bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of Distance Learners’ Demographic Variables on Motivation and Performance in Flipped Classroom
Authors: Egbedokun A. O. , Afolabi O. M.
Published: March 2023

This study was carried out to find out the influence of demographic variables of distance learners on their motivation to learn and perform in a flipped classroom. The study employed a survey research method. The sample for the study consisted of three hundred (300) distance learners who were selected from two distance learning centres. The centers were selected using purposive sampling procedure because the two distance learning centers have deployed flipped learning mode. A questionnaire 4-point Likert scale questionnaire tagged: Flipped Classroom Motivation and Perception Questionnaire (FCMAPQ) was used in data collection. The results showed that only sex variable (T=2.487, p<0.05) significantly contributed to the motivation of the learners in the flipped classroom. Results further showed that demographic variables of sex, work status, age and marital status (F=.15, p>0.05) have no influence on the performance of distance learners’ in the flipped classroom. The study concluded that though demographic variable have no influence on motivation and performance of distance learners, it is rather a plausible innovation in distance learning.
Persistence of Fipronil Residues in the Cabbage Field Soil under Semi-arid Region of Rajasthan
Authors: Jat Hansa Kumari , Jakhar Bhanwar Lal , Sharma Badri Narayan , Yadav Raj Veer , Swati  , Rathore Balwant Singh , Deora Akansha
Published: March 2023

Fipronil is a phenyl pyrazole insecticide, which is used as broad spectrum insecticide to control pest such as diamondback moth, thrips, stem borers, leaf folder, termites, borers, jassids, and bollworms.  A study was undertaken at Rajasthan Agricultural Research Institute, Durgapura, Jaipur during Rabi, 2020 to find out the persistence of fipronil  residues in the soil of cabbage field, when sprayed at its recommended dose (5%  SC, 40 g. a. i. ha-1) and double of the recommended dose (5%  SC, 80 g. a. i. ha-1). The samples were extracted with acetonitrile and cleaned up using a modified QuEChERS method and the residues were analyzed by Gas Chromatography. The residue level of fipronil  in cabbage field soil collected at harvest time of cabbage crop were below the detectable level (BDL) at the recommended dose and double of the recommended dose, respectively.
Phytochemical Profiling Using GC-MS, Antioxidant and Antidepressant Properties of Methanolic Leaf Extract of Clerodendrum polycephalum Baker in Swiss Mice
Authors: Momoh Johnson Oshiobugie , Oshin Taiwo Toyin
Published: March 2023

Background: Clerodendrum polycephalum Baker is used by the traditional people in southwest Nigeria for arresting bleeding from cuts and treating bacterial infections especially wound infection without scientific proof of its efficacy. The plant was investigated in animal models for its antidepressant activity in Swiss mice.

Methods: The GC-MS, phytochemical analyses, antioxidant activities, tail suspension test, forced swimming test and oxidative stress parameters were determined using standard procedures.

Results: A total of 27 compounds were identified consisting of five prominent compounds and 22 minor compounds. The five prominent compounds constitute 63.99% of the Clerodendrum polycephalum Baker plant. The five major compounds and their percentage abundance are: Bicyclo[3.1.1]heptane, 2,6,6-trime thyl-  (21.36%), Squalene (18.69%), Neophytadiene (10.71%), 2-Tridecanol (6.66%) and 2-Dodecanol (6.57%). The phytochemicals present in the methanolic leaf extract of C polycephalum are: flavonoids, steroid, tannins, saponins, anthraquinones, alkaloids, terpenoids, anthocyanin, phenolic compounds and carbohydrate The extract has the ability to scavenge DPPH activity and it contains other components like: total proanthocyanidine (1.369±0.184), flavonoids (2.4%), β-Carotene (0.1336± 0.45 µg) and lycopene (0.0340±0.053µg/g). The antidepressant result showed robust and dose-dependent antidepressant-like activity of Clerodendrum polycephalum Baker. There are statistically significant (P<0.0001) reductions in the duration of immobility time both in the tail suspension and forced swimming test. Clerodendrum polycephalum extract produced significant (P<0.0001-0.0044) increase in total protein of the plasma, liver and kidney homogenate of the treated groups (Group C, D and E) compared to the untreated mice in group B. The level of antioxidant parameter such as catalase, and superoxide dismutase were significant increased (P<0.0001) and MDA values significantly reduce (P<0.0001-0.0014) in the treated groups administered with the extract and imipramine compared to the untreated mice in group B.   

Conclusion: The results show that methanolic leaf extract of the Clerodendrum polycephalum Baker has potential antioxidant and antidepressant activities and further studies should be conducted to identify, isolate and evaluate its potential active compound responsible for such effect.
Study on the Correlation between Some Physical and Microbial Quality Characteristics of Peeled Sweet Orange (Citrus sinensis)
Authors: Izah Sylvester Chibueze , Odubo Tamaraukepreye Catherine
Published: March 2023

Background /Aim: The consumption of ready-to-eat fruits such as oranges has increased in recent times. This study assessed the correlation between some physical and microbial quality characteristics of peeled sweet orange (Citrus sinensis) sold in Yenagoa, the capital of Bayelsa State, Nigeria.

Methodology: A total of thirty peeled oranges were purchased, three from each location. Standard microbiological methods were applied for the analysis.

