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This paper examines the velocity distribution function and cyclotron resonance conditions for a beam of electrons moving in a magnetic field which gradually changes with time. A spatial gradient of magnetic field is known to result in an unstable horseshoe distribution of electrons. The field gradient in time adds additional effects due to an induced electric field. The resultant anisotropic velocity distribution function, which we call a Luvdisk distribution, has some distinctive properties when compared to the horseshoe. Fitting the cyclotron resonance condition circle shows that the frequency of the resultant emission is under the local cyclotron frequency. While the spatial gradient results in the emission coming almost perpendicularly to the field, the direction of the radiation under a time-changing field has more variability. The Luvdisk distribution also arises when the magnetic field has a gradient both in space and time. The beam can be unstable if those gradients are added or subtracted from each other (if the gradients are of equal or different sign), which occurs even when the total change of magnetic field is negative. While the frequency of the emission is related to the final magnetic field value, its direction is indicative of the field’s history which produced the instability.
The recognition of ‘fetal origins of adult disease’ has placed new responsibilities on the obstetrician, as antenatal care is no longer simply about ensuring good perinatal outcomes, but also needs to plan for optimal long-term health for mother and baby. Recently, it has become clear that the intrauterine environment has a broad and long-lasting impact, influencing fetal and childhood growth and development as well as future cardiovascular health, non-communicable disease risk and fertility. This article looks specifically at the importance of the developmental origins of ovarian reserve and ageing, the role of the placenta and maternal nutrition before and during pregnancy. It also reviews recent insights in developmental medicine of relevance to the obstetrician, and outlines emerging evidence supporting a proactive clinical approach to optimizing periconceptional as well as antenatal care aimed to protect newborns against long-term disease susceptibility.
In May 1992. a small, circumscribed community outbreak of infection due to verotoxin-producing Escherichia coli O157 phage type 49 occurred in a semi-rural area of south-east Scotland. On the basis of stool cultures, six cases were identified, one of whom was asymptomatic. One child developed the haemolytic uraemic syndrome. Although the source of infection of the index case was not established nor could the extent of person-to-person spread be fully determined, the clinical, microbiological and epidemiological evidence available indicated that a children's paddling pool served as the focal point in the transmission of infection causing the outbreak.
Pre-tracheal air cysts or aeroceles are rare complications of tracheostomy. This is believed to be the first reported case in an adult presenting with a pre-tracheal air cyst 12 years after a tracheostomy. Only three case reports in children have been reported to date. The pathogenesis and treatment options are discussed and a method of managing this condition is suggested.
Interface reinforcement brought about by addition of a γ-amino-propyl-triethoxy-silane (γ-APS) adhesion promoter layer between a silicon wafer and a spun-on benzocyclobutene polymer (BCB) is investigated. Combining cross-sectional TEM and XPS, crack growth is shown to occur along the γ-APS/BCB interface. Ion etching and in-situ XPS are further employed to study chain orientation and chemical bonding variations through the silane layer. A tendency of the amide group to orient away from the wafer is documented and Si-O-Si siloxane bonding at the γ- APS/SiO2 interface is hypothesized as an important mechanism for adhesion strength enhancement.
Poly-SiGe stacked gates with Ge content ([Ge])varying between zero and 100% have been fabricated using an industriel single-wafer machine. These poly-SiGe layers were characterised and fully integrated in a 0.18 pm CMOS process. Interdiffusion of Si and Ge upon subsequent annealing of the structure has been observed and studied. This interdiffusion effect was found to be responsible for the discrepancy observed between theoretical and practical values of the Ge workfunction ɸms evaluated from our electrical measurements and from those of different authors. A technique for the limitation of this interdiffusion effect has then been developped and is described.
Integrated planar circulator circuits operating at frequencies near 10 GHz have been fabricated using transferred film techniques. A 100 micrometer thick film of single-crystal yttrium iron garnet was transferred onto a metallized silicon die using a bond and lap-back process. The alloy bond layer was formed through a low-temperature solid-liquid interdiffusion process using films of indium and gold. This bond layer proved of sufficient strength to permit removal of the gadolinium gallium garnet substrate through grinding, but chemical analysis of the bond shows that interdiffusion occurs between the bond and metallization layers. These integrated gamet/silicon circulator circuits show good device performance.
