Surface Si Atoms Research Articles

Chlorine extraction by atomic hydrogen on Si(111)-7×7 surfaces

We have studied atomic-hydrogen-induced chlorine extraction on Si(111)-7×7 surfaces using photoemission spectroscopy and scanning tunneling microscopy (STM). We exposed the surface with mono- and polychloride Si to atomic hydrogen at room temperature. Photoemissions from Si 2 p core level and Cl 3 s level were measured before and after the atomic hydrogen dosage on the surfaces. Signals with both silicon-chlorides and Cl atoms decrease with increasing atomic hydrogen dosage. After annealing the hydrogen-exposed surface at 720 K, the STM images are very different from those of chlorine adsorbed Si(111) surface, and similar to those of hydrogen-covered Si(111) surfaces after the annealing. We conclude that chlorine atoms are extracted from the Cl/Si(111) surface by atomic hydrogen, and the surface Si atoms are terminated by hydrogens.

Temperature dependence of the electronic structure ofC60films adsorbed onSi(001)−(2×1)andSi(111)−(7×7)surfaces

We report here the temperature-dependent measurements of the valence spectra, the C 1s and the Si 2p core level spectra of the one monolayer ${\mathrm{C}}_{60}$ film adsorbed on Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$ and Si(111)-$(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces, using photoelectron spectroscopy. At 300 K, most ${\mathrm{C}}_{60}$ molecules are physisorbed with the coexistence of minority chemisorbed species on both Si(001)-$(2\ifmmode\times\else\texttimes\fi{}1)$ and Si(111)-$(7\ifmmode\times\else\texttimes\fi{}7)$ surfaces. After annealing the samples at 670 K, ${\mathrm{C}}_{60}$ molecules change the bonding nature to a chemisorption that has both covalent and ionic characters. The covalent bonding orbital is observed at a binding energy of 2.10 eV on both Si surfaces. The amount of charge transfer is estimated to be 0.19 electrons per ${\mathrm{C}}_{60}$ molecule on the Si(001) surface, and to be 0.21 electrons per molecule on the Si(111) surface. We consider the origin of the change in bonding nature to the different distance between two dangling bonds that results from the rearrangement of the surface Si atoms. After annealing at 1070 K, ${\mathrm{C}}_{60}$ molecules decompose and the SiC formation takes progress at the interface. On the Si(001) surface, the molecular orbitals (MO's) disappear at 1120 K and the binding energies of peaks observed in the valence spectra indicate the formation of SiC islands at this temperature. On the Si(111) surface, the disappearance of MO's and the formation of SiC islands are verified at 1170 K. The difference in formation temperature is attributed to the different surface structure.

Theoretical studies on the adsorption of SiCl 4 on the Si (100) 2×1 surface

Using a Si 9H 12 model for the Si (100) 2×1 surface and employing Hartree–Fock and density functional methods, we have calculated equilibrium structures, binding energies and electron attachment energies for products of the adsorption of H 2O, Cl 2 or SiCl 4 on the Si (100) surface. Two local equilibrium structures are found for SiCl 4 adsorption, corresponding, respectively, to a less stable structure with an intact SiCl 4 weakly bonded to a surface Si and to a more stable dissociated SiCl 4 structure, with –Cl and –SiCl 3 groups bonded to adjacent surface Si atoms. Vibrational and NMR spectra have been calculated for both structures. Negative ions formed from these adsorbed molecules are usually more stable by several electron-volts than those produced from the corresponding gas-phase molecules.

Influences of Impurities on Oxidation Processes of Si(100) Substrates

We have investigated the influences of impurities in Si substrates on the oxidation processes and local bonding structures of silicon dioxide (SiO2) films on deuterium- (D-) terminated Si(100) surfaces by high-resolution electron energy loss spectroscopy. It is found that the oxidation processes such as the oxidation rate and the preferential adsorption of oxygen (O) atoms on one of the two back bonds of a surface Si atom are hardly influenced by the impurity element and the impurity concentration. Moreover, boron (B) atoms promote the relaxation of SiO2 films by changing the Si-O force constant. This effect of B atoms is considered to originate from a long-range interaction such as the Coulomb interaction.

Influences of deuterium atoms on local bonding structures of SiO 2 studied by HREELS

We have investigated the initial oxidation processes on D-terminated Si(100)-2 × 1 and -1 × 1 surfaces and influences of Si-D bonding on SiO 2 local bonding structures by high-resolution electron energy loss spectroscopy. It was found from experimental and simulated results that O atoms preferentially adsorb on one of two Si-Si back bonds of a surface Si atom. However, the preferential adsorption on the D-terminated Si(100)-2 × 1 surface is not as significant as the 1 × 1 surface. In addition, from the evaluation by a central-force network model, the length of Si-O bonds formed on the D-terminated 1 × 1 surface is more relaxed than on the 2 × 1 surface. This fact suggests that the existence of Si-Si dimer bonding changes the bonding states of the topmost Si atoms and then the structural change in Si-O-Si formed at the Si back bond occurs.

