Behavior of W and WSi(x) Contact Metallization on n- and p- Type GaN

Sputter-deposited W-based contacts on p-GaN (N{sub A} {approximately} 10{sup 18} cm{sup {minus}3}) display non-ohmic behavior independent of annealing temperature when measured at 25 C. The transition to ohmic behavior occurs above {approximately} 250 C as more of the acceptors become ionized. The optimum annealing temperature is {approximately} 700 C under these conditions. These contacts are much more thermally stable than the conventional Ni/Au metallization, which shows a severely degraded morphology even at 700 C. W-based contacts may be ohmic as-deposited on very heavily doped n-GaN, and the specific contact resistance improves with annealing up to {approximately} 900 C

Bidimensional spectroelectrochemistry: application of a new device in the study of a o-vanillin-copper(II) complex

A new bidimensional spectroelectrochemistry setup for UV-Vis absorption measurements has been developed. The new device has been used to follow electrochemical reactions using two different arrangements: 1) a near-normal configuration that supplies information about the processes taking place both on the electrode surface and in the solution adjacent to it, and 2) a long-optical-pathway configuration based on a mobile slit that controls the position of a light beam passing parallel and adjacent to the electrode surface providing information only about the processes taking place in solution during the electrochemical reaction. The new setup has been validated using o-tolidine, a typical reference system for spectroelectrochemistry. The electrochemical mechanism of oxidation/reduction of Cu(o-Va)2(H2O)2 complex (o-Va = o-Vanillin = 2-hydroxy-3-methoxybenzaldehyde) has been studied using bidimensional UV-Vis absorption spectroelectrochemistry. This Cu(II) complex exhibits antimutagenic, anticarcinogenic and superoxide dismutase mimic properties.Junta de Castilla y León (BU033U16), and Ministerio de Economía y Competitividad (CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015-71955-REDT)CONICET, UNLP, Junta de Castilla y León (BU033U16), and Ministerio de Economía y Competitividad (CTQ2014-55583-R, CTQ2014-61914-EXP, CTQ2015-71955-REDT

Time-frequency distribution of interferograms from a frequency comb in dispersive media

We investigate general properties of the interferograms from a frequency comb laser in a non-linear dispersive medium. The focus is on interferograms at large delay distances and in particular on their broadening, the fringe formation and shape. It is observed that at large delay distances the interferograms spread linearly and that its shape is determined by the source spectral profile. It is also shown that each intensity point of the interferogram is formed by the contribution of one dominant stationary frequency. This stationary frequency is seen to vary as a function of the path length difference even within the interferogram. We also show that the contributing stationary frequency remains constant if the evolution of a particular fringe is followed in the successive interferograms found periodically at different path length differences. This can be used to measure very large distances in dispersive media.IST/Imaging Science and TechnologyApplied Science

Stationary phase based asymptotic analysis of inter-pulse interference from a frequency comb source in dispersive media

We introduce a continuous model to study interpulse interference from a femtosecond frequency comb laser in dispersive media. We extend this model to pulse propagation for longer distances in dispersive media and compare the results to measurements for up to 200 m in air

Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable core

Four novel π-conjugated chromophores with an azobenzene core (1-4) have been synthesized exploiting Pd-catalysed cross-coupling reactions between ethynyl-bearing azobenzene cores and suitably-designed peripheral groups. While in molecules 2 and 3 the azobenzene core is equipped, respectively, with ethynyl and 1,3-butadiyne spacers terminated with a substituted aniline, molecule 4 is an homologue of derivative 2 in which the terminal moieties are replaced by meso-substituted Zn-porphyrins. X-Ray crystallographic studies of substituted azobenzene 2 reveal a nearly planar arrangement of the four phenyl rings and the trans configuration of the NN central unit. The UV-Vis absorption spectrum of molecule 1 in cyclohexane (CHX) is very similar to that of unsubstituted azobenzenes; upon irradiation at the maximum of the intense π-π absorption feature (360 nm), 1 undergoes trans → cis photoisomerization reaching a photostationary state. The process is fully reversible both photochemically and thermally (ca. 120 min in the dark). The UV-Vis electronic absorption features of 2-4 are dramatically different compared to those of 1, but the photochemical process can still be traced and exhibits full reversibility in CHX. Also in the case of compound 4, where the photoreactive azobenzene excited states might be quenched by the low-lying porphyrin electronic levels, the photoreaction does occur. Extensive STM investigations of self-assembled monolayers (SAMs) of 2 and 3 at the solid/liquid interface were performed by means of scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite (HOPG). It is evidenced that only the trans isomer can be physisorbed on the surface whereas the cis form, either produced under illumination in situ or prepared by irradiation of the solution prior to deposition (ex-situ), is never observed on the surface. The smallest azobenzene 1 and the bisporphyrin system 4 did not physisorb onto the surface because of the very small size and the bulky 3,5-di(tert-butyl) phenyl groups hindering flat adsorption on HOPG, respectively. © 2009 The Royal Society of Chemistry

