Growth Pause Research Articles

Impaired Development of Somatotropes, Lactotropes and Thyrotropes in Growth-Retarded (grt) Mice

Congenitally primary hypothyroid growth-retarded (grt) mice exhibit a characteristic growth pause followed by delayed onset of pubertal growth. We characterized the developmental pattern of somatotropes, lactotropes and thyrotropes in the anterior pituitary, as well as plasma levels of their secretory hormones, in grt mice. Compared with normal mice, the weight of grt pituitary gland was similar at 8 weeks of age but significantly heavier after 12 weeks of age. Compared with normal mice, there were significantly fewer somatotropes in the grt pituitary until 8 weeks of age, but the number gradually increased up to 48 weeks. The number of lactotropes in grt mice was consistently lower than that in normal mice from 2 through 48 weeks, whereas the number of thyrotropes in the grt pituitary was consistently higher than in the normal pituitary. Thyrotropes in the grt pituitary exhibited hypertrophy and hyperplasia with less intensive thyroid-stimulating hormone (TSH) immunoreactivity than normal thyrotropes. In normal mice, the sum of the relative proportions of these cells plateaued at 8 weeks, where it remained up to 48 weeks of age. In grt mice, these proportions almost reached normal levels at 12 weeks of age but gradually declined after 24 weeks. Plasma growth hormone concentrations did not differ between grt and normal mice until 24 weeks of age. Compared with normal mice, grt mice exhibited significantly lower plasma prolactin and thyroxine levels but higher TSH levels. These findings indicate that development of somatotropes, lactotropes and thyrotropes in grt mice is impaired, being followed by altered hormone secretion.

A comprehensive study of the effect of in situ annealing at high growth temperature on the morphological and optical properties of self-assembled InAs/GaAs QDs

We investigate the effect of in situ annealing during growth pause on the morphological and optical properties of self-assembled InAs/GaAs quantum dots (QDs). The islands were grown at different growth rates and having different monolayer coverage. The results were explained on the basis of atomic force microscopy (AFM) and photo-luminescence (PL) measurements. The studies show the occurrence of ripening-like phenomenon, observed in strained semiconductor system. Agglomeration of the self-assembled QDs takes place during dot pause leading to an equilibrium size distribution. The PL properties of the QDs are affected by the Indium desorption from the surface of the QDs during dot pause annealing at high growth temperature (520°C) subsiding the effect of the narrowing of the dot size distribution with growth pause. The samples having high monolayer coverage (3.4 ML) and grown at a slower growth rate (0.032 ML s−1) manifested two different QD families. Among the islands the smaller are coherent defect-free in nature, whereas the larger dots are plastically relaxed and hence optically inactive. Indium desorption from the island surface during the in situ annealing and inhomogeneous morphology as the dots agglomerate during the growth pause, also affects the PL emission from these dot assemblies.

Tunability of Photoluminescence of InAs/GaAs Quantum Dots by Growth Pause Introduced Ripening

The effect of dot ripening pause on self organized InAs/GaAs QD's grown by MBE at two different growth rates and the resulting tunability of emission properties were studied with the help of PL and AFM experiments. We found the evolution of larger coherent islands with dot-pause due to high surface mobility of the In adatoms at the growth temperature resulting in a redshift in the PL spectra. A small blue shift in the emission is observed in case of the islands grown at higher growth rate and being allowed to ripen for sufficient time due to In desorption at high growth temperature.

Variation of the morphology of strained AlGaInAs quantum wells with substrate orientation

Abstract Strain-compensated AlGaInAs quantum wells/barriers were grown by OMVPE on InP substrates of different orientations: (1 0 0), (1 0 0) 3° miscut toward (1 1 1)A, (5 1 1)A and (5 1 1)B, (3 1 1)A and (3 1 1)B. Smooth surfaces and narrow photoluminescence lines were obtained on (1 0 0) and (3 1 1)B, even for the highest compressive strain tested, +2%. Under high growth rate, (5 1 1)A was found to yield the roughest surfaces and widest photoluminescence lines. When growth pauses were inserted in the quantum wells, (5 1 1)A, (5 1 1)B, and (1 0 0) 3° miscut toward (1 1 1)A showed very rough surfaces.

