bluuebunny/crossref_metadata_2025 huggingface dataset:
Cysteine-less glycosylphosphatidylinositol-specific phospholipase C is inhibited competitively by a thiol reagent: evidence for glyco-mimicry by p-chloromercuriphenylsulphonate
Authors: STANTON Julie D. , RASHID Mohammad B. , MENSA-WILMOT Kojo
Published: July 2002

Glycosylphosphatidylinositol (GPI)-specific phospholipases are highly valuable for studying the structure and function of GPIs. GPI-specific phospholipase C (GPI-PLC) from Trypanosoma brucei and phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus are the most widely studied of this class of phospholipases C. Inhibition of protein activity by thiol reagents is indicative of the participation of cysteine residues in biochemical events. The thiol reagent p-chloromercuriphenylsulphonate (pCMPS) inhibits T. brucei GPI-PLC, which has eight cysteine residues. Surprisingly, we found that the activity of B. cereus PI-PLC is also blocked by pCMPS, although the protein does not contain cysteine residues. Inhibition of B. cereus PI-PLC was reversed when pCMPS was size-separated from a preformed pCMPS·PI-PLC complex. In contrast, no activity was recovered when T. brucei GPI-PLC was subjected to a similar protocol. Equimolar β-mercaptoethanol (β-ME) reversed the inhibition of PI-PLC activity in a pCMPS·PI-PLC complex. For T. brucei GPI-PLC, however, ultrafiltration of the pCMPS·GI-PLC complex and addition of a large excess of β-ME was necessary for partial recovery of enzyme activity. Thus T. brucei GPI-PLC is susceptible to inactivation by covalent modification with pCMPS, whereas PI-PLC is not. Kinetic analysis indicated that pCMPS was a competitive inhibitor of PI-PLC when a GPI was a substrate. Curiously, with phosphatidylinositol as substrate, inhibition was no longer competitive. These data suggest that pCMPS is a glyco-mimetic that occupies the glycan binding site of PI-PLC, from where, depending on the substrate, it inhibits catalysis allosterically or competitively.
Identification and characterization of the gene encoding the human phosphopantetheine adenylyltransferase and dephospho-CoA kinase bifunctional enzyme (CoA synthase)
Authors: AGHAJANIAN Suren , WORRALL D.Margaret
Published: July 2002

The final two enzymes in the CoA biosynthetic pathway, phosphopantetheine adenylyltransferase (PPAT; EC 2.7.7.3) and dephospho-CoA kinase (DPCK; EC 2.7.1.24), are separate proteins in prokaryotes, but exist as a bifunctional enzyme in pig liver. In the present study we have obtained sequence information from purified pig-liver enzyme, and identified the corresponding cDNA in a number of species. The human gene localizes to chromosome 17q12-21 and contains regions with sequence similarity to the monofunctional Escherichia coli DPCK and PPAT. The recombinant 564-amino-acid human protein confirmed the associated transferase and kinase activities, and gave similar kinetic properties to the wild-type pig enzyme.
Insulin‐like growth factor I administration induces fluid and sodium retention in healthy adults: possible involvement of renin and atrial natriuretic factor
Authors: Møller Jens , Jørgensen Jens O. L. , Marqversen Jørgen , Frandsen Erik , Christiansen Jens S.
Published: July 2002

OBJECTIVE

Growth hormone induces fluid and sodium retention. The underlying mechanism is, however, incompletely understood. A possible mediator could be IGF‐I. To investigate the impact of IGF‐I administration on body fluid distribution and sodium homeostasis in healthy subjects, we examined normal subjects during six days IGF‐I treatment and during a six‐day control period.

DESIGN AND MEASUREMENTS

Eight normal male subjects aged 23–30 years were randomised to receive IGF‐I 50 μg/kg subcutaneously thrice daily during a six day study period, and to a six day control period. After each study period, extracellular volume and plasma volume (ECV, PV) were determined using 82Br and 125I‐albumin. Blood samples, urinary sodium excretion, and bioimpedance were measured every second day of each study period.

RESULTS

Serum IGF‐I (μg/l) increased during active treatment (control, 293 ± 9; IGF‐I, 628 ± 42; P < 0.01). ECV (l) was expanded by IGF‐I (control, 18.42 ± 0.28; IGF‐I, 19.72 ± 0.50; P < 0.05) whereas PV (l) remained unaffected (control, 3.76 ± 0.11; IGF‐I, 3.80 ± 0.16; n.s.). Likewise, bioimpedance and body weight were unchanged by IGF‐I. Plasma renin (mU/l) increased but not significantly during IGF‐I (control, 28.7 ± 2.7; IGF‐I, 39.9 ± 4.3; P = 0.08), and plasma aldosterone was unaffected by IGF‐I. N‐Terminal proANF (pmol/l) was suppressed during IGF‐I administration (control, 422 ± 32; IGF‐I, 330 ± 20; P < 0.05). Diurnal sodium excretion (mmol) was reduced during IGF‐I administration (control, 151 ± 8; IGF‐I, 124 ± 7; P < 0.05).

CONCLUSION

IGF‐I treatment causes fluid and sodium retention. This may be mediated by increased renin release and suppression of atrial natriuretic factor. The present data suggest that the fluid and sodium retaining effect of GH is at least partly mediated through IGF‐I.
GANGRENOUS CHOLECYSTITIS IN THE LAPAROSCOPIC ERA
Authors: Hunt D. R. H. , Chu F. C. K.
Published: July 2002

Background: We reviewed our data of laparoscopic cholecystectomies between 1990 and 1997 with reference to gangrenous cholecystitis.

Methods: In a consecutive series of 1304 patients having laparoscopic cholecystectomies, prospective data collection has permitted analysis of the relationship between gangrenous cholecystitis (GC), acute (non‐gangrenous) cholecystitis (AC) and non‐acute cholecystectomies (NAC).

