Ken Hattori

Initial stage of Ag growth on Ge(001) surfaces at room temperature

Abstract Using scanning tunneling microscopy, we studied Ge(001) surfaces covered with an average of approximately 0.8 monolayers of silver. In the deposition at room temperature, we observed that Ag predominantly grew in a three-dimensional (3D) mode on bare Ge substrates, corresponding to Volmer–Weber growth. At the same time, we found two-dimensional (2D) Ag islands elongated in the dimer-row direction with one monolayer height, though the density of the 2D islands was smaller than one-fifth of that of the 3D islands. Images with atomic resolution showed stripe patterns on the 2D islands and enhancement of asymmetry of the Ge dimers on both sides of the islands at high positive bias voltages.

Development of High-Energy-Resolution Display-Type Photoelectron Spectrometer in the Ultraviolet Photoelectron Spectroscopy Region

We have built a newly designed 2D-photoelectron spectroscopy (PES) analyzer for the measurement of wide-angle photoelectron angular distribution patterns in the ultraviolet photoelectron spectroscopy (UPS) region with no distortion. The spherical electric field inside the analyzer is achieved by the 158 tin obstacle rings placed on the inner surface of the machinable ceramic outer hemisphere. The inner surface of the outer hemisphere is covered with graphite powder to avoid charging. Hence, the ideal shape and the smooth change of the potential on the surface of the hemisphere could be realized. The energy resolution (ΔE/ EP) is estimated to be 0.43% (73 meV) at a kinetic energy of 16.85 eV and 0.16% at a kinetic energy of 300 eV.

Formation of ferromagnetic interface between β-FeSi2 and Si(111) substrate

Epitaxial β-FeSi2 thin films were grown on Si(111)7×7 clean surfaces by solid phase epitaxy in ultrahigh vacuum: iron deposition at low temperature and subsequent annealing. We found that a ferromagnetic interface layer of iron-rich silicides forms between a β-FeSi2 surface layer and a Si(111) substrate spontaneously from transmission electron microscopy observations and magnetization measurements.

Light-induced monolayer modification of chlorinated silicon (111) surfaces studied with a scanning tunneling microscope

Abstract Chlorine adsorbed silicon (111) surfaces are studied with a scanning tunneling microscope before and after weak irradiation of ultraviolet, visible and infrared light. We found that di- and tri- chlorinated silicon adatoms are desorbed out after ultraviolet light irradiation, resulting in the formation of a 7×7 surface structure resembling the so-called rest-atom layer . On the other hand, mono-chlorinated silicon adatoms are hardly moved by irradiation. The modification induced by the visible and infrared light are qualitatively similar to that by ultraviolet light. The obtained surfaces are different from the chlorinated surfaces after annealing. These results indicate that the modification is initiated by light-induced electronic excitation at the surface.

Deduction of atomic orbitals in a valence band by two-dimensional angular distribution of photoelectrons

The method to deduce the atomic orbitals constituting a valence band from two-dimensional photoelectron angular distribution (PEAD) (called ODAD: orbital determination from angular distribution) is fully discussed. The two-dimensional PEAD pattern obtained by using linearly polarized light is characteristic of the initial atomic orbitals. If the initial electronic state is localized and a wide-angle two-dimensional PEAD pattern is observed, the component ratio of atomic orbitals can be deduced by fitting the observed PEAD with calculated patterns. When only a limited area is observed by the effect of band dispersion, we can change the polar angle θ p or the azimuthal angle o p of the sample with respect to the incident photon so that we can get enough information. Even and odd symmetries are easily distinguished when we measure the o p dependence of the photoelectron intensity. The θ p dependence at a special direction (θ k , o k )=(45, 35) is proposed to distinguish all atomic orbitals.

A comparative study of photoluminescence of Zn-polar and O-polar faces in single crystal ZnO using moment analysis

We report on differences in photoluminescence (PL) spectra between Zn-polar and O-polar faces in single crystal ZnO. The PL intensity ratios of one phonon to two phonon replicas of free-excitons in the two polar faces were found to be the same. This result clearly indicates that exciton-phonon coupling strengths in both faces are the same. From moment analysis, however, the relative PL intensity of the zero-phonon free-excitons in the O-polar face was larger than that in the Zn-polar face. We propose that the opposite band bending at the two polar faces causes the difference in the PL properties.

Enhancement of photoluminescence efficiency from GaN(0001) by surface treatments

We investigated the photoluminescence (PL) efficiency of GaN(0001) single crystals with clean and well-defined surfaces using the PL technique in ultrahigh vacuum in situ. We found typical degradation factors: native oxides at the top surface, damaged layers in the subsurface, and hydrogenated non-radiative states inside bulk GaN. By eliminating the degradation factors, a band-to-band PL intensity of approximately 120 times higher than that of the as-received samples was achieved. The PL efficiency enhancement mechanism is discussed, and the role of hydrogen in GaN crystals is proposed.

Laser-induced desorption from silicon (111) surfaces with adsorbed chlorine atoms

We have studied the initial stage of the laser-induced reaction of silicon surfaces with adsorbed chlorine atoms in ultrahigh vacuum, by measuring the species desorbing from the surfaces. In particular, our studies have focused on photo-chemical etching without laser-induced thermal heating. We found that the primary species desorbing from Cl-saturated Si(111) surfaces is the molecule and that the desorption efficiency with 2.3, 3.5 and 4.7 eV photons is significantly enhanced with respect to that for 1.2 eV photons. The results of previous STM studies are discussed and a possible mechanism for the photo-chemical etching is proposed.

Application of atomic stereomicroscope to surface science

A stereograph of atomic arrangement was displayed directly on the screen of display-type spherical-mirror analyzer without any computer-aided conversion process for the first time. This stereoscopic photograph enables viewing three-dimensional atomic arrangement. This technique was realized taking advantage of the phenomenon of circular dichroism in photoelectron angular distribution. The azimuthal shifts of forward focusing peaks in a photoelectron angular distribution taken with left and right helicity light in a special arrangement are the same as the parallaxes in a stereo view of atoms. Hence a stereoscopic recognition of three-dimensional atomic arrangement is possible, when the left eye and the right eye respectively view the two images obtained by left and right helicity light simultaneously.

GaP reconstructed surface studied with STM and LEED

Abstract We have studied GaP ( 1 1 1 ) reconstructed clean surface with scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). We found that the surface consists of six equivalent mirror-symmetric domains and each domain has stripe structure. The stripe directions tilt from three 〈 1 1 2〉 directions clockwise and counterclockwise at the same angle, resulting in six domains. The stripe period and the tilt angle are 1.30 nm and 6.8°, respectively. Magnified STM images revealed that all stripes in one domain have the same protrusion unit along the stripe direction, and that the origin of the protrusion unit is arranged by two vectors for the inter-stripe direction. The same unit-vector in the stripe direction and the two unit-vectors in the inter-stripe direction constitute two different surface-reconstruction units, namely units 1 and 2. We assigned the reconstruction matrices of units 1 and 2 in one domain to 3 −1 2 5 and 4 1 2 5 , respectively. A trial structure model assuming the same elements located at the protrusions well explains observed LEED patterns.