bluuebunny/crossref_metadata_2025 huggingface dataset:
PATHOGENESIS OF CONGENITAL HYPOMYELINATION REVEALED BY P0 OVEREXPRESSING MICE
Authors: Previtali S. , Quattrini A. , Imperiale D. , Canal N. , Trapp B. , Messing A. , Feltri M.L. , Wrabetz L.
Published: July 2002

Extra copies of the myelin protein zero (Mpz) gene cause dysmyelinating neuropathy in transgenic mice. Clinical and pathological findings are dose dependent, ranging from transient perinatal hypomyelination (30% mRNA overexpression), to arrested nerve development with impaired sorting of axons by Schwann cells (700% mRNA overexpression). Mpz overexpression promotes inappropriate trafficking of P0 protein to surface membranes of promyelinating Schwann cells. Ultrastructural immunocytochemistry demonstrates that mistargeting of P0 to the membranes of the advancing mesaxon (the lip of Schwann cell membrane that enwraps the axon) activates P0 homophilic adhesion, arresting spiral wrapping and myelination.

Severely reduced nerve conduction velocities, paucity of active myelin destruction or onion bulbs, and redundant basement membrane formation in P0‐overexpressor mice resemble those present in human congenital hypomyelination (CHN). CHN is a subtype of demyelinating neuropathy due to MPZ mutation. Although increased MPZ has not yet been described in CHN, increased myelin gene dosage (PMP22 and PLP) can cause hereditary myelinopathy in both rodent and human (Charcot‐Marie‐Tooth IA and Pelitzaus Merzbacher disease). Of note, mutations in the Krox 20 transcription factor, encoded by EGR2, are also associated with CHN. Since one of these EGR2 mutations likely disinhibits Krox 20, and Krox 20 may regulate P0 expression, it is possible that MPZ overexpression contributes to the pathogenesis of this EGR2 mutation.
PERIPHERAL NEUROPATHIES AND CHRONIC HEPATITIS C
Authors: Primavera A. , Cocito L. , Reni L. , Caponnetto C. , Abbruzzese M.
Published: July 2002

Objectives: Recent interest has been expressed in peripheral neuropathies in hepatitis C virus (HCV) patients. The aim of this prospective study was to evaluate the prevalence of peripheral neuropathies associated with chronic hepatitis and their clinical manifestations.

Patients and method: Ninety anti‐HCV‐positive patients were consecutively interviewed and examined by the same operator. Forty‐five patients with end‐stage liver disease and awaiting liver transplantation were evaluated at the Liver Transplantation Center (Group 1). Further 45 patients were referred for neurological consultation during hospitalisation in the Department of Medicine (Group 2), where they had been admitted for different clinical reasons. One patient from group 1 and 5 patients from group 2 were excluded from the study because of previous neurological diseases. All patients underwent a standardized neurological evaluation, including history, neurological examination, mini mental state examination, neuropathy symptom score, and neurological disability score. In presence of symptoms and signs of peripheral neuropathy, an electrophysiological evaluation was performed.

Results: Signs or history of encephalopathy were found in 23/44 patients of group 1 (52.2%) and in 8/40 patients of group 2 (20%). Clinical manifestations of neuropathy, confirmed by electrophysiological examination, were found in 11 subjects of group 1 (25%) and in 17 patients of group 2 (42.5%). Most patients had minor symptoms; sensory disturbances occurred more frequently than motor and autonomic dysfunctions. In group 1, peripheral neuropathy was associated with systemic illness (diabetes, renal failure, liver neoplasm, previous alcohol abuse) in 4/11 patients (36.3%); conversely, in all patients from group 2, other possible etiological factors were present: alcohol abuse in 3; diabetes in 4; renal failure in 1; mixed cryoglobulinemia in 7; and neoplasm in 2.

Conclusions: Symptomatic but not disabling neuropathies were found in 33% of HCV in patients. Sensory involvement was prevalent. Systemic illness and mixed cryoglobulinemia were more frequently present in Group 2 than in Group 1. This finding suggests that the metabolic dysfunctions caused by liver disease may be the primary determinant of peripheral system damage only in patients with end‐stage liver disease.
EXPANDING THE NOSOLOGY OF HYPERMYELINATING NEUROPATHIES: DESCRIPTION OF TWO NEW ENTITIES
Authors: Quaranta L. , Sabatelli M. , Madia F. , Ricci E.  , Lippi G. , Conte A. , Gazzetta F. , Tonali P.
Published: July 2002

We describe two patients affected by encephalo‐neuro‐myopathy with partial deficiency of merosin and one patient with familial spastic paraparesis and sensory neuropathy of uncertain nosology, whose sural nerve biopsy disclosed quite unusual hypermyelination features.

Both patients with merosin deficiency showed mental retardation, epilepsy, myopathy and peripheral neuropathy. Brain MRI demonstrated diffuse white matter involvement and nerve conduction studies disclosed reduction of conduction velocities, consistent with a “demyelinating” neuropathy. However nerve biopsy failed to reveal segmental demyelination. The most striking feature, in both of them, was a peculiar hypermyelination, characterized by “globular” thickening of the myelin sheath only at the paranodal regions.

The third patient was affected by spastic paraparesis since early infancy; a brother was affected by similar disease and both parents were normal. In view of the above, an autosomal recessive trasmission seems likely. MRI of the brain and spinal cord was normal. Electrophysiological examination disclosed a “demyelinating” motor and sensory neuropathy. Sural nerve biopsy disclosed a normal density of myelinated fibers. At teased fibers analysis we didn't find any focal hypermyelination nor segmental demyelination. On transverse sections several fibers showed a marked thickening of the myelin sheath.