Results: Twenty‐five patients had gangrenous cholecystitis and 238 had acute cholecystitis. We found that patients with GC were significantly older (65.4 years vs 56.1 years (AC) and 52.7 years (NAC), P < 0.05) and had a higher M : F ratio (1.5:1 vs 1:2.6 (AC) and 1:2.8 (NAC), P < 0.05). Cardiac disease was found to be a significant factor but not diabetes. Preoperative ultrasonography correctly identified only 17 patients with acute inflammatory changes. Seven patients had an absent sonographic Murphy’s sign. The gall bladder wall was generally thicker (4.11 mm vs 3.8 mm (AC) and 2.7 mm (NAC), P < 0.05) but there was marked overlap between the three groups. The common bile duct (CBD) was more dilated (6.1 mm vs 4.8 mm (AC) and 4.6 mm (NAC), P < 0.006) and there was increased incidence of CBD stones in the GC group. Our conversion rate was 8.7% with minimal complications and no operative mortality.

Conclusion: Patients with GC were generally older, more likely to be male and had increased incidence of cardiovascular disease. Preoperative ultrasound cannot accurately identify those patients with gangrenous cholecystitis, but with conversion rates of 8.7% and no operative mortality, they can generally be managed safely with laparoscopic surgical techniques.
A preliminary investigation into the aetiology of Meares–Irlen syndrome
Authors: Evans B. J. W. , Wilkins A. J. , Brown J. , Busby A. , Wingfield A. , Jeanes R. , Bald J.
Published: July 2002

A recent double‐masked placebo‐controlled trial has confirmed that some children experience a reduction in symptoms of eyestrain and headache when they read through individually prescribed coloured filters and has shown that this benefit cannot be solely attributed to a placebo effect. People who are helped by coloured filters in this way have been described as having ‘Meares–Irlen syndrome’. We investigated the mechanism of this benefit by studying the optometric and visual perceptual characteristics of the children in the double‐masked study. This population had normal refractive errors and heterophorias (none of the subjects had strabismus). They demonstrated slightly, but significantly, reduced amplitudes of accommodation and vergence and poor stereo‐acuity. However, these factors seemed to be correlates of Meares‐Irlen syndrome rather than the underlying cause. Pattern glare, a sensitivity to striped patterns (e.g. lines of text), was prevalent in our sample and was significantly associated with the subjects' symptoms. The spatial contrast sensitivity function was normal.
Diversity of Rock‐Inhabiting Cyanobacterial Lichens: Studies on Granite Inselbergs along the Orinoco and in Guyana
Authors: Schultz M. , Porembski S. , Büdel B.
Published: July 2002

Abstract: A survey was undertaken of specialized saxicolous cyanobacterial lichens growing on exposed granitic rock surfaces of inselbergs in Venezuela and additionally in Guyana and French Guyana. The study focused on taxonomy and species composition. Twenty‐three cyanobacterial lichens were found, four of which are new to science: Peltula auriculata, Phylliscum vermiformis, Psorotichia polyspora and Pterygiopsis guyanensis. Besides the new species, almost all taxa are absent from the available lists of lichen‐forming fungi from the Guyana region. Although free‐living cyanobacteria are the dominant group in biological crusts covering the inselbergs, the number of cyanobacterial lichens is relatively high and their distribution is homogenous over long distances and macro‐climatic gradients. As inselberg rock faces undergo relatively fast weathering under moist tropical climates, numerous micro‐habitats are formed. Among these, xeric micro‐habitats are favourable for colonization by cyanobacterial lichens, mainly of the Lichinaceae and Peltulaceae. It is concluded that granite inselbergs in the savannas of the Orinoco lowland and the Guyana region are local centres of diversity for saxicolous cyanobacterial lichens.
Strain and misorientation in GaAs grown on Si(001) by organometallic epitaxy
Authors: Ghandhi S. K. , Ayers J. E.
Published: July 2002

Epitaxial GaAs grown on Si(001) by organometallic vapor phase epitaxy has been characterized for strain and misorientation, using double-crystal x-ray diffraction. (004) and (115) rocking curves indicate that the GaAs is under biaxial tensile stress, σ∥≊2.1×109 dyn/cm2, and is tetragonally distorted with unit cell dimensions 5.6424±0.0012 Å and 5.6656±0.0012 Å. The dislocation density in the GaAs, determined from the rocking curve width, is less than 4.2×107 cm−2. The GaAs(001) axis is tilted with respect to the Si(001), toward the substrate normal. This misorientation is a strain relief mechanism and is caused by shear strain energy in the nucleating GaAs.
Resonant tunneling in AlSb/InAs/AlSb double-barrier heterostructures
Authors: Luo L. F. , Beresford R. , Wang W. I.
Published: July 2002

We report the first observations of resonant tunneling in the AlSb/InAs material system, with a maximum peak-to-valley current ratio of 1.8:1 at room temperature and 9:1 at 77 K. The large AlSb/InAs barrier height of 1.8 eV for electrons and high-mobility InAs will be advantageous in device applications. In particular, the small electron effective mass in InAs makes it possible to demonstrate quantum effects in a 24 nm well, the longest coherence distance reported for double-barrier tunneling structures. We estimate that an AlSb/InAs resonant tunneling transistor can significantly outperform similar devices based on AlGaAs/GaAs.
Superhard material comparable in hardness to diamond
Authors: Badzian Andrzej R.
Published: July 2002

