Kan Nakatsuji

Electron-stimulated desorption (ESD) of the O2⧸Si(111) surface

Abstract We have studied the adsorption states of O 2 on the Si (111)7 × 7 clean surface at room temperature by means of ESD. For ESD at low exposures, desorbed H + , O + and O 2 + were observed, whereas O 2 + was not observed at high exposures. The kinetic energy distribution of the desorbed ions showed peaks around 4.0, 4.0 and 2.0 eV for H + , O + and O 2 + , respectively. The profile of the kinetic energy distribution of O + was almost the same as that reported by Nishijima et al. [J. Appl. Phys. 46 (1975) 3089], although they did not observe O 2 + . With increasing coverage, the O 2 + signal disappeared. It is therefore concluded that the oxygen adsorbs only dissociatively at high exposure, and molecular surface species survive only at low coverage at room temperature. The ESDIAD patterns are reconciled with the model in which the molecular surface species is adsorbed at the on-top site, and the atomic O is adsorbed at both the on-top and bridge sites for low exposure, and only at the bridge site for high exposure.

Adsorption and desorption processes of Cl on a Si(100) surface

Abstract The adsorption and desorption processes of Cl on a Si(100) surface were investigated by means of LEED (low energy electron diffraction), AES (Auger electron spectroscopy), and ESD (electron stimulated desorption). Experimental results suggest that the two processes are greatly influenced by the creation of surface dipoles. The Cl adsorption rate was constant until saturation coverage. The 2 × 1 LEED pattern remained even at high coverages, suggesting that the dimer was not destroyed by the chlorine adsorption. We observed only Cl + ions in the ESD study. The excitation energy dependence of the Cl + ion yield showed a threshold energy at 15–20 eV, suggesting that its desorption mechanism is explained by the Auger stimulated desorption model exciting the Cl 3s core hole. The coverage dependence of the kinetic energy distribution of desorbed ions from the Cl covered surface suggests that the “energy barrier” for desorption of the Cl + ions is lowered by the creation of discrete surface dipoles. The dipole length on this Si(100) surface was estimated to be much greater than the SiCl bond length, which is in contrast to the case on the Si(111) surface where the dipole length was similar to the bond length. This difference was ascribed to the semiconducting character of the Si(100) surface in contrast to the metallic character of the Si(111) surface.

Graphene nanoribbons on vicinal SiC surfaces by molecular beam epitaxy

We present a new method of producing a densely ordered array of epitaxial graphene nanoribbons (GNRs) using vicinal SiC surfaces as a template, which consist of ordered pairs of (0001) terraces and nanofacets. Controlled selective growth of graphene on approximately 10 nm wide of (0001) terraces with 10 nm spatial intervals allows GNR formation. By selecting the vicinal direction of SiC substrate, [1-100], well-ordered GNRs with predominantly armchair edges are obtained. These structures, the high density GNRs, enable us to observe the electronic structure at K-points by angle-resolved photoemission spectroscopy, showing clear band-gap opening of at least 0.14 eV.

Surfactant role of Ag atoms in the growth of Si layers on Si(111) √3×√3-Ag substrates

The growth of Si layers on Si(111) √3×√3-Ag substrates was studied for coverages of up to a few mono-layers. Atomically flat islands were observed to nucleate in the growth at 570 K. The top surfaces of the islands were covered in Ag atoms and exhibited a √3×√3 reconstruction with the same surface state dispersions as Si(111) √3×√3-Ag substrates. These results indicate that the Ag atoms on the substrate always hop up to the top of the Si layers.

Absence of Luttinger liquid behavior in Au-Ge wires: A high-resolution scanning tunneling microscopy and spectroscopy study

Au-induced atomic wires on the Ge(001) surface were recently claimed to be an ideal 1D metal and their tunneling spectra were analyzed as the manifestation of a Tomonaga-Luttinger liquid (TLL) state. We reinvestigate this system for atomically well-ordered areas of the surface with high resolution scanning tunneling microscopy and spectroscopy (STS). The local density-of-states maps do not provide any evidence of a metallic 1D electron channel along the wires. Moreover, the atomically resolved tunneling spectra near the Fermi energy are dominated by local density-of-states features, deviating qualitatively from the power-law behavior. On the other hand, the defects strongly affect the tunneling spectra near the Fermi level. These results do not support the possibility of a TLL state for this system. An 1D metallic system with well-defined 1D bands and without defects are required for the STS study of a TLL state.