Results: Results showed that the in situ characteristics and microbiological load of peeled oranges sold in Yenagoa, Nigeria were in the ranges of 1.86 to 3.15 Log CFU/g (total fungi), 1.56 to 2.86 Log CFU/g (total heterotrophic bacteria), 3.14 to – 4.19 (pH), 1.84 to 3.03 ppt (salinity), 3.27 to 5.00 mS/cm (conductivity) and 2.56 to 4.10 ppt (total dissolved solid). There was statistical deviation (p<0.05) for the in situ characteristics only. There was a positive significant association between the total dissolved solids and conductivity, and total heterotrophic bacteria and total fungi. Salinity negatively correlates with total heterotrophic bacteria and total fungi. The microbial isolates found in the samples include Staphylococcus aureus, Escherichia coli, Enterobacter, Bacillus, Proteus, and Micrococcus species, as well as Aspergillus, Penicillium, Mucor, Fusarium, Saccharomyces cerevisiae, and Trichoderma species.

Conclusion: However, with improved handling and hygiene levels among orange vendors, the microbial load could be reduced. Furthermore, the study showed that similar factors affected the total dissolved solids, conductivity, total heterotrophic bacteria, and total fungi. While the negative correlations that exists between salinity and total heterotrophic bacteria and total fungi revealed that diverse factors affect their concentrations in the orange.
A Pheochromocytoma with Liver Metastasis: What about Curative Surgery?
Authors: Elm’hadi Choukri , Zerrik Meryem , Benameur Yassir , Taleb Imad , Tanz Rachid
Published: March 2023

Introduction: 35% of pheochromocytomas are malignant and generally have a poor prognosis.Visceral metastases are associated with shorter survival. Treatments for advanced pheochromocytomas are essentially palliative. Surgery is potentially curative; however, tumor extension limits the chance for a curative resection.

Case Presentation: A 21-year-old women presented to our hospital with severe hypertension associated with Ménard's triad and heaviness of the left flank.The abdominal CT scan discovered a left adrenal mass with liver metastases. The diagnosis of pheochromocytoma was confirmed biochemically by the elevated catecholamine and metanephrine levels in the blood and urine. After preparation by the alpha-adrenergic blockade, she underwent left adrenalectomy and resection of left liver metastases followed by right hepatectomy. A pathology examination was performed and the final report was a metastatic pheochromocytoma with negative margins. She was discharged on postoperative without complications, and no evidence of recurrence occurred 48 months postoperatively.

Conclusion: complete resection of liver metastases from metastatic pheochromocytoma can improve survival and should also be considered in the event of predominant secreting symptoms.
Does Oral Administration of Fresh and Dry Ginger (Zingiber officinale) Juice have Effect on the Lipid Profile of Albino Rats
Authors: Waribo Helen Anthony , Ani Emmanuel Chukwu , Osuosa Atoukaritou Omodi
Published: March 2023

Aim: This study compares the effect of fresh and dry ginger juice on the lipid profile of albino rats.

Methodology: Fifteen(15) albino rats with average weight of 150g divided into three groups of five (5) rats each were used for the study.5ml of the ginger juice was administered orally for 21 days using the gavage oral tube to the experimental animals. Blood samples were collected via cardiac puncture after a chloroform anesthesia was given to the animals to a heparinized sample bottle. Plasma samples were analyzed for total cholesterol, triglyceride, high density lipoprotein using the colorimeter while and low-density lipoprotein level was calculated.

Results: There was no significant difference in cholesterol, triglyceride, high density lipoprotein and low-density lipoprotein levels (P>0.05) when the group 1(control group) was compared with group 2 (dry ginger group) and group 3 (fresh ginger group).

Conclusion: Ginger juice in different preparation forms does not alter the lipid profiling in the experimental animals over a period of 21 days.
Internodal cells of the giant green alga Chara as an expression system for ion channels
Authors: Lühring Hinrich , Witzemann Veit
Published: July 2002

Internodal cells of the giant green alga Chara corallina were utilized as an expression system for two nicotinic acetylcholine receptor subtypes (nAChR) derived from rat muscle. From Chara internodes that were pressure‐injected with the respective cRNA, cytoplasmic droplets were formed, and functional expression of channel proteins in the membrane delineating the droplets was confirmed by patch‐clamp techniques. The droplet membrane was recently identified as the original tonoplast, patch‐clamp recordings on cytoplasmic droplets were easily performed and single channel activity could be measured with high resolution. The properties of the recombinant nAChR observed in this membrane were similar to those reported for the channel expressed in Xenopus oocytes, and for the native channel recorded in situ. The Chara expression system is, therefore, suitable for functional expression of animal messenger RNAs, without the risk of signal contamination by intrinsic ion channels. It offers a low‐cost and practical alternative to the use of Xenopus oocytes for the investigation of heterologously expressed ion channels.
Effects of Culture Conditions on Enhancement of 2,4‐Dinitrotoluene Degradation by BurkholderiaEngineered with the Vitreoscilla Hemoglobin Gene
Authors: Nasr Mohamed A. , Hwang Kwang‐Woo , Akbas Meltem , Webster Dale A. , Stark Benjamin C.
Published: July 2002

Growth and degradation of 2,4‐dinitrotoluene (2,4‐DNT) were compared in liquid cultures in shake flasks for Burkholderiasp. strain DNT and strain DNT engineered to produce Vitreoscilla (bacterial) hemoglobin (strain YV1). Parameters varied included aeration rate, initial 2,4‐DNT concentration (50 and 200 ppm), and concentration and type of cosubstrate (yeast extract, succinate, casamino acids, and tryptic soy broth). 2,4‐DNT degradation increased with increasing cosubstrate concentration and was greater for strain YV1 than strain DNT under most conditions tested; the greatest advantages of YV1 (up to 3.5‐fold) occurred under limited aeration. A third strain (YV1m), derived from YV1 by repeated growth on 2,4‐DNT‐containing medium, demonstrated increased 2,4‐DNT degradation (up to 1.3‐fold compared to YV1) at 200 ppm 2,4‐DNT. The growth profiles of the three strains with respect to each other were in general similar to those of the degradation patterns of 2,4‐DNT.
Dispersed Organic Solvent To Enhance the Overall Gas/Water Mass Transfer Coefficient of Apolar Compounds in the Biological Waste‐Gas Treatment. Modeling and Evaluation
Authors: Cesário Maria T. , de Wit Hugo L. , Tramper Johannes , Beeftink Hendrik H.
Published: July 2002