Superhard boron suboxides, with hardness close to that of diamond, were synthesized from boron/boron oxide mixtures. Such hardness is expected when a material’s molar volume approaches the value characteristic for diamond. These materials consist of boron-rich phases belonging to the boron-oxygen system. The phase which contains 4 at. % oxygen and a crystal structure related to β-rhombohedral boron can scratch diamond faces. During scratching of diamond the suboxide is worn also, and the wear debris is amorphized. Wear on the {100} diamond faces results from a cleavage mechanism which leaves a rough surface covered with cleaved {111} microfaces. The {100} faces are more easily abraded than the {111} diamond faces. Wear on {111} faces consumes much more energy and leaves grooves of plastically deformed diamond.
Chromium-doped forsterite laser pumped with 1.06 μm radiation
Authors: Verdun Horacio R. , Thomas Leonard M. , Andrauskas Donna M. , McCollum Tom , Pinto Albert
Published: July 2002

Laser action in chromium-doped forsterite (Cr:Mg2SiO4), pumped with a 1.064 μm Nd:YAG laser, was demonstrated for the first time. The free-running laser emission is centered at 1.221 μm and has a spectral width of 28 nm. An efficiency of 8% has been achieved with an uncoated crystal produced by the laser-heated-pedestal growth method. The available evidence suggests that the laser action is due to Cr4+ in tetrahedral sites.
Cathodoluminescence measurement of an orientation dependent aluminum concentration in AlxGa1−xAs epilayers grown by molecular beam epitaxy on a nonplanar substrate
Authors: Hoenk Michael E. , Chen Howard Z. , Yariv Amnon , Morkoç Hadis , Vahala Kerry J.
Published: July 2002

Cathodoluminescence scanning electron microscopy is used to study AlxGa1−x As epilayers grown on a nonplanar substrate by molecular beam epitaxy. Grooves parallel to the [011̄] direction were etched in an undoped GaAs substrate. Growth on such grooves proceeds on particular facet planes. We find that the aluminum concentration in the epilayers is dependent on the facet orientation, changing by as much as 35% from the value in the unpatterned areas. The transition in the aluminum concentration at a boundary between two facets is observed to be very abrupt.
Molecular beam epitaxial growth and characterization of InSb on Si
Authors: Chyi J.-I. , Biswas D. , Iyer S. V. , Kumar N. S. , Morkoç H. , Bean R. , Zanio K. , Lee H.-Y. , Chen Haydn
Published: July 2002

Epitaxial layers of InSb have been grown on Si substrates by molecular beam epitaxy. Room-temperature electron mobilities are 48 000 and 39 000 cm2/V s for 3.2 μm-thick InSb with and without a thin GaAs buffer, respectively. The corresponding carrier concentrations are 2.2×1016 and 2.7×1016 cm−3. A sample with an InSb thickness of 8 μm exhibited room-temperature mobilities as high as 55 000 cm2/V s with carrier concentrations of about 2.0×1016 cm−3. A sharp band-edge transmission spectrum is observed at room temperature for the 8 μm layer.
Growth and characterization of (111) oriented GaInAs/GaAs strained-layer superlattices
Authors: Beery J. G. , Laurich B. K. , Maggiore C. J. , Smith D. L. , Elcess K. , Fonstad C. G. , Mailhiot C.
Published: July 2002

We describe the growth, ion beam, and photoluminescence characterization of Ga1−xInxAs/GaAs strained-layer superlattices grown along the [111] axis. The layer thicknesses and composition are determined by Rutherford backscattering. Normal incidence channeling gives a minimum channeling yield of 5.7%. Strain conditions are found by off-normal incidence channeling using the angular scan method. Comparison of the photoluminescence spectrum of the superlattice with theoretical calculations provides strong evidence for the existence of strain-generated electric fields in [111] growth axis strained-layer superlattices.
High quality quantum wells of InGaP/GaAs grown by molecular beam epitaxy
Authors: Hafich M. J. , Quigley J. H. , Owens R. E. , Robinson G. Y. , Li Du , Otsuka N.
Published: July 2002

High quality quantum wells of GaAs confined by barriers of InGaP have been grown by gas-source molecular beam epitaxy. High-resolution lattice images obtained with transmission electron microscopy of single quantum wells reveal high quality interfaces for both the normal InGaP/GaAs and the inverted GaAs/InGaP interface. Multiple-line low-temperature photoluminescence emission is observed for the thinnest GaAs quantum well. The range of well thicknesses examined was 0.6–5.2 nm, with the smallest well producing a quantum confinement energy shift of over 410 meV, corresponding to photoluminescence emission at 640 nm (1.94 eV) from GaAs.
Anomalous temperature dependence of the ordered Ga0.5In0.5P photoluminescence spectrum
Authors: Kondow Masahiko , Minagawa Shigekazu , Inoue Youji , Nishino Taneo , Hamakawa Yoshihiro
Published: July 2002

The temperature dependence of the photoluminescence spectrum of Ga0.5 In0.5 P/GaAs grown by organometallic vapor phase epitaxy is measured. Samples with a highly long-range ordered structure show anomalous behavior, where peak energy changes with temperature exhibiting Z-shape dependence. In the range between 100 and 30 K, peak energy decreases monotonously with decreasing temperature, and below 30 K, begins to increase, splitting into two peaks. The most probable cause of this behavior is crystal defects related to the long-range ordered structure.
Micropolarizer made of the anodized alumina film
Authors: Saito M. , Kirihara M. , Taniguchi T. , Miyagi M.
Published: July 2002

A novel micropolarizer has been fabricated from alumina and nickel by means of anodization and electroplating techniques. Making use of the anisotropic microstructure of the anodized alumina film, a lattice of nickel columns is easily constructed in the film, which works as a wire grid type polarizer. The fabricated polarizer has achieved an extinction ratio larger than 30 dB at the wavelength of 1.3 μm.
Extremely high damage threshold of a new nonlinear crystal L-arginine phosphate and its deuterium compound
Authors: Yokotani Atsushi , Sasaki Takatomo , Yoshida Kunio , Nakai Sadao
Published: July 2002

