Ga Fraction Research Articles

Infrared Dielectric Properties of In1-xGaxAs Epilayers on InP (100)

ABSTRACTThe concentration dependence of optical phonons in strained In1-xGaxAs epilayers grown on InP (100) by chemical beam epitaxy has been characterized with oblique angle polarized far-infrared reflectivity measurements. In this powerful method, the reflectance spectra contain sharp Berreman peaks exactly at the optical phonon frequencies. For radiation polarized in the plane of incidence (p-polarized), peaks for both the TO and LO phonons were observed. For s-polarization only the TO modes were observed. For heavily doped substrates the TO film phonons were observed as reflectance minima, whereas for lightly doped substrates they were seen as maxima. The measured spectra were curve resolved to separate the effects of the various modes which included GaAs-like longitudinal and transverse optic (LO and TO), a disorder induced, and InAs-like LO and TO phonons. The dielectric response function and phonon frequency dependences for all modes were obtained versus Ga fraction for x from 0.25 to 0.75 and the latter showed a quadratic dependence on x over this range. The effects of strain on the phonon frequencies could then be evaluated.

Infrared dielectric response function of strained In1−xGaxAs/InP epilayers

Polarized infrared reflectivity measurements have been performed on strained In1−xGaxAs epilayers grown by chemical beam epitaxy on semi-insulating InP(100) with x varied from 0.314 to 0.556. Fitting of the reflectance spectra yielded excellent agreement with three longitudinal and transverse optical phonon pairs, corresponding to GaAs-like, disorder-induced, and InAs-like modes. The dielectric response function and phonon frequency dependences for all modes were obtained versus Ga fraction.

Preparation, Characterization and Biodegradable Characteristics of Poly(1,3-trimethylene carbonate-co-glycolide)

Poly(1,3-trimethylene carbonate-co-glycolide) (PGCA) has been synthesized by ring-opening polymerization of 1,3-trimethylene carbonate (CA) and glycolide (GA) with stannous octoate as catalyst. The copolymers were characterized by 1 H nuclear magnetic resonance (NMR), 13 C NMR and differential scanning calorimetry. Water content and static contact angle of distilled water on the polymer surface were used to evaluate the hydrophobicity of the copolymers. There was no apparent difference in hydrophobicities of copolymers containing up to 30mol% GA. The biodegradation of PGCA was conducted in phosphate buffer solution at 37°C and in rats. The results indicated that the degradation rates of PGCA were higher than that of PCA and depended on the GA fraction in the copolymers. Furthermore, degradation occurred in the bulk when the GA content exceeded 20mol%. With less GA units the degradation became a surface reaction both in vitro and in vivo. These properties of PGCA may be useful in protein delivery systems.

Effects of alloy composition on the As desorption from and adsorption on strained In xGa 1− xAs surfaces

The specular reflectivity of strained In x Ga 1− x As surfaces grown by molecular beam epitaxy on InAs (100) substrates is measured with reflection high-energy electron diffraction (RHEED). A discontinuous change in the surface reflectivity is observed as the substrate temperature is increased above the transition point where As desorbs from the surface. A clear hysteresis loop is revealed as the substrate temperature is decreased. The substrate temperature required for desorption of surface As increases with Ga composition. A comparison between experimental results and theoretical calculations based on a Monte Carlo simulation shows that the average vertical interaction is increasing with Ga fraction. Fluctuations in alloy composition across the surface result in In-rich domains from which As is preferentially desorbed. The sudden loss of As, corresponding to a first order phase transition, occurs when the As desorbed domains attain a critical size. The metastability of the phase transition is shown to be a minimum for In 0.5Ga 0.5As layers.

Polarization-independent strained InGaAs/InGaAlAs quantum-well phase modulators

Polarization-independent phase modulation in In/sub 1-x/Ga/sub x/As/InGaAlAs multiple-quantum-well waveguides is demonstrated for the first time. It is shown that by increasing the Ga fraction and hence the tensile strain in the quantum well the electric-field-induced refractive index change in the TM polarization Delta n/sub TM/ can be made to approach that in the TE polarization Delta n/sub TE/. At 1.523 mu m, the ratio Delta n/sub TM// Delta n/sub TE/=1 for x=0.7 with a phase shift coefficient of 17.4 degrees /V-mm was achieved. Polarization independence was maintained over the entire range of reverse bias voltage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Endogenous Gibberellins and Cytokinins in Spear Tips of Asparagus officinalis in Relation to Sex Expression

Abstract Endogenous gibberellins (GA) and cytokinins (CK) were extracted from asparagus (Asparagus officinalis L.) spear tips, purified, and determined by lettuce hypocotyl and amaranthus bioassays, respectively. There was no quantitative difference in GA-Iike activity between heterogametic male and female spears. The major GA fraction in asparagus spears has 1 OH group. Asparagus spears contain 3 major fractions of CK-like activity. Fraction 1 eluted from Sephadex LH-20 and C18 HPLC columns with or before zeatin-riboside. Fractions 2 and 3 eluted in a similar pattern to IPA-riboside and IPA, respectively. There were higher levels of CK fraction 2 and trends toward higher levels of fraction 1 and total CK in female than in heterogametic male spears. There were also higher CK:GA ratios in female than in heterogametic male spears. The data support the hypothesis that sex in asparagus is controlled in part by CK levels or by CK:GA ratios.