In the biological treatment of waste gases, the contaminants are first transferred from the gas to the water phase and, subsequently, converted by the microorganisms. If the compound to be removed is poorly water‐soluble, the biological degradation is often limited by the slow transport from the gas to the aqueous phase. This transport limitation can be reduced by dispersing an apolar water‐immiscible organic solvent with a high affinity for the pollutants in the aqueous medium. A mathematical model was derived to be able to predict theoretically for which gaseous compounds and at which conditions a relevant intensification of the gas/water transfer can be expected. A characteristic composite gas/water mass transfer coefficient was obtained which is a function of the amount of solvent in the system and of the affinity of the compound for the water and for the solvent. Validation of the model predictions was done by carrying out steady‐state experiments in a stirred‐tank reactor in the absence of biological degradation. Two compounds were tested namely toluene and oxygen, being moderately and poorly water‐soluble, respectively. An increase of the mass transfer coefficient with increasing solvent volume fractions was found. While for oxygen a 2.2‐fold increase was found at 15% (v/v) FC40, an enhancement of circa 1.15 was observed for toluene at the same solvent volume fraction. The experimentally determined values of the enhancement factor defined as the ratio between the mass transfer coefficient in the systems with and without solvent compared well with the theoretical model predictions.
Estimation of O2 and CO2 Solubility in Microbial Culture Media
Authors: Gros J. B. , Dussap C. G. , Catté M.
Published: July 2002

A simple model for predicting gas solubilities of O2 and CO2 at low pressure and near ambient temperature in solutions of salts, sugars, and organic solvents (alcohols, ketones, ethers, aldehydes, etc.) is proposed. It is derived from the Van Laar assumptions and takes account of size differences between molecules in solution by their volumetric fraction. It is a group contribution model where anions and cations are considered as groups and other molecules are treated as in the UNIFAC (UNIQUAC Functional group Activity Coefficients) procedure. Pseudo‐Henry's constants for groups were determined using solubility data in aqueous solutions containing only one salt, one sugar, or one organic compound at 25 °C. The predictive performance of the model was evaluated by comparison with experimental data using multicomponent aqueous salt−sugar mixtures. The model was used to estimate solubilities of oxygen and carbon dioxide in fermentation media.
Seed dispersal of Acacia erioloba by African bush elephants in Hwange National Park, Zimbabwe
Authors: Dudley Joseph P.
Published: July 2002

Approximately two‐thirds (64%) of all dry season samples of elephant dung analysed during a 3‐year study in the Main Camp subregion of Hwange National Park, Zimbabwe, contained seed and/or pod materials from Acacia erioloba. Most seeds were recovered intact and actively germinating seeds were not uncommon. Very little pod mass relative to seed mass was recovered in most instances, with pod fragments recorded from only 56% of all exhaustively sampled elephant dung piles containing A. erioloba fruit materials. Nonetheless, large pod fragments and even entire intact pods were recovered occasionally from elephant dung. Seeds and pods of A. erioloba may comprise 12% or more of total wet‐weight dung mass; individual dung piles were found which contained > 5000 A. erioloba seeds. Birds and smaller mammals search out and consume A. erioloba seeds present within elephant dung piles. The findings of this study indicate that potential digestibility of A. erioloba seeds for bush elephants (Loxodonta africana africana) may be much higher than expected from previous studies. In controlled feeding trials with captive bush elephants (age 11–15 years old) maintained on predominantly free‐range dry season diets, the estimated efficiency of digestion for A. erioloba seeds consumed in pods was 81% to 96%, with a gut‐transit time of between 24.5 and 36.0 h. On the basis of throughput times determined in experimental feeding trials, potential elephant‐dispersal distances of 20–50 km are predicted for A. erioloba in the Kalahari Sands landscapes of southern central Africa.
Autocytotoxic T-cell clones in lichen planus
Authors: Sugerman P.B. , Satterwhite K. , Bigby M.
Published: July 2002

We examined the in vitro cytotoxic activity of cutaneous T-cell lines and clones from lichen planus (LP) patients against autologous epidermal keratinocytes. T cells were cultured from LP lesions and adjacent clinically normal skin and cloned by limiting dilution. Keratinocytes were cultured from LP lesions and adjacent clinically normal skin and immortalized by transfection with the E6 and E7 genes from human papillomavirus 16 (HPV16). The lesional T-cell line from one LP patient contained 27% γδ+ T cells and was significantly more cytotoxic against autologous lesional keratinocytes than the T-cell line from clinically normal skin. Clones isolated from the lesional T-cell line were significantly more cytotoxic against autologous lesional keratinocytes than clones isolated from the non-lesional T-cell line. Most cytotoxic clones from LP lesions were CD8+ and most non-cytotoxic clones from LP lesions were CD4+. One cytotoxic clone was CD4– and CD8– and expressed the γδ T-cell receptor. Two CD8+ LP lesional T-cell clones showed dose-dependent killing of HPV16 E6/E7-immortalized autologous lesional and normal keratinocytes, but no cytotoxic activity against Epstein–Barr virus-transformed autologous B-cell blasts. The cytotoxic activity of CD8+ lesional T-cell clones against autologous lesional keratinocytes was partially blocked with anti-major histocompatibility complex (MHC) class I monoclonal antibodies. These data support the hypothesis that CD8+ lesional T cells recognize an antigen associated with MHC class I on lesional keratinocytes and that CD8+ cytotoxic T cells lyse keratinocytes in LP lesions.
The relative power of heat‐precipitation nephelometric and clottable (Clauss) fibrinogen in the prediction of ischaemic heart disease: the Caerphilly and Speedwell studies
Authors: Sweetnam P. M. , Yarnell J. W. G. , Lowe G. D. O. , Baker I. A. , O'Brien J. R. , Rumley A. , Etherington M. D. , Whitehead P. J. , Elwood P. C.
Published: July 2002

