Shozo Suto

HREELS, STM, and STS study of CH3-terminated Si(111)-(1×1) surface

An ideally (1x1)-CH(3)(methyl)-terminated Si(111) surface was composed by Grignard reaction of photochlorinated Si(111) and the surface structure was for the first time confirmed by Auger electron spectroscopy, low energy electron diffraction, high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). HREELS revealed the vibration modes associated to the CH(3)-group as well as the C-Si bond. STM discerned an adlattice with (1x1) periodicity on Si(111) composed of protrusions with internal features, covering all surface terraces. The surface structure was confirmed to be stable at temperatures below 600 K. STS showed that an occupied-state band exists at gap voltage of -1.57 eV, generated by the surface CH(3) adlattice. This CH(3):Si(111)-(1x1) adlayer with high stability and unique electronic property is prospective for applications such as nanoscale lithography and advanced electrochemistry.

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].

Millimeter Wave Absorption of OH- in NaCl Crystal

The absorption spectrum and its temperature dependence of the off-center OH - ion in NaCl crystal has been observed in the millimeter wave region from 1.1 to 2.5 cm -1 ( λ=4 to 9 mm). Two absorption bands due to the motion of the center of mass of the OH - ion have been observed at 1.85 and 1.56 cm -1 and are assigned to the transitions between A 1g and T 1u states and between T 1u and E g states, respectively. The transition dipole moments of the former and the latter transitions are estimated as 1.6 and 3.7 debye. The ratio of the two transition energies is 1.19 and this value is different from the ratio of 2 predicted by the tight well tunneling model. The energy levels are explained well by the LCAO-MO calculation based on a three dimentional adiabatic potential proposed by Gomez, Bowen and Krumhansl.

SiC islands grown on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces by C60 precursor

Abstract We have investigated the formation processes and characteristics of SiC islands grown on the Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces by C 60 precursor using high resolution electron energy loss spectroscopy (HREELS). The SiC islands are prepared by heating the C 60 adsorbed Si surfaces and confirmed by the observation of the optical surface phonon, Fucks-Kliewer mode. We found that SiC islands are formed at 1170 K and 1120 K on the Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces, respectively. We attribute the difference in the formation temperature to the different amount of charge which is transferred from Si surface to C 60 molecule at 500 K. The difference in the scattering cross-section of the Fucks-Kliewer mode indicates the different shape of SiC islands on each surface.

Interaction of C60 with Si(111)7×7 and Si(100)2×1 surfaces studied by STM, PES and HREELS: annealing effect

We report here measurements of the valence band spectra, the C 1s core level spectra and the vibrational excitation spectra of a C 60 monolayer (ML) film on Si(111)7 × 7 and Si(100)2 × 1 surfaces, using photoelectron spectroscopy and high-resolution electron energy loss spectroscopy (HREELS) in combination with scanning tunneling microscopy. 1 ML films are formed after annealing the 5 ML films at 670 K. The bonding states between C 60 molecules and Si substrates are clearly observed at 2.1 eV on both surfaces in the valence band spectra. The shift of binding energies of the molecular orbitals and the C Is core level spectra indicate the charge transfer. The softening of several vibrational modes is observed on the Si(111)7 × 7 and Si(100)2 × 1 surfaces with HREELS. The charge transfer scheme explains the softening well. We discuss the bonding nature in terms of hybridization of C 60 molecular orbitals with the surface states.

Theory and numerical study of exciton dynamics in a disordered linear chain

We have formulated the exciton dynamics in a disordered linear chain with exciton wave functions given by the one-dimensional Frenkel exciton Hamiltonian with disorder. It is assumed that exciton–phonon coupling is weak and that the dynamics is governed by the competing processes of phonon scattering and radiative decay. The phonon scattering rate is given on the assumption that excitons do not change the site by the scattering. The strength of exciton–phonon coupling and the density of phonon states are independent of energy. The radiative decay rate is given by the Einstein’s A coefficient. The detail of the numerical procedure is also described. Absorption spectra, luminescence spectra, the time response of luminescence intensity, and temperature dependence are calculated for the model system of poly(di-n-hexylsilane) film. It is discussed that long-range dipole–dipole interaction is responsible for the luminescence depolarization.