L-arginine phosphate monohydrate (LAP) and deuterated LAP (DLAP) are new organic nonlinear optical materials useful for higher harmonics of radiation from high-power lasers. We measured the bulk laser damage threshold of these crystals using light from a 1.05 μm laser with 1 and 25 ns pulse widths and 0.53 μm laser light with 0.6 and 20 ns pulse widths. In every case, these crystals show much higher thresholds than potassium dihydrogen phosphate (KDP and fused silica. These crystals are very interesting not only as a frequency converter but also as other optical components of high-power lasers, because of their extremely high damage threshold.
Zinc delta doping of GaAs by organometallic vapor phase epitaxy
Authors: Hobson W. S. , Pearton S. J. , Schubert E. F. , Cabaniss G.
Published: July 2002

Zinc delta-doped layers have been grown by atmospheric pressure organometallic vapor phase epitaxy. Secondary-ion mass spectroscopy and electrochemical capacitance-voltage profiling were used to measure the spatial distribution of the Zn for both as-grown and annealed samples. The narrowest atomic profiles had full width at half maxima of 70 Å for peak Zn concentrations of ≤3×1018 cm−3. The as-grown width of these profiles is attributed to a combination of dopant memory effect and growth-related diffusion during the actual formation of the delta-doped layer. An effective diffusion coefficient D of ≤7×10−17 cm2/s is estimated for a growth temperature of 625 °C. Rapid thermal annealing at 900 °C for 5 s of several samples grown under various conditions led to calculated values of D in the range 0.5–1.0×10−12 cm2/s.
Compositional modulation in AlxGa1−xAs epilayers grown by molecular beam epitaxy on the (111) facets of grooves in a nonplanar substrate
Authors: Hoenk Michael E. , Nieh C. W. , Chen Howard Z. , Vahala Kerry J.
Published: July 2002

We report the first observation of a lateral junction formed in an alloy due to an abrupt transition from segregated to random AlGaAs alloy compositions. Al0.25Ga0.75As epilayers were grown by molecular beam epitaxy on [011̄] oriented grooves in a nonplanar (100) GaAs substrate. A quasi-periodic modulation of the aluminum concentration occurs spontaneously in material grown on the (111) facets of the groove, with a period of 50–70 Å along the [111] direction. The compositional modulation is associated with a reduction of the band gap by 130 meV, with respect to the random alloy. While segregation of the AlGaAs alloy has been seen previously, this is the first observation of segregation of AlGaAs grown on a (111) surface. The compositional modulation terminates abruptly at the boundaries of the (111) facet, forming abrupt lateral junctions in the AlGaAs layers grown on a groove.
Growth characterization of YBa2Cu3O7−x thin films on (100) MgO
Authors: Li Q. , Meyer O. , Xi X. X. , Geerk J. , Linker G.
Published: July 2002

The growth features of YBa2Cu3O7−x thin films on (100) MgO substrates were studied by He ion channeling and x-ray diffraction measurements. A minimum yield value of 7% at 2 MeV He ion energy and a standard deviation of the crystallite misorientation of only 0.1° show that epitaxial growth (c-axis oriented) is achieved despite a large lattice mismatch of about 9% between the film and the substrate. Detailed studies of the energy dependence of the dechanneling yield at the film-substrate interface for films of different thickness reveal the presence of dislocations probably formed by strain relief in the initial state of growth. Stacking faults appear as the main defect structure in the bulk of the films.
Entropy measurements on slow Si/SiO2 interface states
Authors: Cobden D. H. , Uren M. J. , Kirton M. J.
Published: July 2002

Using telegraph noise measurements on small n- and p-channel metal-oxide-silicon field-effect transistors, we have measured the entropy change associated with the change of the charge state of individual slow Si/SiO2 surface states. In n-channel devices we find that the entropy change is positive on electron emission to the silicon conduction band, while in p-channel devices it is positive on hole emission to the valence band. The results suggest that the slow states in the upper and lower regions of the silicon band gap are of a different type.
Epitaxial growth of high-mobility GaAs using tertiarybutylarsine and triethylgallium
Authors: Haacke G. , Watkins S. P. , Burkhard H.
Published: July 2002

Epitaxial layers of nominally undoped GaAs have been grown by metalorganic chemical vapor deposition using liquid tertiarybutylarsine and triethylgallium. n-type layers were obtained having total residual shallow acceptor concentrations of ∼1013 cm−3 and Hall mobilities comparable to those obtained with arsine and triethylgallium in the same reactor. Liquid-nitrogen Hall mobilities up to 116 000 cm2 /V s were observed.
Formation of silicon tips with <1 nm radius
Authors: Marcus R. B. , Ravi T. S. , Gmitter T. , Chin K. , Liu D. , Orvis W. J. , Ciarlo D. R. , Hunt C. E. , Trujillo J.
Published: July 2002

Electron emitters in vacuum microelectronic devices need sharp tips in order to permit electron emission at moderate voltages. A method has been found for preparing uniform silicon tips with a radius of curvature less than 1 nm. These tips are formed by oxidation of 5-μm-high silicon cones through exploitation of a known oxidation inhibition of silicon at regions of high curvature.
Absence of negative ion effects during on-axis single target sputter depositions of Y-Ba-Cu-O thin films on Si (100)
Authors: Migliuolo M. , Belan R. M. , Brewer J. A.
Published: July 2002

Stoichiometric thin films of YBa2Cu3O7−δ have been deposited on (100) silicon substrates by on-axis single target magnetron sputtering. The effect of oxygen resputtering was minimized through the use of much stronger than usual magnetic assemblies in the source. A magnet assembly incorporating NdB and NdFeB magnets produced a magnetic field above the target twice as large as the one produced by a standard SmCo magnet assembly. This allows for the use of lower operating voltages and a better electron racetrack confinement, resulting in little or no oxygen resputtering. We have obtained stoichiometric or near-stoichiometric films on silicon both by dc and rf magnetron techniques at a variety of sputtering pressures and target to substrate distances.
Real-time observation of molecular beam epitaxy growth on mesa-etched GaAs substrates by scanning microprobe reflection high-energy electron diffraction
Authors: Hata M. , Isu T. , Watanabe A. , Katayama Y.
Published: July 2002

