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To evaluate graphene grown by a new method based on gas-source molecular beam epitaxy (MBE), in which a cracked-ethanol source is employed, from the macroscopic viewpoint, we investigated crystal truncation rod scattering by x-ray reflectivity measurement. From the analysis of the x-ray reflectivity data, we found that the MBE-grown graphene forms a layered atomic structure from the macroscopic view, too. The average spacing of the MBE-grown graphene was 3.39 A, which is larger than the interlayer spacing values for bulk crystalline graphite, and the height distributions of the MBE-grown graphene were relatively large. The occupancies of the graphene are smaller than unity and are smaller than that of its underlying graphene, suggesting that graphene did not grow in a layer-by-layer manner but in a three dimensional one, while each layer grew laterally. These results indicate that our new approach is feasible for the formation of wafer-scale graphene, although further improvement of the quality of the graphene by optimizing the growth condition is needed. [DOI: 10.1380/ejssnt.2011.58]

We describe the behavior of iron and cobalt catalysts on silicon substrates in chemical vapor deposition (CVD) of carbon nanotubes. Nanoparticles of iron and cobalt exhibited a melting point drop in methane ambient. Nanoparticles present after nanotube growth are identified as Fe3C and Co3C, for iron and cobalt, respectively. These findings indicate that a eutectic compound of metal and carbon is formed in the methane ambient, resulting in the phase separation of graphite (nanotubes) as the carbon uptake in the catalyst melt increases. This supports the vapor-liquid-solid mechanism for the nanotube growth by CVD. Iron- or cobalt-silicide formation causes poisoning of the catalysts. However, the coexistence of oxygen due to native oxide on the silicon surface or the metal surface causes formation of a SiO2 base that can prevent silicidation of iron particles.

We describe the development of an equipment which can be employed for examining various surface phenomena in real-time by performing time-resolved core-level photoelectron spectroscopy. We demonstrate this unique capability by examining the desorption of Sb atoms occurring from GaSb(001) as well as Sb/GaAs(001) in the temperature range of 440oC∼580oC, which is the usual growth temperature. The time dependence of core-level photoelectron spectra revealed that the activation energy for Sb desorption varies from 0.7 eV to 2.5 eV due to the difference in local bonding structures. In addition, experiments on alternating growth of GaAs suggest that there are two Ga adsorption sites, the occupation of which is dependent on the Ga coverages. This finding is in agreement with theoretical studies. These results demonstrate the potential of the technique to elucidate the surface process by real-time analysis.

The effects of crystal defects near the surface on the position of surface Fermi level (EFS) are investigated using photoluminescence (PL) measurements and synchrotron radiation photoelectron spectroscopy (SRPES). For the lightly Sidoped GaAs (001) surface, PL measurements reveal that after heating to 500°C a layer with lower PL peak intensities related to gallium vacancies than those of the bulk exists just under the thermal degraded layer. SRPES shows that EFS moves upward to 1.1-1.17 eV above the valence band maximum when this thermal degraded layer is removed by chemical etching. The excess arsenic on the surface, which is formed by rinsing the etched surface with deoxygenated and deionized water, is evaporated by heating in ultra-high vacuum (UHV). After evaporation of excess arsenic on the surface by heating, the etching-depth dependence of EFS for a sample pre-heated in UHV correlates with the existence of this defect concentration layer. These results suggest that the position of EFS for GaAs (001) surface is strongly influenced by crystal defects near the surface.

Deposition of Al and Au was performed on a Se treated GaAs(100) surface. The deposition of Al at low coverages results in a significant reduction in band bending that is interpreted in terms of further passivation of the Se treated GaAs surface. At intermediate coverages, band bending is consistent with what one would expect on the basis of Schottky limit. In this intermediate thickness regime, the formation of metallic Ga resulting from a Ga-Al exchange reaction producing a thermodynamically stable AlxSey interlayer is observed. At higher Al coverages, band bending is increased and is accompanied by Se surface segregation and a band bending which is slightly higher than expected from an ideal Al/GaAs interface. In a manner similar to Al deposition, deposition of Au results in segregation of only one Se species and provides further evidence on the structure of the Se/GaAs surface. Finally, Schottky barrier heights of 0.3 and 0.9 eV were realized for AI/Se/GaAs and Au/Se/GaAs respectively. This result suggests the Se passivated surface provides a wide degree of freedom in the fabrication of GaAs based Schottky barrier diodes.