Scielo RSS <![CDATA[Superficies y vacío]]> vol. 25 num. 4 lang. pt <![CDATA[SciELO Logo]]> <![CDATA[<b>Influencia del tiempo de gelificación sobre las propiedades estructurales, vibracionales y</b> <b>eléctricas del compósito PEG<sub>0.2</sub>/V<sub>2</sub>O<sub>5</sub></b>]]> Se estudiaron las propiedades estructurales, vibracionales y eléctricas de películas V2O5·nH2O y PEG0.2/V2O5 en función del tiempo de gelificación. Las películas fueron sinterizadas por sol-gel mediante la ruta del pentóxido de vanadio sobre sustratos de vidrio previamente tratados. La caracterización se llevo a cabo mediante Difracción de Rayos-X, Espectroscopia Raman y el método de las cuatro puntas convencional de van der Pauw. Los resultados indican que el aumento en el tiempo de gelificación del sol precursor (V2O5.nH2O) incrementa la calidad cristalina, el grado de hidratación, y la concentración de iones V5+ en las películas. En el compósito híbrido (PEG0.2/V2O5) los iones VO2+ actúan como enlazantes entre las cadenas poliméricas haciéndolas más compactas, lo cual facilita la movilidad de los portadores de carga eléctrica entre los iones vanadios. Los resultados mostraron una transición de fase semiconductor-metal alrededor de 34°C, una transición óptica del tipo indirecta (para V2O5·nH2O) y una transición vítrea alrededor de Tg=-69°C.<hr/>Structural, vibrational and electrical properties of V2O5·nH2O and PEG0.2/V2O5 films as a function of gelation time were studied. The films were sintered by sol-gel using vanadium pentoxide route on glass substrates previously treated. Films were characterized by X-Ray Diffraction, Raman Spectroscopy and the four-point van der Pauw conventional method. The results indicated that when the gelation time of the precursor (V2O5·nH2O) is increased, the crystalline quality, the degree of hydration, and V5+ ion concentration are increased. In the hybrid composite (PEG02/V2O5), VO²+ ions act as bonding between the polymer chains, making them more compact to facilitate the mobility of electric charge carriers between vanadium ions. The results showed a semiconductor-metal transition about 34°C, an indirect optical transition (V2O5·nH2O) and a vitreous transition around Tg=-69°C. <![CDATA[<b>Transient heating of solids of finite size under bulk absorption of a light pulse</b>]]> The transient temperature associated with the bulk absorption of a rectangular laser pulse in a solid-state sample of finite size is calculated analytically and analyzed. Radiation is incident on the frontal surface with an arbitrary surface thermal conductivity. The opposite surface is thermostatically controlled and maintained at a constant equilibrium temperature. The general solution is obtained for pulses of arbitrary duration. The pulse duration is determined with respect to the relaxation time of the nonstationary thermal diffusion. This latter is the characteristic time of the problem. <![CDATA[<b>Au doping of CdS polycrystalline films prepared by co-sputtering of CdS-Cd-Au targets</b>]]> Au doped CdS polycrystalline films were grown on Corning glass substrates at room temperature by co-sputtering from a CdS-Cd-Au target. Elemental Cd and Au were placed onto the CdS target covering small areas. The electrical, structural, and optical properties were analyzed as a function of Au content. The Au doped CdS polycrystalline films showed a p-type semiconductor nature. It was found that the electrical resistivity drops and the carrier concentration increases as a consequence of Au incorporation within the CdS lattice. In both cases, the changes were of several orders of magnitude. <![CDATA[<b>Defectos en estructuras de índice de refracción periódico con gap omnidireccional</b>]]> Se hace un estudio de los modos de tunelamiento generados por la introducción de una impureza en un cristal fotónico unidimensional, que se construye a partir de alternar capas de dieléctrico y metamaterial. En este trabajo la impureza se genera al cambiar el índice de refracción o el ancho del dieléctrico de la capa central del cristal fotónico. Se observa que los modos de tunelamiento que se encuentran dentro de los gaps, presentan un comportamiento omnidireccional cuando se cumple la condición n1d1 +n2d2 =0. Esta condición hace que los modos de tunelamiento presenten insensibilidad ante el tipo de polarización, ángulo de incidencia, y escalamiento de la celda unitaria. Ante la variación del índice de refracción de la impureza no se generan modos de tunelamiento en ninguno de los gaps. Sin embargo cuando se hace variar el espesor de la impureza se producen cambios en los modos de tunelamiento en ambos gaps.