Microscopic distribution of growth rates on mesa-etched GaAs(001) wafers was measured in real time during molecular beam epitaxy growth by scanning microprobe reflection high-energy electron diffraction. It has been observed that the growth rate on the GaAs(001) surface near the edge of (111)A surfaces becomes larger. The exponential variation of the growth rate as a function of the distance from the edge reflects surface diffusion of Ga atoms. The diffusion length on the (001) surface is estimated to be about 1 μm at 560 °C. The relatively larger diffusion length suggests that the incorporation rate of migrating Ga atoms by steps is much smaller than unity.
Formation of arsenic precipitates in GaAs buffer layers grown by molecular beam epitaxy at low substrate temperatures
Authors: Melloch M. R. , Otsuka N. , Woodall J. M. , Warren A. C. , Freeouf J. L.
Published: July 2002

We have grown film structures by molecular beam epitaxy which include GaAs buffer layers grown at low substrate temperatures (250 °C). The film structures have been examined using transmission electron microscopy. The layers grown at normal temperatures (600 °C) were free of defects or clusters. In contrast, the layer which was grown at low substrate temperatures contained precipitates which have been identified as hexagonal arsenic. The density of the arsenic precipitates is found to be very sensitive to the substrate temperature during growth.
Resonant interband tunneling in InAs/GaSb/AlSb/InAs and GaSb/InAs/AlSb/GaSb heterostructures
Authors: Longenbach K. F. , Luo L. F. , Wang W. I.
Published: July 2002

Tunneling in InAs/GaSb/AlSb/InAs and GaSb/InAs/AlSb/GaSb structures has been observed for the first time and shown to offer large peak-to-valley ratios at higher peak current densities than previous double-barrier and single-barrier polytype interband tunneling results. Room-temperature peak-to-valley ratios as high as 20:1 were observed at peak current densities of 28 kA/cm2. The highest peak-to-valley ratio observed at 80 K was 80:1 at 1.2 kA/cm2. The large peak-to-valley ratios are attributed to resonant interband tunneling with a confined state and band-gap blocking of nonresonant currents. The operation of these devices is similar to asymmetric double-barrier structures.
Growth of GaAs quantum wire arrays by organometallic chemical vapor deposition on submicron gratings
Authors: Colas E. , Simhony S. , Kapon E. , Bhat R. , Hwang D. M. , Lin P. S. D.
Published: July 2002

We report on the growth of dense lateral arrays of GaAs quantum wire structures, obtained by organometallic chemical vapor deposition (OMCVD) on GaAs substrates where a submicron grating has been lithographically defined and etched prior to deposition. The experiments were performed simultaneously on (100) oriented substrates, where the wires are virtually ‘‘isolated’’ from each other, and on substrates that are vicinal with respect to the (100) orientation, where the wires are ‘‘smoothly connected’’ by quantum wells. Transmission electron microscopy investigations allowed the study of the morphology of the resulting structures, which was related to the microscopic step nature of the starting surfaces and revealed important basic aspects of growth dynamics.
Does the two-dimensional electron gas effect contribute to high-frequency and high-speed performance of field-effect transistors?
Authors: Feng M. , Lau C. L. , Eu V. , Ito C.
Published: July 2002

We present experimental evidence that current gain cutoff frequency (ft) values equal to or greater than those achieved with high electron mobility transistors (HEMTs) and pseudomorphic HEMTs can also be achieved by ion-implanted GaAs and InGaAs metal-semiconductor field-effect transistors. These measured ft results clearly suggest that the average electron velocity under the gate is determined primarily by the high-field electron velocity rather than the low-field electron mobility. Hence, we conclude that the transport properties of the two-dimensional electron gas in HEMTs and pseudomorphic HEMTs do not make a significant contribution to the high-frequency and high-speed performance of these devices.
Quantum beats of light hole and heavy hole excitons in quantum wells
Authors: Leo Karl , Damen Theo C. , Shah Jagdeep , Göbel Ernst O. , Köhler Klaus
Published: July 2002

We report the first observation of quantum beats due to the interference of the polarization decay of heavy hole and light hole excitons in semiconductor quantum wells.
Effect of photodeposited iron oxide and tin oxide on the consolidation of porous Vycor glass
Authors: Mendoza E. A. , Sunil D. , Wolkow E. , Gafney H. D. , Rafailovich M. H. , Sokolov J. , Long G. G. , Jemian P. R. , Schwartz S. A. , Wilkens B. J.
Published: July 2002

Iron oxide and tin oxide have been photodeposited in porous Vycor glass and examined before and after consolidation of the glass. Scanning electron microscopy reveals that the iron oxide particles are larger than the tin oxide particles. However, small-angle x-ray scattering and Rutherford backscattering show that the glass consolidates about the iron oxide but not about the tin oxide. Photodeposition of tin oxide, which appears to chemically modify the glass surface and to prevent its consolidation, offers a means of producing highly resolved regions of porosity in the otherwise consolidated glass.
Improvement of rare-gas solid moderators by using conical geometry
Authors: Khatri R. , Charlton M. , Sferlazzo P. , Lynn K. G. , Mills A. P. , Roellig L. O.
Published: July 2002