Abnormal myelin proliferation is a distinctive pathological feature in HNPP and CMT4B, two hereditary neuropathies in which sausage‐shaped thickenings of the myelin sheath (tomacula) and very irregular focal outfoldings, respectively, are always associated with segmental demyelination. Our patients showed morphological changes very different from these conditions. The two patients with partial merosin deficiency showed a focal, “globular” myelin overgrowth confined to paranodal regions, while the patient with spastic paraparesis disclosed a diffuse thickening of the myelin sheath. Furthermore, no demyelination neither hypomyelination were observed in our three patients. Based on these findings it may be inferred that hypermyelination may per se be responsible for the slowing of conduction velocities.
FUNCTIONAL EVALUATION OF PATIENTS AFFECTED WITH CHARCOT‐MARIE‐TOOTH DISEASE
Authors: Vinci P. , Perelli S.L. , Esposito C. , Villa L.M.
Published: July 2002

Charcot‐Marie‐Tooth disease is a genetic neuropathy causing progressive muscle weakness and wasting and sensory reduction, starting from the distal regions of the limbs. Typical is the variability in clinical severity, which depends on the distoproximal extension of muscle weakening and its biomechanical consequences. Since the rehabilitative intervention varies according to the disease severity, we have previously suggested to grade patients in seven levels (or stages) as to the lower limbs and four levels as to the upper limbs (1). Patients are fitted in the classification on the basis of the results of a functional evaluation, which includes:

The muscle testing is to be repeated after orthotic and physiotherapeutic treatment (2), since muscle strength can improve as the result of an increased mobility and a better muscle action line.

The range‐of‐motion testing is also to be repeated to monitor the efficacy of the stretching program and, in children, to discover new joint alterations as soon as they appear.

This methodology is recommended not only for rehabilitative purposes but also to monitor the natural evolution of the disease in an easy and objective way.
ACUTE PREVALENTLY MOTOR AXONAL NEUROPATHY DURING DISULFIRAM TREATMENT
Authors: Volpi N. , Alessandrini C. , Venturini E. , Vatti G. , Bibbò G. , Giannini F.
Published: July 2002

Disulfiram (Di) presents hepatic and central nervous system toxicity as main side effects. Di occasionally produces peripheral neuropathy consisting of a chronic axonal sensory pattern. A 34‐year‐old woman, after a five‐year history of moderate abuse, started Di therapy (400 mg per day) and stopped alcohol intake. Three months later, painful tingling and burning sensation occurred on distal lower limbs, rapidly followed by a foot drop within three days and severe weakness spreading to proximal segments and to upper limbs within two weeks. The patient presented with moderate distal multimodal hypoaesthesia, complete loss of deep tendon reflexes, mild distal muscle hypotrophy and severe motor disability (Rankin grade 4). Electrophysiological examination showed a typical prevalently motor axonal neuropathy with inexcitability of motor responses in lower limbs, but normal latencies of “F” responses in upper limbs. Standard cerebrospinal fluid (CSF) examination was normal and investigations for cytomegalovirus, herpes viruses and Campylobacter Jejuni (CJ) were negative. Anti GM1 antibodies (Ab) were negative in serum and CSF. Sural nerve biopsy revealed axonal degeneration with moderate loss of large myelinated fibres, frequent endoneurial foamy macrophages and milder degeneration of unmyelinated axons. No axonal swellings with filamentous deposits were detected. Rows of myelin ovoids were present on teased fibres, whereas rare short remyelinated internodes and no segmental demyelination were observed. Follow‐up, three months later, shows a significant increase of general strength and sensory function (Rankin grade 3), though heels and toes gait is not recovered and painful/burning paraesthesias still require gabapentin treatment. The main diagnostic problem of this case is distinction from acute axonal Guillain Barré syndrome. Although there is no consensus regarding electrophysiological diagnostic criteria for AMSAN, significant abnormalities of “F” waves are generally observed on affected nerves. On the immunological side, anti GM1 Ab increased titer and evidence of CJ infection, though not mandatory, are very frequent in AMSAN. Moreover, pathological changes distinctive of AMSAN, such as peri‐axonal or intra‐axonal macrophages, were absent in our case. On the other hand, a similar acute onset of a sensory—motor axonal neuropathy during Di treatment has been previously described (Nukada, 1981).
Growth and Parasite Defence in Plants; the Balance between Resource Sequestration and Retention: In Lieu of a Guest Editorial
Authors: Matyssek R. , Schnyder H. , Elstner E.‐F. , Munch J.‐C. , Pretzsch H. , Sandermann H.
Published: July 2002

Abstract: A hypothesis on regulation of the balance between growth and parasite defence in plants is formulated, namely that plants regulate their resource allocation in a way where stress tolerance and resistance inherently lead to constraints on growth and competitiveness. Seven reviews and the subsequent article in this issue of Plant Biology contributing to this problem are briefly introduced in context.
Near-room-temperature operation of Pb1−xSrxSe infrared diode lasers using molecular beam epitaxy growth techniques
Authors: Spanger Beate , Schiessl U. , Lambrecht A. , Böttner H. , Tacke M.
Published: July 2002

Double-Heterostructure lasers have been fabricated with an active layer of PbSe sandwiched between cladding layers of Pb1−x Srx Se. They were operated up to T=290 K (17 °C) in pulsed and T=169 K in cw mode. This is the highest operational temperature in pulsed mode reported for lead salt lasers in the mid-infrared. The emission wavelength tunes with temperature from 8.0 μm (T=20 K) to 4.4 μm (T=285 K).
Phonon-assisted stimulated emission in thin (<55 Å) AlGaAs-GaAs-AlGaAs single quantum wells
Authors: Lo Y. C. , Kolbas R. M.
Published: July 2002