<hr/>We analyze tunneling modes generated by the introduction of an impurity in a one-dimensional photonic crystal, which is constructed from alternating layers of dielectric and metamaterial. In this work the impurity is generated by changing the refractive index or width the dielectric of the central layer the photonic crystal. It is noted that the tunneling modes which are within the gaps, exhibit an omnidirectional behavior when the condition n1d1 +n2d2 =0. This condition causes tunneling modes is insensitivity to the type of polarization, incidence angle, scaling of the unit cell. Given the variation of the refractive index of the impurity tunneling modes are generated in any of the gaps. But when you vary the thickness of the impurity are changes in modes of tunneling in both gaps. <![CDATA[<b>Photoluminescence studies of Mg-doped gallium nitride films grown by metalorganic chemical vapor deposition</b>]]> Mg-doped GaN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) have been investigated by Photoluminescence, Raman scattering and X-ray Diffraction in the as-grown condition and after annealing. The photoluminescence measurement showed 2.9 eV band associated with the optical transitions from the conduction band to the Mg acceptor. Raman spectra shows local vibrational modes at 647.73 cm-1 and 265 cm-1 related to a local vibrational mode (LVM) of magnesium in GaN and as evidence of p-type character, respectively. The XRD analysis was employed to study the structure of the films. <![CDATA[<b>Morphological and optical properties of porous silicon annealed in atomic hydrogen</b>]]> In this work, optical and morphological changes on hydrogen-annealed porous silicon layers (PSL) are reported. The PSL were annealed in a hot filament chemical vapor deposition (HFCVD) system at a temperature of 1000 °C. Annealing time, 10, 20, 60, 90, 180 and 360 sec was taken as parameter to changes the properties of PSL. After treatment, the samples were characterized by scanning electron microscopy (SEM), Fourier Transform infrared (FTIR) spectroscopy, reflectance and photoluminescence (PL) measurements. It was found that silicon matrix, in the PSL, was etched off by their interaction with atomic radicals causing changes in its morphology, porosity and thickness. The reflectance of the film, which is dependent on the porosity, also decreased to values near to 0% in the range of 400 to 850 nm even for short annealing times. The emission properties of the films, in the visible range, shifted to blue region for longer processing time. <![CDATA[<b>Optical properties of Si nanocrystals self-nucleated onto SiO<sub>2</sub> surfaces</b>]]> SiO2/Si heterolayers were synthesized employing a RF magnetron sputtering system. For thin Si layers, it is shown that intrinsic voids on the SiO2 surface act as nucleation sites, promoting the formation of nanocrystals (NCs). As the deposition time of the Si layer increases it is observed that the influence of the SiO2 texture on the nucleation of Si nanocrystals diminishes. Studies carried out on samples grew on glass substrates shown that control in the NCs sizes allows to control the absorption edges of the synthesized samples between 300 and 700 nm. As-growth samples shown two photoluminescence bands around 570 and 660 nm, a band in the infrared region shows up and shifts toward the visible region as the Si layer thickness decreases. <![CDATA[<b>Modelling of GaAsP/InGaAs/GaAs strain-balanced multiple-quantum well solar cells</b>]]> A design of de GaAsP/InGaAs/GaAs solar cell is presented that allows to model high efficiency devices. The stress, tensile and compressive, are considered in order to compute the electron and hole dispersion relation E(k) in conduction and valence band. Similarly, the optical transitions in quantum well and barriers were evaluated to calculate the quantum internal efficiency and the photocurrent. GaAsP/InGaAs/GaAs solar cell is optimized to reach the maximum performance by means of J-V relation. Our model was used to determine the highest efficiencies for cells containing quantum wells under varying degrees of strain, showing that cells with strained quantum wells achieve high efficiencies.