Effects of Gas switching sequences on GaAs/GaAs 1−ySb y superlattices

The interfacial structure of GaAs/GaAs 1− y Sb y superlattices (SL) was modified through changes in the interlayer gas switching sequence. Trimethyl gallium, trimethyl antimony, and arsine were used as the precursors in the growth of thin-layer SL structures. Sufficient antimony introduction during the growth pause prior to GaAs 1− y Sb y growth was required for obtaining abrupt interfaces with a minimal amount of compositional grading as determined by X-ray diffraction, as was the arsine introduction after alloy layer growth. In both cases, the specific amount of Sb or As deposition was found to produce an optimal structure with minimal interfacial grading. The antimony composition of the pseudomorphic alloy layers appear to be constrained by thermodynamic and kinetic limitations.

L1/Laminin modulation of growth cone response to EphB triggers growth pauses and regulates the microtubule destabilizing protein SCG10.

During development, EphB proteins serve as axon guidance molecules for retinal ganglion cell axon pathfinding toward the optic nerve head and in midbrain targets. To better understand the mechanisms by which EphB proteins influence retinal growth cone behavior, we investigated how axon responses to EphB were modulated by laminin and L1, two guidance molecules that retinal axons encounter during in vivo pathfinding. Unlike EphB stimulation in the presence of laminin, which triggers typical growth cone collapse, growth cones co-stimulated by L1 did not respond to EphB. Moreover, EphB exposure in the presence of both laminin and L1 resulted in a novel growth cone inhibition manifested as a pause in axon elongation with maintenance of normal growth cone morphology and filopodial activity. Pauses were not associated with loss of growth cone actin but were accompanied by a redistribution of the microtubule cytoskeleton with increased numbers of microtubules extending into filopodia and to the peripheral edge of the growth cone. This phenomenon was accompanied by reduced levels of the growth cone microtubule destabilizing protein SCG10. Antibody blockade of SCG10 function in growth cones resulted in both changes in microtubule distribution and pause responses mirroring those elicited by EphB in the presence of laminin and L1. These results demonstrate that retinal growth cone responsiveness to EphB is regulated by co-impinging signals from other axon guidance molecules. Furthermore, the results are consistent with EphB-mediated axon guidance mechanisms that involve the SCG10-mediated regulation of the growth cone microtubule cytoskeleton.

Effects of growth pause on the structural and optical properties of InGaAsN–InGaAsP quantum well

Optical and structural characterizations were conducted on an InGaAsN quantum well (QW) with large energy bandgap barrier material consisting of InGaAsP ( E g=1.62 eV) grown by metalorganic chemical vapor deposition. A growth pause annealing technique substantially improves both the structural and the optical quality of the QW. With an optimum growth pause of 14 s, surface roughness reduces by 50% and photoluminescence intensity increases 25 times compared with the sample undergoing a 3-s growth pause. No significant emission wavelength blue shift was observed, indicating this growth pause annealing process involves a mechanism different from that of the conventional post-growth thermal annealing of InGaAsN QW. Preliminary results on InGaAsN-active diode lasers employing InGaAsP barrier material indicate performance is limited by nonradiative recombination.

Improved photoluminescence of InGaAsN–(In)GaAsP quantum well by organometallic vapor phase epitaxy using growth pause annealing

The metalorganic chemical vapor deposition of a highly strained InGaAsN quantum-well (QW) surrounded by (In)GaAsP direct barrier layers is investigated. We found that growth pause annealing with AsH3, performed immediately before and after the growth of the QW, significantly improves the optical quality of InGaAsN QW with (In)GaAsP direct barriers. The utilization of larger band gap barrier materials, such as InGaAsP or GaAsP, will potentially lead to reduced carrier leakage from the QW laser structures.

