Anisotropic thermal conductivity of AlGaN/GaN superlattices.

High thermal conductivity is an important parameter for nitride-based power electronic and deep-UV light emitters. Especially in the latter case short period superlattices and multicomponent alloys are used and the knowledge of the thermal properties of the binary compounds is sufficient. In-plane and cross-plane thermal conductivity of AlGaN/GaN superlattices were measured by differential two-wire 3ω method in the temperature range from 147 to 325 K. Samples were grown by metalorganic vapor phase epitaxy; the structure quality and accuracy of superlattice structures preparation were verified by means of HRXRD and transmission electron microscopy. It was observed, that value of thermal conductivities decrease with decreasing period thickness, while temperature dependencies differ from each other-in-plane thermal conductivity decreases, and cross-plane-increases with increasing temperature. Callaway method was used for thermal conductivity calculation; dependence of boundary scattering rate on the phonon wavelength was taken into account. Minimum thermal conductivity was added to calculated values to include the influence of high frequency acoustic phonons and optical phonons on the heat transport. Calculations are in good agreement with experimental results.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods.

We studied the emission of bare and aluminum quinoline (Alq3)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 µm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq3 coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq3 spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq3 spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Direct observation of correlations between individual photon emission events of a microcavity laser.

Lasers are recognized for coherent light emission, the onset of which is reflected in a change in the photon statistics. For many years, attempts have been made to directly measure correlations in the individual photon emission events of semiconductor lasers. Previously, the temporal decay of these correlations below or at the lasing threshold was considerably faster than could be measured with the time resolution provided by the Hanbury Brown/Twiss measurement set-up used. Here we demonstrate a measurement technique using a streak camera that overcomes this limitation and provides a record of the arrival times of individual photons. This allows us to investigate the dynamical evolution of correlations between the individual photon emission events. We apply our studies to micropillar lasers with semiconductor quantum dots as the active material, operating in the regime of cavity quantum electrodynamics. For laser resonators with a low cavity quality factor, Q, a smooth transition from photon bunching to uncorrelated emission with increasing pumping is observed; for high-Q resonators, we see a non-monotonic dependence around the threshold where quantum light emission can occur. We identify regimes of dynamical anti-bunching of photons in agreement with the predictions of a microscopic theory that includes semiconductor-specific effects.

A structural investigation of highly ordered catalyst- and mask-free GaN nanorods.

GaN nanorods were grown on r-plane sapphire substrates by a two-step approach. Nucleation sites for the nanorods were provided by the formation of AlN islands during nitridation in a metal organic vapor phase system. These islands are a-plane oriented as expected for nitride growth on r-plane sapphire. The nanorods themselves were grown by plasma assisted molecular beam epitaxy. The nanorods show an inclination towards the surface normal of 28.3° and are highly ordered. Studies with high resolution x-ray diffraction polar plots reveal the epitaxial relationship between the substrate and nanorods as a c-direction growth on inclined m-plane facets of the nitridated islands. The determined lattice constants show nanorods which are strain free. The growth direction of the nanorods has been confirmed in a transmission electron microscope by convergent beam electron diffraction patterns to be in the N-polar [Formula: see text] direction.

Light-emitting diode based on mask- and catalyst-free grown N-polar GaN nanorods.

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.

Electroluminescence from a single InGaN quantum dot in the green spectral region up to 150 K.

We present electrically driven luminescence from single InGaN quantum dots embedded into a light emitting diode structure grown by metal-organic vapor-phase epitaxy. Single sharp emission lines in the green spectral region can be identified. Temperature dependent measurements demonstrate thermal stability of the emission of a single quantum dot up to 150 K. These results are an important step towards applications like electrically driven single-photon emitters, which are a basis for applications incorporating plastic optical fibers as well as for modern concepts of free space quantum cryptography.

Local structure of uncapped and capped InGaN/GaN quantum dots.

The local structure around the indium atoms in uncapped and capped In(x)Ga(1-x)N quantum dots has been studied by In K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The samples were grown by metal organic vapour phase epitaxy. The EXAFS was successfully applied to study the structural properties of buried quantum dots which are not optically active. The analysis revealed that capping the quantum dots with GaN does not affect the bond distances of the In-N and In-Ga, but makes the In-In distance shorter by 0.04 A.