Since the initial report of phonon-assisted stimulated emission in AlGaAs-GaAs quantum wells, several laboratories have performed similar experiments which have produced a wide range of experimental data and interpretations. We present photopumped laser data (77 K) from three different AlGaAs-GaAs-AlGaAs single quantum well heterostructures grown by molecular beam epitaxy. Closely spaced end-to-end laser modes are observed one phonon energy (≊36 meV) below the n=1′ electron to light hole confined-particle state which we attribute to phonon-assisted stimulated emission. Also, the data are inconsistent with impurity related luminescence or optical absorption losses which have been suggested as alternative explanations. These results are important because these are the thinnest quantum wells to exhibit phonon-assisted stimulated emission and the first independent observation and confirmation of this important phenomenon.
Pseudomorphic InGaAs base ballistic hot-electron device
Authors: Seo K. , Heiblum M. , Knoedler C. M. , Hong W-P. , Bhattacharya P.
Published: July 2002

We report the first successful incorporation of a pseudomorphic InGaAs base in a ballistic hot-electron device. The device, with a 28-nm-thick In0.15Ga0.85As base, had a collector-base breakdown voltage of 0.55 V and a maximum current transfer ratio of 0.89 at 4.2 K, considerably higher than the 0.75 in a comparable GaAs-base device. Electron energy spectroscopy measurements revealed that at least 30% of the injected electrons traversed the InGaAs base ballistically, causing a strong modulation in the injected currents into the quantized base. The Γ-L valley separation in the strained In0.15Ga0.85As was estimated to be about 410 meV.
Nonlinear subgap photoconductivity of polycrystalline silicon
Authors: Bock W. , Prettl W.
Published: July 2002

The photoconductance of polycrystalline silicon films at photon energies smaller than the band gap has been measured as a function of intensity applying a 1.3 μm wavelength semiconductor laser. The observed photosignal increases superlinearly at low intensities and saturates above about 1.5 W/cm−2. This distinct nonlinearity is caused by a significant energy dependence of optical to thermal cross sections of trap states in the band gap. Assuming a three-level-rate equation model, grain boundary trap densities were evaluated.
Observation of complex domain walls in KTiOPO4
Authors: Loiacono G. M. , Stolzenberger R. A.
Published: July 2002

Optical nonuniformities observed in second-harmonic generation experiments on large KTiOPO4 crystals (dark line defects) are shown to be directly related to the presence of complex domain walls. It is unlikely that these domains are ferroelectric; however, their polar nature suggests they may be similar to the Dauphiné twins observed in quartz.
Interfacial reaction of BN/Ni3Al
Authors: Chou T. C.
Published: July 2002

Interfacial reaction between BN and Ni3Al was studied by diffusion bonding at 1000 °C. Error function type diffusion profiles of Ni and Al were observed. Al enrichment at the interface was detected. Kirkendall voids [Trans. Am. Inst. Min. Eng. 171, 130 (1947)] with faceted interfaces were observed in the interdiffusion zone of Ni3Al side. In situ fractured interface showed a typical heterogeneous interfacial reaction. Three-dimensional Ni clusters were observed to grow into islands, and unevenly distributed in an alumina matrix of whisker form. The intriguing pattern of the outdiffusion and clustering of Ni atoms, as compared to Al, during the chemical interdiffusion between BN and Ni3Al is discussed based upon grain boundary diffusion, diffusion environment, and thermodynamic driving force under oxygen environment.
Reactivity and passivation for Bi adatoms on YBa2Cu3O6.9 and Bi2Ca1+xSr2−xCu2O8+y
Authors: Meyer H. M. , Hill D. M. , Weaver J. H. , Nelson D. L. , Goretta K. C.
Published: July 2002

X-ray photoemission spectroscopy was used to examine the reactivity of clean YBa2Cu3O6.9 and Bi2Ca1+xSr2−xCu2O8+y surfaces with adatoms of vapor-deposited bismuth. For YBa2Cu3O6.9, depositions ≤8 Å of Bi induce O 1s, Cu 2p3/2, and Bi 4f line-shape changes due to BiO bonding, limited oxygen loss from the substrate, partial conversion of Cu2+ to Cu1+, and growth of Bi metal. For coverages greater than 8 Å, there is no reaction and only Bi metal growth is observed. For Bi deposition onto Bi1Ca1+xSr2−xCu2O8+y, there is minimal reaction with only subtle O 1s and Cu 2p3/2 line-shape changes and the nucleation and growth of Bi metal starting at low coverages (≥2 Å). Comparison shows that Bi is less reactive than other metals, except Ag and Au, and that Bi2Ca1+xSr2−xCu2O8+y is more stable toward Bi deposition than YBa2Cu3O6.9.
Growth and microstructure of Bi-Sr-Ca-Cu-O thin films
Authors: Shah S. I. , Jones G. A.
Published: July 2002

Thin superconducting Bi-Sr-Ca-Cu-O films have been grown by reactive magnetron sputtering with an average cation ratio of 2:2:1:2 on single-crystal (100)MgO. Films show a superconducting transition onset at 117 K, but do not go to zero resistance until 83–84 K. Secondary electron microscopy and electron beam microprobe showed two major phases with different compositions and morphologies. The dominant phase was a lamellar phase with composition very close to the 2:2:1:2 cation ratio. The second phase had a needle-like morphology, which was deficient in Bi, and had excess Cu. The nonsuperconducting Sr14−xCaxCu24O41 phase had been known to exist in bulk; therefore, the needle phase could be an intermediate step in the formation of Sr14−xCaxCu24O41 phase as Bi deficiency in the needles increased with increasing annealing temperature and/or time.
Electronic structure of ceramics and thin-film samples of high T
c Bi2Sr2CaCu2O8+δ superconductors: Effects of Ar+ sputtering, O2 exposure, and Rb deposition
Authors: Lindberg P. A. P. , Soukiassian P. , Shen Z.-X. , Eom C. B. , Lindau I. , Spicer W. E. , Geballe T. H.
Published: July 2002