Self-assembled GaInAs quantum wire heterostructure design for temperature stabilized emission wavelength

The strain-induced lateral-layer ordering process is used to fabricate GaInAs quantum wire (QWR) heterostructures on (1 0 0)-InP via molecular beam epitaxy. Using photoluminescence (PL) spectroscopy, the effect of the barrier material between QWR layers on the peak PL behavior with respect to temperature is investigated. In addition, two types of growth interrupt schemes were used to optimize the PL characteristics of the QWR heterostructure. Growth pauses during the deposition of the QWR layer were explored as well as growth pauses in between the QWR layer and barrier material. Consequently, peak PL wavelengths stabilized to less than 1 Å/°C above room temperature have been obtained for GaInAs QWR heterostructures using nominally lattice-matched AlGaInAs barriers and QWR-barrier growth interrupts to improve PL intensity. It is shown that strain is the main factor behind this PL behavior with respect to temperature.

Effect of Photoperiod and Nitrogen Supply on Basal Shoots Development in Rhododendron Catawbiense

In order to control young plant form by modifying culture conditions, plants of Rhododendron catawbiense from in vitro culture were grown in a greenhouse under different photoperiodic treatments (long or short days) combined or not with a several-week nitrogen starvation. After 12 weeks of culture under long days (16 h) with nitrogen supply, plants showed a rhythmic acrotonous development. When long days were combined with a six-week nitrogen starvation, the apical growth pause was extended leading to an increase of the number of acrotonous lateral ramifications. Short-day (8 h) treatment affected distal burst potential and moreover when a concomitant nitrogen starvation was applied. This lack of distal development allowed basal buds swelling, leading to basitonous plants. When plants were returned back to long days after 2, 4 or 6 weeks under short days, distal buds resumption competed with basal shoots development. Durable basitonous plants were obtained by a 12-week short days treatment combined with a 6-week nitrogen starvation.

Burst Potential Characterisation by Capacity for Nucleotide Accumulation in Rhododendron Catawbiense Apical Buds

Rhododendron catawbiense cv. Album propagated in vitro were transferred ex vitro and grown in a greenhouse, under long or short days. Under long days, the rhythmic growth led to an acrotonous development. In contrast, under short days, the upper buds were unable to burst, allowing basitony. In both photoperiodic conditions, the apical buds were sampled at different stages of the experiment. Growth capacities of the isolated buds were estimated by measuring their abilities to increase and diversify their non-adenylic nucleotide pool (NTP) after supplying adenosine as a precursor. Under long days, during the growth pause, the apical buds were able to increase and diversify their NTP pool. Under short days, adenosine was used to produce important quantities of ATP, while NTP pool increase became weaker. Nevertheless, during this long growth pause, apical bud tissues retained capacities to increase their NTP pool until the basal shoots developed.

Growth and reproduction of the endemic cruciferous species Pringlea antiscorbutica in Kerguelen Islands

This paper presents the first results from a 7-year monitoring of Pringlea plants established naturally from seed at Kerguelen at two sites with different microenvironmental characteristics. The field growth and reproductive traits of Pringlea are reported for the first time. Pringlea plants grow much faster than was previously believed, attaining around 50 cm diameter in 4 years. The growth pause in winter is short. Pringlea first flowers mainly in its 3rd or 4th year of growth and, as such, this species can be described as an early-flowering perennial. Inter-individual variability for all growth and reproductive parameters was generally higher than inter-site variability. These biological traits are compared to other subantarctic phanerogams and are discussed in terms of adaptation to subantarctic climate and ecological distribution of Pringlea antiscorbutica.