A CdSe quantum dot based resonant cavity light-emitting diode showing single line emission up to 90 K.

A II-VI wide-bandgap resonant cavity light-emitting diode is presented. The active region consists of CdSe quantum dots embedded in ZnSSe/MgS barriers, resulting in improved quantum efficiency at elevated temperatures. The resonant cavity is formed by a 14-period bottom distributed Bragg reflector and the semiconductor to air interface on top of the structure. Temperature dependent micro-electroluminescence measurements reveal emission of a single quantum dot up to 90 K. The turn-on voltages are 6 V at 4 K and 4 V at room temperature. These results are promising for the realization of green surface-emitting devices in general, and especially for an electrically driven prospective single photon source operating at room temperature.

Highly ordered catalyst-free and mask-free GaN nanorods on r-plane sapphire.

Self-organized and highly ordered GaN nanorods were grown without catalyst on r-plane sapphire using a combination of molecular beam epitaxy and metal-organic vapor-phase epitaxy. AlN nucleation centers for the nanorods were prepared by nitridation of the sapphire in a metal-organic vapor-phase epitaxy reactor, while the nanorods were grown by molecular beam epitaxy. A coalesced two-dimensional GaN layer was observed between the nanorods. The nanorods are inclined by 62 degrees towards the [Formula: see text]-directions of the a-plane GaN layer. The high degree of ordering and the structural perfection were confirmed by micro-photoluminescence measurements.

Lonomia caterpillar envenoming in French Guiana reversed by the Brazilian antivenom: A successful case of international cooperation for a rare but deadly tropical hazard.

In the American continent, larval forms (caterpillars) of the Lonomia genus can cause systemic reactions in human beings. In this Paper, we report the third case of Lonomia envenoming recorded in French Guiana in 25 years, and the first in which specific antivenom was administered. Severe symptoms of the envenoming were observed in our patient including pain; coagulopathy and systemic hemorrhage. They are caused by skin contact with caterpillars. Recovery, however, was quite satisfactory thanks to the international cooperation of the health authorities in both France and Brazil.

Fermi level and bands offsets determination in insulating (Ga,Mn)N/GaN structures.

The Fermi level position in (Ga,Mn)N has been determined from the period-analysis of GaN-related Franz-Keldysh oscillation obtained by contactless electroreflectance in a series of carefully prepared by molecular beam epitaxy GaN/Ga1-xMnxN/GaN(template) bilayers of various Mn concentration x. It is shown that the Fermi level in (Ga,Mn)N is strongly pinned in the middle of the band gap and the thickness of the depletion layer is negligibly small. For x > 0.1% the Fermi level is located about 1.25-1.55 eV above the valence band, that is very close to, but visibly below the Mn-related Mn2+/Mn3+ impurity band. The accumulated data allows us to estimate the Mn-related band offsets at the (Ga,Mn)N/GaN interface. It is found that most of the band gap change in (Ga,Mn)N takes place in the valence band on the absolute scale and amounts to -0.028 ± 0.008 eV/% Mn. The strong Fermi level pinning in the middle of the band gap, no carrier conductivity within the Mn-related impurity band, and a good homogeneity enable a novel functionality of (Ga,Mn)N as a semi-insulating buffer layers for applications in GaN-based heterostuctures.

Comparison of magneto-optical properties of various excitonic complexes in CdTe and CdSe self-assembled quantum dots.

We present a comparative study of two self-assembled quantum dot (QD) systems based on II-VI compounds: CdTe/ZnTe and CdSe/ZnSe. Using magneto-optical techniques we investigated a large population of individual QDs. The systematic photoluminescence studies of emission lines related to the recombination of neutral exciton X, biexciton XX, and singly charged excitons (X(+), X(-)) allowed us to determine average parameters describing CdTe QDs (CdSe QDs): X-XX transition energy difference 12 meV (24 meV); fine-structure splitting δ1=0.14 meV (δ1=0.47 meV); g-factor g = 2.12 (g = 1.71); diamagnetic shift γ=2.5 µeV T(-2) (γ =1.3 µeV T(-2)). We find also statistically significant correlations between various parameters describing internal structure of excitonic complexes.

AIDS-related non-Hodgkin's lymphoma: the outcome and efficacy of radiation therapy.