Hydrogen-terminated Si Surfaces. Role of Hydrogen Atoms in the Initial Oxidation Processes of H-Terminated Si(100) Surfaces.

Initial oxidation processes and local bonding structures of hydrogen-terminated Si(100)-2×1 and H2O-terminated Si(100)-2×1 surfaces have been examined by high-resolution energy loss spectroscopy (HREELS). The hydrogen adsorption on Si(100) surfaces suppress the oxidation of dimer bond sites, and oxygen atoms preferentially adsorb on one of the two back-bond sites of surface Si atoms. On the other hand, oxidation proceeds randomly up to an oxygen coverage of 3 ML on H-terminated Si(111) surfaces. The Si-O-Si bonds formed on H-terminated Si(100) surfaces are more relaxed than those on clean Si(100) surfaces, which is considered to originate from the change in bond angles of Si-O-Si bonds. The H2O-terminated Si(100)-2×1 surfaces are also stable for the adsorption of oxygen molecules. However, the uptake of oxygen atoms of Si-OH species into back-bond sites occurs even at room temperature by the reaction of H2O-terminated Si(100) surfaces with atomic hydrogen.

Atomic‐Order Thermal Nitridation of Silicon at Low Temperatures

Atomic‐order nitridation of Si(100) in an environment (124–1400 Pa) at 300–650°C has been investigated using an ultraclean low pressure hot‐wall reactor system. At 500°C or higher, the N atom concentration (nN) initially increases and tends to saturate to a certain value . At 400°C or lower, on the H‐terminated Si surface, the Si‐hydride decreases with increasing exposure time and becomes hardly observed when nN reaches nearly to the surface Si atom concentration . On the H‐free Si surface, nN increases up to with the appearance of the Si‐hydride instantly after exposure. It is expected that dissociatively adsorbs on the Si dangling bonds. It is found that nN is well described by Langmuir‐type physical adsorption and reaction of on the Si surface. The ultrathin nitride film shows very good characteristics as a mask against oxidation.

Diffusion mechanisms of a Si adatom on H-terminated Si(100) surfaces

We present features in the adsorption and diffusion of a Si adatom on H-terminated Si(100) surfaces based on first-principles total-energy calculations. We find that the adatom spontaneously substitutes for a H atom upon adsorption. The resulting diffusion species on the surface is no longer a single atom and a typical approach based on a single potential energy surface is invalid. As an alternative, we trace the diffusion pathways of the adatom-H unit, simultaneously examining the effect of H release and capture by the adatom. Extensive investigations on the $2\ifmmode\times\else\texttimes\fi{}1$ phase show that the adatom releases the H atom bonded to it and captures another H atom as it migrates on the surface. In addition, the adatom penetrates into the surface, breaking a surface backbond. This results in exchange between the adatom and a surface Si atom. The mechanisms of H release and capture and of adatom exchange are also expected on other phases of the H-terminated Si(100) surfaces.

Surface Structure and Properties of Calcium Hydroxyapatite Modified by Hexamethyldisilazane

The surface of synthetic calcium hydroxyapatite Ca10(PO4)6(OH)2(CaHAP) particles was treated by repeated modification with hexamethyldisilazane [(CH3)3Si]2NH (HMDS) in hexane and thermal treatment and the surface of the modified CaHAP was characterized by various means. No remarkable change in XRD patterns or in particle shape by the modification was observed. The width of the CaHAP particles gradually increased with repeating the modification. FTIR results indicated that HMDS reacted with surface P–OH groups of CaHAP to yield surface Si–(CH3)3groups. The surface of the modified CaHAP was hydrophobic. The surface Si–(CH3)3groups turned to three kinds of surface Si-OH groups by treating the modified materials at 500°C in air. These formed surface Si–OH groups and the remaining surface P–OH groups reacted with HMDS by repeating the modification, resulted in the increase of the surface Si atoms. The modified material having surface Si–(CH3)3or Si–OH groups adsorbed much less CO2than the unmodified one.

Unusual Ti adsorption on Si(001) and subsequent activation of Si ejection

The epitaxy of Ti on Si(001) exhibits a profound intermixing of Ti and Si atoms giving rise to the formation of titanium silicide. This phenomenon differs considerably from typical epitaxial growth and is not understood. Using first-principles total-energy calculations we examined the reaction of a Ti adatom with a Si(001) surface. We found that the penetration of the Ti adatom into a near-surface interstitial site and the subsequent ejection of its neighboring surface Si atoms onto a terrace is kinematically favored with respect to the ``normal'' hopping diffusion on a Si surface. These reactive processes provide the microscopic mechanism of an initial stage of transition-metal silicidation.