A slow positron beam with narrow energy spread can be obtained by moderating the β+ decay of a radioactive source. We report here the improvement in the efficiency of the rare-gas solid positron moderators by using a different geometry. The efficiency for slow positrons, ε, was measured for the cylindrical and the cone configurations of the moderator with the latter yielding ε of (4.6±0.2)×10−3 for solid neon deposited on an encapsulated radioactive 22Na source deposited on a 5 μm Cu98/Be2 window. No corrections were applied for the attenuation caused by the window. The ε for the conical configuration is (2.7 ± 0.2) times higher than that for the cylindrical configuration. Other rare-gas solids (e.g., Ar, Xe, Kr) yielded lower ε as compared to that for the solid neon in agreement with the earlier measured efficiencies of Mills and Gullikson [Appl. Phys. Lett. 49, 1121 (1986)].
p-type ZnSe by nitrogen atom beam doping during molecular beam epitaxial growth
Authors: Park R. M. , Troffer M. B. , Rouleau C. M. , DePuydt J. M. , Haase M. A.
Published: July 2002

A novel approach to producing p-type ZnSe epitaxial layers is reported which involves nitrogen atom beam doping during molecular beam epitaxial growth. Net acceptor concentrations as large as 3.4×1017 cm−3 have been measured in nitrogen atom beam doped ZnSe/GaAs heteroepitaxial layers which represents the highest acceptor concentration reported to date for ZnSe:N epitaxial material grown by molecular beam epitaxy. In addition, light-emitting diodes based on ZnSe:N/ZnSe:Cl, p-n homojunctions have been found to exhibit dominant electroluminescence in the blue region of the visible spectrum at room temperature.
ZnSe light-emitting diodes
Authors: Ren J. , Bowers K. A. , Sneed B. , Dreifus D. L. , Cook J. W. , Schetzina J. F. , Kolbas R. M.
Published: July 2002

We report the successful fabrication of ZnSe p-n junction light-emitting diodes in which Li and Cl are used as p-type and n-type dopants, respectively.
Demonstration of resonant transmission in InAs/GaSb/InAs interband tunneling devices
Authors: Yu E. T. , Collins D. A. , Ting D. Z.-Y. , Chow D. H. , McGill T. C.
Published: July 2002

We have performed a theoretical and experimental analysis of current transport in InAs/GaSb/InAs interband tunneling devices as a function of GaSb layer width. Our results demonstrate that current transport in these devices occurs not through simple ohmic conduction, as had been previously proposed, but via light-hole-like resonances in the GaSb valence band formed due to the imperfect matching of InAs conduction-band and GaSb valence-band wave functions at the InAs/GaSb interfaces. These resonances produce a strong dependence of the current-voltage characteristics on GaSb layer width that is both predicted theoretically and observed experimentally. Our results also suggest that coupling between InAs conduction-band and GaSb heavy-hole valence-band states is relatively unimportant in these devices. In addition, we have been able to obtain peak current densities of ∼9×104 A/cm2, significantly higher than any previously reported current densities for this structure.
Maximum density of Pb centers at the (111) Si/SiO2 interface after vacuum anneal
Authors: Stesmans A. , Van Gorp G.
Published: July 2002

The density of interfacial [111]Pb centers, i.e., 0Si≡Si3 defects with unpaired bond along [111], has been accurately determined by K-band electron spin resonance at 4.3 K on (111)Si/SiO2 structures using various oxidation conditions. Reversible defect depassivation (dehydrogenation) has been monitored by post-oxidation vacuum annealing in the temperature range 752–835 °C. It was found that the density of both ESR active and passivated Pb defects is (11.4±0.6)×1012 cm−2 on all (111)Si/SiO2 interfaces prepared by oxidation in dry O2 at 920–1000 °C. This number appears to be a natural constant predominantly set by the oxidation temperature; variations in oxidation oxygen pressure (1.5×10−5–840 Torr) and time have little influence. The Pb’s are found to account for all the fast interface states.
Enhanced diffusion of Sb-doped layers during Co and Ti reactions with Si
Authors: Honeycutt J. W. , Rozgonyi G. A.
Published: July 2002

The effects of Co and Ti thin-film reactions with Si on diffusion of buried Sb-doped layers have been investigated. Sb profile analysis by secondary-ion mass spectrometry shows that greatly enhanced, nonuniform Sb diffusion occurs during reactions of various thicknesses (30–300 nm) of Co and Ti by rapid thermal annealing. A simple nonequilibrium intrinsic diffusion model is invoked to estimate time-average excess vacancy concentrations. Vacancy concentrations of about 107 times equilibrium values are shown to exist during CoSi2 formation by reaction of a 30 nm Co film at 700 °C for 5 min in Ar. Diffusion enhancements at large distances from the silicide edge are observed by bevel and etch techniques.
Some photoeffect roles in rapid isothermal processing
Authors: Singh R. , Sinha S. , Thakur R. P. S. , Chou P.
Published: July 2002

When compared to furnace processing, for identical and lower substrate temperatures, more photons are available in the visible and ultraviolet regions for rapid isothermal processing (RIP) based on incoherent radiation as the energy source. In this letter, we provide experimental evidence for photoeffects in RIP for a wide variety of materials. As compared to furnace processed samples, rapid isothermal annealed phosphosilicate glass films on Si substrate show a higher value of refractive index, a lower flatband charge density, and a lower thermal stress. High-temperature superconducting thin films on Y-Ba-Cu-O deposited by RIP assisted metalorganic chemical vapor deposition on yttrium stabilized zirconia substrate show a larger grain size, a higher value of the transition temperature than their furnace counterpart. The microscopic understanding of a particular deposition or annealing process is necessary to take full advantage of photoeffects in RIP.
Excitonic optical nonlinearity induced by internal field screening in (211) oriented strained-layer superlattices
Authors: Sela I. , Watkins D. E. , Laurich B. K. , Smith D. L. , Subbanna S. , Kroemer H.
Published: July 2002

