, and H/Si(110) substrates, indicating that the hydrogen barrier layer does not prevent silicide formation.

Photoemission studies of the initial interface formation of ultrathin MgO dielectric layers on the Si(111) surface

This study investigates the interface formation between a magnesium oxide dielectric overlayer and an ultrathin SiO 2 layer (∼ 0.3 nm) grown on the atomically clean p-type Si(111) surface in ultra high vacuum. Both soft X-ray synchrotron radiation based photoemission and conventional X-ray photoelectron spectroscopy have been used to characterise the evolution of the interface and monitor the change in the interfacial oxide thickness. As the MgO film grows, there is an increase in the intensity of the silicon oxide features indicating the growth of the interfacial oxide which saturates at a thickness of approximately 0.7 nm. Spectra acquired at the surface sensitive 130 eV photon energy, reveal the emergence of a chemically shifted component on the low binding energy side of the substrate peak which is attributed to atomic displacement of silicon atoms from the substrate to the interfacial oxide at room temperature. This evidence of atomic disruption at the high dielectric constant material (high-κ) and silicon interface would be expected to contribute to charge carrier scattering mechanisms in the silicon and could account for the generally observed mobility degradation in high-κ stacks. Thermal annealing studies of deposited MgO films show that dissociation begins to occur above 600 °C with desorption of Mg and the growth of a silicon oxide.

Electric-field effects on the diffusion of Si and Ge adatoms on Si(001) studied by density functional simulations

We present results of density functional theory (DFT) calculations of the electric-field effects on the clean Si(001) surface and the diffusion of Si or Ge adatoms on Si(001) surface. Our results indicate that the electric field only slightly affected the dimer bond lengths and buckling angles of the clean Si(001), implying that the electric field of scanning tunneling microscopy tip should not be responsible for the observation of symmetric dimers and the flip-flop motion of the buckling dimers. Also, the electric field mainly influences the diffusion along the dimer row, and the diffusion barrier could be reduced greatly under a positive electric field. It can be expected that the positive field should make the diffusion of Si or Ge adatoms on the Si(001) surface become more anisotropic.

First-principles study of identical [formula omitted] clusters on the Si(0 0 1) surface

The first-principles calculations have been performed on the adsorption of tetrahedron- and rhombus- Nb 4 clusters on the Si ( 0 0 1 ) - ( 4 × 2 ) . The results show that both Nb 4 configurations can be stably adsorbed on the Si(0 0 1) surface, and the adsorption of bidimensional Nb 4 clusters is more stable than those of tetrahedron- Nb 4 . The energy barriers are very high which indicates that it is difficult for the Nb 4 clusters to diffuse. The work functions of the most stable adsorption sites decrease compared with that of the clean Si(0 0 1) surface, which demonstrates that the electronic charges transfer from the Nb 4 clusters to the Si(0 0 1) surface.

The interaction between atoms of Au and Cu with clean Si(1 1 1) surface: A study combining synchrotron radiation grazing incidence X-ray fluorescence analysis and theoretical calculations

In order to evaluate the interactions between Au/Cu atoms and clean Si(1 1 1) surface, we used synchrotron radiation grazing incidence X-ray fluorescence analysis and theoretical calculations. Optimized geometries and energies on different adsorption sites indicate that the binding energies at different adsorption sites are high, suggesting a strong interaction between metal atom and silicon surface. The Au atom showed higher interaction than Cu atom. The theoretical and experimental data showed good agreement.

