| 1. |
- Hammar, M., et al.
(författare)
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In situ ultrahigh vacuum transmission electron microscopy studies of hetero-epitaxial growth I. Si(001)/Ge
- 1996
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 349:2, s. 129-144
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Tidskriftsartikel (refereegranskat)abstract
- We use ultrahigh vacuum transmission electron microscopy (UHV-TEM) to study the growth of Ge on Si(001) in real time at different temperatures and for coverages ranging from the initial monolayers to the development and relaxation of 3D islands. During growth of the first monolayers the surface gradually changes from a disordered missing-dimer structure to a rather well ordered (2 × 8) reconstruction, an evolution clearly resolved by the TEM. As the coverage is increased 3D islands starts to form. The growth and relaxation of these islands are shown to depend significantly on the temperature, e.g. with different dislocations formed at high and low temperatures. We interpret this difference in terms of the brittle-ductile transition in Ge, below which dislocation glide is frozen out. An interesting observation is that islands grown at low temperatures are more fully relaxed than those grown at higher temperatures. At high enough temperature the islands are initially, up to a specific size, coherent with the substrate and further growth occurs in a remarkably oscillatory fashion with the introduction of each (60°-type) dislocation, where the core of the island, of about 2000 Å in diameter, remains fully strained. However, in the low-temperature regime the islands grow relaxed from the outset with pure edge dislocations continuously being introduced in the moving edges. For temperatures less than 600̊C the transition from 2D to 3D growth occurs via the formation of small and strained 3D islands, so-called "hut clusters". We monitor the nucleation and characteristics of these clusters and discuss their possible role in the formation of relaxed 3D islands. The different growth mechanisms are discussed in terms of a simple model for the energetics of strain-relaxed islands, leading to a qualitative description of the temperature-dependent growth modes.
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| 2. |
- LeGoues, F. K., et al.
(författare)
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In situ TEM study of the growth of Ge on Si(111)
- 1996
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 349:3, s. 249-266
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Tidskriftsartikel (refereegranskat)abstract
- We have used the UHV-TEM to study the growth of Ge on Si(111) in situ, from the deposition of the first monolayer to complete relaxation by the introduction of dislocations. We show that, at 650°C, the growth of Ge islands is dominated by steps on the surface. Indeed, islands nucleated on steps that run along the 〈112〉 direction have a very high aspect ratio, while other islands are roughly triangular in shape. Dislocations form initially exclusively by coalescence of these smaller islands. Further growth involves a complicated competition between surrounding strained islands and the relaxed island. At lower temperature (350°C), steps still are a preferred site for island formation, but the limited surface diffusion makes it possible for islands to also nucleate on flat terraces. In this case, an island grows by incorporating dislocations at its edge, which results in a completely relaxed island from the start, and a very uniform network of dislocations. These in situ electron microscopy studies reveal a much more complicated and rich growth process than previously imagined, and tie previously obtained results together into a single picture.
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| 3. |
- Chen, W., et al.
(författare)
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Redox properties of titanium oxides on Pt3Ti
- 1995
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 99:34, s. 12892-12895
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Tidskriftsartikel (refereegranskat)abstract
- The morphology and electronic structure of surface-segregated titanium oxides on Pt3Ti(111) are presented. Core level photoemission spectra at grazing emission reveal two states of oxidation: a dominant and reducible four-valent oxide together with a small amount of a three-valent oxide is produced by oxidation in 0 2 at and below 400°C; an irreducible three-valent oxide by oxidation in 02 at and above 450 °C. The ratio between the active four-valent and the inactive three-valent oxides decreases with increasing oxidation temperature. The probability for reduction by CO is almost unity for the Ti 4+ oxide, and the conclusion must be that the four-valent oxide plays an active role for catalytic reactions. Scanning tunneling measurements relate these observations to changes in the dispersion and nucleation of the oxide overlayer. The four-valent oxide grows as islands with remaining areas open for CO adsorption while the three-valent oxide spreads on and blocks the crystal surface. Photoemission spectra relate these dispersion effects to an electronic interaction between the Ti 3+ oxide and adjacent Pt atoms. The above observations are in accordance with the common picture of dispersion effects in titania-supported SMSI catalysts and prove that interfacial energies play a crucial role whether the dominant phase is metallic or an oxide.
