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W Jager

Publications and source records attributed to W Jager.

5 recordsLinked to original sources
PubMed2000

Quantitative HRTEM analysis of semiconductor quantum dots

Elastic strains and layer compositions of semiconductor quantum dots are quantified by the measurement of lattice fringe spacings from high-resolution micrographs. Analyses of simulated images, taking thin-specimen relaxation into account by application of finite element simulations, demonstrate that the local slopes of these strain profiles may contain severe artefacts mainly caused by local crystal tilts. Nevertheless, average strain values may be measured with sufficient accuracy and can be used to obtain an estimate on average layer compositions by application of the continuum theory of elasticity when analysing experimental micrographs. Focusing on In(x)Ga1-xAs/GaAs and Ge(x)Si1-x/Si heterostructures, it is demonstrated that elastic strains of nanoscale coherent islands are severely decreased due to an elastic relaxation mechanism compared to fully strained two-dimensional layers. The analysis of self-assembled quantum dots and two-dimensional wetting layers buried by capping layers gives clear evidence for a substantial reduction of the lattice strains compared to the values expected for the nominal layer stoichiometries. This observation originates most presumably from a compositional intermixing during epitaxial growth.

Journal Article↗
PubMed1990

The influence of maternal exercise on the pulsatility index of the umbilical artery blood velocity waveform.

Doppler bloodflow velocity measurements in the umbilical arteries are quite generally used as a parameter for obstetrical management, notwithstanding the fact that restricted knowledge is available about the physiological and pathophysiological mechanisms of human umbilical circulation. The possible effects of maternal haemodynamic changes on the fetal circulation are only scarcely investigated. In a prospective study the effect of maternal exercise on the Pulsatility Index (PI) values measured in the umbilical artery with a pulsed Doppler device was assessed to investigate the need of standardization with respect to maternal exertion. The studied group consisted of 23 women with uncomplicated pregnancies of various gestational age, who underwent a moderate strain. Statistical analysis showed no significant difference in FHR and PI values before and after exercise, so it is concluded that it is not necessary to introduce a period of rest to precede a Doppler investigation.

Adult↗
PubMed1990

Characteristics of the umbilical artery velocity waveform as function of measurement site.

In 30 uncomplicated singleton pregnancies, varying in duration between 24 and 40 weeks, the variability of the flow velocity waveform (FVW) along the course of the umbilical artery was investigated. Blood flow velocities were recorded at 4 locations in the vessel: within the fetal abdomen, 0-5 cm from the origin of the umbilical cord, in the free-floating part, and 0-5 cm from its insertion in the placenta. From the Doppler signals recorded, the pulsatility index (PI) and a parameter for the frequency distribution index (FDI) were calculated. PI values differed among the locations, but no unequivocal tendency could be demonstrated. Statistical analysis, including multiple regression analysis for maternal and menstrual age and fetal heart rate, showed no significant difference in PI and FDI values for any of the 4 locations. It can be concluded that in uncomplicated pregnancies, possible changes in FVW (quantified by PI) along the course of the umbilical artery have no clinical relevance. Therefore, standardization for the sampling site when measuring PI in this vessel seems to be unnecessary.

Adult↗