Selective doping in a surface band and atomic structures of the Ge(111)

Atomic and electronic structures of the Ge(111) (√3 × √3)R30°-Au surface with two metallic bands are studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). The bias-voltage-dependent periodic structure observed by STM is consistent with the electronic structure calculated for an optimized conjugate honeycomb chained trimer (CHCT) model. Electrons are selectively doped to the electron-like surface metallic band by excess Au atoms, which form triangle structures with the Au trimers of the CHCT model. The discrepancy for the bottom energy of the electron-like band between the ARPES results and those of the calculation is attributed to the doping. The triangle structure is mobile at room temperature, but stable at 80 K. Both Au and Ge atoms deposited at room temperature on the Ge(111) (√3 × √3)R30°-Au surface dope electrons to the electron-like surface metallic band. Moreover, the Au atoms increase the spin-orbit interaction at the surface, and thus make the splitting of the spin-polarized band due to the interaction larger than that before the deposition.

(\sqrt{3}\times \sqrt{3})\mathrm{R}3 0^{\circ}

Abstract We have directly observed the Fermi surface of Bi2Sr2CaCu2O8(Bi(2212)) by a display-type angle-resolved photoelectron spectrometer. The experiment was done at room temperature by using linearly-polarized synchrotron radiation (SR). The photon energy was 32 eV and the incident angle of the photon was perpendicular to the sample surface. Photoelectron angular distribution (PEAD) patterns were measured at various azimuthal angles φ from 0° to 135° in steps of 22.5°, where φ is the angle between the direction of the electric vector of the photon and the sample Cu O bond direction. The observed shape of the Fermi surface is consistent with the band calculation by the FLAPW method [S. Massidda et al., Physica C 152 (1998) 251]. We confirmed the symmetry of the atomic orbital constituting the Fermi surface to bedx2−y2 by ‘orbital determination by angular distribution’ (ODAD).

–Au surface

Abstract Angle-resolved photoelectron spectroscopy on1 T -TaS 2 has been performed in full solid angles by using a display-type hemispherical mirror analyzer. We obtained a very clear dependence of the photoelectron angular distribution (PEAD) pattern on the incident photon polarization. The difference between the observed and calculated PEAD patterns is discussed. In addition, we have investigated the effect of Ta 5p-5d resonance photoemission process on the photoelectron angular distribution patterns. Some new strong photoemission peaks were observed in the resonance photoemission condition.

Symmetry analysis of the Fermi surface of Bi2Sr2CaCu2O8 by display analyzer

Abstract The adsorption process and desorption mechanism of Cl on the Si(111) 7 × 7 surface were studied by means of LEED, AES, ESD (electron stimulated desorption) and ESDIAD (electron stimulated desorption ion angular distribution) methods. The saturation behavior in the adsorption process revealed by means of AES with a low electron flux was found to follow the Langmuirs adsorption equation. The LEED result suggested that the surface corrosion did not take place during the Cl exposure. In the ESD experiment, only Cl + was detected as the positive ion species. In the ESDIAD study, Cl + ions were predominantly observed in a cone of about 10° around the direction perpendicular to the surface. The dependence of the Cl + desorption yield on the incident electron energy showed two threshold energies (10–20 and 200 eV). These two thresholds correspond to two different excitation mechanisms for ion desorption. The higher (lower) threshold is ascribed to the Cl 2p (Cl3s) excitation. However, we could not detect any noticeable difference in the kinetic energy distribution of the desorbed ions through the whole excitation energy range. Hence, it is revealed that these two excitations result in the same final state for desorption as suggested by the KF model.

TWO-DIMENSIONAL ANGLE-RESOLVED RESONANCE PHOTOELECTRON SPECTROSCOPY OF 1T-TAS2

Abstract We have observed the two-dimensional photoelectron angular distribution (PEAD) patterns of the valence band of the 2 H -TaSe 2 using a display type photoelectron spectrometer. The PEAD patterns which correspond to the two-dimensional band map show the various changes with the change of the binding energies, reflecting the complex band structure of 2 H -TaSe 2 . The intensity suppression area of the pattern at the Ta 5d band indicates that the half-filled Ta 5d band consists mainly of d z 2 -like orbitals as predicted by previous band calculation.