Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications.

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Structural properties of self-organized semiconductor nanostructures

Although silicon has long been the material of choice for most microelectronic applications, it is a poor emitter of light (a consequence of having an ‘indirect’ bandgap), so hampering the development of integrated silicon optoelectronic devices. This problem has motivated numerous attempts to develop silicon-based structures with good light-emission characteristics1, particularly at wavelengths (∼1.5 μm) relevant to optical fibre communication. For example, silicon–germanium superlattice structures2 can result in a material with a pseudo-direct bandgap that emits at ∼1.5 μm, and doping silicon with erbium3 introduces an internal optical transition having a similar emission wavelength, although neither approach has led to practical devices. In this context, β-iron disilicide has attracted recent interest4,5,6,7,8,9,10,11,12 as an optically active, direct-bandgap material th might be compatible with existing silicon processing technology. Here we report the realization of a light-emitting device operating at 1.5 μm that incorporates β-FeSi2 into a conventional silicon bipolar junction. We argue that this result demonstrates the potential of β-FeSi2 as an important candidate for a silicon-based optoelectronic technology.

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A silicon/iron-disilicide light-emitting diode operating at a wavelength of 1.5 μm

The combinatorial optimization simulated annealing algorithm is applied to the analysis of Rutherford backscattering data. The analysis is fully automatic, i.e., it does not require time-consuming human intervention. The algorithm is tested on a complex iron-cobalt silicide spectrum, and all the relevant features are successfully determined. The total analysis time using a PC 486 processor running at 100 MHz is comparable to the data collection time, which opens the way for on-line automatic analysis.

Simulated annealing analysis of Rutherford backscattering data

Physiological and molecular determinants of embryo implantation

The solid phase crystallization of chemical vapor deposited amorphous silicon films onto oxidized silicon wafers, induced either by thermal annealing or by ion beam irradiation at high substrate temperatures, has been extensively developed and it is reviewed here. We report and discuss a large variety of processing conditions. The structural and thermodynamical properties of the starting phase are emphasized. The morphological evolution of the amorphous towards the polycrystalline phase is investigated by transmission electron microscopy and it is interpreted in terms of a physical model containing few free parameters related to the thermodynamical properties of amorphous silicon and to the kinetical mechanisms of crystal grain growth. A direct extension of this model explains also the data concerning the ion-assisted crystal grain nucleation.

Crystal grain nucleation in amorphous silicon

Ion beam synthesized polycrystalline semiconducting FeSi2 on Si(001) has been investigated by transmission measurements at temperatures between 10 and 300 K. The existence of a minimum direct band gap was demonstrated and its variation with the temperature was studied by means of a three‐parameter thermodynamic model and the Einstein model. Band tail states and states on a shallow impurity level were found to give rise to the absorption below the fundamental edge. The presence of an Urbach exponential edge was shown and the temperature dependence of the Urbach tail width was also studied based on the Einstein model. A strong structural disorder associated with grain boundaries between and within the FeSi2 grains and their related defects was found to be the dominant contribution at room temperature.

Optical absorption study of ion beam synthesized polycrystalline semiconducting FeSi2

Grazing-incidence x-ray diffraction has been utilized to give a direct measure of the lateral strain distribution during in situ molecular-beam epitaxy deposition of Ge onto Si(001). The results demonstrate that the critical thickness for strain relaxation is 3–4 monolayers (ML), which coincides with that at which islanding is observed. Strain relief is gradual and depends strongly on growth conditions and annealing procedure. At a coverage of ∼10 ML the strain distribution exhibits two components, one of which is almost fully relaxed and the other having a range of lattice spacings intermediate between those for bulk Si and Ge. Concurrent specular reflectivity measurements have been used to monitor intermixing at the interface, showing that the island formation onset is at 3–4 ML, with a rapid increase in island height beyond a coverage of 6 ML. Comparison with electron-microscopy results indicates that strain relaxation is intimately related to islanding on the Ge surface.

Strain relaxation during the initial stages of growth in Ge/Si(001)

motherapy for patients who have received treatment in the past. Our recommendation to treat patients with impending immunosuppression (from human immunodeficiency virus infection, induction for organ transplant, or other etiologies) who have not previously received antitrypanosomal therapy should be interpreted in the context of our overall BII recommendation for T cruzi–infected adults up to age 50 years without advanced heart disease. The major objective of etiologic treatment in this setting is to prevent development or progression of cardiomyopathy. Our intention in explicitly listing anticipated immunosuppressionasapatientcategoryfortreatmentwastolendadditional weight to the recommendation since we assume that antitrypanosomal therapywouldbebetter tolerated,andtheoretically more effective, before immunosuppression develops or is induced.Althoughdataarelackingonthispoint,asecondarytheoreticalbenefit is apossible reduction in the riskof reactivation. Advancedrenal,hepatic,orcardiacdysfunctionwouldcertainly complicate therapy and constitutes a contraindication. We agree with Altclas et al that posttransplantation monitoring is indicatedwhetherornotacourseofantitrypanosomal treatment was completed prior to transplant. As stated in the Clinical Review, we recommend that treatment decisions be individualized for all adult patients with Chagas disease, balancing the potential benefit vs the risk of drug toxicity.

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Plasma anandamide concentration and pregnancy outcome in women with threatened miscarriage.

Background
In early pregnancy, increased plasma levels of the endocannabinoid anandamide (AEA) are associated with miscarriage through mechanisms that might affect the developing placenta or maternal decidua.

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Modulation of the endocannabinoid system in viable and non-viable first trimester pregnancies by pregnancy-related hormones

We have investigated by the use of surface x‐ray diffraction the initial strain relaxation of Ge on Si(001) when a ‘‘surfactant’’ layer of Sb [0.7 monolayers (ML)] is present. Due to the high sensitivity of the technique to lateral strain in the overlayer, we have been able to observe directly the onset of strain relaxation at a coverage of 9–10 ML. This strain relief proceeds gradually as a function of coverage, but unlike the case without an Sb surfactant, it was not possible to relax the overlayer fully. No bulklike Ge was seen even up to a coverage of ≊55 ML. Concurrent specular reflectivity measurements also showed that the overlayer formed in a layer‐by‐layer mode up to the same level. These results differ quite markedly from those obtained without the surfactant layer, where several stages of island formation and strain relaxation are seen. They show dramatically how the modification of surface energies by the presence of a surfactant can affect the morphology of, and strain in, a lattice mismatched overlayer. In addition, they are direct confirmation that the initial strain relaxation mechanism in operation when a surfactant is not present is the formation of coherent Ge islands, rather than that of dislocations.

M. S. Finney

Papers

Optical absorption study of ion beam synthesized polycrystalline semiconducting FeSi2

Strain relaxation during the initial stages of growth in Ge/Si(001)

Plasma anandamide concentration and pregnancy outcome in women with threatened miscarriage.

Modulation of the endocannabinoid system in viable and non-viable first trimester pregnancies by pregnancy-related hormones

The influence of Sb as a surfactant on the strain relaxation of Ge/Si(001)