Microscopic (AFM) and resonant photoemission study of Gd/Si(1 1 1) interface

Atomic force microscopy (AFM) and Fano-type resonant photoemission spectroscopy (RPS) of Gd atoms deposited on Si(1 1 1) surface are presented. Layers of thicknesses 0.2, 2 and 15 nm were deposited under UHV conditions. Anomalously deep craters were observed on the Si(1 1 1) surface side of the interface below the Gd layer. We expect that anomalously high chemical activity of Gd and a dominating diffusion of Si atoms are responsible for the effect. Application of synchrotron radiation in the energy region hν corresponding to the Gd 4d–4f transition (140–170 eV) enabled Fano-type resonant photoemission spectra. Si(1 1 1) surface valence band spectra with contributions from Gd 4f electrons were detected for a 0.2-nm-thick Gd layer deposited onto a clean Si(1 1 1) surface. The photoemission study revealed a Fano resonance shape of the curve with resonance and antiresonance energies hv=151.8 and 146.8 eV, respectively. The Gd 4f localized electrons contribute to the valence band density of states located at 9.8 eV below the valence band edge.

Regular step systems on clean Si(hhm)7 × 7 surfaces

The preparation conditions and atomic structure of regular step systems on clean Si(hhm) surfaces are studied. Regular single- and triple-step structures with different periodicities have been fabricated in ultrahigh vacuum using various heat treatment procedures. The atomically resolved scanning tunneling microscopy data demonstrate several stable triple-step configurations on vicinal Si(hhm) surfaces.

Ab initiostudy of the diffusion and decomposition pathways ofSiHxspecies on Si(100)

Diffusion and decomposition of ${\text{SiH}}_{x}$ species adsorbed on the clean Si(100) surface are processes of relevance for the growth of crystalline silicon by plasma-enhanced chemical vapor deposition. In this work, we report an extensive search of diffusion and decomposition pathways for ${\text{SiH}}_{3}$, ${\text{SiH}}_{2}$, and SiH by means of combined ab initio metadynamics simulations and optimization of minimum-energy reactions paths. We find that on the clean surface ${\text{SiH}}_{3}$ undergoes stepwise decompositions into Si and H adatoms according to ${\text{SiH}}_{3}\ensuremath{\rightarrow}{\text{SiH}}_{2}+\text{H}\ensuremath{\rightarrow}\text{SiH}+2\text{H}\ensuremath{\rightarrow}\text{Si}+3\text{H}$ with an overall reaction barrier of the order of 0.8 eV, consistent with the scenario inferred from secondary ion mass spectroscopy data. The lifetime of ${\text{SiH}}_{3}$ at room temperature calculated within transition state theory in the harmonic approximation is in agreement with experiments. The lifetime of ${\text{SiH}}_{2}$ turns out to be similar to that of ${\text{SiH}}_{3}$. Possible trap states for ${\text{SiH}}_{2}$ are proposed, based on energetics and by comparing calculated scanning tunneling microscope images with experimental data.

Conducting atomic force microscopy studies of nanoscale cobalt silicide Schottky barriers on Si(111) and Si(100)

Cobalt silicide (CoSi2) islands have been formed by the deposition of thin films (∼0.1–0.3 nm) of cobalt on clean Si(111) and Si(100) substrates in ultrahigh vacuum (UHV) followed by annealing to ∼880 °C. Conducting atomic force microscopy has been performed on these islands to characterize and measure their current-voltage (I-V) characteristics. Current-voltage curves were analyzed using standard thermionic emission theory to obtain the Schottky barrier heights and ideality factors between the silicide islands and the silicon substrates. Current-voltage measurements were performed ex situ for one set of samples (termed “passivated surfaces”) where the silicon surface surrounding the islands was passivated with a native oxide. Other samples (termed “clean surfaces”) remained in UHV, while I-V curves were recorded. By comparing the barrier heights and ideality factors for islands on passivated surfaces and clean surfaces, the effects of the nonpassivated surfaces on conduction have been studied. The barrier heights measured from CoSi2 islands on clean surfaces are found to be ∼0.2–0.3 eV below barrier heights measured from similar islands on passivated surfaces. The main cause of the reduced Schottky barrier in the clean surface samples is attributed to Fermi level pinning by nonpassivated surface states of the clean silicon surface. However, the measured barrier heights of the islands are equivalent on both clean Si(111) and Si(100) surfaces, suggesting that the nonpassivated surface is influenced by cobalt impurities. Furthermore, the barrier heights of islands on the clean surfaces are lower than what can be explained by Fermi level pinning alone, suggesting the presence of additional reductions in the Schottky barrier heights. These variations are greater than what can be attributed to experimental error, and the additional barrier height lowering is primarily attributed to spreading resistance effects. Schottky barrier inhomogeneity is also identified as a possible cause of the additional barrier height lowering and nonideality in the Schottky contacts. Current-voltage measurements of the clean surface samples were also obtained at several temperatures. The barrier heights were found to decrease, and the ideality factors were found to increase with decreasing temperature. The dependence of the barrier height is attributed to the temperature variation of the Fermi level.