Surface structure of cesium adsorption on the Si(001)2×1surface

The detailed surface structure of Cs adsorption on the Si(001) $2\ifmmode\times\else\texttimes\fi{}1$ surface was investigated by means of tensor low-energy electron diffraction analysis. Various adsorption sites were considered including the recently proposed possibility of asymmetric dimer reconstruction of the Si top layer. We confirm the structure model with double-layer adsorbates and symmetric Si dimers underneath for the saturation coverage Cs adsorption. The major structural parameters within this structure are discussed in terms of the covalent bonding between the adsorbates and Si surface atoms.

Formation of periodic step and terrace structure on Si(100) surface during annealing in hydrogen diluted with inert gas

Annealing of a Czochralski Si(100) substrate in a gas flow in which H2 is diluted to a concentration of 3% in He was investigated. The surfaces annealed at 900 °C or above showed well-developed terraces with mono-atomic steps that alternate between the straight A-step and the zigzag B-step configurations, whereas no morphological improvement occurred in the surfaces annealed at temperatures below 900 °C, as confirmed by atomic force microscopy. Reflection high-energy electron diffraction observations and Fourier-transform infrared attenuated total reflection spectra of the flat surfaces clarified that the surface Si atoms make dimers with their dangling bonds terminated by H atoms, i.e., the surface reconstruction is Si(100)2×1/1×2-H. We show that the depth of the etching was not sufficient to account for the observed smoothening of the surface during the annealing at high temperatures, and conclude that the main mechanism of surface flattening is surface migration of the Si atoms.

New model for Si(111)-(3×1)Li through determination of its surface Si atom density with the use of scanning tunneling microscopy

We report on surface Si atom density in a Si(111)-(3×1)Li structure determined from scanning tunneling microscopy observation. During a Li adsorption process on Si(111) surfaces, the imbalance of surface Si atom density between the clean (7×7) and Li-induced (3×1) structures causes the nucleation of Si islands. From measurements of the coverage of nucleated islands, we determine surface Si atom density in the Si(111)-(3×1)Li as four atoms per (3×1) unit cell and propose a structural model accounting for the present results.

SiC film formation and growth by the thermal reaction of aC60film adsorbed on a Si(111)-(7×7) surface: Bonding nature ofC60molecules and SiC-film surface phonons

We report here measurements of temperature-dependent vibrational excitations of ${\mathrm{C}}_{60}$ molecules adsorbed on a Si(111)-(7$\ifmmode\times\else\texttimes\fi{}$7) surface, and the formation of a SiC film by thermal reaction using high-resolution electron-energy-loss spectroscopy (HREELS). The interactions of ${\mathrm{C}}_{60}$ molecules with the Si surface are judged from the charge states of ${\mathrm{C}}_{60}$ molecules, determined quantitatively by the energy shifts of the vibrational modes. Most ${\mathrm{C}}_{60}$ molecules interact weakly by van der Waals force at room temperature. At 670 K, two adsorption states, i.e., ionic and covalent bonds, are formed under the rearrangement of surface Si atoms. The amount of charge transfer is estimated to be (4$\ifmmode\pm\else\textpm\fi{}$1) electrons per ${\mathrm{C}}_{60}$ molecule for the ionic bond. At 1070 K, covalent bonds between ${\mathrm{C}}_{60}$ molecules are formed, and at 1170 K $3C$-SiC(111) islands are formed. The formation of $3C$-SiC(111) is verified by the observation of the surface-optical-phonon Fuchs-Kliewer mode. We have grown the $3C$-SiC(111) film, repeating the adsorption of ${\mathrm{C}}_{60}$ molecules, and annealing the sample. Well-oriented films with low step density are obtained. The lower-energy shift of the Fuchs-Kliewer mode, observed for $3C$-SiC(111) films thinner than 30 nm, indicates the softening of the Si-C bond caused by the buffer layer.

Effects of atomic hydrogen on growth behavior of Si films by Si 2H 6-source molecular beam epitaxy

The effect of hydrogen atoms on the segregation of Ge atoms has been investigated in the Si overlayer growth on Ge/Si(100) substrates by gas source molecular beam epitaxy with additional atomic hydrogen. The Ge composition on the growing surfaces decreases exponentially with increasing Si overlayer thickness. The irradiation of excess atomic hydrogen reduces the decay length of the Ge composition, which directly indicates that hydrogen atoms adsorbed on the growing surface play an important role in suppressing the surface segregation of Ge atoms. A unified linear correlation between the decay length of the Ge segregation in Si/Ge/Si(100) and the hydrogen coverage on Si atoms obtained from the Si/Si(100) homoepitaxial growth with additional atomic hydrogen has been obtained. This fact suggests that the exchange between surface Si atoms and subsurface Ge atoms is suppressed by the hydrogen atoms on surface Si atoms.

