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  • 1
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 582-584 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We present gain and spontaneous emission measurements in In0.35Ga0.65As/GaAs multiple quantum well lasers. First, the gain is extracted from measured amplified spontaneous emission through a laser facet. Second, unamplified spontaneous emission is detected through the transparent GaAs-substrate. Taking advantage of the fundamental relationship between gain and spontaneous emission, the separation of the electron and hole quasi-Fermi levels, ΔEF, at various bias currents below laser threshold is accurately determined. Theoretically, ΔEF is calculated utilizing a simple two-subband structure model. Fitting the theoretical predictions to the experimental data allows us to determine the threshold carrier density (nth=2.0×1018 cm−3), the in-plane hole effective mass (mv1=0.21m0), and the amount of band gap shrinkage at threshold (ΔEg,th=33 meV). © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    Standort Signatur Erwartet Verfügbarkeit
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  • 2
    Digitale Medien
    Digitale Medien
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 65 (1994), S. 2223-2225 
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We report on theoretical and experimental results on a novel metal-semiconductor-metal (MSM) photodetector with a backgate provided by a p-doped layer. The backgate allows extremely short sweep-out times for the holes, due to strong electric fields normal to the surface. Thus, long tails due to slow moving holes and screening of the external drift fields by hole space charge accumulation at high optical power, which lead to a degradation of the time response of conventional MSM photodetectors, are avoided. The high frequency performance measured up to 8 GHz in the time and frequency domain showed a significant reduction of the bandwidth limiting hole tail compared to standard MSM photodetectors. © 1994 American Institute of Physics.
    Materialart: Digitale Medien
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  • 3
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 8277-8283 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Diffusion of ammonia was studied over the smooth Re(001) surface utilizing optical second harmonic diffraction from surface coverage grating. The decay of the first order diffraction peak as a function of surface temperature for different initial coverages was measured and then simulated by numerical solution of Fick's second diffusion equation, employing a coverage dependent diffusion constant. For the first time the second order diffracted second harmonic signal was monitored during the diffusion process, supporting the calculated initial coverage profile and the diffusion model. The resulting diffusivity D(θ)=D0exp[−Em(θ)/RT], is defined by a barrier for diffusion Em(θ)=E0−ωZθ, with activation energy at zero coverage E0=3.4±0.6 kcal/mol, D0=2.8×10−3 cm2 sec−1 and the repulsion energy between the pair of nearest neighbor molecules ω=0.2±0.06 kcal/mol. The magnitude and effect of the repulsive interactions on diffusion are compared to their strong effect on the activation energy for desorption.
    Materialart: Digitale Medien
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  • 4
    Digitale Medien
    Digitale Medien
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 4040-4043 
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Real time monitoring of laser induced thermal desorption of NH3 from Re(0001) at ns time scale was performed utilizing optical SHG. Coverage dependent desorption kinetic parameters determined under equilibrium conditions were found to correctly describe the 8 orders of magnitude faster desorption rates. The role of strong repulsive interactions is discussed.
    Materialart: Digitale Medien
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  • 5
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 2 (1995), S. 4127-4133 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Analytical criteria for supercritical and asymptotic stability and for subcritical and superexponential instability are obtained for slab-like, spherical, and cylindrical thermal structures. It is assumed that both, the thermal conductivity κ(T) and the gain/loss function Q(T), can be written as continuous functions of the temperature and they have continuous derivatives. Conditions on κ and Q under which the symmetry of the structure determines the kind of instability (or stability) are obtained. Additionally, it is found that the response of the structure not only depends on the amplitude of the disturbance, but also on whether the disturbance increases or decreases the initial steady temperature. In particular, the threshold value for the amplitude of the disturbances beyond which a linearly stable configuration destabilizes, and explicit conditions for catastrophic heating or cooling are obtained. Applications to structures constituted by atomic and molecular gas are outlined. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
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  • 6
    Digitale Medien
    Digitale Medien
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1376-1383 
    ISSN: 1089-7666
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In this paper, the direct observation of wake-field self-focusing of an electron beam in plasma is reported. The dynamics of beam self-pinching and the fast collisionless evolution of a Bennett-like, near-equilibrium profile are examined theoretically and computationally. The experimental results are compared to predictions of the analysis and discussed in the context of application to the plasma lens and the plasma wake-field accelerator.