Growth of single-crystal SiO2 clusters on Si(001) surface

Crystalline SiO2 clusters of nearly square shape were fabricated from amorphous silicon deposited on a clean Si(001) surface, where the deposited silicon was oxidized in the presence of atomic hydrogen and annealed in an O2 atmosphere. The side of these clusters was aligned along the [110] and [11¯0] azimuths of the Si(001) substrate. The crystallinity of the clusters was verified by grazing incidence x-ray diffraction and low-energy electron diffraction. The crystal structure of SiO2 clusters was estimated to be β-tridymite, whose [101¯0] and [0001] axes were parallel to the ⟨110⟩ axes of the substrate Si(001). When the Si(001) surface without deposited amorphous silicon was oxidized by the method described above, single-crystal SiO2 clusters were also prepared. However, the shape, the size, and the number density of the clusters were different from those of the clusters made from the amorphous silicon.

Surface characteristics and protein adsorption on combinatorial binary Ti‐M (Cr, Al, Ni) and Al‐M (Ta, Zr) library films

Systematic studies of protein adsorption onto metallic biomaterial surfaces are generally lacking. Here, combinatorial binary library films with compositional gradients of Ti(1-x)Cr(x), Ti(1-x)Al(x), Ti(1-x)Ni(x) and Al(1-x)Ta(x), (0 <x < 1) and Al(1-y)Zr(y) (0 < y <0.5) as well as corresponding pure metal films were sputtered onto clean Si surfaces. Bulk and surface chemistry, film microstructure, and surface roughness were subsequently correlated to fibrinogen or albumin adsorption measured using a high throughput wavelength dispersive spectroscopy technique. X-ray diffraction revealed these binary films to have crystalline phases present primarily at either extreme of the compositional library and an amorphous zone dominating along the gradient. These mirror-like films were generally found by atomic force microscopy to have a roughness of less than 8 nm, with any relative increases in roughness consistent with the development of crystalline phases. Surface chemistry by quantitative high-resolution X-ray photoelectron spectroscopy differed significantly from bulk film composition as measured by electron microprobe, with TiO(2) and Al(2)O(3) preferentially forming on the binary film surfaces. Correspondingly, protein adsorption onto these films closely correlated with their surface oxide fractions. Aluminum deposited as either a constant-composition film or as part of a binary library consistently adsorbed the least amount of albumin and fibrinogen, with alumina-enrichment of the surface oxide correlating with this adsorption. Overall, this combinatorial materials approach coupled with high-throughput surface analytical methods provides an efficient method of screening potential metallic biomaterials that may enable as well systematic studies of surface properties driving protein adsorption on these metal / metal oxide systems.

Regular stepped structures on clean Si(hhm)7×7 surfaces

Regular single and triple step arrays with different periodicities have been fabricated in ultrahigh vacuum on clean Si(557) surfaces at various thermal treatment procedures. The atomic structure of the triple step staircases has been studied with high resolution scanning tunneling microscopy (STM). The results of atomically resolved STM experiments demonstrate a number of possible triple step configurations on Si(hhm) surfaces. The triple step models consistent with atomically resolved STM data obtained on regular Si(223) and Si(556) triple step staircases are presented. Possible driving forces for self-assembling regular step arrays on large scale areas are discussed.

