Fabrication and Optical Properties of a Fully Hybrid Epitaxial ZnO-Based Microcavity in the Strong Coupling Regime

In order to achieve polariton lasing at room temperature, a new fabrication methodology for planar microcavities is proposed: a ZnO-based microcavity in which the active region is epitaxially grown on an AlGaN/AlN/Si substrate and in which two dielectric mirrors are used. This approach allows as to simultaneously obtain a high-quality active layer together with a high photonic confinement as demonstrated through macro-, and micro-photoluminescence ({\mu}-PL) and reflectivity experiments. A quality factor of 675 and a maximum PL emission at k=0 are evidenced thanks to {\mu}-PL, revealing an efficient polaritonic relaxation even at low excitation power.Comment: 12 pages, 3 figure

Valence band offset of the ZnO/AlN heterojunction determined by X-ray photoemission spectroscopy

The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values

On the quantum and classical scattering times due to charged dislocations in an impure electron gas

We derive the ratio of transport and single particle relaxation times in three and two - dimensional electron gases due to scattering from charged dislocations in semiconductors. The results are compared to the respective relaxation times due to randomly placed charged impurities. We find that the ratio is larger than the case of ionized impurity scattering in both three and two-dimensional electron transport.Comment: 4 pages, 3 figure

ESTUDIO CITOLÓGICO, EXO Y ENDOMORFOLÓGICO EN ATRIPLEX LAMPA (MOQ.) D. DIETR. (CHENOPODIACEAE)

Atriplex lampa (Moq.) D. Dietr. is a halophytic, dioecious shrub native to Patagonia,Argentina. Although relatively neglected byscience, it is important for sand-stabilizationand for biomass and fodder production dueto its nutritional characteristics and acceptable percentage of gross protein.The objective of this study is to provide cytological and morpho-anatomical informationabout A. lampa, which could be useful whenapplied in recovery and repopulation plansintending to use this species in areas degraded by petroleum industry activity.Natural populations in the Province of Chubut (45°-46° S; 67°- 68° W), Argentina, werestudied. Karyotypes and meiotic behaviorwere analyzed according to conventionalprotocols, as were anatomical and morphological comparisons. Analysis of variancewas used to test morphometrical differencesbetween populations and cytotypes. Voucherspecimens are deposited at the Facultad deCiencias Naturales, Universidad NacionalSan Juan Bosco de Patagonia, Argentina.Our results show that A. lampa has two cytotypes: diploids (2n = 2x = 18) and tetraploids(2n = 4x = 36). The stems and roots haveanomalous secondary growth, and the leavesare amphistomatic, with a Kranz structure,and are covered with dense glandular trichomes. The leaves and fruit bracteoles aredifferent sizes in the two cytotypes.Atriplex lampa (Moq.) D. Dietr. es una especie que habita en ambientes áridos y suelossalinos, produce considerable cantidad debiomasa, aportando forraje de buena calidaden épocas de receso invernal. En Argentina, Provincia de Chubut, Departamento deEscalante, se observaron dos poblacionesnaturales fenotípicamente diferentes.El objetivo de este trabajo fue estudiarlascitológica, exo y endomorfológicamente afin de comprobar si las variantes fenotípicas responden a variaciones en el númerosomático.Por primera vez se presenta el análisiscariotípico y meiótico de la especie.Hasta el momento la especie era descritacomo diploide, 2 n = 2 x = 18, en este estudiose encontró un citotipo tetraploide 2n = 4x =36. Las diferencias entre ambos citotipos sebasan en el número somático, la morfologíacromosómica, el comportamiento meiótico,el porte arbustivo y el tamaño de hojas ybractéolas fructíferas.La fórmula cariotípica, longitud genómicatotal y rango de variación en longitud cromosómica no se alejan de los valores observados para otras especies del género.Durante la meiosis se observa formaciónde tetravalentes, evidenciando alta homología entre los genomas lo que justifica suorigen autopoliploide, que aún no alcanzóla diploidización. La formación de puentesy fragmentos durante anafase I sugiere laexistencia de dos pares de cromosomashomólogos, heterocigóticos estructurales,para inversiones paracéntricas. La presenciade puentes en la segunda división indicarecombinación meiótica en un amplio segmento invertido.Las observaciones y mediciones exomorfológicas dan resultados significativos para eltetraploide. Los caracters endomorfológicosse mantienen constantes en ambos niveles deploidía al igual que el espesor de la hoja

Atomic resolution interface structure and vertical current injection in highly uniform MoS2MoS_{2} heterojunctions with bulk GaN

The integration of two-dimensional $MoS_{2}$ with $GaN$ recently attracted significant interest for future electronic/optoelectronic applications. However, the reported studies have been mainly carried out using heteroepitaxial $GaN$ templates on sapphire substrates, whereas the growth of $MoS_{2}$ on low-dislocation-density bulk GaN can be strategic for the realization of truly vertical devices. In this paper, we report the growth of ultrathin $MoS_{2}$ films, mostly composed by single-layers ($1L$), onto homoepitaxial $n-GaN$ on $n^{+}$ bulk substrates by sulfurization of a pre-deposited $MoO_{x}$ film. Highly uniform and conformal coverage of the $GaN$ surface was demonstrated by atomic force microscopy, while very low tensile strain (0.05%) and a significant $p^{+}$-type doping ($4.5 \times 10^{12} cm^{-2}$) of $1L-MoS_{2}$ was evaluated by Raman mapping. Atomic resolution structural and compositional analyses by aberration-corrected electron microscopy revealed a nearly-ideal van der Waals interface between $MoS_{2}$ and the $Ga$-terminated $GaN$ crystal, where only the topmost $Ga$ atoms are affected by oxidation. Furthermore, the relevant lattice parameters of the $MoS_{2}/GaN$ heterojunction, such as the van der Waals gap, were measured with high precision. Finally, the vertical current injection across this 2D/3D heterojunction has been investigated by nanoscale current-voltage analyses performed by conductive atomic force microscopy, showing a rectifying behavior with an average turn-on voltage $V_{on}=1.7 V$ under forward bias, consistent with the expected band alignment at the interface between $p^{+}$ doped $1L-MoS_{2}$ and $n-GaN$.Comment: 21 pages, 6 figure

