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See detailHigh-power SiO2/AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors
Kordoš, P.; Heidelberger, G.; Bernát, J. et al

in Applied Physics Letters (2005), 87(14), 143501-143504

We report on SiO2 /AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors MOSHFETs , which exhibit a 6.7 W/mm power density at 7 GHz. Unpassivated and SiO2-passivated heterostructure ... [more ▼]

We report on SiO2 /AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors MOSHFETs , which exhibit a 6.7 W/mm power density at 7 GHz. Unpassivated and SiO2-passivated heterostructure field-effect transistors HFETs were also investigated for comparison. Deposited 12 nm thick SiO2 yielded an increase of the sheet carrier density from 7.6 .10^12 to 9.2 .10^12 cm−2 and a subsequent increase of the static drain saturation current from 0.75 to 1.09 A/mm. The small-signal rf characterization of the MOSHFETs showed an extrinsic current gain cutoff frequency fT of 24 GHz and a maximum frequency of oscillation fmax of 40 GHz. The output power of 6.7 W/mm of the MOSHFETs measured at 7 GHz is about two times larger than that of HFETs. The results obtained demonstrate the suitability of GaN-based MOSHFETs for high-power electronics. [less ▲]

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See detailUltrafast metal-semiconductor-metal photodetectors on low-temperature-grown GaN
Mikulics, M.; Marso, Michel UL; Javorka, P. et al

in Applied Physics Letters (2005), 86(21), 211110

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown sLTd GaN. The photodetector devices exhibit up to 200 kV/cm electric breakdown fields ... [more ▼]

We have fabricated and characterized ultrafast metal-semiconductor-metal photodetectors based on low-temperature-grown sLTd GaN. The photodetector devices exhibit up to 200 kV/cm electric breakdown fields and subpicosecond carrier lifetime. We recorded as short as 1.4-ps-wide electrical transients using 360-nm-wavelength and 100-fs-duration laser pulses, that is corresponding to the carrier lifetime of 720 fs in our LT GaN material. [less ▲]

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See detailGrowth and properties of GaN and AlN layers on silver substrates
Mikulics, Martin; Kočan, Martin; Rizzi, Angela et al

in Applied Physics Letters (2005), 87

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with ... [more ▼]

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN 11-22 orientation. An intermetallic phase of Ga3Ag is found at the GaN/Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of 0002 . Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current 10−3 A/cm2 . These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices. [less ▲]

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See detailLow Current Dispersion and Low Bias-stress Degradation of Unpassivated GaN/AlGaN/GaN/SiC HEMTs
Bernát, J.; Pierobon, R.; Marso, Michel UL et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2005), 2(7), 2676-2679

The performance of unpassivated GaN/AlGaN/GaN/SiC HEMTs with intentionally undoped and doped barrier structures are extensively studied. No drain current dispersion between DC and 50 ns pulses on doped ... [more ▼]

The performance of unpassivated GaN/AlGaN/GaN/SiC HEMTs with intentionally undoped and doped barrier structures are extensively studied. No drain current dispersion between DC and 50 ns pulses on doped devices and less than 10% dispersion on undoped ones is observed. The full (100%) current recovery on undoped sample was measured in 1μs. The drain current extrapolated from 2 GHz large signal measurements corresponds to the measured static drain current confirming negligible current dispersion of our devices. Insignificant (<5 %) degradation in overall device performance parameters (IDs, gm, fT, fmax, Pout) on both undoped and doped structures after 12-hour-long bias stress was obtained. These results documents that suitable device performances can be obtained also on unpassivated GaN-based HEMTs. [less ▲]

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See detailPhotomixers fabricated on nitrogen-ion-implanted GaAs
Mikulics, M.; Marso, Michel UL; Cámara Mayorga, I. et al

in Applied Physics Letters (2005), 87(4), 41106-1-3

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion ... [more ▼]

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N+ ions into GaAs substrates with an ion concentration of 3 1012 cm−2. The resulting material exhibited 110 fs carrier lifetime due to implantation-induced defects. Our photomixers were fabricated as metal-semiconductor-metal devices, placed at the feed point of a broadband antenna. Optoelectronic measurements were performed in the wavelength range between 350 nm and 950 nm. In comparison to their counterparts photomixers fabricated on low-temperature-grown GaAs the N+-implanted GaAs photomixers exhibit improvements on both the output power and responsivity. A maximal responsivity of above 100 mA/W was achieved and we did not observe any dependence of the mixer cut-off frequency on the bias voltage. These characteristics make N+-implanted GaAs the material of choice for efficient optoelectronic photomixers. [less ▲]

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See detailLarge-area traveling-wave photonic mixers for increased continuous terahertz power
Michael, E. A.; Vowinkel, B.; Schieder, R. et al

in Applied Physics Letters (2005), 86(11), 111120-111123

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature ... [more ▼]