Bonding nature of C60 adsorbed on Si(111)7×7 and Si(100)2×1 surfaces studied by HREELS and PES

We report here the measurements of vibrational excitation spectra, valence band spectra and C Is core level spectra of C 60 molecules adsorbed on Si(111)7 × 7 and Si(100)2 × 1 surfaces, using high-resolution electron energy loss spectroscopy (HREELS) and photoelectron spectroscopy (PES). At a coverage between 1 and 0.25 ML, HREELS and PES measurements show that the interaction is as weak as the van der Waals interaction on both surfaces. At a coverage lower than 0.25 ML, some contradictory results are obtained by HREELS and PES. With HREELS, the softening of several vibrational modes is observed on the Si(111)7 × 7 surface, but no indication of softening is observed on the Si(100)2 × 1 surface. The charge transfer scheme explains the softening well. These results by HREELS indicate that the nature of the bonding is ionic on the Si( 111)7 x 7 surface and of the van der Waals type on the Si(100)2 × 1 surface. In contrast, the bonding states between C 60 molecules and Si substrates are clearly observed at 2.4 eV on both surfaces with PES. The PES measurements indicate that the interaction has a covalent character on the Si(111)7 × 7 surface, and that it is in between covalent and ionic, i.e. not purely ionic or at the covalent limit, on the Si(100)2 x 1 surface. We will discuss the nature of the interaction manifested by HREELS and PES measurements in comparison with the spectra of C 60 molecules adsorbed on metal surfaces.

Molecular precursor of oxygen on Si(111)7 × 7 surface

Abstract We have investigated the molecular precursor of oxygen on Si(111)7 × 7 surface at 1 and 2 L coverage using high resolution electron energy loss spectroscopy (HREELS) at room temperature. The time dependence of the energy loss intensity due to the OO stretching mode at 154 meV shows that the lifetime of the molecular precursor of oxygen is 530 min. Taking into account the ESDIAD pattern of the molecular precursor and the same lifetime measured by Sakamoto et al. [Surf Sci. 306 (1994) 93], we conclude that the molecular precursor adsorbs at the on-top site of the Si(111) surface. We also discuss the origin of the intrinsic lifetime.

SiC film formation from C60 monolayer on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces studied by HREELS-STM

Abstract We have investigated the thermal reaction of C 60 molecules and the formation of SiC films on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces by the combined measurements of high-resolution electron-energy-loss spectroscopy (HREELS) and scanning tunnelling microscopy (STM), HREELS-STM. The surface phonon energy of SiC film, formed by heating the 1 ML film of C 60 adsorbed on Si(111)-(7 × 7) and Si(001)-(2 × 1) surfaces up to 900°C, is observed at 114 meV. Moreover, new peaks are measured at 91 and 102 meV in high resolution measurements. Taking into account the cross sections and the angular profiles of the scattered electrons, we attribute the energy loss peaks at 91, 102 and 114 meV to the SiC vibration at the SiC surfaces, the low frequency Fuchs-Kliewer phonon mode and the high-frequency one, respectively. STM images of SiC films on both surfaces show the presence of many islands which surface areas are 10 × 10–35 nm 2 . The well characterized SiC films are formed at approximately 900°C on both surfaces.

Initial stage of SiC film growth on Si(111)7×7 and Si(100)2×1 surfaces using C 60 as a precursor studied by STM and HRTEM

We have investigated the structure and the growth mechanism of cubic silicon carbide (3C–SiC) films formed by thermal reaction of Si(111) and Si(100) substrates with C60 molecules, using scanning tunneling microscopy (STM) and high-resolution transmission electron microscopy (HRTEM). The cross-sectional HRTEM images show the good epitaxial growth of SiC films on the Si(111) surface and the 3×3 surface reconstruction is observed by STM. In contrast, many dislocations are formed in the SiC films grown on the Si(100) surface and no surface structure is observed by STM. We find that the amorphous buffer layers, which have relevance to release the strain due to the lattice mismatching, are formed at the interface between SiC films and the Si(111) substrate.