UHVCVD-grown SiO.s 7Ge0.13/Si heterostructures have been implanted with erbium, and photoluminescence and electroluminescence centred on 1.54ýim have been studied. Implantation conditions were chosen so that the erbium concentration profile was flat over the spatial location of the SiGe quantum well region. We demonstrate that the technology of implantation and regrowth is feasible even when Si/SiGe interfaces are present. We have obtained more intense photoluminescence from erbium implanted SiGe heterostructures than that from a silicon layer implanted with a higher erbium dose. We report forward bias electroluminescence from the Er doped SiGe/Si heterostructures; the photoluminescence and electroluminescence from these structures demonstrates that the detailed mechanism of excitation is different from the Er:Si case.
Thin heteroepitaxial Si1-yCy films have been grown on Si (100) by Ultrahigh Vacuum Chemical Vapor Deposition (UHV/CVD) using silane and methylsilane as silicon and carbon precursors. Carbon incorporation has been studied in the growth temperature range of 550°C to 650°C. The layers have been characterized using high resolution X-ray diffraction and secondary ion mass spectrometry. The total carbon content of the alloys increases linearly with the methylsilane partial pressure and a methylsilane sticking coefficient approximately 2 times higher than that of silane was extracted. Layers with up to 1.34 % substitutional carbon have been obtained at the lowest growth temperature. Fully substitutional carbon can be obtained for levels up to 0.65%. Variations of the growth rate with temperature and carbon content are also discussed.
A new approach for one-dimensional photonic bandgap formation is introduced. The method consists of wafer bonding and delamination, which is capable of stacking single crystalline semiconductor layers on non-crystalline insulator layers. Si and SiO2 layers with sub-wavelength periodicity are successfully stacked to form photonic crystals consisting of 3 pairs without a defect layer and of 4.5 pairs with a defect layer. The transmittance spectra are well reproduced by transfer matrix calculations. This clearly verifies the potential of the wafer bonding and delamination method.
Single crystal Gd2O3 dielectric thin films were epitaxially grown on GaAs. The Gd2O3 film has a cubic structure isomorphic to Mn2O3, and is (110) oriented in single domain on the (100) GaAs surface. The oxide film has low leakage current densities ˜ 10–9 – 10–10 A/cmT2 at zero bias. Typical breakdown field is 4 MV/cm for an oxide film 185 Å thick, and >10 MV/cm for an oxide less than 50 Å thick. Both accumulation and inversion layers were observed in the Gd2O3-GaAs metal oxide semiconductor (MOS) diodes using capacitance-voltage (C-V) measurements, with an interfacial density of states around 1011 cm–2 eV–1.
Recent low-temperature photoluminescence (PL) studies will be discussed for coherent Si1-xGex. and Si1-xGexCy alloy multiple quantum wells on Si (001) substrates grown by either ultra-high vacuum chemical vapour deposition or solid source molecular beam epitaxy. An in-plane applied-stress technique will be described which removes systematically band edge degeneracies revealing the lower, PL-active CB. Applied-stress data taken with this technique at ultra-low excitation intensity proved intrinsic type II CB alignment in SiGe on Si (001). Apparent type I alignment observed at higher intensity will also be discussed. New applied stress PL results are presented for Si1-x-yGexCy quantum wells under various grown-in stress condition
We present the studies of the thermal stability of various metal including Au, Ti, Pt, Pd and Pt/Ti/Pt/Au Schottky contacts on strained Ga0.2In0.8P/InP semiconductors. Auger electron spectroscopy (AES) analysis and cross-sectional TEM of the thermally annealed Schottky diode were performed to investigate the failure mechanism. For Pt/Ti/Pt/Au schottky contacts on strained GalnP/InP, no significant change was found for samples annealed up to 350°C. However, a drastic degradation of the barrier height and the ideality factor was observed in samples annealed at 400°C, which may be caused by the interdiffusion and penetration of metals into the semiconductor. Finally InGaAs/InP doped channel heterojunction FET's (DC-HFET's) with a GaInP Schottky barrier enhancement layer (SBEL) were grown and fabricated. The 0.25 μm gate-length devices showed excellent DC and RF performance, with anfi of 117 GHz and an fmax of 168 GHz.