A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs sLT-GaAsd. It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 µW at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices. [less ▲]

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See detailWedge-shaped layers from porous silicon: the basics of laterally graded interference filters
Bohn, H. G.; Marso, Michel UL

in Physica Status Solidi A. Applied Research (2005), 202(8), 1437-1442

The process of making laterally graded interference filters has been reanalyzed. A simple and consistent picture arises if one applies a constant electric field perpendicular to the etch current instead ... [more ▼]

The process of making laterally graded interference filters has been reanalyzed. A simple and consistent picture arises if one applies a constant electric field perpendicular to the etch current instead of a constant current. The system is quantitatively modeled by means of a pure ohmic equivalent circuit model. [less ▲]

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See detailUltrafast Low-Temperature-Grown Epitaxial GaAs Photodetectors Transferred on Flexible Plastic Substrates
Mikulics, M.; Adam, R.; Marso, Michel UL et al

in IEEE Photonics Technology Letters (2005), 17(8), 1725-1727

We demonstrate low-temperature (LT)-grown GaAs photodetectors transferred on flexible polyethylene terephthalate (PET) plastic substrates. The LT-GaAs layer was patterned into 20 20 m2 chips, which after ... [more ▼]

We demonstrate low-temperature (LT)-grown GaAs photodetectors transferred on flexible polyethylene terephthalate (PET) plastic substrates. The LT-GaAs layer was patterned into 20 20 m2 chips, which after placing on the PET substrates were integrated with coplanar strip transmission lines. The devices exhibit low dark currents ( 2 10 8 A), subpicosecond photoresponse time, and signal amplitudes up to 0.9 V at the bias voltage of 80 V and under laser beam excitation power of 8 mW at 810-nm wavelength. At the highest bias ( 80 V) level, an increase of the response time (up to 1.3 ps) was observed and attributed to the influence of heating effects due to low thermal conductivity of PET. Our LT-GaAs-on-PET photodetectors withstand hundredfold mechanical bending of the substrate and are intended for applications in hybrid optoelectronic circuits fabricated on noncrystalline substrates, in terahertz imaging, and in biology-related current-excitation tests. [less ▲]

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See detailA quantitative analysis of fish consumption and stroke risk.
Bouzan, C.; Cohen, J. T.; Connor, W. E. et al

in American Journal of Preventive Medicine (2005), 29(4), 347-352

Although a rich source of n-3 polyunsaturated fatty acids (PUFAs) that may confer multiple health benefits, some fish contain methyl mercury (MeHg), which may harm the developing fetus. U.S. government ... [more ▼]

Although a rich source of n-3 polyunsaturated fatty acids (PUFAs) that may confer multiple health benefits, some fish contain methyl mercury (MeHg), which may harm the developing fetus. U.S. government recommendations for women of childbearing age are to modify consumption of high-MeHg fish to reduce MeHg exposure, while recommendations encourage fish consumption among the general population because of the nutritional benefits. The Harvard Center for Risk Analysis convened an expert panel (see acknowledgements) to quantify the net impact of resulting hypothetical changes in fish consumption across the population. This paper estimates the impact of fish consumption on stroke risk. Other papers quantify coronary heart disease mortality risk and the impacts of both prenatal MeHg exposure and maternal intake of n-3 PUFAs on cognitive development. This analysis identified articles in a recent qualitative literature review that are appropriate for the development of a dose-response relationship between fish consumption and stroke risk. Studies had to satisfy quality criteria, quantify fish intake, and report the precision of the relative risk estimates. The analysis combined the relative risk results, weighting each proportionately to its precision. Six studies were identified as appropriate for inclusion in this analysis, including five prospective cohort studies and one case-control study (total of 24 exposure groups). Our analysis indicates that any fish consumption confers substantial relative risk reduction compared to no fish consumption (12% for the linear model), with the possibility that additional consumption confers incremental benefits (central estimate of 2.0% per serving per week). [less ▲]

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See detailInfluence of carrier supply doping on the RF properties of AlGaN/GaN/SiC high-electron-mobility transistors
Marso, Michel UL; Bernát, J.; Javorka, P. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2005), 2(7), 2611-2614

We have fabricated undoped as well as modulation-doped AlGaN/GaN HEMTs on SiC substrate. The influence of the carrier supply layer on device performance is investigated by Hall, channel conductivity ... [more ▼]

We have fabricated undoped as well as modulation-doped AlGaN/GaN HEMTs on SiC substrate. The influence of the carrier supply layer on device performance is investigated by Hall, channel conductivity, small signal RF and delay time evaluation. While the doping layer improves the DC performance it degrades the RF behaviour of the device with the highest carrier supply doping of 5x1018 cm–3. The channel conductivity measurements show identical dependence of the mobility on carrier concentration for all samples. The saturation velocity, extracted by evaluation of the total delay time as function of the inverse drain current, decreases from 0.86x107 cm/s for the undoped device to 0.7x107 cm/s for the highest doped HEMT. This result shows that the degradation of RF performance is due to the reduction of the effective saturation velocity caused by the carrier supply layer. [less ▲]