The Caerphilly and Speedwell studies have previously reported the predictive power of heat‐precipitation, nephelometric, fibrinogen for 10‐year incidence of ischaemic heart disease. A Clauss, clotting time, fibrinogen was also measured at baseline, but has not previously been reported. The predictive power of the two assays is compared. Both methods were employed on fasting blood samples from a total of 4391 men aged 45–63 years. Over the following 10 years 533 (12%) developed major ischaemic heart disease. Nephelometric fibrinogen was higher by 0.33 g/l among the men who developed disease; clottable fibrinogen was higher by 0.20 g/l. This difference is statistically significant (P = 0.01). Relative odds of developing heart disease increased steadily to 3.53 (P < 0.0001) in the 20% of men with the highest nephelometric fibrinogen; for clottable fibrinogen the corresponding relative odds increased to 2.24 (P < 0.0001). When both measures of fibrinogen were included in logistic regression models together with age and smoking habit, the trend for incidence to increase with increasing nephelometric fibrinogen remained highly significant (P < 0.0001), whereas for the Clauss fibrinogen the trend almost entirely disappeared (P = 0.37).

We conclude that functional assays of clottable fibrinogen may not reflect all of the mechanisms which mediate the association between fibrinogen and cardiovascular disease and that assays of both ‘heat precipitable’ and ‘clottable’ fibrinogen should be included in all future prospective studies.
Effect of lens wear on corneal stroma: preliminary findings
Authors: Jalbert Isabelle , McOptom Fiona Stapleton
Published: July 2002

Purpose: The aim of the study was to measure the effects of long‐term extended wear of hydrogel lenses.

Methods: Nine subjects wearing disposable hydrogel lenses and age‐ and sex‐matched non‐lens‐wearing control subjects were enrolled. Using confocal microscopy, stromal keratocyte cell densities were measured.

Results: Anterior stromal keratocyte density was reduced in the lens‐wearing group (544 ± 206 cells/mm2) compared to the non‐lens‐wearing group (804 ± 145 cells/mm2) (P < 0.01). Posterior stromal keratocyte density was lower in the lens‐wearing group (514 ± 111 cells/mm2) than in the control group (628 ± 101 cells/mm2) (P < 0.05).

Conclusions: These preliminary findings indicate that extended wear of hydrogel lenses reduces stromal keratocyte density. The mechanisms for this alteration may include hypoxic, cytokine‐mediated or mechanical effects.
The lens paradigm in experimental myopia: oculomotor, optical and neurophysiological considerations
Authors: Flitcroft D. I.
Published: July 2002

The lens‐rearing paradigm has developed great importance in the field of experimental myopia. Although an apparently simple paradigm, the results of any experiment can be influenced by a variety of factors including habitual viewing distance, ocular refraction, oculomotor performance and the spatial sensitivity of the retinal elements involved in retinal image assessment. Computer modelling has been used to evaluate the expected impact when lenses are placed in front of a primate eye as a function of the above parameters. Spatial band‐pass responses of the mechanisms responsible for emmetropisation are predicted to lead to a limited range of retinal defocus over which compensation to lenses could occur. Even assuming an equal ability to detect hyperopic and myopic defocus, it is predicted that primate eyes should be able to compensate for a much larger range of minus lenses than plus lenses. This derives from the ability of the accommodation system to keep retinal defocus within this operating range over a wider range of minus lenses than plus lenses. The range over which compensation can occur will depend on the spatial tuning of the elements responsible for detecting retinal defocus and capabilities of the accommodation system. The observed asymmetry in responses of the primate eye to rearing with plus and minus lenses and observed differences between primates and chickens in lens rearing studies may therefore be partly attributable to optical and neurophysiological considerations.
The Reproductive Biology of Two Species of Diplusodon Pohl (Lythraceae) from Serra do Cipó, Southeastern Brazil
Authors: Jacobi C. M. , do Carmo R. M. , Oliveira R. S.
Published: July 2002