The nonlinear optical properties of a new class of strained-layer superlattices (intrinsic Stark effect superlattices) have been investigated. Specifically, we have compared the nonlinear transmission of Ga1−xInxAs-GaAs strained-layer superlattices grown along the (211) axis to identical superlattices grown along the (100) axis, and found that the optical nonlinearity in the (211) sample is about one order of magnitude greater than in the (100) sample. A blue shift of the exciton resonance and an increase in the exciton absorption strength in the (211) sample with increasing light intensity was observed (attributed to screening of the intrinsic Stark effect fields by photogenerated carriers), resulting in the stronger optical nonlinearity. The maximum of the nonlinear absorption index, ‖α2‖, in the (211) sample was 54 cm/W (‖Im χ3‖=0.33 esu) whereas in the (100) sample the maximum of ‖α2‖ was 6.9 cm/W (‖Im χ3‖=0.042 esu). The measured carrier recovery time in both samples was 2 ns.
Role of heavy-hole states in interband tunnel structures
Authors: Ting D. Z.-Y. , Yu E. T. , McGill T. C.
Published: July 2002

We report on the first calculation of transport in InAs/GaSb/AlSb-based interband tunnel structures which uses a realistic band structure model. The results are compared with calculations using a two-band model which includes only the lowest conduction band and the light-hole band. It is found that heavy-hole states can introduce substantial hole-mixing effects in device structures containing GaSb quantum wells, and should have a significant influence on current-voltage characteristics interband devices.
Sub-30 nm lithography in a negative electron beam resist with a vacuum scanning tunneling microscope
Authors: Dobisz E. A. , Marrian C. R. K.
Published: July 2002

We report studies of minimum feature sizes in 50 nm films of the high-resolution negative electron beam resist, SAL-601-ER7 from the Shipley Corporation. Developed linewidths of 27 nm and line spacing of 55 nm, from center to center, were produced by lithography with a vacuum scanning tunneling microscope (STM). In contrast, a minimum linewidth of 95 nm was obtained from exposure with a 17 nm (1/e diameter) 50 kV electron beam. Patterns written in the STM at electron energies down to 15 eV were visible in the developed resist. The limit at 15 eV is related to the operation of the STM and does not represent an exposure threshold energy for the resist.
In situ buried GaInAs/InP quantum dot arrays by selective area metalorganic vapor phase epitaxy
Authors: Galeuchet Yvan D. , Rothuizen Hugo , Roentgen Peter
Published: July 2002

Using metalorganic vapor phase epitaxy to grow GaInAs/InP layers on masked InP substrates patterned with submicron square holes, we have fabricated in situ buried quantum dot arrays for the first time. Starting with mask openings ≥150 nm × 150 nm and utilizing the natural crystal habits to form low-index plane facetted pyramids inside the holes, highly regular GaInAs quantum dots embedded in InP are obtained in a single growth step. As verified by cathodoluminescence, the dots exhibit very high luminescence efficiencies, even at room temperature, owing to the absence of air-exposed or etch-damaged heterointerfaces.
Effects of impurities Si and Be on the creation of Ga vacancies and Ga interstitials in GaAs grown by molecular beam epitaxy
Authors: Lee Jong-Lam , Wei Long , Tanigawa Shoichiro , Kawabe Mitsuo
Published: July 2002

Two type of top layers, namely, undoped and Si-doped GaAs layers, were grown, respectively, on a Be-doped epilayer by molecular beam epitaxy, and the effects of impurities of Be and Si on the creation of Ga interstitials IGa and Ga vacancies VGa during the annealing of the specimens were investigated by a slow positron beam. The concentration of VGa created from the surface of the undoped GaAs during the annealing decreases drastically when the annealed GaAs was kept at room temperature for one month. This implies that the Be atoms diffusing to the undoped GaAs during the growth and/or the annealing cause the creation of IGa in the undoped layer and it makes the recombination with VGa. On the other hand, no decrease in the concentration of VGa was observed in the Si-doped GaAs. These support the creation of IGa and of VGa, respectively, in the Be-diffused GaAs and in Si-diffused GaAs.
Carrier gas-free chemical vapor deposition technique for in situ preparation of high quality YBa2Cu3O7−δ thin films
Authors: Schulte B. , Maul M. , Becker W. , Schlosser E. G. , Elschner S. , Häussler P. , Adrian H.
Published: July 2002

A new carrier gas-free metalorganic chemical vapor deposition technique was developed yielding a high deposition rate. The β-dicetonate precursor compounds were synthesized with reproducible water content which is of great importance for their vapor pressure. With the technique described above c-axis oriented YBa2Cu3O7−δ thin films were prepared with Tc(R=0)=91.8 K and jc(77 K)=1.3×106 A/cm2 on (100)SrTiO3 substrates and Tc(R=0)=91.1 K and jc(77 K)=3×105 A/cm2 on (100)MgO substrates at B=0 T. The influence of the metal composition on the formation of precipitation was studied by SEM investigations.
Low threshold pulsed and continuous-wave laser action in optically pumped (Zn,Cd)Se/ZnSe multiple quantum well lasers in the blue-green
Authors: Jeon H. , Ding J. , Nurmikko A. V. , Luo H. , Samarth N. , Furdyna J.
Published: July 2002

Laser action in the blue-green at room temperature has been achieved in pulsed optically pumped (Zn,Cd)Se/ZnSe multiple quantum well structures at threshold intensities Ith≊30 kW/cm2. Continuous-wave operation in a II-VI semiconductor laser has also been demonstrated for the first time, here above 100 K. The role of excitons is found to be of importance in defining the lasing mechanism up to room temperature in these quasi-two-dimensional wide-gap heterostructures.
Zn1−yCdySe1−xTex quaternary wide band-gap alloys: Molecular beam epitaxial growth and optical properties
Authors: Brasil Maria J. S. P. , Tamargo Maria C. , Nahory R. E. , Gilchrist H. L. , Martin R. J.
Published: July 2002

