Library

Notes: In this article, nitrogen-rich silicon nitride plasma deposited under conditions minimizing Si—H bonding is shown to possess extremely low bulk electron trapping rates which are as low or lower than plasma-deposited oxides produced using He dilution. The bulk trap density, measured by avalanche injection decreases as the rf power is decreased. The total charge trapped within these silicon nitrides reaches a saturation value determined by high field detrapping in thick nitride films.

Notes: Boron modulation-doped Si/SiGe heterojunctions have been grown by molecular beam epitaxy. The two-dimensional hole gas formed along the heterojunction, just inside the alloy, has a sheet density in the range 2–5×1011 cm−2 and a typical mobility at 5 K of 2000 cm2 V−1 s−1. An explanation for the magnitude of the mobility is sought by considering likely scattering mechanisms, namely those due to remote impurities, interface roughness, alloy disorder, and interface impurities. A self-consistent model is used to determine the sheet density in terms of structural and energy parameters and dopant concentrations in the heterostructure. It is shown that the presence of negatively charged impurities at the heterojunction provides the basis for a consistent interpretation of the experimental results.

Notes: The chemical mechanisms involved in the decomposition of boron trichloride and the concomitant incorporation of elemental boron into Si(111) were elucidated. The reaction between BCl3 and Si(111) is quit complex due to the presence of Si, B, and Cl in a number of chemically distinct environments simultaneously. Annealing the sample to 570 °C effectively desorbs all molecularly adsorbed BCl3. Additional anneals to 710 and 870 °C largely reduce the BCl2 and BCl3 moeities to form the subsurface-boron reconstructed surface.

Notes: The interaction between B2H6 and SiO2 was investigated by soft x-ray photoemission. Thermally activated, autocatalytic dissociative chemisorption of B2H6 on SiO2 to form elemental films was discovered at temperatures exceeding 550 °C. These surfaces are shown to be quite reactive towards SiH2Cl2. This process is thus a mechanism whereby the selectivity of chemical vapor deposition exhibited by SiH2Cl2 towards SiO2 can be rapidly degraded.

Notes: Many vibration–rotation components of the ν2 fundamental of the SiH+3 cation have been measured using diode laser velocity modulation spectroscopy. By comparing the intensities of these components with their nuclear statistical weights, the ion is shown to be planar (D3h). The position of the ν2 band center is 838.0674(7) cm−1 and the ground state rotational constant is B0=5.2153(1) cm−1, r0=0.1462 nm. These results are compared with ab initio calculations.

Notes: A selected ion flow tube study has been carried out at 300 K of the reactions of some atomic and molecular positive and negative halogen ions with Cl2 and Br2 from which the rate coefficients k and ion product distributions have been determined. For the energetic F+ ion reactions, dissociative charge transfer is the dominant process, while for the Cl+ ions, only nondissociative charge transfer occurs. For the less energetic Br+ and I+ reactions, dihalogen molecular ions are important products. All these positive ion reactions proceed quite efficiently, i.e., the k are appreciable fractions of kc, their respective collisional rate coefficients, except for the reactions of Cl2 with the lower energy ions of the spin–orbit triplet of I+, i.e., I+(3P1,0), for which k∼0.07kc, this being due to the endothermicities of the reactions. The molecular ion Cl2+ undergoes rapid nondissociative charge transfer with Br2, a process which is, of course, endothermic for the reaction of Br2+ with Cl2 and so no reaction is observed. The less-energetic atomic negative ion reactions proceed—via atom exchange—in which the atomic negative ion of the reactant molecular species and a dihalogen molecule are produced. For those reactions that are exothermic, the k are, within error, equal to (2/3)kc, implying that they proceed via complexes which separate statistically back to reactants (1/3) and forward to products (2/3). Both the Br−+Cl2 and Cl−+Br2 reactions are somewhat less efficient (i.e., k〈2/3kc), a result of the slight endothermicities of the reactions. Of the molecular negative ion reactions, electron transfer is the major process in the Cl2− reaction with Br2, whereas the reaction of Br2− with Cl2 proceeds relatively slowly producing the triatomic ion BrCl2−.