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See detailUltrafast Phenomena in Freestanding LT-GaAs Devices
Marso, Michel UL; Mikulics, M.; Adam, R. et al

in Acta Physica Polonica A (2005), VOL 107; PART 1

We report on the fabrication and high-frequency performance of our photodetectors and photomixers based on freestanding low-temperature-grown GaAs. The MBE-grown low-temperature GaAs layers are lifted ... [more ▼]

We report on the fabrication and high-frequency performance of our photodetectors and photomixers based on freestanding low-temperature-grown GaAs. The MBE-grown low-temperature GaAs layers are lifted from the native GaAs substrate and transferred on top of variety of host substrates. The freestanding devices exhibit breakdown electrical ¯elds above 200 kV/cm and dark currents below 3 £ 10¡7 A at 100 V bias. Device photoresponse shows 0.55 ps wide electrical transients with voltage amplitudes up to 1.3 V, measured using an electro-optical sampling technique with 100 fs wide laser pulses. Photomixing experiments at 460 GHz yield a 9 times higher output power for the freestanding device on Si/SiO2 host substrate compared to the native substrate. [less ▲]

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See detailInfluence of passivation-induced stress on the performance of AlGaN/GaN HEMTs
Gregusova, Dagmar; Bernát, J.; Drzik, M. et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2005), 2(7), 2619-2622

This paper reports on properties of intentionally undoped AlGaN/GaN/sapphire-based high electron mobility transistors (HEMTs) before and after passivation with SiO2 and Si3N4. Our results indicate that ... [more ▼]

This paper reports on properties of intentionally undoped AlGaN/GaN/sapphire-based high electron mobility transistors (HEMTs) before and after passivation with SiO2 and Si3N4. Our results indicate that the DC performance of the AlGaN/GaN HEMTs improved significantly as the stress in the passivation layer increased from compressive to tensile. It corresponded to changes in the sheet carrier concentration. Unlike the DC properties, RF properties of the HEMTs were less sensitive to the stress. [less ▲]

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See detailLes représentations sociales de la médiation au Luxembourg
Houssemand, Claude UL; Liviero, K

Report (2005)

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See detailGeneration of frustrated liquid crystal phases by mixing an achiral n–smc mesogen with an antiferroelectric chiral smectic liquid crystal
Lagerwall, Jan UL; Giesselmann, Frank; Selbmann, Christine et al

in Journal of Chemical Physics (2005), 122(14), 144906

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric ... [more ▼]

By mixing the achiral liquid crystal HOAB, exhibiting a nematic (N)-smectic-C (SmC) mesophase sequence, with the chiral antiferroelectric liquid crystal (AFLC) (S,S)-M7BBM7, forming the antiferroelectric SmCa phase, at least seven different mesophases have been induced which neither component forms on its own: a twist-grain-boundary (TGB) phase, two or three blue phases, the untilted SmA phase, as well as all three chiral smectic-C-type ``subphases,'' SmCalpha, SmCbeta, and SmCgamma. The nature of the induced phases and the transitions between them were determined by means of optical and electro-optical investigations, dielectric spectroscopy, and differential scanning calorimetry. The induced phases can to a large extent be understood as a result of frustration, TGB at the border between nematic and smectic, the subphases between syn and anticlinic tilted smectic organization. X ray scattering experiments reveal that the smectic layer spacing as well as the degree of smectic order is relatively constant in the whole mixture composition range in which AFLC behavior prevails, whereas both these parameters rapidly decrease as the amount of HOAB is increased to such an extent that no other smectic-C-type phase than SmC/SmC exists. By tailoring the composition we are able to produce liquid crystal mixtures exhibiting unusual phase sequences, e.g., with a direct isotropic-SmCa transition or a temperature range of the SmCbeta subphase of about 50 K. (C) 2005 American Institute of Physics. [less ▲]

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See detailElectrolyte effects on the nematic-isotropic phase transition in lyotropic liquid crystals
Mukherjee, Prabir K.; Lagerwall, Jan UL; Giesselmann, Frank

in Liquid Crystals (2005), 32(10), 1301-1306

A phenomenological approach to the description of the electrolyte effect on the nematic– isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the electrolyte is discussed ... [more ▼]

