Serge Rudaz - Academia.edu (original) (raw)

Papers by Serge Rudaz

Light-Emitting Diodes: Research, Manufacturing, and Applications, 1997

Hyperfine Interactions, 1984

The depolarization rate of anomalous muonium, Mu*, in germanium isotopically enriched in 74Ge (I ... more The depolarization rate of anomalous muonium, Mu*, in germanium isotopically enriched in 74Ge (I = 0) was measured as a function of field. The concentration of 73Ge (I = 9/2) was about 9 times less than natural abundance. The depolarization rate at 10 K in this isotopically enriched crystal for both lines of those Mu* centers whose symmetry axes make an angle of 90 ~ to the field is less than 1 #sec-~ at all fields down to the lowest one measured, 14.5 gauss. This is in sharp contrast to the wide lines reported at low field in germanium having natural isotopic abundance. The spectrum of Mu* in the isotopically enriched Ge crystal was also seen at zero field. These results confirm that the increased depolarization rate for Mu* at low fields arises from unresolved nuclear hyperfine structure. The depolarization rates observed were consistent with an average hyperfine interaction with a single 73Ge nucleus of 2.5 MHz, a value requiring nearly 1% of the spin density to be on a typical atom.

The 15th Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2002

Abstract There are numerous materials challenges involved in the production of high-efficiency II... more Abstract There are numerous materials challenges involved in the production of high-efficiency III-nitride lasers and LEDs, some of which can be mitigated by epitaxy and device physics. The lack of a suitable lattice-matched substrate for epitaxy of AlInGaN films ...

The ('13)C Nuclear Magnetic Resonance lines, spin -lattice relaxation times T(,1) and spin-sp... more The ('13)C Nuclear Magnetic Resonance lines, spin -lattice relaxation times T(,1) and spin-spin relaxation times T(,2) of CO chemisorbed on the surface layer of small particles of Pt metal have been observed by the method of spin echoes as a function of magnetic field, temperature, CO coverage and Pt dispersion. The samples are industrial catalysts: Pt supported on eta-alumina, with Pt dispersions ranging from 4% to 76%. After cleaning, the samples are exposed to CO isotopically enriched to 90% ('13)C. The ('13)C line shapes and spin-lattice relaxation times establish the presence of a ('13)C Knight shift (DELTA)H. The spin -lattice relaxation times are independent of the applied field H(,0) and follow the Korringa relation, T(,1)T((DELTA)H/H(,0))('2) = constant. From 77 K to 300 K, the relaxation of the magnetization is multiexponential: the distribution in T(,1)'s reflects a distribution in Knight shifts arising from a distribution of CO chemisorption sites...

Journal of Molecular Catalysis, 1983

NMR studies of alumina-supported platinum catalysts are presented. For the samples studied, the d... more NMR studies of alumina-supported platinum catalysts are presented. For the samples studied, the dispersion (fraction of Pt atoms on the surface) ranged from 4% to 76%. The lQ5Pt absorption line-shapes exhibit dramatic differences among samples. Certain features of these line-shapes have been assigned to various kinds of Pt sites in the Pt particles. In samples with adsorbed CO (i3C enriched) , 13C-195Pt double resonance was used to resolve that part of the Pt line-shape which arises from surface Pt bonded to the CO molecules. The results are compared with NMR data taken on Pt carbonyls.

physica status solidi (a), 2002

High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an i... more High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an inverted, or "flip-chip", configuration are described. These devices are mounted in specially designed high-power (%1-5 W) packages and exhibit high extraction efficiency and low operating voltage. In the blue wavelength regime, output powers greater than 250 mW (1 Â 1 mm 2 device) and 1 W (2 Â 2 mm 2 device) are delivered at standard operating current densities (%50 A/cm 2), corresponding to "wall-plug" efficiencies of 22%-23%. Employing phosphors for the generation of white light, these same devices achieve luminous efficiencies greater than 30 lm/W.

Light-Emitting Diodes: Research, Manufacturing, and Applications IV, 2000

Physical Review B, 1985

The authors present the first observation of muon spin rotation for normal (Mu) and anomalous (Mu... more The authors present the first observation of muon spin rotation for normal (Mu) and anomalous (Mu * ) muonium centers in compound semiconductors, specifically GaP and GaAs. As in the elemental semiconductors, the muonium defect centers are characterized by a large ...

