The effect of substrate on high-temperature annealing of GaN epilayers: Si versus sapphire

We have studied the effects of rapid thermal annealing at 1300¿°C on GaN epilayers grown on AlN buffered Si(111) and on sapphire substrates. After annealing, the epilayers grown on Si display visible alterations with craterlike morphology scattered over the surface. The annealed GaN/Si layers were characterized by a range of experimental techniques: scanning electron microscopy, optical confocal imaging, energy dispersive x-ray microanalysis, Raman scattering, and cathodoluminescence. A substantial Si migration to the GaN epilayer was observed in the crater regions, where decomposition of GaN and formation of Si3N4 crystallites as well as metallic Ga droplets and Si nanocrystals have occurred. The average diameter of the Si nanocrystals was estimated from Raman scattering to be around 3¿nm. Such annealing effects, which are not observed in GaN grown on sapphire, are a significant issue for applications of GaN grown on Si(111) substrates when subsequent high-temperature processing is required.

Nucleation and growth of GaN nanorods on Si (111) surfaces by plasma-assisted molecular beam epitaxy - The influence of Si- and Mg-doping

The self-assembled growth of GaN nanorods on Si (111) substrates by plasma-assisted molecular beam epitaxy under nitrogen-rich conditions is investigated. An amorphous silicon nitride layer is formed in the initial stage of growth that prevents the formation of a GaN wetting layer. The nucleation time was found to be strongly influenced by the substrate temperature and was more than 30 min for the applied growth conditions. The observed tapering and reduced length of silicon-doped nanorods is explained by enhanced nucleation on nonpolar facets and proves Ga-adatom diffusion on nanorod sidewalls as one contribution to the axial growth. The presence of Mg leads to an increased radial growth rate with a simultaneous decrease of the nanorod length and reduces the nucleation time for high Mg concentrations.

Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

Síntesi i estudi de les propietats de fotoluminescència de nanoestructures de sulfur de cadmi

S’han sintetitzat estructures de CdS a partir de sílice mesoporosa (SBA-15, SBA-16 i KIT-6) pel mètode del motlle rígid i s’han caracteritzat mitjançant TEM i XRD, i se n’han estudiat les seves propietats de fotoluminescència mitjançant CSLM. Els resultats obtinguts s’han comparat amb una mostra de CdS comercial. El CdS SBA-15 ha conservat l’estructura mesoporosa del motlle, mentre que en els altres dos casos s’ha mantingut només de manera parcial. Per a totes les mostres, la mida de partícula es troba en el rang dels 3-10 nm. Pel que fa a l’estructura, s’ha detectat fase wurtzita i zinc-blenda, en diferents percentatges entre elles. La formació de wurtzita pot ser en part responsable del col·lapse parcial de la mesostructura. El pic de màxima emissió en l’espectre de fotoluminescència es desplaça entre les diferents estructures segons la mida de partícula. L’amplada del pic varia en funció del grau de dispersió de partícula, de manera que s’observa que aquesta és inferior en les mostres sintetitzades respecte de la comercial.

Gestión de un proyecto de implantación de un repositorio de objetos de aprendizaje

El presente Trabajo Final de Carrera (TFC) está centrado en la Gestión de un Proyecto de Implantación de un Repositorio de Objetos Digitales de Aprendizaje en una Universidad, y queda englobado en el área de Gestión de Proyectos de la Ingeniería Técnica Informática de Gestión.

Influence of the (111) twinning on the formation of diamond cubic/diamond hexagonal heterostructures in Cu-catalyzed Si nanowires

The occurrence of heterostructures of cubic silicon/hexagonal silicon as disks defined along the nanowire (111) growth direction is reviewed in detail for Si nanowires obtained using Cu as catalyst. Detailed measurements on the structural properties of both semiconductor phases and their interface are presented. We observe that during growth, lamellar twinning on the cubic phase along the (111) direction is generated. Consecutive presence of twins along the (111) growth direction was found to be correlated with the origin of the local formation of the hexagonal Si segments along the nanowires, which define quantum wells of hexagonal Si diamond. Finally, we evaluate and comment on the consequences of the twins and wurtzite in the final electronic properties of the wires with the help of the predicted energy band diagram.

Influence of premetallization surface treatment on the formation of Schottky Au-nGaN contacts

The influence of premetallization surface preparation on the structural, chemical, and electrical properties of Au-nGaN interfaces has been investigated by x-ray photoemission spectroscopy (XPS), current-voltage measurement (I-V) and cross-section transmission electron microscopy (TEM). XPS analysis showed that the three GaN substrate treatments investigated i.e., ex situ hydrofluoric acid etch, in situ anneal in ultrahigh-vacuum (UHV), and in situ Ga reflux cleaning in UHV result in surfaces increasingly free of oxygen contamination. XPS and TEM characterization of Au-nGaN formed after the three premetallization surface treatments show that HF etching and UHV annealing produce abrupt, well-defined interfaces. Conversely, GaN substrate cleaning in a Ga flux results in Au/GaN intermixing. I-V characterization of Au¿nGaN contacts yields a Schottky barrier height of 1.25 eV with a very low-ideality factor and very good contact uniformity for the premetallization UHV anneal, while the Ga reflux cleaning results in a much lower barrier (0.85 eV), with poor ideality and uniformity. I-V and XPS results suggest a high density of acceptor states at the surface, which is further enhanced by UHV annealing. These results are discussed in the context of current models of Schottky barrier formation.

Electronic structure and properties of AlN

The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.

Electronic structure and properties of AlN

The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.

El derecho de huelga y sus límites legales

Se dice que la huelga es un hecho natural en su fundamento que nace como reacción del ser humano ante determinados comportamientos que considera injustificados, perjudiciales para el o que simplemente no le gustan, y es por ello que la mayoría de doctos1 en la mate-ria concluyen que la huelga, no puede sin más, ser prohibida, pues las limitaciones legales no conseguirán eliminarlas de la vida laboral aunque se las podrá encauzar, condicionar a determinados requisitos, etc., pero cuantos más límites y frenos a su libre ejercicio se pon-gan, más vulneración existirá de la normativa reguladora.

El Cascanueces en 3D', versión libre de la adaptación del cuento de Hoffmann

Las navidades pasadas se presentó en las salas de cine El Cascanueces en 3D. Se trata de una adaptación libre del cuento de hadas de Navidad El Cascanueces y el rey de los ratones,2 creado por un escritor, músico, pintor, dramaturgo y jurista alemán llamado Ernst Theodor Amadeus Hoffmann3 (1776-1822). Hoffmann, gran admirador de Mozart, cambió su tercer nombre por el del músico austriaco y durante muchos años se ganó la vida como crítico musical,compositor y director de orquesta

El cervell té mecanismes per a les metàfores

Al cervell li costa entendre i generar metàfores. Comprendre el llenguatge figuratiu implica necessàriament la utilització simultània de diversos circuits neurals, que han d"extreure informació no literal a partir de paraules que tanmateix també tenen un significat concret. Per entendre aquesta facultat exclusivament humana, Nira Mashal i els seus col·laboradors, de la Universitat Ben-Ilan de Ramat Gan i del centre de salut mental Aviv-Brull de Tel Aviv, a Israel, han monitoritzat el funcionament del cervell en un grup de voluntaris mentre comprenien metàfores o expressions literals, i l"han comparat amb el de malalts d"esquizofrènia, una patologia que es caracteritza, entre altres aspectes, per la incapacitat d"entendre-les.