Overgrowth of InGaAs quantum dots formed by alternating molecular beam epitaxy

Abstract Alternating molecular beam epitaxy is used to grow InGaAs quantum dots on (1 0 0) GaAs by utilizing the Stranski-Krastanow growth transition. A growth pause during the overgrowth with GaAs that is used to bury the quantum dots improves the photoluminescence intensity and line width compared to continuous growth of GaAs. The optimum thickness of GaAs to grow at the quantum dot growth temperature of 515°C is 9.0 nm. Multiple layers of quantum dots have been grown, and transmission electron micrographs show vertical alignment between the layers. Intensity-dependent, room-temperature photoluminescence shows that excited-state luminescence can be reduced by using multiple layers of quantum dots, but growing too many layers leads to degradation of the photoluminescence intensity.

Evolution of surface topography during metalorganic vapor phase epitaxy of InP/InGaAs/InP quantum well heterostructures

The evolution of surface topography during epitaxial growth of lattice matched InP/InGaAs/InP on (100) InP substrate is observed using in situ elastic light scattering supported by ex situ atomic force microscopy. A topographically smooth growth transition from InP to InGaAs is observed. However, the InP-on-InGaAs interface exhibits three-dimensional nucleation followed by planarization and two-dimensional epitaxy. The three-dimensional phase is a result of the high surface energy of InP relative to InGaAs. A growth pause after the InGaAs QW increases the transient roughness of the InP surface and increases the thickness of InP required for planarization.

A novel hypothyroid 'growth-retarded' mouse derived from Snell's dwarf mouse

This paper describes a novel mutant mouse that has been spontaneously derived from the Snell's dwarf (DW/J) mouse. It was named the 'growth-retarded mouse' because of a characteristic growth pause followed by the delayed onset of pubertal growth. The onset of the increase in pituitary GH content that normally occurs concomitant with pubertal growth was also delayed in the growth-retarded mice. The serum concentration of thyroxine was very low in these mice from the neonatal period through adulthood, and a supplement of tri-iodothyronine was effective in shortening the growth pause and commencing the suppressed pubertal growth. Histological and immunohistochemical studies revealed that the anterior pituitary gland of the growth-retarded mouse contains clustered unusual chromophobic cells which are not reactive to various antisera against anterior pituitary hormones and the gland becomes enlarged with age. Breeding data indicated that these characteristics of the mice show an autosomal recessive inheritance and the gene responsible was designated as 'grm'. Partial linkage analysis utilizing microsatellite polymorphism demonstrated that the grm gene does not identify with the lit or hyt genes. Based on comparison of the hormonal status and growth pattern between growth-retarded, dwarf and normal mice, we have suggested the existence of a mutual interaction, possibly positive feedback regulation, between the pituitary and thyroid glands, that develops or matures the hormonal network which is responsible for rapid somatic growth and metabolic changes at puberty in mice.

The influence of weight, sex, birthdate and maternal age on the growth of weanling mink

Mink kits (Mustela vison) are weaned at a mean age of 32·2 days. Litters born later in the season are weaned at a younger age than those born early, but are not significantly lighter in weight, suggesting that their development has been accelerated. In some litters the transition to reliance on solid food is preceded by a period of arrested growth. This does not appear to be because the mother's milk supply has been over-stretched, as larger, heavier or male-biased litters are no more likely to experience a growth pause, nor to enter it earlier in development. However, in litters with arrested growth, the greater the individual mean kit weight, the earlier the growth pause occurs. This suggests that certain mothers deny milk to their young when they reach a particular stage of development. Multiparous females are much more likely than primipares to have litters which experience a period of arrested growth. Multiparous females are also more likely to have illness in their litters, and appear less protective of their young. The growth of male kits seems to be more vulnerable than females; their growth pauses tend to last longer, and in this sex the presence of such a pause results in them being lighter at weaning than other males. Male kits also grow faster than females, and are heavier at weaning. However, there is no significant sex difference in the age at weaning.

