M.B.P. Zakia

Metal mesh resonant filters for terahertz frequencies

The interest in terahertz photometric and imaging measurements has motivated the development of bandpass resonant filters to be coupled to multiple-pixel devices such as bolometer arrays. Resonant grids are relatively simple to fabricate, exhibiting high transmission at the central frequency, a narrow bandpass, and good rejection of the side frequencies of the spectrum. We have fabricated filters centered at different frequencies between 0.4 and 10 THz, using photolithography and electroforming techniques. Transmission measurements have shown center frequencies and bandwidths close to the design predictions. The performance of the filters was found not to be critically dependent on small physical deformations in the mesh, becoming more noticeable at higher frequencies (i.e., for smaller physical sizes). Wider bandwidths, needed to attain higher sensitivities in the continuum, were obtained by changing the design parameters for filters at 2 and 3 THz.

New telescopes for ground-based solar observations at submillimeter and mid-infrared

The solar submillimeter-wave telescope (SST) is the only one of its kind dedicated to solar continuous observations. Two radiometers at 0.740 mm (405 GHz), and four at 1.415 mm (212 GHz) are placed in the Cassegrain focal plane of the 1.5-m dish at El Leoncito high altitude site, San Juan, Argentina. The aperture efficiencies are close to design predictions: 20% and 35% for 2 and 4 arcminutes beam sizes at 405 and 212 GHz, respectively. The positioner absolute pointing accuracy is 10 arcseconds. Spectral coverage is complemented by ground-based mid-infrared telescopes developed for high cadence observations in the continuum 10 micron band (30 THz), using small apertures and room-temperature microbolometer cameras. Using the system, a new solar burst spectral component was discovered, exhibiting fluxes increasing for smaller wavelengths, separated from the well known microwave component. Rapid sub-second pulsations are common for all bursts. The pulsations onset times of appear to be connected to the launch times of CMEs. Active regions are brighter for shorter submillimeter-waves. Mid-IR bright regions are found closely associated with calcium plages and magnetic structures near the solar photosphere. Intense and rapid 10 micron brightening was detected on active centers in association with weak flares. These results raise challenging difficulties for interpretation.

Preparation and characterization of semiconducting polymeric blends. Photochemical synthesis of poly(3-alkylthiophenes) using host microporous matrices of poly(vinylidene fluoride)

Chemical and electrochemical synthesis techniques have been the principal methods of obtaining polymers in industry and scientific research laboratories. However, during the last two decades, photochemical synthesis, although poorly studied, has been reported to present several advantages, in that it is a fast and cheap technique, and it is not aggressive to the environment. The technique has been applied to the production of some conducting polymers. In this study, semiconducting polymeric blends composed of PT3AA-K–PVDF and PT3MA–PVDF were respectively obtained by photochemical polymerization in aqueous solutions of 3-thiophene acetic acid and 3-thiophene methyl acetate monomers using PVDF microporous matrices and potassium dichromate as catalyst. The percentage of products and by-products incorporated in the host matrix was obtained by gravimetric analysis. The chemical structures of the polymers synthesized were analyzed by FTIR, UV-vis and 1H NMR. GPC analysis indicated the formation of oligomers composed of 5–6 mers. The morphology of the matrices and polymeric blends was observed by SEM-EDS and their electric behavior evaluated by measures of electric conductivity. The SEM images show the presence of polythiophene in the pores of the PVDF microporous membrane. The thermal properties of the polymers and their blends were evaluated by DSC and TGA. Thermal analysis by DSC demonstrated an increase in melting temperature of the blends, attributed to the confinement of PVDF crystalline phases for the polymer photosynthesized. The results of volumetric conductivity measurements of polymeric blends show an increase in conductivity in the matrices from 10−15 to 10−11 S cm−1.

SOLAR-T : Terahertz Photometers to Observe Solar Flare Emission on Stratospheric Balloon Flights

A new solar flare spectral component has been found with intensities increasing for larger sub-THz frequencies, spectrally separated from the well known microwaves component, bringing challenging constraints for interpretation. Higher THz frequencies observations are needed to understand the nature of the mechanisms occurring in flares. A twofrequency THz photometer system was developed to observe outside the terrestrial atmosphere on stratospheric balloons or satellites, or at exceptionally transparent ground stations. 76 mm diameter telescopes were designed to observe the whole solar disk detecting small relative changes in input temperature caused by flares at localized positions at 3 and 7 THz. Golay cell detectors are preceded by low-pass filters to suppress visible and near IR radiation, band-pass filters, and choppers. It can detect temperature variations smaller than 1 K with time resolution of a fraction of a second, corresponding to small burst intensities. The telescopes are being assembled in a thermal controlled box to which a data conditioning and acquisition unit is coupled. While all observations are stored on board, a telemetry system will forward solar activity compact data to the ground station. The experiment is planned to fly on board of long-duration stratospheric balloon flights some time in 2013-2015. One will be coupled to the GRIPS gamma-ray experiment in cooperation with University of California, Berkeley, USA. One engineering flight will be flown in the USA, and a 2 weeks flight is planned over Antarctica in southern hemisphere summer. Another long duration stratospheric balloon flight over Russia (one week) is planned in cooperation with the Lebedev Physics Institute, Moscow, in northern hemisphere summer.

Near-band-gap refractive index of GaSb

We have measured the refractive index of GaSb by ellipsometry for the wavelength region below band gap ranging from 1.75 to 2.5 μm. As far as we know, this is the first measurement for this wavelength region using this technique. There is a lack of agreement amongst the previous values reported in the literature for this material, where other techniques have been used. We describe the approach used here to obtain the refractive index from the ellipsometry parameters and then compare our results with the ones previously reported.

Morphological study of polycrystalline SiGe alloy deposited by vertical LPCVD

As device dimensions shrink to the deep-submicron scale, new challenges arises from the very small scale used and even poly crystalline silicon (poly-Si) presents problems as gate electrode. The use of SiGe as gate material can present many advantages over the poly-Si, as it leads to a lower boron penetration and gate depletion. In this paper authors present some morphological studies of polycrystalline SiGe thin films deposited in a vertical LPCVD (Low Pressure Chemical Vapor Deposition) reactor for using as MOS (Metal Oxide Semiconductor) gate electrode.

Proton radiation hardening of silicon oxynitride gate nMOSFETs formed by nitrogen implantation into silicon prior to oxidation

Silicon oxynitride (SiO/sub x/N/sub y/) insulators have been obtained by low-energy nitrogen ion implantation into Si substrates prior to conventional or rapid thermal oxidation. Theses films have been used as gate insulators in enhancement nMOSFETs and MOS capacitors. MOS capacitors were used to obtain capacitance-voltage (C-V) measurements. A relative dielectric constant of 3.9 was adopted to extract the Equivalent Oxide Thickness (EOT) of films from C-V curves, resulting in values between 5 nm and 12 nm. nMOSFETs were bombarded with H/sup +/ ion beams (energy of 0.17 MeV and doses of 0, 10/sup 12/, 10/sup 13/ and 10/sup 14/ protonS/cm/sup 2/) to investigate radiation hardening. nMOSFET electrical characteristics, such as threshold voltage (V/sub T/), transconductances (Gm) and sub-threshold slope (S), were extracted before and after proton radiation. For high dose bombardment, VT, S are increased and Gm is reduced. These oxynitride gate device performance degradation was significant only for doses >10/sup 12/ protons/cm/sup 2/.