W and WSi(x) Ohmic Contacts on p- And n-Type GaN

W and WSi ohmic contacts on both p- and n-type GaN have been annealed at temperatures from 300-1000 *C. There is minimal reaction (< 100 ~ broadening of the metal/GaN interface) even at 1000 *C. Specific contact resistances in the 10-5 f2-cm2 range are obtained for WSiX on Si-implanted GaN with a peak doping concentration of- 5 x 1020 cm-3, after annealing at 950 `C. On p-GaN, leaky Schottky diode behavior is observed for W, WSiX and Ni/Au contacts at room temperature, but true ohmic characteristics are obtained at 250 - 300 *C, where the specific contact resistances are typically in the 10-2 K2-cm2 range. The best contacts for W and WSiX are obtained after 700 *C annealing for periods of 30- 120 sees. The formation of &WzN interracial phases appear to be important in determining the contact quality

Photoinduced structural modifications in multicomponent architectures containing azobenzene moieties as photoswitchable cores

Four novel pi-conjugated chromophores with an azobenzene core (1-4) have been synthesized exploiting Pd-catalysed cross-coupling reactions between ethynyl-bearing azobenzene cores and suitably-designed peripheral groups. While in molecules 2 and 3 the azobenzene core is equipped, respectively, with ethynyl and 1,3-butadiyne spacers terminated with a substituted aniline, molecule 4 is an homologue of derivative 2 in which the terminal moieties are replaced by meso-substituted Zn-porphyrins. X-Ray crystallographic studies of substituted azobenzene 2 reveal a nearly planar arrangement of the four phenyl rings and the trans configuration of the N=N central unit. The UV-Vis absorption spectrum of molecule 1 in cyclohexane (CHX) is very similar to that of unsubstituted azobenzenes; upon irradiation at the maximum of the intense pi-pi absorption feature (360 nm), 1 undergoes trans -> cis photoisomerization reaching a photostationary state. The process is fully reversible both photochemically and thermally (ca. 120 min in the dark). The UV-Vis electronic absorption features of 2-4 are dramatically different compared to those of 1, but the photochemical process can still be traced and exhibits full reversibility in CHX. Also in the case of compound 4, where the photoreactive azobenzene excited states might be quenched by the low-lying porphyrin electronic levels, the photoreaction does occur. Extensive STM investigations of self-assembled monolayers (SAMs) of 2 and 3 at the solid/liquid interface were performed by means of scanning tunneling microscopy (STM) on highly oriented pyrolytic graphite (HOPG). It is evidenced that only the trans isomer can be physisorbed on the surface whereas the cis form, either produced under illumination in situ or prepared by irradiation of the solution prior to deposition (ex-situ), is never observed on the surface. The smallest azobenzene 1 and the bisporphyrin system 4 did not physisorb onto the surface because of the very small size and the bulky 3,5-di(tert-butyl)phenyl groups hindering flat adsorption on HOPG, respectively

High temperature stable WSi{sub x} ohmic contacts on GaN

The authors have sputter-deposited 500--1200{angstrom} thick WSi{sub 0.45} metallization onto n{sup +} GaN (n{ge}10{sup 19}cm{sup {minus}3}) doped either during MOCVD growth or by direct Si{sup +} ion implantation (5{times}10{sup 15}cm{sup {minus}2}, 100 keV) activated by RTA at 1,100 C for 30 secs. In the epi samples R{sub C} values of {approximately}10{sup {minus}14}{Omega}cm{sup 2} were obtained, and were stable to {approximately}1000 C. The annealing treatments up to 600 C had little effect on the WSi{sub x}/GaN interface, but the {beta}-W{sub 2}N phase formed between 700--800 C, concomitant with a strong reduction in near-surface crystalline defects in the GaN. Spiking of the metallization down the threading and misfit dislocations was observed at 800 C, extending >5,000{angstrom} in some cases. This can create junction shorting in bipolar or thyristor devices, R{sub C} values of <10{sup {minus}6}{Omega}cm{sup 2} were obtained on the implanted samples for 950 C annealing, with values of after 1050 C anneals. The lower R{sub C} values compared to epi samples appear to be a result of the higher peak doping achieved. The authors observed wide spreads in R{sub C} values over a wafer surface, with the values on 950 C annealed material ranging from 10{sup {minus}7} to 10{sup {minus}4}{Omega}cm{sup 2}. There appear to be highly nonuniform doping regions in the GaN, perhaps associated with the high defect density in heteroepitaxial material, and this may contribute to the variations observed. They believe that near-surface stoichiometry is variable in much of the GaN currently produced due to the relative ease of preferential N{sub 2} loss and the common use of H{sub 2}-containing growth (and cool-down) ambients. Finally, the ohmic contact behavior of WSi{sub x} on abrupt and graded composition In{sub x}Al{sub 1{minus}x}N layers has been studied as a function of growth temperature, InN mole fraction (x = 0.5--1) and post WSi{sub x} deposition annealing treatment