Generation of fast-switching As2 and P2 beams from AsH3 and PH3 for gas-source molecular-beam epitaxial growth of InGaAs/InP multiple quantum well and superlattice structures

In an attempt to maximize the cracking and dimerization efficiencies with the use of Ta baffling, a severe memory effect was observed which necessitated excessive growth pause durations for group V beam switching. This memory effect was significantly reduced by the implementation of separate Ta-tube catalytic hydride crackers for each group V source. A comparison of W and Ta tubes revealed that the catalytic effect which was observed for the decomposition of AsH3 and PH3 on a Ta surface was not observed for a W surface. The improved high-conductance design yielded cracking and dimerization efficiencies on the order of 99% and 90%, respectively. Switching characteristics have been studied using time-resolved quadrupole mass spectrometry, and switching times of <1 s have been achieved for As2 and P2 with significantly longer switching times observed for As4. Multiple layer heterostructures have been grown with the baffled/common cracker and the separate open-tube crackers for evaluation of the memory effects by secondary ion mass spectrometry. InGaAs/InP quantum well structures have been grown with the fast-switching crackers. Double crystal x-ray diffraction, transmission electron microscopy, and low temperature photoluminescence (PL) characterization suggest that the interfaces obtained with the fast-switching crackers are abrupt to within ∼5 Å when short growth interuptions (∼10 s) are used. However, low temperature (5 K) PL excitation spectra indicate that an interface asymmetry is present. These data suggest that despite verification of fast As2 and P2 switching using time-resolved quadrupole mass spectrometry, growth pause optimization, and group III flux stabilization are still required in order to obtain optimal interface quality for InGaAs/InP heterostructures.

Optimization of interfaces in arsenide–phosphide compounds grown by gas source molecular-beam epitaxy

A novel source switching procedure in gas source molecular-beam epitaxy to achieve abrupt interfaces in lattice-matched InP/GaInAs has been demonstrated. In this process, a two-step gas source switching procedure was used at the InP-to-GaInAs interface and a single growth pause was employed at the GaInAs-to-InP interface. Gas source switching procedures were optimized through characterizing multiple quantum well samples using photoluminescence and double crystal x-ray diffraction measurements. Single quantum wells grown using the optimized switching procedure displayed the narrowest luminescence linewidth values compared to other documented samples grown by gas source molecular-beam epitaxy.

Morphology and optical properties of strained InGaAs quantum wires

Strained InGaAs quantum wires have been grown on holographically-patterned (100) GaAs substrates using alternating molecular beam epitaxy. Transmission electron micrographs reveal that the initial grating has rough sidewalls from wet-etching, but they are smoothed by the epilayers within about 100 Å. The growth conditions favor the slow-growth plane controlling the rate of planarization of the grating. The low temperature photoluminescence from the quantum wire sample exhibits two peaks; the high intensity peak centered at 1.417 eV is attributed to the quantum wires at the bottom of the grating, and the lower intensity peak at 1.445 eV is attributed to the sidewall quantum wells. This assignment is verified by growing two additional samples that had identical growth conditions with the exception of a high temperature growth pause after the deposition of the InGaAs layer to promote surface migration and/or desoprtion from the sidewalls. The quantum wire/quantum well peak intensity ratio is 40:1 for the sample with the growth pause compared to 6:1 for the sample without a growth pause.

Improved InGaP/GaAs hetetointerfaces during gas-source MBE growth

The conditions for obtaining InGaP/GaAs heterointerfaces which are laterally uniform and atomically abrupt when grown by gas-source MBE are reported. Using in-situ reflection high-energy electron diffraction, double crystal X-ray diffraction, and transmission electron microscopy, the effects of a growth pause on the interfacial structure and composition of lattice-matched InGaP/GaAs multiple quantum well structures were studied. It was found that by providing sufficient time for annealing of the growth surface, interfacial regions which correspond to an intrinsic strain model of ideal InGaP/GaAs heteroepitaxial interfaces could be obtained.