Sakura Takeda

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.

Bi induced superstructures on Si(110)

The authors report new surface reconstructions that are produced when Bi is adsorbed on Si(110) surfaces held at high substrate temperatures. At 600 °C and a Bi coverage of 0.1 ML, reflection high energy electron diffraction (RHEED) patterns reveal the formation of a 3×″4″-Bi structure with b1 = 3a1 and b2=−a1+4a2. At 630 °C and 0.2 ML Bi coverage, a 3×″6″-Bi structure is observed, with b1=3a1 and b2=−a1+6a2. The coexistence of streaks and spots in the RHEED patterns of the 3×″4″ and 3×″6″ structures indicate a high density of antiphase domains, with poor structural correlation along the [001] direction. The authors propose structural models based on rows of Bi adatoms aligned along the [1¯10] direction. The models proposed can also be applied to similar reconstructions on Si(110) adsorbed with other metals.

Structure determination of the clean (001) surface of strained Si on Si1−xGex

The surface structure of the strained Si(001) (thickness of 20 nm) on Si1−xGex (x = 0.1, 0.2, and 0.3) was studied by low-energy electron diffraction (LEED). LEED intensity-energy spectra of the 2 × 1 reconstructed clean surfaces showed a systematic change that indicates the lattice contraction along the [001] direction remains even at the surfaces. The atomic structures were quantitatively determined, and they were compared with the unstrained pristine Si. The differences in the atomic position almost follow the difference in the bulk lattice constant determined by X-ray diffraction measurements. The results indicate that the strain produced at the Si/Si1−xGex interface remains unchanged up to the surface layer.

System to measure accurate temperature dependence of electric conductivity down to 20 K in ultrahigh vacuum.

We have developed the new in situ electrical-conductivity measurement system which can be operated in ultrahigh vacuum (UHV) with accurate temperature measurement down to 20 K. This system is mainly composed of a new sample-holder fixing mechanism, a new movable conductivity-measurement mechanism, a cryostat, and two receptors for sample- and four-probe holders. Sample-holder is pushed strongly against the receptor, which is connected to a cryostat, by using this new sample-holder fixing mechanism to obtain high thermal conductivity. Test pieces on the sample-holders have been cooled down to about 20 K using this fixing mechanism, although they were cooled down to only about 60 K without this mechanism. Four probes are able to be touched to a sample surface using this new movable conductivity-measurement mechanism for measuring electrical conductivity after making film on substrates or obtaining clean surfaces by cleavage, flashing, and so on. Accurate temperature measurement is possible since the sample can be transferred with a thermocouple and∕or diode being attached directly to the sample. A single crystal of Bi-based copper oxide high-Tc superconductor (HTSC) was cleaved in UHV to obtain clean surface, and its superconducting critical temperature has been successfully measured in situ. The importance of in situ measurement of resistance in UHV was demonstrated for this HTSC before and after cesium (Cs) adsorption on its surface. The Tc onset increase and the Tc offset decrease by Cs adsorption were observed.

Interband interaction between bulk and surface resonance bands of a Pb-adsorbed Ge(001) surface

We investigated the valence band structure of a Pb-adsorbed Ge(001) surface by angle-resolved photoelectron spectroscopy. Three Ge bands, G1, G2, and G3, were observed in a Ge(001) 2 × 1 clean surface. In addition to these three bands, a fourth band (R band) is found on the surface with 2 ML of Pb. The R band continuously appeared even when the surface superstructure was changed. The position of the R band does not depend on Pb coverage. These results indicate that the R band derives from Ge subsurface states, known as surface resonance states. Furthermore, the effective mass of G3 is significantly reduced when the R band exists. We found that this reduction of G3 effective mass was explained by the interaction of the G3 and R bands. Consequently, the surface resonance band is considered to penetrate into the Ge subsurface region affecting the Ge bulk states. We determine the hybridization energy to be 0.068 eV by fitting the observed bands.

Unusual energy separation of subbands in Si(111) p-channels induced by In adsorption

We investigated the dispersion structure of the hole subbands in vicinal Si(111)4 × 1-In surfaces directly by angle-resolved photoelectron spectroscopy. In this study, three inversion layers with different impurity concentrations and numbers of times of flash annealing (FA) were investigated. We observed wider energy separations of subbands levels for the sample with a less number of FA and a higher impurity concentration. However, the observed energy levels and separations had smaller binding energy than those calculated by triangle potential approximation. We found that the discrepancies were due to the out-diffusion of Arsenic atoms from the sub-surface region of silicon via high temperature FA. The possible potential profile of the space charge layer after considering the out-diffusion of Arsenic dopant atoms at the sub-surface region was proposed.