A phenomenological approach to the description of the electrolyte effect on the nematic– isotropic phase transition in lyotropic liquid crystals is proposed. The influence of the electrolyte is discussed by varying the coupling between the concentration variables and the orientational order parameter. The analysis shows that the discontinuity in the first order nematic–isotropic phase transition as measured by TNI{T0 increases as a function of the NI weight fraction of the electrolyte. Here TNI is the first order nematic–isotropic phase transition temperature and TN0I is the extrapolated supercooling limit. The electrolyte dependence of the Cotton–Mouton coefficient and the non-linear dielectric effect in the isotropic phase above the nematic–isotropic phase transition are calculated. The theoretical predictions are found to be in good agreement with experimental results. [less ▲]

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See detailDifferences between smectic homo- and copolysiloxanes as a consequence of microphase separation
Rössle, Martin; Braun, L.; Schollmeyer, D. et al

in Liquid Crystals (2005), 32(5), 533-538

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the ... [more ▼]

This paper compares smectic phases formed from LC-homo- and LC-co-polysiloxanes. In the homopolysiloxane, each repeating unit of the polymer chain is substituted with a mesogen, whereas in the copolysiloxanes mesogenic repeating units are separated by dimethylsiloxane units. Despite a rather similiar phase sequence of the homo- and co-polysiloxanes—higher ordered smectic, smectic C* (SmC*), smectic A (SmA) and isotropic—the nature of their phases differs strongly. For the copolymers the phase transition SmC* to SmA is second order and of the ‘de Vries’ type with a very small thickness change of the smectic layers. Inside the SmA phase, however, the smectic thickness decreases strongly on approaching the isotropic phase. For the homopolymer the phase transition SmC* to SmA is first order with a significant thickness change, indicating that this phase is not of the ‘de Vries’ type. This difference in the nature of the smectic phases is probably a consequence of microphase separation in the copolymer, which facilitates a loss of the tilt angle correlation between different smectic layers. This has consequences for the mechanical properties of LC- elastomers formed from homo- and co-polymers. For the elastomers from homopolymers the smectic layer compression seems to be rather high, while it seems to be rather small for the copolymers. [less ▲]

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See detailChiral smectic C subphases induced by mixing a bistereogenic antiferroelectric liquid crystal with a non-chiral liquid crystal
Lagerwall, Jan UL; Giesselmann, Frank; Rauch, Sebastian et al

in Ferroelectrics (2005), 315

By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ and SmCa mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type ... [more ▼]

By mixing a bistereogenic antiferroelectric liquid crystal (AFLC) compound, exhibiting only the SmQ and SmCa mesophases, with the achiral N-SmC liquid crystal HOAB we could induce all three AFLC SmC-type subphases, SmCalpha, SmCbeta and SmCgamma. This seems to be in contradiction with two recent postulations regarding the subphase stability, one of which suggests that the subphases appear as a result of strong chiral interactions, the other that these phases require high smectic order something one would generally not expect in mixtures. We have studied the helical pitch, optical tilt angle, spontaneous polarization and the x-ray diffraction due to the smectic layering, as a function of mixing ratio in order to better understand the relation between phase sequence and mixture composition. The smectic layer spacing shows a strongly non-linear behavior suggesting that the basic structure of the pure AFLC substance is retained up to a HOAB content of about 75\%. [less ▲]

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See detailA study of a bistereogenic mesogen for the development of orthoconic antiferroelectric liquid crystal materials
Lagerwall, Jan UL; Yates, Chris; Rauch, Sebastian et al

in Ferroelectrics (2005), 315

The first orthoconic antiferroetectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90 degrees between neighboring layers, were mixtures of partially fluorinated ... [more ▼]

The first orthoconic antiferroetectric liquid crystals (OAFLCs), i.e. smectics where the optical director changes direction by 90 degrees between neighboring layers, were mixtures of partially fluorinated monostereogenic compounds. They have successfully demonstrated the orthoconic properties (orientation-independent dark state between crossed polarizers) but suffer from too high polarization and too short helical pitch, necessitating very thin samples. Using an (S,R) bistereogenic OAFLC we have obtained the orientation-independent dark state in rather thick samples, but several other problems arise with this compound. The strongly first-order SmA-SmCa transition produces defects leading to light leakage. In order to be switchable the sample must furthermore be mixed with chiral dopants, generally reducing the tilt angle as well as a shortening the helical pitch. Finally, a SmC phase often appears in the phase sequence of the mixture, strongly dominating over the desired SmCa phase in thin cells. [less ▲]

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See detailCoopération transfrontalière des universités dans l’espace SaarLorLux
Lehners, Jean-Paul UL

in Schmeling, Manfred; Veith, Michael (Eds.) Universitäten in europäischen Grenzräumen / Universités et Frontières en Europe. Frankreich-Forum. Jahrbuch des Frankreichzentrums der Universität des Saarlandes (2005)

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See detailSozialwissenschaftliche Forschung in Österreich: Kärnten
Hoenig, Barbara UL

Article for general public (2005)

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