1998 High-Temperature Electronic Materials, Devices and Sensors Conference (Cat. No.98EX132), 1998

ABSTRACT The past ten years have seen a virtual revolution for the optoelectronics industry deali... more ABSTRACT The past ten years have seen a virtual revolution for the optoelectronics industry dealing with LEDs. With the development of new III-V materials, such as AlGaAs, AlInGaP, and InGaN, and epitaxial structures capable of very efficient visible light generation, a vast new field of applications for LEDs has opened. The most recent development has been the introduction of bright blue and green LEDs based on InGaN. This now makes coverage of the entire color spectrum possible, from red to violet at brightness and efficiency levels exceeding conventional filament light sources. Full-color large screen video displays with excellent color rendition and brightness and even white light LEDs are being produced. Semiconductor lasers have also benefitted from progress with InGaN technology, and have been demonstrated with emission wavelengths around 400 nm. The primary importance of these devices is in the area of CD data storage, where the short wavelength increases storage density by approximately a factor of four over current systems using a red AlInGaP or infrared AlGaAs laser. Despite these advances, we have barely begun to see the possibilities for LEDs. Continuing improvements in materials and device efficiency and light extraction techniques are set to raise performance limits by at least a factor of two for InGaN and AlInGaP devices over the next few years. This presentation focuses on the advances that have been achieved with InGaN materials technology and the types of devices that have been created. Current applications and possible future use for high performance blue LEDs and lasers are also discussed

Light-Emitting Diodes: Research, Manufacturing, and Applications V, 2001

Physical Review B, 1982

The authors report measurement of ' Pt spin-lattice relaxation times Tl and spin-spin relaxation ... more The authors report measurement of ' Pt spin-lattice relaxation times Tl and spin-spin relaxation times T2 of small particles of Pt supported on alumina. Tl and T2 were measured at various static fields H p, for frequencies vp of 45, 55, and 74 MHz, and at temperatures of 4.2, 77, and 300 K. Though strong functions of H p/vp at any given vp, the relaxation times Tl and T2 at fixed Hp/vp are independent of particle size. Tl is longest at the position (Hp/vp) corresponding to the "surface peak" described in paper I (the preceding paper), indicating that conduction-electron spins are largely tied up for surface Pt atoms. The peak in Tl shifts position with change in surface coating exactly as does the peak in NMR echo amplitude, showing that the change in Hp/vp of the surface peak as a function of surface coating is most likely a chemical shift.

Physical Review B, 1986

ABSTRACT Double electron-muon resonance (DEMUR) is reported on anomalous muonium centers in silic... more ABSTRACT Double electron-muon resonance (DEMUR) is reported on anomalous muonium centers in silicon. Resonance of EPR transitions otherwise non-observable is detected by characteristic structure in the muon-spin-rotation (μSR) spectra. In order to explain the nature of this structure, the theory of DEMUR is extended to include inhomogeneous broadening of the EPR and μSR transitions owing to nuclear hyperfine interactions with the 29Si nuclei present. The principal effect of this extension is that only one of the two μSR lines is split rather than both as predicted when nuclear hyperfine effects are neglected.

Science, 1986

In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possi... more In recent years, improvements in the sensitivity of nuclear magnetic resonance have made it possible to detect progressively smaller numbers of nuclei. Experiments and studies previously thought to be impractical can now be undertaken, for example, the study of phenomena at surfaces. Nuclear magnetic resonance has been applied to study simple molecules (carbon monoxide, acetylene, and ethylene) adsorbed on metal surfaces (ruthenium, rhodium, palladium, osmium, iridium, and platinum). The metals, in the form of clusters 10 to 50 angstroms in diameter, supported on alumina, are typical of real catalysts. The experiments provide information about the bonding of the molecules to the metal, the structures the molecules assume after adsorption, the motion of molecules on the surface, the breakup of molecules induced by heating, and the products of such breakup.

Physical Review Letters, 1985

The authors report 13 C NMR studies of 13 CO adsorbed on small Pt particles and show that the dat... more The authors report 13 C NMR studies of 13 CO adsorbed on small Pt particles and show that the data yield quantitative information about the CO-Pt bond. They find that the 13 C resonance position is shifted 200 ppm to higher frequency than in CO gas. They prove that the shift ...