The electronic structure of bulk, and, for the first time, thin-film samples of the new class of high Tc Bi2Sr2CaCu2O8+δ superconductors, is investigated by photoemission spectroscopy using synchrotron radiation. Ar+ sputtering and Rb deposition result in disrupted Bi  O bonds and subsequent change in the valency of Bi, while oxygen adsorption or annealing in oxygen is found to restore the Bi  O bonds. The results also show that adsorbates of O2 and/or Rb readily give rise to a number of new oxygen states in the valence band of Bi2Sr2CaCu2O8+δ. The Ar+-sputtered film is found to be more sensitive to adsorbates of O2 and Rb than the scraped bulk sample, as monitored by the Bi 5d core shifts and the oxygen-induced valence-band states.
Transverse-mode-stabilized ridge stripe AlGaInP semiconductor lasers incorporating a thin GaAs etch-stop layer
Authors: Tanaka T. , Minagawa S. , Kajimura T.
Published: July 2002

Transverse-mode-stabilized ridge stripe AlGaInP semiconductor lasers fabricated using a thin GaAs layer as an etch stop are reproducibly realized. The ridge stripe structure is fabricated utilizing the large difference in etching rates between GaAs/AlGaAs and AlGaInP layers. As a result, the thickness control of the cladding layer adjacent to light absorbing layers is significantly improved. Results show that the GaAs layer is effective as an etch-stop layer even if the thickness is only 1–2 nm. The total internal loss of the lasers is estimated to be about 20 cm−1 and loss due to the insertion of the GaAs layer is almost negligible. A threshold current of about 50 mA and a stable fundamental transverse mode up to an output power of about 10 mW is attained at room temperature under cw operation. Similar results were obtained when an AlGaAs layer was used instead of GaAs.
Atomic resolution imaging of electrode surfaces in solutions containing reversible redox species
Authors: Heben Michael J. , Penner Reginald M. , Lewis Nathan S. , Dovek Moris M. , Quate Calvin F.
Published: July 2002

Procedures are described for insulating metal scanning tunneling microscope (STM) tips with either glass or polymer coatings. In solutions containing 0.10 M of a reversible redox couple, Fe(CN)−3/−46, the faradaic limiting current to polymer coated tips was 200–500 pA and that for glass coated tips was <10 pA. For polymer insulated tips, steady-state currents of 10–100 pA were observed at tip-sample displacements less than 0.3 μm. The suppression of faradaic current achieved by these coating procedures enabled the collection of the first atomic resolution STM images of highly ordered pyrolytic graphite electrodes in contact with redox-active electrolytes. Preliminary data for the in situ electrochemical characterization of these tips are also discussed.
Subpicosecond electrical pulse generation using photoconductive switches with long carrier lifetimes
Authors: Krökel D. , Grischkowsky D. , Ketchen M. B.
Published: July 2002

Using a new mechanism of electrical pulse generation, we have generated 350 fs (full width at half maximum) electrical pulses on a coplanar transmission line, fabricated on an unimplanted silicon-on-sapphire substrate.
Optically enhanced photoconductivity in semi-insulating gallium arsenide
Authors: Desnica U. V. , Šantić B.
Published: July 2002

Time evolution of photoconductivity of semi-insulated gallium arsenide illuminated at low temperatures with monochromatic 0.7–1.8 eV photons was studied. For low light intensity the photosensitivity increases with time by several orders of magnitude and exhibits different dynamics for above-the-gap and below-the-gap energy photons. For various photosensitivity stages the concentration of charge trapped in deep traps was determined by measuring thermally stimulated current. Six deep traps in the 85–250 K temperature range were observed. Good correlation between the photosensitivity and the total concentration of trapped charge was found. The increase of photosensitivity is interpreted as the increase of lifetime of optically created free holes due to the trapping of electrons, which therefore are not available for recombination. The same interpretation also gives a new insight in previously published results on increased photosensitivity in gallium arsenide.
Shallow impurity neutralization in GaP by atomic hydrogen
Authors: Singh Mandeep , Weber Jörg
Published: July 2002

Hydrogen neutralizes both donors and acceptors in GaP by a mechanism that is independent of the site of the impurity in the lattice, but is dependent on the identity of the impurity. Hydrogen also passivates an isoelectronic trap nitrogen, a phenomenon first encountered here in GaP. We propose a simple model for the NH binding based on a polarized N−H+ bond. This model accounts for the relative passivation of a series of NN pairs, where the passivation depends critically on the NN separation distance. Our results also support the recently revised correlation of spectral lines with NN pairs of varying separations.
Spatial mode structure of broad-area semiconductor quantum well lasers
Authors: Chang-Hasnain C. J. , Kapon E. , Bhat R.
Published: July 2002

The spatial mode characteristics of gain-guided broad-area quantum well lasers grown by organometallic chemical vapor deposition were investigated experimentally. GaAs/AlGaAs quantum well lasers grown on 6°-off (100) oriented substrates exhibit excellent material uniformity, which allows study of their modal behavior. Gain-guided broad-area lasers fabricated on such uniform material demonstrate nearly ideal gain-guiding modal behavior. These lasers tend to lase in the fundamental mode near threshold and emit single-lobed far-field patterns. In these well-behaved broad-area lasers, we have identified the mechanism for degradation in the spatial coherence at high pumping levels as the onset of higher order lateral modes.
High-power, high-efficiency 1.3 μm superluminescent diode with a buried bent absorbing guide structure
Authors: Nagai Haruo , Noguchi Yoshio , Sudo Shoichi
Published: July 2002