Western European Populations of the Ichneumonid Wasp Hyposoter didymator Belong to a Single Taxon

Hyposoter didymator (Hymenoptera, Ichneumonidae) is a generalist solitary endoparasitoid of noctuid larvae. In the present work, we tested whether populations of H. didymator were divided in several genetically distinct taxa as described for many other generalist parasitoid species, and whether differences in H. didymator parasitism rates were explained by the insect host species and/or by the plant on which these hosts were feeding on. The genetic analysis of natural populations collected in different regions in France and Spain on seven different insect hosts and seven different host plants (775 individuals) showed that H. didymator populations belong to a unique single taxon. However, H. didymator seems to be somewhat specialized. Indeed, in the fields it more often parasitized Helicoverpa armigera compared to the other host species collected in the present work. Also, H. didymator parasitism rates in field conditions and semi-field experimental studies were dependent on the host plants on which H. armigera larvae are feeding. Still, H. didymator can occur occasionally on non-preferred noctuid species. One hypothesis explaining the ability of H. didymator to switch hosts in natura could be related to fluctuating densities of the preferred host over the year; this strategy would allow the parasitoid to avoid seasonal population collapses

Cleaved-facet violet laser diodes with lattice-matched Al0.82In0.18N/GaN multilayers as n-cladding

Electrically injected, edge-emitting cleaved-facet violet laser diodes were realized using a 480 nm thick lattice matched Si doped Al0.82In0.18N/GaN multilayer as the cladding on the n-side of the waveguide. Far-field measurements verify strong mode confinement to the waveguide. An extra voltage is measured and investigated using separate mesa structures with a single AlInN insertion. This showed that the electron current has a small thermally activated shunt resistance with a barrier of 0.135 eV and a current which scales according to V-n, where n similar to 3 at current densities appropriate to laser operation. (C) 2011 American Institute of Physics. (doi:10.1063/1.3589974

Extreme ultraviolet detection using AlGaN-on-Si inverted Schottky photodiodes

We report on the fabrication of aluminum gallium nitride (AlGaN) Schottky diodes for extreme ultraviolet (EUV) detection. AlGaN layers were grown on silicon wafers by molecular beam epitaxy with the conventional and inverted Schottky structure, where the undoped, active layer was grown before or after the n-doped layer, respectively. Different current mechanisms were observed in the two structures. The inverted Schottky diode was designed for the optimized backside sensitivity in the hybrid imagers. A cut-off wavelength of 280 nm was observed with three orders of magnitude intrinsic rejection ratio of the visible radiation. Furthermore, the inverted structure was characterized using a EUV source based on helium discharge and an open electrode design was used to improve the sensitivity. The characteristic He I and He II emission lines were observed at the wavelengths of 58.4 nm and 30.4 nm, respectively, proving the feasibility of using the inverted layer stack for EUV detectio

High Electron Mobility in AlGaN/GaN Heterostructures Grown on Bulk GaN Substrates

Dislocation-free high-quality AlGaN/GaN heterostructures have been grown by molecular-beam epitaxy on semi-insulating bulk GaN substrates. Hall measurements performed in the 300 K–50 mK range show a low-temperature electron mobility exceeding 60 000 cm2/V s for an electron sheet density of 2.4×1012 cm−2. Magnetotransport experiments performed up to 15 T exhibit well-defined quantum Hall-effect features. The structures corresponding to the cyclotron and spin splitting were clearly resolved. From an analysis of the Shubnikov de Hass oscillations and the low-temperature mobility we found the quantum and transport scattering times to be 0.4 and 8.2 ps, respectively. The high ratio of the scattering to quantum relaxation time indicates that the main scattering mechanisms, at low temperatures, are due to long-range potentials, such as Coulomb potentials of ionized impurities

GaN-AlGaN Heterostructure Field-Effect Transistors over Bulk GaN Substrates

We report on AlGaN/GaN heterostructures and heterostructurefield-effect transistors(HFETs) fabricated on high-pressure-grown bulk GaN substrates. The 2delectron gas channel exhibits excellent electronic properties with room-temperature electron Hall mobility as high as μ=1650 cm2/V s combined with a very large electron sheet density ns≈1.4×1013 cm−2.The HFET devices demonstrated better linearity of transconductance and low gate leakage, especially at elevated temperatures. We also present the comparative study of high-current AlGaN/GaN HFETs(nsμ\u3e2×1016 V−1 s−1) grown on bulk GaN, sapphire, and SiC substrates under the same conditions. We demonstrate that in the high-power regime, the self-heating effects, and not a dislocation density, is the dominant factor determining the device behavior