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A trend toward greater body size in dizygotic (DZ) than in monozygotic (MZ) twins has been suggested by some but not all studies, and this difference may also vary by age. We analyzed zygosity differences in mean values and variances of height and body mass index (BMI) among male and female twins from infancy to old age. Data were derived from an international database of 54 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), and included 842,951 height and BMI measurements from twins aged 1 to 102 years. The results showed that DZ twins were consistently taller than MZ twins, with differences of up to 2.0 cm in childhood and adolescence and up to 0.9 cm in adulthood. Similarly, a greater mean BMI of up to 0.3 kg/m2 in childhood and adolescence and up to 0.2 kg/m2 in adulthood was observed in DZ twins, although the pattern was less consistent. DZ twins presented up to 1.7% greater height and 1.9% greater BMI than MZ twins; these percentage differences were largest in middle and late childhood and decreased with age in both sexes. The variance of height was similar in MZ and DZ twins at most ages. In contrast, the variance of BMI was significantly higher in DZ than in MZ twins, particularly in childhood. In conclusion, DZ twins were generally taller and had greater BMI than MZ twins, but the differences decreased with age in both sexes.
We present the recent progress in the intermixing of InGaAs/GaAs quantum dot (QD) material. Quantum dot intermixing (QDI) allows the tuning of the energy bandgap in selected areas of the wafer or optoelectronic device, thus modifying its emission or absorption properties, in much the same way as in quantum well (QW) material. QDI has recently received increasing interest, as it combines bandgap engineering with the predicted advantages that quantum dots offer, such as low temperature-sensitivity of threshold current, high modulation frequency and low chirp.
We have applied the dielectric-cap-based techniques that were originally developed for QW structures, to the intermixing of InGaAs/GaAs/AlGaAs QD material with an emission wavelength of 1280 nm. Intermixing was achieved by sputtering a QDI-enhancing cap in some areas, and a QDI-suppressing cap in other areas, followed by a high-temperature anneal cycle. Extremely large bandgap blue-shifts of up to 280 nm have been obtained with an anneal temperature of 800 °C. The shifts were inferred from the photoluminescence (PL) spectra measured at 77 K under red-laser excitation.
To be of use in many applications, QDI must be able to provide a multiplicity of bandgaps on a single substrate. Multiple bandgaps can be created by varying the thickness of the QDI-enhancing cap, repeating the anneal cycle several times, or varying the coverage density of QDI-enhancing features over that of QDI-suppressing ones. The latter approach, termed selective intermixing in selected areas (SISA), involves the deposition of QDI-enhancing patterns of various area fill factors, which, upon annealing, will cause different degrees of intermixing in the underlying regions.
To demonstrate the SISA process in the QD material, we defined patterns containing lines and squares of various sizes (3 - 100 μm) and area fill factors (5% - 95%). The wafer was then annealed at 725 °C for 1 minute. As expected, the observed bandgap shifts were commensurate with the fill factor, with a 5% coverage providing a minimum shift (0 - 10 nm) and ∼40% a maximum one (∼ 120 nm). At fill factors above 40%, the shifts appeared to saturate and even decrease slightly. The effect of the feature size and shape was not very pronounced, with smaller features generating somewhat larger shifts. This may be due to the fabrication-related size bias that will have the strongest effect on the fill factor of smaller features. The PL spectra measured from patterns of large-size features (20 μm or more) often had a lopsided shape and broader peak width, which may be attributed to the limited spatial resolution of the measurement probe.
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