The records of all 16 patients with AIDS-related lymphoma treated with radiation therapy at our institutions were reviewed. All patients were male with a median age of 32 years, and all but one had biopsy proven high-grade NHL. Eleven had lymphoma involving the central nervous system and five had lymphoma involving other sites. Seven of the 11 patients with CNS involvement had primary CNS lymphoma. All patients were treated with megavoltage X rays to doses ranging from 1050 cGy in 1 1/2 weeks to 5037 cGy in 6 weeks. Of those patients with CNS lymphoma, only one responded completely and four responded partially to irradiation. All patients died within a range of 0.2 to 5.3 months (median survival = 2.2 months) from starting radiation therapy. In contrast, 3 of 5 patients (60%) with NHL outside the CNS responded completely and 1 responded partially to involved-field irradiation. These patients survived a median of 12.6 months with one achieving long-term lymphoma-free survival at 40 months. This long-term survivor presented with Stage IE lymphoma as his only manifestation of AIDS. We conclude that AIDS-related lymphomas respond less favorably to radiation therapy than lymphomas in non-immunosuppressed patients. Furthermore, CNS lymphomatous involvement is an ominous occurrence in the AIDS patient. In our experience, cranial irradiation failed to provide significant palliation or survival prolongation in this group of patients. Instead, long-term survival is possible in AIDS patients with limited NHL outside the CNS, and it is in these patients that combination chemotherapy plus involved-field radiation therapy may play a curative role.

Plasmodium falciparum parasites in French Guiana: limited genetic diversity and high selfing rate.

The genetic characteristics of Plasmodium falciparum isolates collected in French Guiana, where malaria transmission is low and occurs in isolated foci, were studied. Blood samples were collected from 142 patients with symptomatic malaria and typed using a polymerase chain reaction-based strategy for merozoite surface protein-(MSP-1) block 2, the MSP-2 central domain, and glutamate-rich protein (GLURP) repeat domain polymorphism. This showed that the parasite population circulating in French Guiana presented a limited number of allelic forms (4, 2, and 3 for MSP-1 block 2, MSP-1, and GLURP, respectively) and a small number of mixed infections, contrasting with the large genetic diversity of parasite populations and infection complexity reported for Africa, Asia, and other parts of South America. Two groups of isolates displaying identical 3 loci allele combinations were further studied for the Pf332 antigen, histidine-rich protein-1, thrombospondin-related anonymous protein, and Pf60 multigene family polymorphism. Within each group, most isolates were identical for all markers tested. This suggests a high rate of self-fertilization of P. falciparum parasites in French Guiana, resulting in homogenization of the population. The implications of these findings for malaria control in areas of low endemicity are discussed.

Tonate virus infection in French Guiana: clinical aspects and seroepidemiologic study.

Two recent cases of human infection with Tonate virus, one of which was a fatal case of encephalitis, have renewed interest in these viruses in French Guiana. The clinical aspects of confirmed and probable cases of infection with this virus indicate that it has pathogenic properties in humans similar to those of other viruses of the Venezuelan equine encephalitis complex. To determine the prevalence of antibodies to Tonate virus in the various ethnic groups and areas of French Guiana, 3,516 human sera were tested with a hemagglutination inhibition test. Of these, 11.9% were positive for the virus, but significant differences in seroprevalence were found by age, with an increase with age. After adjustment for age, significant differences were found between places of residence. The prevalence of antibody to Tonate virus was higher in savannah areas, especially in the Bas Maroni (odds ratio [OR] = 22.2, 95% confidence interval [CI] = 15.2-32.4) and Bas Oyapock areas (OR = 13.4; 95% CI = 9.8-18.4). The ethnic differences observed in this study were due mainly to differences in place of residence, except that whites were significantly less frequently infected than other ethnic groups. This study indicates that Tonate virus infection is highly prevalent in French Guiana, especially in savannah areas.

Response to radiotherapy of head and neck tumors in AIDS patients.