A superluminescent diode (SLD) operating in the 1.3 μm wavelength region has been fabricated by the liquid phase epitaxial growth technique. Lasing is effectively suppressed by incorporating an unpumped buried bent guide structure for SLD operation. Devices with an antireflection coating on the front facet emit a high optical power of 11.5 mW at an injection current of dc 200 mA. The Fabry–Perot mode has been sufficiently suppressed over the entire emission spectral bandwidth of 40 nm. The transverse mode is stable and a beam divergence of 48°×33° has been achieved. A coupling power of 1 mW was achieved for the 1.9% Δn, and 2.8-μm-diam core single-mode fiber by butting.
Implantation damage and anomalous diffusion of implanted boron in silicon
Authors: Guo Qiang , Bao Ximao , Hong Jianming , Yan Yong , Feng Duan
Published: July 2002

The anomalous diffusion of implanted boron in silicon damaged by self-implantation has been investigated by cross-section transmission electron microscopy and secondary-ion mass spectroscopy. During rapid thermal annealing, bulges in the boron profile are observed at the edges of the defected band and the profile broadening is accompanied by the shrinking of the band. The decay time became unobvious due to the emission of point defects from the edges of the defected band. It is suggested that the anomalous diffusion of boron is related to the point defects produced by implantation and emitted from the defected band.
Observation of transient absorption by using the thermal lens method
Authors: Terazima Masahide , Azumi Tohru
Published: July 2002

The application of thermal lens spectroscopy to the absorption measurement of the transient species is demonstrated. The unique advantages of this method are discussed.
Low-loss Ti:LiNbO3 intersecting waveguides
Authors: Agrawal Niraj , McCaughan L.
Published: July 2002

Interactions between guided and radiation modes are studied experimentally in Ti:LiNbO3 single-mode, single-Δn intersecting waveguides with small angles. These measurements, which cannot be interpreted in terms of any previous theory for intersecting waveguides, can be understood in terms of a recently developed multiple scattering analysis. Furthermore, it is found that the radiation losses in optical switches and crossovers can be dramatically reduced by a fractional doping of the intersection region.
Electronic states in lateral structures on modulation-doped heterointerfaces
Authors: Tokura Y. , Tsubaki K. , Susa N.
Published: July 2002

The electronic states in an AlGaAs-GaAs modulation-doped heterointerface with a lateral structure of order 10 nm are analyzed self-consistently. The structure is realized by using a fractional layer superlattice. Resonant gaps appear in the electron state band diagram because the potential cannot be expressed as a sum of the functions of two directions. The electron channel location strongly depends on which subband the electron belongs to.
Integrated external cavity GaAs/AlGaAs lasers using selective quantum well disordering
Authors: Werner J. , Kapon E. , Stoffel N. G. , Colas E. , Schwarz S. A. , Schwartz C. L. , Andreadakis N.
Published: July 2002

Integrated external cavity GaAs/AlGaAs single quantum well lasers were fabricated by selective quantum well disordering. Lasers with 2.07-mm-long passive sections and 0.48-mm-long active sections had threshold currents of 33 mA, compared to 9.8 mA for lasers without passive sections. Lasing data indicate a residual modal loss of 11 cm−1 in the passive sections, consistent with direct waveguide loss measurements. Control composite structures with a nondisordered quantum well in the passive sections showed significantly higher threshold currents and a large red shift of as much as 11.4 nm in the lasing wavelength compared to lasers without a passive cavity. This red shift is the main reason for the reduced resonant losses in integrated external cavity lasers with a nondisordered quantum well in the passive section.
Schottky diodes on hydrogen plasma treated n-GaAs surfaces
Authors: Paccagnella A. , Callegari A. , Latta E. , Gasser M.
Published: July 2002

The characteristics of Schottky diodes on n-GaAs fabricated after an in situ low-pressure rf H2 plasma treatment have been investigated as a function of the substrate temperature during the plasma treatment. Degraded rectifying characteristics result after room-temperature treatments, while diodes with ideality factor as low as 1.01 were achieved in the temperature range 160–240 °C. An increase in barrier height was also observed with increasing substrate temperature during plasma treatment. The contact properties are correlated to H diffusion in a surface layer of GaAs, which passivates the dopant atoms and defect sites.
Fabrication of microlasers and microresonator optical switches
Authors: Scherer A. , Jewell J. L. , Lee Y. H. , Harbison J. P. , Florez L. T.
Published: July 2002

We have microfabricated low-threshold, high-speed vertical-cavity lasers and optical switches by optimizing the mirror design, crystal growth, and ion etching of microresonators. By minimizing the sidewall ion damage in electrically pumped microlasers, we have defined large arrays of 3-μm-diam surface-emitting devices with threshold currents below 1.5 mA. Ion beam etching was also used to define 0.5–1.5 μm wide all-optical microresonator switches with recovery times as low as 30 ps and controlling energies as low as 0.6 pJ.
Quasi-ballistic resonant tunneling of minority electrons into the excited states of a quantum well
Authors: Vengurlekar A. S. , Capasso F. , Sen S. , Hutchinson A. L. , Chu S. N. G. , Sivco D. , Cho A. Y.
Published: July 2002

Observation of energy-dependent transmission of nonequilibrium minority electrons through a symmetric double-barrier (DB) quantum well heterostructure is reported. The DB is placed in the base of a GaInAs/AlInAs bipolar transistor. The electrons are launched into the DB with variable kinetic energies using a tunnel barrier in the emitter and varying the emitter-base voltage. The resulting peak in the collector current provides for the first time evidence of quasi-ballistic resonant tunneling of minority carriers into the eigenstates of a quantum well. The small peak-to-valley ratio and the broad peak also demonstrate the importance of scattering and of the anisotropy in momentum space of the incident distribution function in the region between the ballistic electron launcher and the DB.
Gallium phosphide microlenses by mass transport
Authors: Liau Z. L. , Diadiuk V. , Walpole J. N. , Mull D. E.
Published: July 2002

