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OBJECTIVE: To investigate whether the blink-startle reflex (BSR) is a good marker of habituation to vibroacoustic stimulation in healthy preterm fetuses. MATERIALS AND METHODS: A total of 22 women with uncomplicated pregnancies at 30-34 weeks participated in the study. Vibroacoustic stimulus was repeatedly applied to the maternal abdomen above the fetal head for a period of 2 s every 10 s. Fetal eye tightening monitored by ultrasound within 2 s of the stimulus was considered a positive response. The habituation rate was defined as the number of stimuli applied before the fetus stopped responding to two consecutive stimuli. Results were compared using the Mann-Whitney U-test. RESULTS: All 22 fetuses showed habituation at a rate that varied from 1 to 9 (mean 4.2+/-1.8). CONCLUSION: BSR is a good marker of habituation in preterm fetuses. Further study is needed to ascertain whether abnormal BSR habituation could be a sign of fetal distress. [ABSTRACT FROM AUTHOR]

B+ ions were implanted in Si at ultra-low energies: 0.25, 0.5 and 1 keV, respectively, and at different doses: 1×1014, 1×1015 cm−2. Lattice distortion and disorder due to the implantation process were investigated by means of a high resolution X-ray diffraction method. Due to the very low implantation depth (a few nm), the X-ray diffraction measurements were carried out by triple-crystal diffractometry. With this experimental configuration it was possible to separate coherent from diffuse scattering, considerably improving the signal-to-noise ratio. For the analysis of the experimental curves, the subsurface region was divided in several thin layers. The layer thickness, the static Debye–Waller factor, which is related to the lattice damage, and the lattice spacing modification (strain) were the parameters of the fitting procedure. Despite the small thickness of the ‘subsurface-damaged area’, it was possible to obtain the main parameters describing the depth distribution of the lattice distortions in the analyzed crystals. Transmission electron microscopy investigations were made on two samples implanted at the lowest energies and the results obtained by the X-ray diffraction were confirmed. [Copyright &y& Elsevier]

High resolution x-ray diffraction and topographic methods have been used to study lattice strain relaxation in the Ga1- XAl XSb/GaSb system. Samples with layer thickness ranging between 0.1 and 6 μm and with Al concentration x=0.402±0.005 have been grown by molecular beam epitaxy at 550 °C on (0 0 1) oriented undoped GaSb LEC substrates. A first critical thickness ( tC1), related to the misfit dislocation generation, has been found to be between 0.16 < tC1 < 0.20 μm. Due to the weak sensitivity of the rocking curve to the onset of relaxation, this result has been obtained by means of a double crystal topographic technique. A “plateau” region in the curve of the residual strain versus thickness has been observed for t ranging between 0.2 and 0.5 μm. The residual strain ɛres shows a dependence close to t–0.5 above a second critical thickness value tC2 slightly larger than 0.5 μm. Finally, in the last region above a layer thickness of 3 μm, strong dislocation interaction effects seem to affect the relaxation. A comparison with theoretical models has been made. [ABSTRACT FROM AUTHOR]

The double crystal X-ray rocking curves of GaAlAs/GaAs laser structures, with both a single and double confinement, have been calculated on the basis of the Takagi-Taupin dynamica theory. It has been demonstrated that very small changes in the thickness and composition of the active and the internal confining layers give rise to dramatic modifications of the rocking curves; this offers in principle a very powerful tool for measuring very precisely thickness and composition of these layers. However, the shape of the Bragg peak of the external confining layers exhibits a nearly period behaviour as a function of the thickness of the active or the internal confining layer; a simple relation between the thickness period and the composition difference of the considered layers has been obtained for the first time. Finally, the effect of the interchange of the confining layers on the rocking curves has been discussed. [ABSTRACT FROM AUTHOR]

A possible unintentional incorporation of As in Al[sub x]Ga[sub 1-x]Sb/GaSb heterostructures grown by molecular beam epitaxy in a chamber where group-III arsenides are also prepared, was investigated by high resolution x-ray diffraction measurements. The incorporation was determined by measuring the lattice mismatch between GaSb substrates and the possibly As-contaminated GaSb buffer layers in several structures containing layers with different Al concentration. The largest As molar fraction value measured is 0.000 43. The effect of the As incorporation on the deviation from Vegard's law, as previously found in the AlGaSb epitaxial system, was considered. From this analysis it was possible to confirm the nonlinear variation of the lattice constant versus the Al content, as previously determined. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Analyzes the lattice strain relaxation in the Ga[sub 1-x]Al[sub x]Sb/GaSb system using high resolution X-ray diffraction. Mechanisms responsible for the strain relaxation; Critical thickness for the strain relaxation; Measurement of the residual strain; Temperature dependence on the layer thickness.