Abstract: The reproductive biology of two shrub species of Diplusodon (Lythraceae), D. hirsutus and D. orbicularis, was studied. Their distribution is restricted, mainly associated with “campos rupestres”, a montane biome in SE Brazil. Both species proved to be self‐compatible but because of herkogamy they are greatly dependent on visitors to effect pollination. The main pollinators are medium‐sized bees. In experiments where pollinators were excluded or discouraged the percentage of abscised fruits was total or extremely high; most of these fruits dropped within the first two weeks after pollination in D. orbicularis and three weeks in D. hirsutus. The percentage of surviving fruits from control or manual pollination was higher in D. orbicularis (ca. 70 %) than in D. hirsutus (ca. 50 %). Seed set was very variable, with slightly lower means in D. hirsutus. Within each species the mean seed set was similar in self‐, cross‐pollinated fruits, and controls. In both species, the similarity between control and manual pollination fruit survival curves suggests that factors other than pollen limitation are the cause of abortion in the study area. The high rate of abscission, which was also observed in unmanipulated fruits, was attributed to resource limitation. In D. hirsutus this was caused by drought during seed development. In D. orbicularis it was enhanced by nutrient drain by shoot‐galling flies. The lack of selective abortion of either selfed fruits or fruits with a small number of seeds may be a useful mechanism to overcome this detrimental effect which is felt later during fruit development.
Dynamic Organization of Microtubules and Microfilaments during Cell Cycle Progression in Higher Plant Cells
Authors: Kumagai F. , Hasezawa S.
Published: July 2002

Abstract: The cytoskeleton, which mainly consists of microtubules (MTs) and actin microfilaments (MFs), plays various significant roles that are indispensable for eukaryotic viability, including determination of cell shape, cell movement, nuclear division, and cytokinesis. In animal cells, MFs appear to be of more importance than MTs, except for spindle formation in nuclear division. In contrast, higher plants have a rigid cell wall around their cells, and have thus evolved elegant systems of MTs to control the direction of cellulose microfibrils (CMFs) deposited in the cell wall, and to divide centrifugally in a physically limited space. Dynamic changes in MTs during cell cycle progression in higher plant cells have been observed over several decades, including cortical MTs (CMTs) during interphase, preprophase bands (PPBs) from late G2 phase to prophase, spindles from prometaphase to anaphase, and phragmoplasts at telophase. The MFs also show some changes not as obvious as MT dynamics. However, questions regarding the process of formation of these arrays, and the precise mechanisms by which they fulfill their roles, remain unsolved. In this article, we present an outline of the changes in the cytoskeleton based on our studies with highly‐synchronized tobacco BY‐2 cells. Some candidate molecules that could play roles in cytoskeletal dynamics are discussed. We also hope to draw attention to recent attempts at visualization of cytoskeletons with molecular techniques, and to some examples of genetic approaches in this field.
Writing nanometer-scale symbols in gold using the scanning tunneling microscope
Authors: Li Y. Z. , Vazquez L. , Piner R. , Andres R. P. , Reifenberger R.
Published: July 2002

The conditions required to electroetch nanometer-sized craters in flat gold substrates with a scanning tunneling microscope operating in air are identified. Reproducible nanometer-scale modifications of the substrate are possible. Letters and complex symbols with linewidths as small as 2 nm have been written. Experiments show that a good tunneling tip is not destroyed by the writing process.
Very high speed GaInAs metal-semiconductor-metal photodiode incorporating an AlInAs/GaInAs graded superlattice
Authors: Wada O. , Nobuhara H. , Hamaguchi H. , Mikawa T. , Tackeuchi A. , Fujii T.
Published: July 2002

A lateral structure metal-semiconductor-metal photodiode has been fabricated on GaInAs, in which an AlInAs/GaInAs graded superlattice has been incorporated. This photodiode has exhibited a dark current lower than 100 nA, an internal quantum efficiency of greater than 80% at a wavelength of 1.3 μm, and a capacitance of 40 fF, all at the bias voltage of 10 V. The response speed of this photodiode has been characterized by electro-optic sampling to exhibit a full width at half maximum of 14.7 ps.
Electron transport in (Cs)Na2KSb photocathodes
Authors: Yang B.
Published: July 2002

Photoelectron transport in (Cs)Na2KSb real space is investigated using the Monte Carlo simulation technique for various photon energies. The quantum yield and modulation transfer function (MTF) of the (Cs)Na2KSb photocathodes, and the energy distribution and transit time spread (TTS) of the emitted electrons have been obtained. In particular, the quantum yield of the photocathode has a good agreement with the measured results. Computed results indicate that the energy distribution spread and TTS of the emitted electrons from a typical (Cs)Na2KSb photocathode are 0.3–0.6 eV and 300–180 fsec, respectively.
Photoluminescence study of nitrogen-oxygen donors in silicon
Authors: Steele A. G. , Lenchyshyn L. C. , Thewalt M. L. W.
Published: July 2002

The series of nitrogen-oxygen donors recently observed in infrared absorption has been studied for the first time using photoluminescence spectroscopy. These complexes are found to bind excitons and multiexcitonic complexes, with an exciton localization energy of 3.9 meV. Conclusive identification of the exciton binding centers with the nitrogen-oxygen donors is provided by the observation of bound-exciton two-electron transitions which leave the donors in their 2s or 2p± excited states.
Near-band-gap photoluminescence of Si1−xGex alloys grown on Si(100) by molecular beam epitaxy
Authors: Terashima Koichi , Tajima Michio , Tatsumi Toru
Published: July 2002

Photoluminescence spectra in the near-band-gap region of Si1−xGex alloys (x=0.04 and 0.15) grown on Si(100) substrates by molecular beam epitaxy have been measured at 4.2 and 12 K. Radiative recombinations of free and bound excitons in thin layers of Si1−xGex alloys have been clearly observed for the first time. No-phonon transitions and transverse-optical (TO) phonon-assisted transitions have been identified.The luminescence lines become broader with an increase in excitation intensity; the broadening is interpreted to be due to the generation of the bound multiexciton complexes (BMECs). The position of the band-edge luminescence lines is determined by the strain in the epitaxial layer as well as the alloy composition. The defect-related L band appears in the case of x=0.15.
Reduction of mirror temperature in GaAs/AlGaAs quantum well laser diodes with segmented contacts
Authors: Herrmann F. U. , Beeck S. , Abstreiter G. , Hanke C. , Hoyler C. , Korte L.
Published: July 2002