Arrays of high quality refractive microlenses have been formed in GaP substrates by mesa etching followed by a heat treatment in which the multistep mesas were smoothed due to surface energy minimization. A smooth lens surface and an accurately controlled lens profile have been obtained. Microlenses of 130 μm diameter and 200 μm focal length have been used to collimate the outputs of GaInAsP/InP and GaAs/GaAlAs diode lasers and have yielded a nearly diffraction-limited beam divergence of 0.68°.
Dependence of optimal growth temperature on carbon content in hydrogenated amorphous silicon carbide alloys
Authors: Winer K.
Published: July 2002

The temperature (TE) above which optimal growth takes place during plasma-enhanced chemical vapor deposition of undoped hydrogenated amorphous silicon carbide alloys from methane/silane gas mixtures is shown to increase with the relative gas-phase mole fraction of methane XCH4. The increase in TE from ≊220 °C for XCH4=0.0 to ≊350 °C for XCH4=0.90 at 2 W rf power is attributed to a corresponding decrease in the rate of hydrogen diffusion with carbon incorporation.
Enhanced hot-electron photoluminescence from heavily carbon-doped GaAs
Authors: Aitchison B. J. , Haegel N. M. , Abernathy C. R. , Pearton S. J.
Published: July 2002

An enhancement of hot-electron photoluminescence due to degenerate conditions in the valence band has been observed in metalorganic molecular beam epitaxial grown GaAs:C with net acceptor concentration of up to 4×1020 cm−3. The photoluminescence (PL) was studied as a function of free-carrier concentration and sample temperature. Comparison of the PL spectra from the heavily doped GaAs to that of undoped material shows a peak shift to lower energy coupled with a greatly enhanced high-energy tail extending into the visible region of the spectrum. At 300 K, luminescence at 1.8 eV is observed at 20% the intensity of the peak luminescence at 1.36 eV.
Selective heteroepitaxial growth of Si1−xGex using gas source molecular beam epitaxy
Authors: Hirayama Hiroyuki , Hiroi Masayuki , Koyama Kazuhisa , Tatsumi Toru
Published: July 2002

Si1−xGex heteroepitaxial layers have been grown on Si(100) surfaces by gas source molecular beam epitaxy. Si2H6 and GeH4 were used as the Si and Ge source gases, respectively. The Ge mole fraction x in the grown film was found to be controlled by the GeH4 flow rate. The growth rate decreased gradually with increasing GeH4 flow rate. Selective epitaxial growth of Si1−xGex using a SiO2 mark on a Si(100) substrate was successfully achieved.
Formation and Schottky barrier height of metal contacts to β-SiC
Authors: Waldrop J. R. , Grant R. W.
Published: July 2002

Formation of Schottky barrier contacts to n-type β-SiC(100) was systematically investigated for several metals with various physical and chemical properties. The metals (Pd, Au, Co, Ti, Ag, Tb, and Al) were deposited onto oxygen terminated (∼1 monolayer) surfaces. Metal/β-SiC interface chemistry and Schottky barrier height φB during contact formation were obtained by x-ray photoemission spectroscopy; the corresponding electrical properties of thick contacts were characterized by capacitance-voltage and current-voltage methods. The metal/β-SiC interface is unreactive at room temperature. X-ray photoemission spectroscopy and electrical measurements demonstrate that these metal contacts exhibit a wide range of φB , 0.95–0.16 eV; within this range an individual contact φB value depends strongly on the metal work function in general accord with the Schottky–Mott limit.
Fabrication of IrSi3/p-Si Schottky diodes by a molecular beam epitaxy technique
Authors: Lin T. L. , Iannelli J. M.
Published: July 2002

IrSi3/p-Si Schottky diodes have been fabricated by a molecular beam epitaxy technique at 630 °C. Good surface morphology was observed for IrSi3 layers grown at temperatures below 680 °C, and an increasing tendency to form islands is observed in samples grown at higher temperatures. Good diode current-voltage characteristics were observed and Schottky barrier heights of 0.14–0.18 eV were determined by activation energy analysis and spectral response measurement.
Butt-coupled InGaAs metal-semiconductor-metal waveguide photodetector formed by selective area regrowth
Authors: Soole J. B. D. , Schumacher H. , LeBlanc H. P. , Bhat R. , Koza M. A.
Published: July 2002

We report the monolithic integration of an InGaAs metal-semiconductor-metal Schottky barrier photodetector with a butt-coupled InP/InGaAsP/InP waveguide, where the latter is formed by selective area regrowth using organometallic chemical vapor deposition. A fast pulse response (60–65 ps full width at half maximum, with no discernible tail) and a high quantum efficiency (∼80%) were observed for 1.3 μm guided light with the detector biased at 7 V. A thin InAlAs Schottky barrier enhancement layer limited the dark current to ∼28 nA at this bias.
Measurements of the three-dimensional impurity profile in Si using chemical etching and scanning tunneling microscopy
Authors: Takigami Takako , Tanimoto Masafumi
Published: July 2002

Three-dimensional boron impurity concentration profiles in silicon substrates have been measured with 10 nm resolution by scanning tunneling microscopy (STM) on chemically etched cleaved silicon surfaces using an impurity sensitive HF-HNO3-H2O solution. Comparisons to depth profiles obtained with secondary-ion mass spectroscopy and spreading resistance methods reveal that our proposed method measures activated boron impurity concentration rather than total boron concentration. Three-dimensional impurity profiling is demonstrated with a metal-oxide-silicon structure, and the lateral junction depth in the test structure is found to be 70% of the junction depth.
Nanometer-scale features produced by electric-field emission
Authors: McBride S. E. , Wetsel G. C.
Published: July 2002