    Materialart: Digitale Medien
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  • 7
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 6208-6210 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A method for the synthesis of gadolinium oxide magnetoliposomes, i.e., nanosized gadolinium oxide magnetic particles coated by a phospholipid membrane, is presented. Magnetoliposomes were prepared by reacting lauric acid coated gadolinium oxide nanoparticles with dimyristoylphosphatidylcholine liposomes prepared using a direct injection method. The gadolinium oxide magnetoliposomes were characterized using transmission electron microscopy imaging, x-ray diffraction, and fluorescence. The magnetic properties of the magnetoliposomes were investigated with a superconducting quantum interference device magnetometer and nuclear magnetic resonance (NMR) spectrometry. Our results indicate that the magnetoliposomes contain approximately spherical nanoparticles averaging 20 nm in diameter. The occurrence of a phospholipid bilayer surrounding the magnetic particles is confirmed both by transmission electron micrographs of samples negatively stained with uranyl acetate and by digital fluorescence imaging microscopy measurements of magnetoliposomes labeled with fluorescein. The particles are paramagnetic at room temperature. NMR measurements show that the ratio between the relaxivities of the particles depends largely on their preparation. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
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  • 8
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 5699-5701 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A new method for the synthesis of magnetoliposomes, i.e., nanosized magnetic particles coated by a phospholipid membrane, is presented. Magnetoliposomes are prepared by directly using the phospholipid vesicles as nanoreactors for the precipitation of the magnetic particles. The magnetoliposomes have been characterized using transmission electron microscopy imaging and x-ray powder diffraction. The magnetic properties of the magnetoliposomes have been investigated with a superconducting quantum interference device magnetometer. Our results indicate that the magnetoliposomes contain approximately spherical maghemite nanoparticles averaging 25 nm in diameter. The occurrence of a phospholipid bilayer surrounding the magnetic particles is confirmed both by transmission electron micrographs of samples negatively stained with uranyl acetate and by digital fluorescence imaging microscopy measurements of magnetoliposomes labeled with fluorescein. The temperature dependence of the zero field cooled and field cooled susceptibilities of the magnetoliposomes is consistent with their expected superparamagnetic nature. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
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  • 9
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 3401-3406 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: In this paper we present an optical method to control the geometry of buried layers in optoelectronic heterostructures. The technique uses an optical microscope equipped for infrared applications and relies on the fact that the different layers of the multilayer structure have different band gaps. Accordingly transmission/absorption, reflectance, and photoluminescence of the inidividual layers exhibit their characteristic near-band-gap spectral variations at different wavelengths. By appropriate selection of the wavelength range used for image formation, any layer of interest can be made visible. As an example we investigated a mushroom-type InGaAsP/InP 1500-nm laser structure with subsequent mass transport. Both technological steps, the formation of the mushroom by underetching, and the regrowth by mass transport represent critical processes in the fabrication of these index-guided lasers. Our results show clearly that the successful accomplishment of the process can be controlled by images of the selected buried layer. Contrast and resolution of the pictures are sufficient to show any irregularity in etching or regrowth. The main advantage of the method is that it offers the possibility of investigating whole wafers without sample preparation and in a nondestructive way, which is in marked contrast to observation with a scanning electron microscope, where only profiles along cleaved facets can be inspected.
    Materialart: Digitale Medien
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  • 10
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 4677-4682 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Thermal degradation in InP has been regarded to be equivalent to surface deformation. By means of luminescence microscopy we discovered a much larger scale of degradation effects. Crystallographic defects of several geometrical characteristics have been observed although the surfaces seemed to be perfectly smooth. The standard methods of protection against degradation used in liquid phase epitaxy turned out to be unreliable concerning the invisible degradation effects. We propose a model which describes the evolution of degradation starting with invisible crystallographic defects. An extreme loss of phosphorus concentrated around dislocations causes local melting representing the final and visible stage of degradation. We suppose that P vacancies which have never been noticed before are responsible for inhomogeneities affecting processing and the reliability of optoelectronic devices.
    Materialart: Digitale Medien
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