We present real time observations of the interaction of dislocations in heteroepitaxial strained layers using a specially modified ultrahigh vacuum transmission electron microscope equipped with in-situ deposition capabilities. These observations have led to delineation of the regime of epilayer thickness and composition where dislocation interactions result in blocking of the propagating threading segment. It is found that both the blocking probability as well as the magnitude of the dislocation interaction force are strongly dependent on the Burgers vectors of the dislocations involved, with the greatest effects observed when the Burgers vectors of the two dislocations are parallel with respect to each other. Frame-by-frame analysis of the motion of the dislocation threading segment during interaction is used to extract the magnitude of the interaction stresses as a function of both the level of heteroepitaxial strain and the dislocation geometry. Finally, by continuing growth following observations of blocking during annealing, we find that blocked dislocations are likely to remain in that configuration until substantial additional heteroepitaxial stresses are incorporated into the layer. These results have direct relevance to the successful integration of strained layer heterostructures into electronic device applications. This is because blocked threading segments result in the introduction of undesired band gap states, enhance impurity diffusion, modify surface morphology and act to limit the dislocation density reductions achievable in graded buffer structures.
Utilizing Marker layer experiments and Z-contrast imaging, we have observed the formation of surface cusps during SixGe1−x alloy growth. The formation of cusps can be understood in terms of stress-driven surface diffusion, and we consider the large stress build-up at the cusp tip as a potential source for the nucleation of misfit dislocations.
We have combined Z-contrast imaging and Ge marker layer experiments to study the evolving surface morphology of SixGel-x alloys grown by molecular beam epitaxy (MBE). Surface cusps are seen to arise as the intersection lines between coherent islands. The potential implications of stress concentrations associated with cusps are considered with a view to strain relaxation in the film via dislocation nucleation.
A reduction in the optical energy gap of more than 65 meV has been observed in In0.53Ga0.47 As grown on (100) InP by atmospheric pressure metalorganic vapor phase epitaxy. The band gap energies were deduced from room temperature photocurrent spectroscopic measurements, accounting for differences in composition and strain. Spontaneous CuPt type ordering of In and Ga atoms on the (111) subplanes of the InGaAs2 was confirmed by transmission electron microscopy. Superlattice signatures in the transmission micrographs were observed only for samples with associated reduced band gap energies, and were confirmed by visible double periodicity in high resolution images. In0.53Ga0.47 As was grown under a variety of conditions, some which promoted ordering. In general, lower growth temperatures and moderate (∼4 μ/hr) growth rates promoted a greater degree of ordering and reduction of the band gap energy. The influence of growth conditions on the ordered structure is considered within the context of current theories.
We report on growth, doping, and characterization studies of GaN films produced by the Electron Cyclotron Resonance microwave plasma assisted Molecular Beam Epitaxy. The films were grown heteroepitaxially on sapphire (0001), whose surface was converted into atomically smooth AIN by plasma nitridation. The GaN films were grown in two temperature steps, a process found to promote the layer-by-layer growth mode. ECR plasma conditions to grow either n-type autodoped or semi-insulating GaN film were identified. The structure and microstructure as well as the electrical properties of these two classes of films are discussed. A systematic dependence between electron mobility and net carrier concentration was found, which predicts that the mobility of GaN with a net carrier concentration of 1014cm−3 is about 104cm2 /V.s. The insulating films were intentionally doped either p-type or n-type by incorporation of Mg or Si during film growth. Hole or electron concentrations at 300K between 1018-1019cm−3 have been obtained without requiring any post-growth treatment.
Photoreflectance (PR) has been performed on a series of undoped and n-type, InGaAs and InAlAs molecular beam epitaxy (MBE) grown layers with different In mole fractions, and epilayer thicknesses on Fe-doped semi-insulating (SI)-InP substrates. From investigations of the temperature dependence, time constant dependence and an additional cw light beam intensity dependence, three substrate peaks are identified as an excitonic transition from the substrate, a free electron transition near the interface which gives a Franz-Keldysh oscillation (KFO), and a transition from the spin-orbit split-off valence band. The results are indicative of a redistribution of charge near the substrate interface in the process of MBE growth; the associated PR signal (phase) could be used for in-situ monitoring of epilayer growth on SI-InP wafers.