Thickness Dependent Formation of Iron Silicides on Clean and Boron Modified Si(001) Surface

Thickness Dependent Formation of Iron Silicides on Clean and Boron Modified Si(001) Surface

First-principle study of Mg adsorption on Si(111) surfaces

We have carried out first-principle calculations of Mg adsorption on Si(111) surfaces. Different adsorption sites and coverage effects have been considered. We found that the threefold hollow adsorption is energy-favoured in each coverage considered, while for the clean Si(111) surface of metallic feature, we found that 0.25 and 0.5 ML Mg adsorption leads to a semiconducting surface. The results for the electronic behaviour suggest a polarized covalent bonding between the Mg adatom and Si(111) surface.

Control of GaP and GaAs Nanowire Morphology through Particle and Substrate Chemical Modification

We demonstrate two very different morphologies for GaP and GaAs nanowires grown by Au-assisted MOVPE on Si(111) substrates: rodlike wires and tapered wires with sharp tips. We show that the morphology is related to the stability of the particles at the wire tips during growth, and we propose that the mechanism of this effect is diffusion of Au away from the tip. Diffusion occurs, leading to tapered wires, only if there is a clean Si surface to act as a reservoir for the Au. Furthermore, the presence of indium in the particles, even at background levels from previous growth runs, inhibits the migration of Au. These results demonstrate a dramatic example of the sensitivity of wire morphology to substrate and particle chemistry, which could provide an important tool to tune nanowire morphology through particle alloying or surface treatment.

Antiferromagnetic coupling between two adjacent dangling bonds on Si(001): Total-energy and force calculations

Scanning tunneling microscopy experiments reported that desorption from the hydrogen- and halogen-terminated Si(001) surfaces exhibits frequently the two types of dangling-bond (DB) configurations. One is the intradimer configuration, where two DBs are within a single Si dimer, and the other is the interdimer configuration, where two DBs are on one side of two adjacent Si dimers. Our spin-polarized density-functional-theory calculations show that the intradimer configuration is nonmagnetic with a buckled-dimer geometry, while the interdimer configuration is antiferromagnetic with two adjacent symmetric dimers. In addition, we show that when the dissociative adsorption of hydrogen molecule occurs across the ends of two adjacent dimers on a clean Si(001) surface, such an antiferromagnetic coupling between two adjacent DBs still exists, thereby giving rise to a structural transformation from buckled to symmetric dimers.

Topologically induced surface electron state on Si(1 1 1) surfaces

First-principle electronic structure calculation reveals the appearance of a new class of surface state on hydrogenated and clean Si(1 1 1) surfaces. The states are found to exhibit different characteristics to conventional surface electron states in terms of the peculiar distribution of the wavefunction depending on the wavenumber. In addition, the state results in flat dispersion bands in a part of the surface Brillouin zone having energy of about 8 eV below the top of the valence band. An analytic expression based on the tight-binding approximation corroborates the surface electron state results from the delicate balance of the electron transfer among the atoms situated near the surface. The obtained results give a possible extension and generalization of the edge state in graphite ribbons with zigzag edges.

Structural conformations and electronic properties of biphenyl adsorbed on the clean and on the partially hydrogenated Si(1 0 0) surface: An ab initio calculation

First principles density functional calculations have been used to investigate the structural conformations and the electronic properties of biphenyl molecule adsorbed on the clean and on the partially hydrogenated Si(1 0 0) surfaces. The results show that the biphenyl can be adsorbed on several stable conformations on the clean Si(1 0 0) surface and on the Si(1 0 0) surface modified with hydrogens. The biphenyl is chemisorbed on the Si surface with adsorption energies very similar to each other for all conformations. The electronic structure results reveal that the occupied states are quite different for each conformation, while the empty states are very similar to each other, making possible to identify the different conformations only throw occupied states topography. The biphenyl in any conformation on the Si surface does not induce surface conductivity, even when the biphenyls on the Si surface are permitted to bond to each other forming a row of biphenyl molecules.