Nanometer-scale features have been formed in air on metallic surfaces with various tip/sample material combinations (W/Au, Au/Au, Au/Pt, and W/Pt) using a scanning tunneling microscope (STM) instrument. The instrument has sufficient stability, computer control, and in-process measurement capability to record important processing signals while creating nanoscale patterns. The sample surface can be quantitatively characterized using both tunneling spectroscopy and imaging before and after pulsing the tip-sample voltage (Vt). Images show that the form of the created features ranges from craters to mounds when Vt exceeds a threshold value, Vc, which is dependent on the tunneling resistance (R). Results of measurement of Vc vs R combined with results of the measurement of tip displacement versus Vt allow the determination of the threshold electric field (Ec). For the W tip and the Pt sample, Ec has been determined to be 0.23 V/Å. Electric fields of this magnitude are sufficient to remove atoms by high-field emission.
Low energy ion-assisted deposition of titanium nitride ohmic contacts on alpha (6H)-silicon carbide
Authors: Glass R. C. , Spellman L. M. , Davis R. F.
Published: July 2002

Low energy (100 eV) ion-assisted reactive evaporation was used to deposit titanium nitride (TiN) onto single crystal α(6H)-silicon carbide (SiC) wafers to investigate the potential of the former for electrical contacts. Theoretical considerations indicate that TiN (work function of 3.74 eV) should form an ohmic contact with SiC [work function of 4.8 eV for the (0001) face] provided an oxide-free interface can be obtained. Activated nitrogen was used to clean the SiC surface prior to deposition, while Auger spectroscopy and current-voltage (I-V) measurements were used to assess oxygen concentration at the interface and contact character, respectively. The contacts were ohmic after deposition. Little change was observed after annealing at 450 and 550 °C for 15 min.
Robust superconducting tapes based on Bi2Sr2Ca2Cu3O10−δ
Authors: Li Qiang , Ostenson J. E. , Fang M. M. , Finnemore D. K.
Published: July 2002

A rugged conductor based on the Bi2Sr2Ca2Cu3O10−δ has been developed by dispersing Ag throughout the matrix on a micrometer scale. The Ag seems to effectively blunt crack tips and provides regions for plastic flow. The performance degrades relatively little with the strains of handling, soldering and quenching to 77 K. Tapes having a critical current density of 4,500 A/cm2 at 77 K and zero field will show over 20,000 A/cm2 at 4.2 K and zero field. At 20 T for the field perpendicular to the tape surface, these conductors show 2,800 A/cm2 at 4.2 K and 2,200 A/cm2 at 20 K. The critical current density falls roughly linearly with strain and decreases from about 22,000 A/cm2 at zero strain to about 10,000 A/cm2 at 1.0% bending strain.
Detection of the EL2 metastability by x-ray rocking-curve measurements at low temperatures
Authors: Kowalski G. , Leszczynski M.
Published: July 2002

X-ray experiments are usually seen as a tool for studying large lattice imperfections. We present for the first time experimental evidence obtained from x-ray measurements that metastable behavior of the EL2 defect in semi-insulating GaAs can be connected with crystal lattice relaxation. New unexpected results showing change of the lattice state while cooling semi-insulating GaAs samples in darkness are also reported.
Effect of oxygen partial pressure on the in situ growth of Y-Ba-Cu-O thin films on SrTiO3
Authors: Zheng J. Q. , Shih M. C. , Williams S. , Lee S. J. , Kajiyama Hiroshi , Wang X. K. , Zhao Z. , Viani K. , Jacobson S. , Dutta P. , Chang R. P. H. , Ketterson J. B. , Roberts T. , Kampwirth R. T. , Gray K. E.
Published: July 2002

The evolution of YBCO film growth with thickness at various oxygen pressures was observed by in situ synchrotron x-ray diffraction in real time. When the films were deposited at 2 Å/s and 730 °C under higher oxygen partial pressures (in an Ar/O2 mixture of 90 mTorr), the nucleation was observed to have c-axis orientation. After the films reached a critical thickness, the growth of the YBCO film changed from c axis to a axis and then propagated epitaxially. This provides evidence that a-axis epitaxial growth nucleates on a c-axis base. The critical thickness reflects the competition between the growth of the c and a axes, which is determined by the oxygen partial pressure in the process of thin-film formation. The a-axis oriented films showed a very sharp rocking curve (less than 0.1°) which indicates a very high structural quality. For very low oxygen partial pressures, the in situ growth process was very similar, but the initial nuclei involve a second phase mixed with a small amount of c-axis ‘‘123’’ phase. The nucleation and growth mechanisms of in situ YBCO films are discussed.
Time-dependent positive charge generation in very thin silicon oxide dielectrics
Authors: Farmer K. R. , Andersson M. O. , Engström O.
Published: July 2002

We have studied the rate at which positive charge is generated starting near the oxide-silicon interface when electrons are injected from the gate through the very thin oxide layer in metal-oxide-(p)silicon tunnel diodes. By varying the oxide thickness, we find that the charging rate is not strongly controlled by the flux of tunneling electrons over a five order of magnitude range in current density. This implies that if the tunneling electrons do participate, then the charge generation in these oxides is at least a two-step process. A comparison of charge generation in aluminum and polycrystalline silicon gate devices suggests that the process does not involve aluminum-related defects. Measurements of the charging rate versus temperature, T, show that it is weakly dependent on T below 150–200 K and apparently thermally activated above this temperature range.
Doping concentration dependence of radiance and optical modulation bandwidth in carbon-doped Ga0.51In0.49P/GaAs light-emitting diodes grown by gas source molecular beam epitaxy
Authors: de Lyon T. J. , Woodall J. M. , McInturff D. T. , Bates R. J. S. , Kash J. A. , Kirchner P. D. , Cardone F.
Published: July 2002