We report the growth of Zn1−yCdySe1−xTex alloys by molecular beam epitaxy on GaAs substrates. The optical properties and the band structure of this new material have been investigated. The compositional dependence of the quaternary band gap was studied using photoconductivity measurements and is well described by a third degree function of x and y. Photoluminescence spectra at low temperature show a single broad band, which narrows with increasing Te composition. We discuss the applicability of this new material for ZnSe/ZnCdSeTe heterostructures and predict a region of optimal band offsets for useful confinement of both electrons and holes.
High quality thin gate oxide prepared by annealing low-pressure chemical vapor deposited SiO2 in N2O
Authors: Ahn J. , Ting W. , Chu T. , Lin S. , Kwong D. L.
Published: July 2002

In this letter, the electrical properties of thin low-pressure chemical vapor deposited (LPCVD) SiO2 annealed in N2O ambient have been studied and compared with thermal oxide of identical thickness. It is shown that N2O-annealed CVD oxide exhibits less interface state generation and less flatband voltage shift under constant current stress than thermal oxide. It also has excellent uniformity and comparable breakdown characteristics. An oxynitride film formation at the Si/SiO2 interface by annealing in N2O is speculated to be the cause of these improvements.
3.06 μm InGaAsSb/InPSb diode lasers grown by organometallic vapor-phase epitaxy
Authors: Menna R. J. , Capewell D. R. , Martinelli Ramon U. , York P. K. , Enstrom R. E.
Published: July 2002

We have observed laser action at λ=3.06 μm in In0.77Ga0.23As0.74Sb0.26/InP0.7Sb0.3 double heterojunction, diode lasers, which were grown by organometallic vapor-phase epitaxy. The maximum operating temperature was T=35 K, and typical threshold current densities were 200–330 A/cm2. At temperatures up to 35 K, the lasing wavelength decreased with increasing temperature owing to a band-filling effect.
Infrared intersubband photoinduced charge polarization in asymmetrical quantum wells
Authors: Levine B. F. , Gunapala S. D. , Hong M.
Published: July 2002

We have studied the long-wavelength photoinduced charge polarization and electron storage produced by infrared intersubband absorption in highly asymmetrical graded quantum wells. At zero bias a large photovoltage is observed, while at high bias the charge transfer probability approaches unity.
Strain relaxation and compensation due to annealing in heavily carbon-doped GaAs
Authors: Hanna M. C. , Majerfeld A. , Szmyd D. M.
Published: July 2002

Heavily C-doped GaAs grown by atmospheric pressure metalorganic vapor phase epitaxy using CCl4 as the C-dopant source has been annealed to study the stability of C acceptors at very high doping levels (p=1018–1020 cm−3). In layers with initial hole densities p≳6×1019 cm−3, 5 min anneals at temperatures ranging from 700 to 850 °C under arsine overpressure caused a significant reduction in the hole density, lattice contraction and photoluminescence intensity, and a smaller reduction in the mobility. For lower doped material, annealing has little effect on the as-grown properties. These changes in the material properties indicate that a compensating recombination center is formed during annealing. Possible compensation mechanisms which explain partially the annealing effects in very heavily C-doped GaAs are analyzed. The results of this study show that there is an upper limit on the hole concentration of p≳6×1019 cm−3 in annealed GaAs:C.
Extremely high electron mobility in Si/GexSi1−x structures grown by molecular beam epitaxy
Authors: Mii Y. J. , Xie Y. H. , Fitzgerald E. A. , Monroe Don , Thiel F. A. , Weir B. E. , Feldman L. C.
Published: July 2002

A modulation-doped Si/GexSi1−x structure was fabricated in which a thin Si layer is employed as the conduction channel for the two-dimensional electron gas. The strained heterostructure is fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The mobility of the two-dimensional electron gas as determined from Hall measurements was 1600 cm2/V s at 300 K and 96 000 cm2/V s at 4.2 K. Recently, a 4.2 K mobility of 125 000 cm2/V s was observed from a similar sample.
Importance of the upper state position in the performance of quantum well intersubband infrared detectors
Authors: Steele A. G. , Liu H. C. , Buchanan M. , Wasilewski Z. R.
Published: July 2002

We present the results of a systematic study of the dependence of intersubband infrared detector performance on the subband energies. By using samples cut from different positions on a single wafer grown with a known thickness variation it is possible to obtain a set of devices for which the well width varies in a controlled manner, but the barrier height remains fixed. Current versus voltage and responsivity measurements demonstrate the importance of the detector performance on the exact location of the upper state: optimum peak response is obtained when the subband is resonant with the top of barrier.
Luminescence characteristics of quantum wires grown by organometallic chemical vapor deposition on nonplanar substrates
Authors: Kapon E. , Kash K. , Clausen E. M. , Hwang D. M. , Colas E.
Published: July 2002

Luminescence properties of GaAs/AlGaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on V-grooved substrates are reported. A model of the crescent-shaped wires yields parabolic QWR potential wells with subbands separated by 21.7, 3.9, and 16.7 meV for electrons, heavy holes, and light holes and effective width of 16 nm for the ground electron state. Spectrally and spatially resolved cathodoluminescence images reveal highly uniform emission from the QWR regions. Photoluminescence excitation spectra exhibit enhanced absorption at the QWR subbands, with subband separations in good agreement with the model.
Visible electroluminescence from porous silicon
Authors: Koshida Nobuyoshi , Koyama Hideki
Published: July 2002

It is demonstrated that photoluminescent porous Si (PS) layers exhibit definitely visible electroluminescence (EL). The PS layers were formed by anodization of single-crystal nondegenerate p-type Si wafers in an HF solution. The experimental EL cells are of the form semitransparent metal/PS layer/p-type Si/Al electrode. These cells show a rectifying junction behavior. When the forward current density reaches a certain value, stable visible (orange) light is uniformly emitted through a semitransparent electrode. A possible explanation of this is the radiative transition due to electron and hole injection into quantized states in PS.