Notes: Objectives 1. To compare the ultrasound biparietal diameter and crown-rump length of fetuses with and without Down's syndrome in the first half of pregnancy; 2. To investigate the effect of estimation of gestational age using either measure on the detection rate of serum screening for Down's syndrome.Design Matched case-control study. Cases were singleton Down's syndrome pregnancies with a biparietal diameter or a crown-rump length recorded. Five controls were matched to each case on: medical centre; the date of the ultrasound scan examination (within two years); gestational age measured as the number of days since the first day of the last menstrual period; and the ultrasound measure used (ie the biparietal diameter (the measure of choice), or the crown-rump length otherwise). If a woman had a serum screening test for Down's syndrome, the biparietal diameter or crown-rump length measurement had to be taken prior to the screening test so that the result of the test could not influence whether a scan was performed.Setting Ten antenatal screening centres in seven countries in Europe and North America.Subjects Two hundred and one women with singleton Down's syndrome pregnancies and 1005 women with unaffected singleton pregnancies.Results The median biparietal diameter of fetuses with Down's syndrome was identical to that among the controls (median difference 0.0mm, 95% confidence intervals (CI)–0.5 to 0.5mm). The estimates of gestational age based on biparietal diameter yielded a median gestational age less than that based on the women's last menstrual period: three days less for cases and two days less for controls; small but statistically significant differences probably reflected a minor systematic difference in the conversion of a biparietal diameter to a gestational age estimate. The median crown-rump length of fetuses with Down's syndrome was also identical to that among controls (median difference 0.0mm, 95% CI–1.5 to 2.0 mm). There was no significant difference between the median gestational age estimate based on crown-rump length and that based on the women's last menstrual period.Conclusion In antenatal screening for Down's syndrome the routine use of an ultrasound biparietal diameter or crown-rump length measurement to estimate gestational age will not adversely affect the detection rate. To avoid differences in gestational age estimates using the last menstrual period and the biparietal diameter influencing screening performance, separate medians should be derived for each serum marker using the two methods of estimating gestational age. The appropriate set of medians can then be used to calculate the multiple of the median value for each woman screened depending on the method used to estimate her gestational age.

Notes: Objective To investigate the effect of using a routine ultrasound estimate of gestational age and maternal weight adjustment on maternal serum alpha-fetoprotein (AFP), unconjugated oestriol (uE3) and human chorionic gonadotrophin (hCG) levels in antenatal screening for Down's syndrome.Design Women with a singleton pregnancy without Down's syndrome were screened using the three serum markers and an estimate of gestational age based on ‘dates’ (time since first day of the last menstrual period) and one based on an ultrasound scan examination was recorded together with maternal weight.Setting Women attending the Homerton Hospital, Hackney, for their antenatal care between February 1989 and January 1990.Subjects 2113 women with a singleton pregnancy without Down's syndrome.Results The use of ultrasound to estimate gestational age (usually based on the biparietal diameter of the fetal skull) led to a significant reduction in the variance of each marker at a given week of pregnancy. The level of each marker was negatively associated with maternal weight, so that adjustment for weight also led to a reduction in variance. These data on gestational age and maternal weight, taken together with published data on pregnancies associated with Down's syndrome, indicate that the routine use of ultrasound to estimate gestational age will increase the detection rate from 58% to 67% while maintaining the false-positive rate at 5%, or reduce the false-positive rate from 5.7% to 3.1% while maintaining the detection rate at 60%. Routine maternal weight adjustment for the serum marker levels was much less useful, increasing the detection rate by about 0.5% for a given false-positive rate, or reducing the false-positive rate about 0.1% for a given detection rate.Conclusion An ultrasound gestational age estimate available at the time of Down's syndrome screening confers a substantial advantage to screening performance with a further small benefit resulting from maternal weight adjustment, which is worth adopting if it can be done without difficulty or extra cost.

Notes: We have demonstrated a 10-dB reduction in the intensity sidelobes of an apodized-interaction-strength guided-wave acousto-optic filter with a center passband of 1530 nm. Acoustic-intensity weighting was achieved by launching a surface acoustic wave (SAW) beam in a straight acoustic waveguide, and gradually transferring this SAW intensity to the active device, and back out, by evanescent-wave coupling across a 20 μm barrier over a 19-mm interaction length. The intensity sidelobe was −5 dB for an unapodized filter with abrupt onset and cutoff of the interaction, but sidelobes were reduced to at most −15 dB for a SAW intensity with raised-cosine weighting. A conversion efficiency of 97% was achieved with 80 mW input power for the 2.0-nm-wide apodized filter.