A serious problem of high-power GaAs/AlGaAs laser diodes is the strong mirror heating, which is caused by the carrier density and the correlated nonradiative surface recombination at the cleaved mirrors. Therefore the top electrode was segmented in three parts and the influence of a separate controllable potential in the mirror region on the temperature has been studied. The local temperature was measured using spatially resolved Raman scattering. A substantial reduction of the mirror temperature is possible by applying a suitable potential to the mirror contacts. This may lead to an improvement in reliability in the case where the high mirror temperature is responsible for the degradation of the laser.
Phase-selective route to high T
c superconducting Tl2Ba2Can−1CunO2n+4 films: Combined metalorganic chemical vapor deposition using an improved barium precursor and stoichiometry-controlled thallium vapor diffusion
Authors: Malandrino Graziella , Richeson Darrin S. , Marks Tobin J. , DeGroot Donald C. , Schindler Jon L. , Kannewurf Carl R.
Published: July 2002

Films of the Tl2Ba2Can−1CunO2n+4 high Tc superconductors (n=2 or 3) can be prepared with a high degree of phase selectivity using a combination of metalorganic chemical vapor deposition (MOCVD) and vapor diffusion. Ba-Ca-Cu-O films are first prepared by MOCVD using the volatile metalorganic precursors Ba(hexafluoroacetyl- acetonate)2(tetraglyme), Ca(dipivaloylmethanate)2, and Cu(acetylacetonate)2. The ‘‘second-generation’’ barium precursor exhibits significantly improved thermal stability and volatility over previously used compounds. Thallium is then incorporated into these films by vapor diffusion using a Tl-Ba-Ca-Cu oxide mixture of controlled composition as the source of volatile thallium oxides. Phase control is achieved by a combination of improved stoichiometry of the deposited film, a result of the new Ba source, and annealing with the appropriate oxide mixture. The resultant films consist predominantly of the Tl2Ba2Ca2Cu3Ox or Tl2Ba2CaCu2Ox phase, each having preferential orientation of the crystallite Cu-O planes parallel to the substrate surface. The films exhibit superconducting onset temperatures of ∼115 K with zero resistance by 93 and 85 K, respectively. Critical current densities for a number of films, estimated by the Bean method, are in the range 104–105 A/cm2 at 5 K.      
Homogeneous hydrogen-terminated Si(111) surface formed using aqueous HF solution and water
Authors: Watanabe Satoru , Nakayama Noriaki , Ito Takashi
Published: July 2002

Aqueous HF etching of silicon surface removes surface oxide, leaving a silicon surface terminated by atomic hydrogen. We studied the effect of the immersion in water, following HF etching, on the surface hydride structure and flatness, by measuring Si-H stretching vibration using infrared absorption spectroscopy. Immersion at 20 °C flattens the Si(111) surface, which is atomically rough just after etching, to some extent. Boiling water (100 °C) produces an atomically flat surface homogeneously covered with silicon monohydride (—SiH) normal to the surface and free of oxidation. The surface has a low defect density of less than 0.5%.
Surface topography changes during the growth of GaAs by molecular beam epitaxy
Authors: Smith G. W. , Pidduck A. J. , Whitehouse C. R. , Glasper J. L. , Keir A. M. , Pickering C.
Published: July 2002

Changes in surface roughness taking place during (001) GaAs molecular beam epitaxy growth have been studied in situ using laser light scattering and ex situ using atomic force microscopy (AFM). Substantial increases in light scattering are found to occur firstly during oxide thermal desorption, associated with surface pit formation, and secondly during continued layer growth, due to the buildup of atomic step arrays. Monolayer height GaAs steps are readily resolved using AFM in air.
Layer-by-layer deposition of La1.85Sr0.15CuOx films by pulsed laser ablation
Authors: Chern M. Y. , Gupta A. , Hussey B. W.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) has been used to monitor the growth of La1.85Sr0.15CuOx (LSCO) thin films on (100) SrTiO3 substrates by pulsed laser deposition. The films are grown using a combination of pulsed molecular oxygen and a continuous source of atomic oxygen, with the average background pressure maintained as low as 1 mTorr. The RHEED pattern is sharp and streaky, and the intensity of the specular beam oscillates during the deposition, indicating a two-dimensional layer-by-layer epitaxial growth. The film thickness measured by x-ray small-angle interference is consistent with the thickness determined by the RHEED oscillation period with a growth unit of half a unit cell. Thin films of YBa2Cu3O7−δ (YBCO) with good RHEED oscillations have also been grown under similar oxygenation conditions. The low-pressure-grown LSCO and YBCO films are superconducting, with zero-resistance temperatures of 15 and 80 K, respectively.
2×2 GaAs asymmetric Mach–Zehnder interferometer switch
Authors: Feng Hao , Li Xihua , Yang Zhuoya , Wang Minghua
Published: July 2002

We report on the first 2×2 GaAs asymmetric Mach–Zehnder interferential switch. The mechanism of this device is analyzed. Instead of using Y branches, two asymmetric X junctions have been utilized in the switch. The device is characterized by a switching voltage of 12.5 V and a crosstalk ratio of less than −22 dB at an operating wavelength of 1.15 μm. It is shown that this device could be applied to switching arrays.
Monitoring ion etching of GaAs/AlGaAs heterostructures by real time spectroscopic ellipsometry: Determination of layer thicknesses, compositions, and surface temperature
Authors: Heyd A. R. , Collins R. W. , Vedam K. , Bose S. S. , Miller D. L.
Published: July 2002