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| 4. |
- Gauthier, Y., et al.
(författare)
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Reconstruction of the Pt50Ni50(100) surface : a LEED and STM study
- 1995
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Ingår i: Surface Science. - 0039-6028. ; 327:1-2, s. 100-120
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Tidskriftsartikel (refereegranskat)abstract
- The structure of the Pt50Ni50(100) surface was investigated by low energy electron diffraction (LEED) and scanning tunnelling spectroscopy (STM). Superstructures corresponding to 12 × 1 and 19 × 1 reconstructions were observed in the LEED pattern, and these reconstructions were identified with atomic resolution by STM. The reconstructed surface layer has a quasi-hexagonal atomic mesh with almost (111) atomic density and is divided into terraces and domains the size of which can be more than a thousand ångströms. The domains are corrugated with protrusions made up by 5 × 1 subunits of the quasi-hexagonal mesh and with valleys containing depressed atoms or 2 × 1 subunits. The 12 × 1 and 19 × 1 periodicities are determined by the sequence of valleys; depending on the thermal treatment, one or the other dominates but both generally coexist on the terrace. Owing to the complexity of the structure, LEED calculations were performed for a simplified model with surface atoms situated in a planar net and uniform quasi-hexagonal meshes. We find that in both reconstructions the two surface layers are significantly enriched with platinum. The top layer enrichment is consistent with previous Auger measurements. © 1995.
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| 5. |
- GOTHELID, M, et al.
(författare)
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ADSORPTION OF TIN ON THE GE(111)-C(2X8) SURFACE STUDIED WITH SCANNING-TUNNELING-MICROSCOPY AND PHOTOELECTRON-SPECTROSCOPY
- 1995
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 328:1-2, s. 80-94
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Tidskriftsartikel (refereegranskat)abstract
- The growth and epitaxy of Sn on Ge(111) have been investigated using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and core level photoelectron spectroscopy for coverages ranging from 0.4 monolayers (ML) to above the critical coverage at 1.6 ML. At the lowest coverage a (root 3 X root 3)R30 degrees reconstruction is formed at an annealing temperature of 250-300 degrees C while an annealing above 500 degrees C creates a dimer-adatom-stacking fault (DAS) (7 X 7) structure. In the (7 X 7) structure we argue that Sn occupies both adatom and dimer sites. A previously suggested difference in the (root 3 X root 3)R30 degrees reconstruction at different coverages could not be revealed in our STM images and it seems likely that the structure is the same both at 0.4 and 0.7 ML Sn coverage. We also report the observation of a new superstructure, a (4 X root 7) reconstruction in the submonolayer regime, which appears as a minority structure in disordered regions adjacent to a (5 X 5) DAS structure, Finally in the post-monolayer region a (3 X 2 root 3) structure, surrounded by vast areas of an amorphous tin overlayer, has been imaged by STM. As the coverage was increased, the amorphous layer completely covered the ordered (3 X 2 root 3) phase, which still could be observed in LEED. Additional room temperature deposition of Sn deteriorated the fractional order LEED spots presumably due to indiffusion of Sn from the interface as the critical coverage was surpassed.
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| 6. |
- GOTHELID, M, et al.
(författare)
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STRUCTURAL AND ELECTRONIC EVOLUTION ON THE GE(111)-AG SURFACE
- 1995
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 52:19, s. 14104-14110
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution core-level photoelectron spectroscopy has been used to study three different silver induced surface reconstructions on the Ge(111) surface. At the lowest coverage a (4 X 4) structure is formed, which displays a similar Ge 3d core-level line shape as the clean c(2 X 8) surface. Details in the spectra are discussed with respect to possible models. The Ge(111)-Ag (root 3 X root 3)R30 degrees structure Ge 3d spectrum is dominated by a very strong contribution assigned to the two topmost Ge layers in a missing top layer structure, similar to the Si(111)-Ag (root 3 X root 3)R30 degrees surface. A weak bulk peak is present on the high-binding-energy side of the spectrum, while a third contribution assigned to Ge in phase boundaries is included in the fit on the lower-binding-energy side. A comparison with results obtained from the Ge(111)-Au root 3 structure points to substantial differences between the two noble-metal-induced root 3 reconstructions on the Ge(111) surface. Finally, after further deposition of silver at room temperature, the root 3 geometry is locally broken creating a (6 X 6) structure and a new surface-related peak emerges on the low-binding-energy side of the Ge 3d spectrum, which was interpreted as being due to Ge atoms floating on top of the outermost surface layer. The valence band also revealed the existence of small metallic silver islands.
