Isao Sumita

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.

Scanning Tunneling Microscope Observation of Si(111) δ7×7 Formed by Si Deposition

Using scanning tunneling microscopy (STM), we study the initial stage of silicon deposition on Si(111)7×7 at room temperature. At less than 0.1 monolayer, three kinds of adsorbed structures are observed. The most common (70%) is a structure which has four maxima around a center dimer in its STM image. The position of the protrusions observed by STM changes drastically in the empty and the filled state. The bias voltage dependence is explained by the molecular orbitals between sp 3 bonds which are formed upon the breaking of backbonds of adatoms. Based on the measured difFusion length on the surface, the molecular beam is also analyzed. Si 2 is suggested to sublimate from the resistivity-heated Si source

Local Ordering and Lateral Growth of Initial Thermal Oxide of Si(001)

The initial stages of the thermal (600 o C) oxide growth of Si(001) clean surfaces were studied by scanning tunneling microscopy (STM). Oxide growth starts from both step edges and terraces. At 120L, almost the entiresurface was covered by oxides. Steps could still be identified, and the apparent roughness had a local minimum at this O 2 exposure. These results indicate that the first layer is oxidized quite uniformly. Some local orderings of the initial oxides were observed on the terraces, which may enhance the oxide growth in the lateral (parallel to the surface) direction

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.

Glass-fritless Cu alloy pastes for silicon solar cells recquiring low temperature sintering

This paper describes a novel Cu paste for low temperature sintering, which is necessary to fabricate electrodes on transparent conductive films in hetero-junction solar cell. To this end, glass fritless Cu alloy pastes containing a low melting point alloy were prepared. The pastes were evaluated in terms of printability, contact resistance, line resistance, adhesive property, and oxidation resistance. As a result, it can be concluded that the pastes would be applicable for silicon solar cells requiring low temperature process.

Giant adatom-like structures observed with scanning tunneling microscopy : super adatoms on Si(331)

Abstract Scanning tunneling microscopy (STM) is used to investigate the atomic structure of Si(331). The (331) terraces have large areas of more than 50 × 50 nm2, and single atomic-height steps of 0.12 nm are observed. No faceting into other planes is observed. The reconstructed structure found on the terraces can explain the LEED pattern. A unit cell consists of adatom-like structures, the diameter of which becomes as much as 1.3 nm. We call this structure an super adatom. Even at domain boundaries, the super adatom seldom breaks into atomic fragments. The discussion is focused on the origin of the huge diameter and the behaviour of the super adatom at a high-temperature 1 × 1 phase.

Screen printed finger electrode with high aspect ratio by single printing for crystal Si solar cell using novel screen mask

We developed novel screen mask with repellent property to organic solvent containing in Ag paste to print finger electrode with high aspect ratio by single printing and to reduce bleeding caused by spreading the paste at the edge of electrode line. The average height of finger electrode is 40.6 μm and average width is 64.0 μm when using the mask with 40 μm thick emulsion layer composed of 30μm hard layer and 10μm soft layer. The designed opening for electrode is 60 μm.

Fine texture for front surface fine lines with screen printing

The fine lines are realized by decreasing the texture size. When the texture size is 10 μm, the line width becomes wider due to the bleeding of the silver paste during the screen printing process. As decreasing the texture size, the jaggy area decreases and the line width becomes narrower. When the texture size is 1μm, the line width becomes almost same as the mask one. The open circuit voltage increases as the texture size becomes smaller. The decrease in the metal area causes the decrease of the recombination at the interface between the metal and semiconductor.

Surface morphology induced by homoepitaxial growth on a vicinal Si(111) surface

Abstract We have studied homoepitaxial growth on a vicinal Si(111) surface consisting of (111) terraces and (331) facets by scanning tunneling microscopy. After the growth around 500°C, there remained (111) terraces and (331) facets. However, above 500°C, the (331) facets changed into individual steps of bilayer height at the upper part; these facets disappear at 600°C. The change of the surface morphology was deduced to be the result of the growth kinetics difference between these two surfaces.

Characterization of Glass Frit in Conductive Paste for N-Type Crystalline Silicon Solar Cells

Contacting silver paste for an emitter of silicon solar cells has been pointed out to create shunting and to increase carrier recombination due to silver-crystallites at the emitter. However, since these observed electrical losses come of not only the crystallites but also multiple effects of constituents in the paste, whether the crystallites mainly cause these losses has still remained unknown. In this study, how glass frit “itself,” which is most important constituent in the paste, affects the electrical losses is investigated by applying the “floating contact method.” Furthermore, the contact interface between the frit and silicon surface is also investigated to elucidate the mechanism of the losses. The metal oxide glass frit itself is found to drastically cause the shunting and the recombination in n-type solar cells, resulting in loss in open-circuit voltage, and creates metal lead crystallites through redox reaction at silicon surface during firing. The shunting originates from the metallic lead, and the recombination is induced by diffused impurities or defects into the silicon emitter through the reaction. The electrical losses in the solar cells are led to not only by the silver-crystallites, but also by the glass frit itself.