A real time spectroscopic ellipsometry (RTSE) study of the GaAs/AlGaAs/GaAs heterostructure, performed during Ar ion beam etching, provides the instantaneous GaAs and AlGaAs layer thicknesses and etch rates, the Al composition for the alloy layer, the evolution of the thickness and composition of the surface damage layer, and the near-surface temperature. In addition, combined effects of ion-induced atomic intermixing and etching inhomogeneity can be assessed as the interfaces are crossed. These results demonstrate the extensive capabilities of the RTSE technique for monitoring semiconductor structures during processing.
Thermosetting nonlinear optical polymer: Polyurethane with disperse red 19 side groups
Authors: Shi Yongqiang , Steier William H. , Chen Mai , Yu Luping , Dalton Larry R.
Published: July 2002

A new thermosetting second-order nonlinear optical polyurethane polymer with disperse red 19 pendant groups was studied by second-harmonic generation and electro-optic modulation methods. The polymer can be crosslinked by thermal curing during electric field poling. The electro-optic coefficient r13 measured with a Mach–Zehnder interferometer ranged from 5 pm/V at 800 nm to 13 pm/V at 633 nm. The polymer crosslinked during poling showed no significant decay of the nonlinearity at room temperature for thousands of hours. A long-term thermal stability of the dipole alignment at an elevated temperature, 90 °C, was demonstrated. The thermosetting polymer showed improved alignment stability under ultraviolet exposure.
Temperature dependence of optical constants for amorphous silicon
Authors: Do Nhan , Klees Leander , Leung P. T. , Tong Frank , Leung Wing P. , Tam Andrew C.
Published: July 2002

The temperature dependence of the optical constants for amorphous silicon (a-Si) is studied for two different sample thicknesses at two infrared wavelengths. It is observed that the extinction coefficient of a-Si can increase significantly with temperature in the strong absorption regime. In addition, using the Mott–Davis formula, results are obtained for the variation of the optical gap energy for a-Si with temperature, with similar feature observed for both amorphous and crystal silicon.
High-power multiple-quantum-well GaInAsSb/AlGaAsSb diode lasers emitting at 2.1 μm with low threshold current density
Authors: Choi H. K. , Eglash S. J.
Published: July 2002

The first GaInAsSb/AlGaAsSb diode lasers with a quantum-well active region have been demonstrated. These devices, which were grown by molecular beam epitaxy, emit at ∼2.1 μm. For room-temperature pulsed operation of lasers 100 μm wide, threshold current density as low as 260 A/cm2 and differential quantum efficiency up to 70% have been obtained for cavity lengths of 2000 and 300 μm, respectively. One device 100 μm wide and 1000 μm long has operated pulsed at heatsink temperatures up to 150 °C, with a characteristic temperature of 113 K between 20 and 40 °C. For another device of the same dimensions, cw output power up to 190 mW/facet has been achieved at a heatsink temperature of 20 °C. These characteristics represent a dramatic improvement in performance over double-heterostructure GaInAsSb/AlGaAsSb diode lasers.
Electrical properties of strontium titanate thin films by multi-ion-beam reactive sputtering technique
Authors: Peng C.-J. , Hu H. , Krupanidhi S. B.
Published: July 2002

Thin films of SrTiO3 were deposited by multi-ion-beam reactive sputtering technique using SrO and Ti-metal targets. The dielectric behavior of the films deposited on Pt-coated Si substrates showed thickness dependence. A dielectric constant of 219 at 100 KHz was found for 1.2 μm thick films. A charge storage density of about 15 fC/μm2 and leakage current density of 106 μA/cm2 at an electric field of 0.17 MV/cm were obtained for 0.3 μm films. The probable interface layers present between film and silicon substrate, in the case of metal-insulator-silicon configuration, appear to influence the electrical behavior. Preliminary analysis showed that the conduction of the films in high field region was bulk-limited Poole–Frenkel emission mechanisms.
Forward-bias characteristics of Si bipolar junctions grown by molecular beam epitaxy at low temperatures
Authors: Jorke H. , Kibbel H. , Strohm K. , Kasper E.
Published: July 2002

Forward-bias current-voltage characteristics of molecular beam epitaxy grown Si p+-i-n+ junctions have been determined at room temperature. At small widths of the i zone (Li=5 and 10 nm) band-to-band tunneling with a maximum peak-to-valley ratio of two is observed. Up to Li=30 nm (trap assisted) forward-bias tunneling is apparent with saturation tunneling current densities somewhat lower than in p-n junctions at comparable widths of the space-charge region WSCR(0). For Li≳30 nm and Tg=500 °C growth temperature surface recombination dominates the low bias range. At Li=35 nm and Tg=325 °C, both surface and bulk recombination is observed. We found evidence that Si molecular beam epitaxy layers grown at this low temperature get an increasing density of crystalline defects with growing thickness.
Effect of superlattice layer elastic moduli on hardness
Authors: Shinn M. , Barnett S. A.
Published: July 2002

Epitaxial V0.6Nb0.4N/NbN superlattices have been deposited on MgO(100) by reactive magnetron sputtering. The V0.6Nb0.4N alloy was chosen for one layer, rather than VN, in order to reduce the lattice mismatch from 5.7% to 3.5%, allowing growth of superlattices with relatively planar layers. The elastic moduli of NbN and V0.6Nb0.4N are nearly equal, in contrast to previously studied high-hardness nitride superlattices where the layer moduli differed substantially. Vickers microhardness measurements showed no measurable deviation (<4 GPa) from alloy values. The results verify that the difference in layer elastic moduli is the primary parameter determining nitride superlattice hardening.
GaAs/AlAs trench-buried quantum wires with nearly rectangular cross sections grown by metalorganic chemical vapor deposition on V-grooved substrates
Authors: Sogawa T. , Ando S. , Kanbe H.
Published: July 2002