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| 7. |
- Hammar, M. L., et al.
(författare)
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Effects of hormonal replacement therapy on the postural balance among postmenopausal women
- 1996
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Ingår i: Obstetrics and Gynecology. - 0029-7844 .- 1873-233X. ; 88:6, s. 955-960
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVE: To establish whether hormone replacement therapy affects postural balance in postmenopausal women. METHODS: Nineteen healthy postmenopausal women with vasomotor symptoms were included. Median age was 54 years, median time since menopause was 3 years. They underwent dynamic posturography before and after 4 and 12 weeks of transdermal estrogen treatment (17 beta-estradiol 50 micrograms/day) as well as after 2 additional weeks of combined estrogen-progestagen treatment. The dynamic posturography method quantifies the amplitude, frequency, and pattern of body sway and tests the visual, vestibular, and somatosensory systems, which together maintain balance. The two most difficult tests either cancel visual and distort somatosensory inputs or give distorted information from both the visual and somatosensory systems. RESULTS: Hormone replacement therapy increased static balance performance assessed by dynamic posturography. A highly significant improvement was seen in the two most difficult tests between the pretreatment test and the test performed after 4 weeks of estrogen therapy (P < .01, P < .001, respectively). This improvement was sustained after 12 weeks and also during the 14th week, with the women on combined estrogen-progestagen treatment. CONCLUSION: Estrogen treatment increased balance performance measured by dynamic posturography, indicating that the beneficial effects from estrogens on postmenopausal fracture risk may include central nervous system effects on balance. Two weeks' addition of gestagen to the treatment regimen did not counteract the estrogen effects.
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| 8. |
- Hammar, M., et al.
(författare)
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Topography dependent doping distribution in selectively regrown InP studied by scanning capacitance microscopy
- 1998
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 72:7, s. 815-817
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Tidskriftsartikel (refereegranskat)abstract
- We have used scanning capacitance microscopy (SCM) to study the dopant distribution in regrown InP with high sensitivity and spatial resolution. Sulfur or iron doped InP was selectively regrown around n-doped InP mesas using hydride vapor phase epitaxy, and the resulting structure was imaged in cross section by SCM. For calibration purposes, reference layers with known doping levels were grown directly on top of the region of interest. Dramatic variations in the carrier concentration around the mesa, as well as pronounced differences in the behavior of S and Fe are observed. We correlate these findings to the growth and doping incorporation mechanisms. © 1998 American Institute of Physics.
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| 9. |
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| 10. |
- Jacks, G., et al.
(författare)
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Tentative nitrogen budget for pit latrines - Eastern Botswana
- 1999
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Ingår i: Environmental Geology. - : Springer Science and Business Media LLC. - 0943-0105 .- 1432-0495. ; 38:3, s. 199-203
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Tidskriftsartikel (refereegranskat)abstract
- A major problem with on-site sanitation is nitrate pollution of the groundwater. A tentative nitrogen budget is established for pit latrines in eastern Botswana. The ammonia volatilisation was found to be negligible while leaching varied largely from about 1 to 50%. Leaching of nitrate was assessed by using chloride as tracer, assuming two sources of chloride, atmospheric deposition and the use of common salt in food. The initial content of nitrogen in excreta was assessed from nutritional data. The residual nitrogen in abandoned latrines as found by analysis, was 15–20%. The remainder should be denitrification which would then be in the order of 30–70%. That denitrification is important is supported by an elevated N-isotope ratio in groundwater and in deep-rooted non-N-fixing trees. The varying leaching rate provides a possibility of checking it by sealing the latrines. Since about 95% of the nitrogen in human excreta is present in the urine, an even more attractive solution would be urine-separating latrines with surface near percolation of the urine in the root zone of the vegetation, utilising it for crop growth. Since such latrines are used elsewhere in the world the problem is not technical but social acceptability.
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