Mosaic Phase Research Articles

Scanning tunneling microscopy study of the Si(111)-(√3 × √3)-Pb mosaic phase

The atomic and electronic structure of the Pb-induced (√3 × √3)R30 mosaic phase on Si(111) substrates has been studied by means of scanning tunneling microscopy (STM). A systematic voltage dependent STM imaging analysis combined with scanning tunneling spectroscopy measurements have enabled us to interpret the different height contrast between Si and Pb adatoms observed on STM images. This contrast difference may be attributed to a large charge-transfer between the different adatom dangling-bond orbitals.

Characterization of the parasexual cycle in the eyespot fungus, Pseudocercosporella herpotrichoides

Parasexual recombination in the eyespot fungus, Pseudocercosporella herpotrichoides , is demonstrated using complementary auxotrophic mutants derived from the W-type isolate 22–20. The parasexual cycle follows a typical pattern consisting of a mosaic heterokaryon phase, a relatively stable diploid phase and various putative aneuploid stages leading to recombinant haploid formation. The heterokaryon can be resolved into its component homokaryons either by isolating single hyphal tips or from single conidia, which remain homokaryotic. The non-sporing diploids can be isolated from random plating of spores and hyphal fragments taken from heterokaryotic colonies, or by regenerating protoplasts isolated from heterokaryotic mycelia. An efficient method for obtaining diploids was developed, which exploits the particularly slow growth rate of heterokaryons formed between complementary histidine auxotrophs ( Hisl, Hien, Hi ). Diploid mycelium grows out as faster growing sectors from such heterokaryons. Segregation yielding recombinant haploids followed exposure to 5-fluorouracil. Various putative aneuploid stages were observed, typified by their very slow growth rate and instability. Spontaneous outgrowths yielding stable recombinant types were recovered from these unstable colonies following maceration of mycelium and plating on a non-selective, complete medium. A cross was performed to investigate the inheritance of benzimidazole and N -phenylcarbamate fungicide resistance. After retesting of resistance phenotypes, cosegregation of carbendazim resistance and increased sensitivity to the phenylcarbamate MDPC was confirmed in most cases. One exception involved a putative recombinant in which sensitivity to both carbendazim and the phenylcarbamate was combined. This recombinant was obtained after exposure of the original diploid to 5-fluorouracil, known to stimulate mitotic crossing-over in other fungi. However the possibility remains that it represents a chance spontaneous mutation either in the resistance gene or at some other locus.

Angle-resolved photoemission study of the submonolayer phases of Pb on Ge(111).

The (√3×√3)R 30 o α and γ submonolayer reconstructions of the Pb/Ge(111) system have been studied with angle-resolved photoemission spectroscopy utilizing synchrotron radiation. The α phase consists of 1/3 monolayer (ML) of Pb adatoms in T 4 sites, whereas the γ phase consists of an equal mix of Pb (1/6 ML) and Ge (1/6 ML) adatoms, forming a mosaic phase. Dispersions of the adatom-induced surface states of the γ and α phases have been probed along the Γ and ΓKM directions of the surface Brillouin zone. Our results indicate that despite their structural differences, the α and γ phases have essentially the same electronic structure

Photoemission study of the Si(111)- sqrt 3 x sqrt 3 -Pb mosaic phase: Observation of a large charge transfer.

The Pb-induced \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 mosaic phase of the Si(111) surface has been studied with Si 2p core-level and angle-resolved photoelectron spectroscopy. The results from these measurements provide evidence for a model with a 1:1 mixture of Pb and Si adatoms for the \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 mosaic phase, which was proposed in a recent scanning tunneling microscopy study. We observe quite large surface core-level shifts which are interpreted as due to a charge transfer between the different adatom dangling-bond orbitals.

Growth and morphology of Pb on Si(111)

Tunneling microscopy, thermal desorption, Rutherford backscattering, and low-energy electron diffraction are used to study the structures and coverages of the phases of Pb on the Si(111)7 × 7 surface. For room-temperature deposition at low coverage on the 7 × 7 surface, Pb atoms occupy sites above the rest atoms and between the Si adatoms (with a preference for the faulted half of the unit cell). At 0.6 ML, the Pb forms an ordered overlayer based on the 7 × 7 unit cell. The Pb grows epitaxially up to 3 ML at which point Pb crystals start to form. On annealed samples at low coverage, Pb atoms occupy Si(111)7 × 7 adatom sites. At 1 6 ML, a new √3 × √3 mosaic phase consisting of alternating chains of Pb and Si adatoms with a high melting point is produced. At 1 3 ML the standard √3 × √3 phase is observed. Between 1 3 and 1 ML, a 1 × 1 Pb overlayer is found on annealed samples, while above 1 ML, a rotated incommensurate phase is observed. As more Pb is added above 1 ML, the 2D Pb density is increased, and the incommensuration is reduced, until island formation begins. We link our discussion of the atomic structure of the interface to the variations in Schottky barrier heights observed for Pb/Si(111) diodes.

Annealing behavior of thin polycrystalline silicon films damaged by silicon ion implantation in the critical amorphization range

Thin polycrystalline silicon films were amorphized by silicon ion implantation at two different angles and subsequently annealed at 525°C. The structure of the as-deposited, as-implanted and annealed films was examined using Raman spectroscopy, transmission electron microscopy and the optical appearance method. Two distinct phases were found to coexist in the as-implanted films: one phase consists of a mosaic of crystallites surrounded by amorphous silicon and the other consists of only amorphous silicon. The mosaic phase was due to ion beam channeling through properly oriented polycrystalline grains. The implantation angle was found to determine whether the film recrystallizes upon annealing by a solid phase epitaxial mechanism using the surviving crystallites as seeds.