GaAs trench-buried quantum wires were fabricated by using U-grooved AlAs trenches grown on V-grooved (001) substrates. These trench structures with vertical (110) sidewalls were formed by the faceting of (110) planes, and lateral growth of these planes reduced the trench width to less than 20 nm. A cross-sectional scanning electron microscope image of these trench-buried structures showed GaAs wires about 20 nm wide and 20 nm thick. The growth of these wires is enhanced by the capture of Ga species into the trenches. Blueshifts and strong anisotropy of photoluminescence confirm two-dimensional quantum confinement.
One-dimensional quantum electron states in n-AlxGa1−xAs/u-GaAs modulation-doped corrugated heterojunctions
Authors: Vacek Karel , Sawada Akemi , Usagawa Toshiyuki
Published: July 2002

New quasi-one-dimensional quantum electron states are proposed in modulation-doped n-AlxGa1−xAs/u-GaAs heterostructures with periodic interface bending. Unlike usual flat heterointerfaces, the investigated system is sawtooth corrugated by bendings with a period of about 850 Å and a bending angle 90°. Quantum-mechanical simulations yield energy spectrum with a single level below the Fermi energy. The probability distribution of electrons shows formation of a densely packed quasi-one-dimensional electron gas in the convex corner of the GaAs region. The ground level is as high as 100 meV above the deepest bottom of the conduction band potential and the excited state is separated by a significant energy gap of 16 meV.
Infrared wavelength and temperature dependence of optically induced terahertz radiation from InSb
Authors: Howells S. C. , Herrera S. D. , Schlie L. A.
Published: July 2002

Results of the temperature and infrared wavelength (0.8, 1.4, and 1.9 μm) dependence of terahertz radiation generated from both undoped and Te-doped InSb irradiated with ≊125 fs laser pulses are reported. Undoped InSb shows a substantial change in the spectral content of the terahertz radiation between 80 and 260 K, while the spectrum of Te-doped InSb remains nearly unchanged, an effect attributed to its mobility being dominated by impurity scattering. Also, the terahertz radiation from undoped InSb at 80 K is dependent on the irradiating wavelength, with both a higher frequency spectrum and much larger amplitude generated at longer wavelengths. No such effect is observed at 260 K.
Ultrafast 1.55-μm photoresponses in low-temperature-grown InGaAs/InAlAs quantum wells
Authors: Takahashi R. , Kawamura Y. , Kagawa T. , Iwamura H.
Published: July 2002

Doping with Be was found to be very effective for shortening of carrier lifetime in InGaAs/InAlAs multiple quantum wells (MQWs) grown at low temperature by molecular beam epitaxy. The MQW materials have carrier lifetimes controllable from a few tens of picoseconds to 1 ps in the 1.55-μm wavelength region, coupled with a large optical nonlinearity due to an excitonic feature, implying applicability to ultrafast optical devices in the fiber-optic communication. The carrier lifetime was measured by a time-resolved pump-probe method using an optical source based on a 1.535-μm semiconductor laser. We also investigated the resistivity, carrier density, and Hall mobility in the MQWs.
Femtosecond laser excitation of the semiconductor-metal phase transition in VO2
Authors: Becker Michael F. , Buckman A. Bruce , Walser Rodger M. , Lépine Thierry , Georges Patrick , Brun Alain
Published: July 2002

We have measured the subpicosecond optical response of a solid-state, semiconductor-to-metal phase transition excited by femtosecond laser pulses. We have determined the dynamic response of the complex refractive index of VO2 thin films by making pump-probe optical transmission and reflection measurements at 780 nm. The phase transition was found to be largely prompt with the optical properties of the high-temperature metallic state being attained within 5 ps. The ultrafast change in complex refractive index enables ultrafast optical switching devices in VO2.
Temperature behavior of a bulk InGaAsP/InP ridge waveguide structure for polarization insensitive optical amplifier operation
Authors: Pajarola S. , Eckner J. , Besse P.-A. , Guekos G. , Syvridis D.
Published: July 2002

The polarization dependent temperature behavior of an InGaAsP/InP ridge waveguide structure designed for polarization insensitive semiconductor optical amplifier operation is investigated by means of measuring the threshold current of the diode in an external cavity configuration. In the wavelength range 1.48 μm<λ<1.57 μm and the temperature range 288 K<T<323 K, TE and TM show practically the same wavelength dependent threshold variation with temperature.
Role of mobile hydrogen in the amorphous silicon recrystallization
Authors: Godet C. , Layadi N. , Roca i Cabarrocas P.
Published: July 2002

The plasma deposition of nanocrystalline silicon thin films is usually performed under a high flux of atomic hydrogen and hydrogenated chemical species. The growth mechanisms are investigated using the layer-by-layer deposition of dense nanocrystalline silicon, obtained at 250 °C by alternating SiH4 and H2 plasmas. In the steady state, a minimum exposure time to the hydrogen plasma is necessary to recrystallize the amorphous top layer (10–85 Å). It is shown that this critical time is determined by the diffusion time of some mobile H through the top a-Si:H layer. The recrystallization is discussed in relation to the diffusion of hydrogen leading to the nanovoid and broken bond formation processes.