Surface structure of hydrogen terminated (1 0 0) Si by medium energy ion scattering

(1 0 0) Si surfaces were terminated with hydrogen atoms by hydrogen fluoride (HF) treatment. X-ray photoelectron spectroscopy (XPS) and blocking measurements with medium energy ion scattering (MEIS) using 100 keV He + were performed to investigate the relaxation of the hydrogen adsorbed Si layers. XPS measurements after HF treatments suggested that hydrogen atoms were directly bonded with top surface Si atoms. Blocking measurements along [1 1 0] and [1 1 1] axes resulted in the shifts in blocking dip position by about 1.17 ± 0.3° and 1.7 ± 0.3°, respectively, indicating the Si layer spacing of about 1.28 ± 0.02 A ̊ between the top and second layers after hydrogen termination. These results indicated that the top Si layer had a relaxation of about 6%, provided that the bulk Si lattice spacing is 1.358 Å.

Hydrogen Exchange Reaction on Hydrogen‐Terminated (100) Si Surface during Storage in Water

The chemical state of hydrogen‐terminated (100) Si surface during storage in water and heavy water , has been investigated in situ and in real‐time using infrared spectroscopy in the multiple internal reflection geometry. We have examined how infrared absorption spectra in the Si‐H stretch vibration region change while the Si surface is immersed into and . We demonstrate that hydrogen exchange reaction takes place on the H‐terminated Si surface during storage in water; that is, the hydrogen atom in the surface Si‒H bond is replaced with a hydrogen atom of the water molecule. We propose that the hydrogen exchange reaction proceeds through the formation of a negatively charged state of the surface Si atom and a hydronium ion, .

Structural determination for H2O adsorption on Si(001)2 × 1 using scanned-energy mode photoelectron diffraction

Using scanned-energy-mode photoelectron diffraction we have determined the local adsorption geometry of the OH fragments adsorbed on Si(100)(2 × 1) surface. On this substrate water is known to adsorb dissociatively even at low temperature (90 K), which gives rise to a surface layer comprising coadsorbed OH and H species. The OH fragments are found to be adsorbed in off-atop sites at a dimerised surface Si atom with OSi bond-lengths of 1.7 ± 0.1Å and bond-angles relative to the surface normal of 22 ± 5°.

Structure determination of using scanned-energy mode photoelectron diffraction

A scanned-energy mode photoelectron diffraction study of the surface with adsorbed has provided quantitative determination of key structural parameters previously only predicted from theoretical calculations. The N atoms are found to occupy off-atop sites at a dimerized surface Si atom with an N - Si bondlength of and bond angle relative to the surface normal of . The positions of Si atoms in the dimer to which the adsorbates are bonded are found to indicate that the marked asymmetry of this dimer on the clean surface is lost as a result of the adsorption. The conclusions are discussed in relation to published results from less direct experimental probes and the results of theoretical model calculations.

Vibrational properties and charge transfer ofC60adsorbed on Si(111)-(7×7)and Si(100)-(2×1)surfaces

We report here the measurements of vibrational excitation spectra and the temperature dependence of ${\mathrm{C}}_{60}$ molecules adsorbed on Si(111)-$(7\ifmmode\times\else\texttimes\fi{}7)$ and Si(100)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surfaces using high-resolution electron-energy-loss spectroscopy in combination with scanning tunneling microscopy. A quantitative determination of the molecular charge state is obtained by the lower energy shifts of vibrational modes of ${\mathrm{C}}_{60}$ upon adsorption. On the Si(111)-$(7\ifmmode\times\else\texttimes\fi{}7)$ surface, the amount of charge transfer from the surface to a ${\mathrm{C}}_{60}$ molecule is estimated to be $(3\ifmmode\pm\else\textpm\fi{}1)$ electrons at the coverage lower than 0.25 monolayer (ML) and $(0.7\ifmmode\pm\else\textpm\fi{}1)$ electrons as an average at 1 ML. On the contrary, no indication of charge transfer is observed on the Si(100)-$(2\ifmmode\times\else\texttimes\fi{}1)$ surface at a coverage lower than 1 ML. The difference in the charge state on both surfaces is attributed to the difference in the nature of surface states. After annealing 1 ML ${\mathrm{C}}_{60}$ film at 500 K, the strong interaction between the surface Si atom and a ${\mathrm{C}}_{60}$ molecule is indicated by the softening of vibrational modes on both surfaces.