Double-heterostructure light-emitting diodes (LEDs) consisting of an n-Ga0.51In0.49P emitter, a carbon-doped p-GaAs active layer, and a p-Al0.30Ga0.70As cladding layer have been grown by gas source molecular beam epitaxy with halomethane carbon doping sources. CCl4 and CHCl3 have been used to vary the active layer C doping level from 1018 to 1020 cm−3. Measurements of LED optical modulation bandwidth indicate that the bandwidth increases with C doping, attaining a record value of 1.6 GHz at 1020 cm−3 C concentration. The LED radiance is observed to decline significantly in the 1019–1020 cm−3 range.
Strain induced optical waveguides in lithium niobate, lithium tantalate, and barium titanate
Authors: Eknoyan O. , Taylor H. F. , Tang Z. , Swenson V. P. , Marx J. M.
Published: July 2002

Channel waveguides have been produced in LiNbO3, LiTaO3, and BaTiO3 ferroelectric crystals by depositing thick SiO2 films at an elevated temperature and patterning them by reactive ion etching. The static strain resulting from the large thermal expansion mismatch between the substrate and film causes a localized increase in the refractive index via the strain-optic effect. In addition, an electro-optic contribution to the index increase is believed to result from a surface charge distribution which compensates the electric field due to the piezoelectric effect. Single-mode waveguides at a wavelength of 0.633 μm for both polarizations have been produced in x-cut LiNbO3, with losses of 0.8 dB/cm for TE polarization and 0.9 dB/cm for TM polarization. The 11-μm-wide channel waveguides were formed by a z-axis strain induced by a 2.8-μm-thick SiO2 film deposited at 300 °C. Electro-optic modulation was also demonstrated in these waveguides. Guiding for both polarizations was also observed in LiTaO3 and BaTiO3 channel waveguides at 0.83 μm wavelength.
High frequency characteristics of in-plane-gate transistors
Authors: McLean J. S. , Wieck A. D. , Bleder M. , Ploog K.
Published: July 2002

Directly written in-plane-gate (IPG) field effect transistors fabricated using focused ion beams have been proposed and measurements of their dc characteristics have been made. Here, we present for the first time ac measurements up to high frequencies on these transistors. The measurements are made using a direct wafer probing technique. While parasitic elements adversely affect the high frequency operation of the current generation of the IPG transistor, promising results have been obtained. We measure a transconductance of 20 μS which is essentially independent of frequency up to 1 GHz.
Development of near-bamboo and bamboo microstructures in thin-film strips
Authors: Walton D. T. , Frost H. J. , Thompson C. V.
Published: July 2002

A two-dimensional grain growth model is used to study microstructural evolution in thin film strips. We focus on the strip’s transformation to the bamboo structure, in which individual grains transverse the width of the strip. We find that the approach to a fully bamboo structure is exponential, and that the rate of transformation is inversely proportional to the square of the strip width. When the simulation is extended to model grain boundary pinning due to grooving at grain boundary−free surface intersections, we find that there exists a maximum strip width to thickness ratio beyond which the transformation to the bamboo structure does not proceed to completion.
Observation of the fractional quantum Hall effect in Si/SiGe heterostructures
Authors: Nelson S. F. , Ismail K. , Nocera J. J. , Fang F. F. , Mendez E. E. , Chu J. O. , Meyerson B. S.
Published: July 2002

We have observed the fractional quantum Hall effect in the magnetotransport properties of a two-dimensional electron gas in an n-type Si/SiGe heterostructure. The effect was observed for filling factors ν=2/3 and ν=4/3 in samples whose mobilities at 1.4 K ranged from 37 000 to 85 000 cm2/V s.
Twofold efficiency improvement in high performance AlGaInP light-emitting diodes in the 555–620 nm spectral region using a thick GaP window layer
Authors: Huang K. H. , Yu J. G. , Kuo C. P. , Fletcher R. M. , Osentowski T. D. , Stinson L. J. , Craford M. G. , Liao A. S. H.
Published: July 2002

AlGaInP light-emitting diodes (LEDs) with external quantum efficiencies ≥6% and luminous performance of 20 lm/W have been fabricated. These LEDs are twice as efficient as previously reported AlGaInP devices throughout the spectral region from green (555 nm) to red-orange (620 nm) owing to a thicker GaP window layer (45 vs 15 μm). Using hydride vapor phase epitaxy, thick GaP window layers were grown on top of AlGaInP double heterostructures grown by organometallic vapor phase epitaxy. The efficiency of the LEDs was found to improve as the thickness of the window layer was increased from 9 to 63 μm. This improvement is predicted by a simple model that considers the benefit of enhanced emission through the sides of the thick window. The effect of emission wavelength on quantum efficiency and luminous performance for AlGaInP LEDs with a 45 μm thick window has been studied.
Spectral blue shift of photoluminescence in strained-layer Si1−xGex/Si quantum well structures grown by gas-source Si molecular beam epitaxy
Authors: Fukatsu S. , Yoshida H. , Fujiwara A. , Takahashi Y. , Shiraki Y. , Ito R.
Published: July 2002

Spectral blue shift of excitonic transition due to quantum confinement in Si0.84Ge0.16/Si strained-layer quantum wells, grown by gas-source Si molecular beam epitaxy, is reported. Intense photoluminescence was observed at elevated temperatures though reduced in intensity due to the thermalization of holes to the Si barriers.