Initial manifestations of AIDS in the head and neck area occur frequently. In fact, up to 40 percent of patients may have involvement of the head and neck. The most common malignancies are Kaposi's sarcoma and non-Hodgkin's lymphoma. Since AIDS-related malignancies are a relatively new problem for radiation oncologists, optimal therapy for these neoplasms is unknown. A retrospective review of AIDS patients treated with radiotherapy has been performed. Fourteen patients were identified. Of these, five were treated for head and neck tumors (four for Kaposi's sarcoma and one for non-Hodgkin's lymphoma). Epidemic Kaposi's sarcoma, as well as non-Hodgkin's lymphoma, were seen to be as radioresponsive as the classical forms, but local control was difficult to achieve. Kaposi's sarcoma tended to recur marginally and within the field. Nonetheless, we believe radiotherapy can offer significant palliation for AIDS patients with head and neck tumors. It is of utmost importance that the head and neck surgeon must be acutely aware of the common patterns of presentation of this disease.

[Re-emergence of Venezuelan equine encephalitis virus in French Guiana. Apropos of 1 confirmed case]. / Ré-émergence d'un virus du complexe de l'encéphalomyélite équine vénézuélienne en Guyane française. A propos d'un cas confirmé.

Venezuelan equine encephalitis (VEE) is a mosquito-borne viral disease that occurs in equine species and in man. The strains can be grouped epidemiologically into two major categories: enzootic and epizootic. Enzootic strains cause sporadic human disease and are not associated with disease among equines. These strains are found throughout Florida. Central America, northern South America and Brazil. Epizootic strains are associated with enormous morbidity and mortality in equine species. In man, VEE virus infections are largely asymptomatic and in children and young adults there is an increased risk of encephalitis and dead. We report the first case in French Guiana of Venezuelan equine encephalitis. Clinical examination and biological studies showed encephalitis, interstitial pneumonia and acute liver failure. Despite an adequate symptomatic treatment, the young patient died five days after her admission in multiple organ dysfunction syndrome. Diagnosis is establishing by virologic test: VEE virus is isolated from the blood. These example of re-emerging infectious disease vividly illustrate that we remains vulnerable and emphasizes the need for an active surveillance system.

[Staphylococcus aureus septicemia producing Panton-Valentine leukocidin. 3 cases]. / Septicémies a Staphylococcus aureus producteurs de leucocidine de Panton-Valentine.

BACKGROUND: A strong association has been observed between furuncles and Panton-Valentine leukocidin-producing Staphylococcus aureus. CASE REPORTS: Within one year, we cared for three men at the Cayenne hospital who had Staphylococcus aureus septicemia with severe pleuropulmonary involvement originating from furuncular lesions. The Staphylococcus aureus strains isolated from the skin lesions and from blood cultures produced Panton-Valentine leukocidin. CONCLUSION: These cases demonstrate the gravity of S. aureus septicemia in young patients with furunculosis. These cases are the first reported with severe S. aureus infections associated with Panton-Valentine leukocidin producing strains.

[Plasma exchange and severe Plasmodium falciparum malaria. Apropos of 2 cases]. / Echange plasmatique et paludisme grave à Plasmodium falciparum. A propos de deux cas.

Severe Falciparum malaria is associated with multiple organ dysfunction and a high rate of fatal outcome. Appropriate antimalarial chemotherapy and symptomatic treatment may be supplemented by early plasma exchange. Two cases are reported in which there were no chemoprophylaxis and a late diagnosis. Initial parasitaemias were 17% and 5%. The two patients had cerebral malaria with in the first case pulmonary oedema and DIC. Plasma exchange was performed and clinical biological symptoms abated quickly. The mechanisms of action and benefits of plasma exchange are discussed.

[Probable effect of hemofiltration on hemodynamics and tissue oxygenation in shock secondary to severe malaria]. / Rôle probable de l'hémofiltration sur l'hémodynamique et l'oxygénation tissulaire lors d'un état de choc au décours d'un paludisme grave.

The authors report a case of severe Plasmodium falciparum malaria in a French Guyana endemic area with secondary myocardial dysfunction treated by usual symptomatic therapy and continuous veno-venous haemofiltration (CVVH). Haemodynamic investigations revealed hyperkinetic shock and oxygen supply dependence. Haemodynamic remained critical under conventional therapy and CVVH was introduced. Haemodynamic parameters improved rapidly with reduced oxygen debt. In the absence of associated bacterial, viral and fungal infections, the systemic inflammatory response syndrome with shock and impairment of consciousness seems to be linked to severe Plasmodium falciparum malaria. The benefits and the probable mechanisms of action of CVVH are discussed.