Isolation and Characterization of Chicken Liver and Hepatoma Mc-29 Endoplasmic Reticulum and Golgi Apparatus Membranes and Biosynthesis of Their Glycoconjugates

Rough and smooth endoplasmic reticulum (ER) membranes and trans-Golgi apparatus (GA) fraction, intermediate GA fraction, and cis-GA fraction from White Leghorn chicken liver and hepatoma Mc-29 were isolated. Their purity was checked by electron microscopy and by determination of the activity of the marker enzymes. Rough and smooth membranes were also identified by calculation of the ratio between the content of RNA and the total phospholipids. It was found that the membrane fractions were pure enough. The tumor exhibited a decreased amount of ER and a small GA when compared when the ER and GA in the liver. The biogenesis of membrane glycoconjugates was followed by in vivo experiments either with [14C]glucosamine or by double labeling with [14C]N-acetylmannosamine and [3H]leucine. The radioisotope studies indicated that the synthesis rate of protein in liver and hepatoma rough ER was nearly the same, but in the tumor cells the glycosylation of the nascent polypeptide chain and the formation of glycolipids were significantly decreased.

Thermodynamic factors that influence the lattice matching of InGaAs to InP

The change in the fraction of the HCl flowing over the gallium boat necessary to maintain the lattice matched composition of InGaAs to InP under equilibrium conditions is computed for different source and deposition temperatures, HCl flow rates over the indium, amounts of unreacted HCl, and AsH 3 flow rates. These results are compared with the experimentally determined trends found in the literature. With the exception of the decreasing HCl(Ga) fraction with increasing AsH 3 flow rates, the experimental results can be explained thermodynamically when the effects of the unreacted HCl are taken into account. Increasing the amount of unreacted HCl substantially decreases the HCl(Ga) fraction necessary for lattice matching. This relatively strong effect can, in part, explain why the lattice matching HCl(Ga) fraction for different systems can be very different for seemingly similar growth conditions. The large differences in the computed values using different thermodynamic data and the effects of ignoring the HCl in equilibrium with the liquid metals, the As 2, AsH 3, InCl 2, InCl 3, and GaCl 3 species are briefly discussed.

The biosynthesis of the gibberellin precursorent-kaurene in cell-free extracts and the endogenous gibberellins of Japanese morning glory in relation to seed development

The endogenous levels of gibberellins (GAs) determined by a combined HPLC-bioassay procedure and the formation ofent-kaurene, an immediate GA precursor, in cell-free extracts were studied in relation to seed development inPharbitis nil Choisy cv. Violet. Three biologically active GA fractions were obtained, tentatively identified as GA3, GA5/ GA20, and a GA fraction, possibly GA19 and/or GA44, which all increased in activity during early seed development and subsequently declined during maturation of the seeds. The total endogenous GA level reached its maximum at 19 days after anthesis, just before the seeds had attained their maximum fresh weight at about 23 days after anthesis. Similarly, theent-kaurene synthesizing capacity showed a rapid increase during the period of rapid growth of the seeds, followed by a decline during maturation.

Heteroepitaxial Thermal Gradient Solution Growth of GaN

The growth of single‐crystal epitaxial layers of onto (0001) oriented sapphire substrates from the liquid phase is shown to require three conditions:(i) the partial pressure of in the H2 ambient, greater than the equilibrium pressure, (ii) the growth melt positioned in a thermal gradient to transport the dissolved to the growth interface, and (iii) the control of nucleation by the addition of Bi to the growth melt. Studies of growth at temperatures from 850° to 1050°C show that the layer thickness, , is approximately linear in time and varies with growth temperature and atom fraction of Ga, in the growth melt. When , internal cracks in the basal plane hexagonal directions develop at the layer‐substrate interface and for they propagate to the surface and are thermally etched. A description is given of the interaction of liquid Ga with as a function of and . The low solubility of in Ga ( fraction at 1150°C) prevents useful growth by slow cooling of the melt. The inability to change the electron concentration, , with Zn doping and the decrease of with in the growth melt support but do not unambiguously confirm the model that the native donor is a nitrogen vacancy