T. Yasue

Secondary ion emission processes of sputtered alkali ions from alkali/Si(100) and Si(111)

Abstract The secondary alkali ion yield vs. the work function change (Δ φ ) of Na, K and Cs/Si(100) and Si(111) was measured to discuss the details of secondary ion emission processes. In the case of alkali/metal systems, the secondary ion emission is explained by the electron tunneling model. In this model, the ionization of the ejected atom occurs as a result of electron resonant tunneling through the potential barrier separating an atom and a metal, and the secondary ion yield depends on exponentially the work function change of metal surface. For alkali/Si(100) systems, the secondary ion emission processes are explained in terms of the electron tunneling model since the secondary alkali ion yield vs. the work function change (Δ φ ) follows the exponential manner. However, it is not easy to apply the simple electron tunneling model to our experimental results for alkali/Si(111) systems. There is the essential difference in surface structures between Si(100) and Si(111). Therefore, it is suggested that the local electronic environment around the adsorbates might be taken into consideration for alkali/Si(111) systems.

Initial stage of Cu growth on Si(111)7 × 7 surface studied by scanning tunneling microscopy

Abstract The initial stage of growth of Cu on Si(111)7 × 7 surface at room temperature is observed with a field ion-scanning tunneling microscope (FI-STM). Cu atoms adsorb on adatoms, especially center adatoms, as well as on rest atoms and some Cu atoms form small clusters at very low coverage. At submonolayer coverage triangular clusters lay on the center of sub-units of the DAS structure. At a higher coverage, nearly circular shaped islands are observed, which is a different shape than those observed by others.

A new aberration correction method for photoemission electron microscopy by means of moving focus

A new aberration correction method has been developed for photoemission electron microscopy (PEEM). In order to correct the spherical and chromatic aberrations, a moving focus method was adopted. Several experimental limitations to achieving optimal resolution have also been overcome. A high brightness Hg lamp system has been developed to overcome the insufficient brightness of the conventional Hg lamp. An improvement of brightness by over 100 times as compared with the conventional lamp was achieved. Image blur was also found due to a weak environmental AC magnetic field caused by essential microscope components, i.e., the power transformer and CCD camera. After implementing the high brightness lamp and eliminating stray magnetic field by proper shielding, preliminary experiments demonstrate that aberration correction by moving focus can improve the PEEM image resolution.

Effect of hydrogen on Cu formation on Si(111)

Abstract Cu formation processes on clean Si(111)-(7 × 7) and hydrogen-terminated Si(111) surfaces have been investigated in situ by means of a medium-energy ion scattering (MEIS), RHEED, AES and observed ex situ with a field emission-scanning electron microscope (FE-SEM). The non-uniform Cu films including Si atoms are formed on the Si(111)-(7 × 7) surface at room temperature and the “5 × 5” incommensurate structure can be observed on the Cu Si (111) at high temperature (130–600°C). On the hydrogen-terminated Si(111) surface at room temperature, the growth processes of Cu are not so different from those on the clean Si(111)-(7 × 7) surface. However, the formation processes of Cu on the hydrogen-terminated Si(111) surface at high temperature (∼300–500°C) are quite different from those on the bare Si(111)-(7 × 7) surface. Cu atoms can easily migrate on the hydrogen-terminated Si(111) surfaces and form tall islands. The hydrogen-terminated surfaces remain, and can be observed by means of RHEED and SEM.

Introduction of photoemission electron microscopes at SPring-8 for nanotechnology support

Two photoemission electron microscopy (PEEM) systems with different characteristics have been introduced in SPring-8 for a nanotechnology support project. One is an easy to use system (ELMITEC PEEMSPECTOR), which is equipped with an electrostatic lens. The other one is a high end system spectroscopic photoemission and low energy electron microscope (ELMITEC LEEM/PEEM III), which is equipped with a magnetic lens and an energy filter. Test experiments have been done using the PEEM systems and high quality x-rays at SPring-8. In this paper, some experimental results will be presented.

Surface structure of CuSi(111) at high temperature

The epitaxial growth and the structure of Cu on Si(111)7 × 7 deposited at high temperature (< 300–600°C) was investigated mainly by medium energy ion scattering (MEIS) and scanning tunneling microscopy (STM). The domain images whose periodicity is about 5.5 ± 0.2 times of the Si bulk unit were observed at high sample bias (VS = 2.5 V). The periodicity coincides with the ‘5 × 5’ incommensurate structure that was observed by reflection high energy electron diffraction (RHEED). The ratio of dark to bright area of ‘5 × 5’ was estimated to guess the Si surface structure in the ‘5 × 5’ incommensurate layer after counting the number of Si atoms. The ratio was about 0.83 and there are about 1 monolayer of Si atoms in the incommensurate layer. The structure of the Si bulk that is just beneath the incommensurate layer might be the double layer and the first layer of the Si bulk might be relaxed inward by 0.01 nm after the measurements of the blocking profiles by MEIS.

Medium energy ion scattering and STM studies on CuSi(111)

Abstract The structure of Cu on Si(111) 7 × 7 deposited at high temperature (300–600°C) was examined by the results of medium energy ion scattering (MEIS) and scanning tunneling microscopy (STM). The structure of the Si bulk that is just under the incommensurate layer, which shows the “5 × 5” electron diffraction pattern, might be the double layer and the first layer of the Si bulk is deduced to be relaxed inward by 0.01 nm after the measurements and the Monte Carlo simulation of the blocking profiles of MEIS. The distance (0.05 nm) between the Cu and Si layer in the incommensurate layer was also estimated using the ultra high depth resolution capability (0.16 nm for Si) of MEIS. The Si structure in the incommensurate layer was deduced by the ratio of dark to bright areas of “5 × 5” after counting the number of Si atoms. The ratio was about 0.83 and there is about 1 ML of Si atoms in the incommensurate layer.

High depth resolution analysis of CuSi(111) “5 × 5” structure with medium energy ion scattering

Abstract High depth resolution measurements under the glancing take-off condition of medium energy ion scattering (MEIS) were carried out in order to determine the interlayer distances of the Cu Si(111) “5 × 5” incommensurate structure. The experimental spectra were analyzed using a Monte Carlo simulation. In the simulation, the single electron excitation and the plasmon excitation were taken into consideration as the inelastic energy loss. The energy straggling was estimated by the calculation of the energy spectra for the Si(111) 7 × 7 surface, whose structure was well established. The distance between two Cu layers in the incommensurate layer was obtained to be about 0.06 nm, and that between Cu and Si layers in the incommensurate layer to be 0.006 nm. Also the distance between the Si layer in the incommensurate layer and the first Si layer of the substrate is determined to be 0.19 nm.

Solid-phase epitaxial growth of Ge on H-terminated and oxidized Si(100) surfaces

Abstract Thin Ge films with a thickness of about 10 nm were deposited on hydrogen-terminated and oxidized Si(100) surfaces using electron beams. We prepared the above two types of surfaces by chemical treatments and measured the amounts of oxygen and hydrogen by ion channeling and nuclear resonant reaction of 1 H( 15 N, αγ) 12 C, respectively. They were estimated to be 1.2 nm (SiO 2 ) and 2.3 ± 10 15 Hcm 2 (about 3 monolayers). The samples were post-annealed at 600, 800, 900 and 1000°C for 5 s in a high vacuum. The hydrogen-terminated Si(100) has a strongly stable hydride structure, which remains at the Ge/Si interface up to 900°C. This very thin and stable Si-hydride surface brings significant effects to improve the surface morphology and crystallinity and to hold steep interfaces compared with the oxidized surface.