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  • 1
    ISSN: 1432-0495
    Keywords: Keywords China ; Geochemistry ; River ; Sediment ; Trace metals
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract  A total of 26 geographically and hydrologically diverse sediment samples were collected from 12 major rivers in eastern China. The 〈63-μm fraction of the sediments was analysed for both total concentrations of Cu, Zn, Pb and Cd, and their associations with various geochemical phases. The geographical variations of sediment-bound trace metals can be related to the bedrock types and weathering processes in the corresponding river basins. The rivers in southern China had notably higher concentrations of trace metals in sediments because of abundant non-ferrous mineral deposits and stronger weathering process in the region. A large proportion of trace metals in these sediments was associated with iron and manganese oxides and organic matter. Relative low levels of trace metals were found in river sediments in northern China, and a significant proportion of the metals was bound to organic matter, carbonates, and the residual fraction. The sediments in the Yellow River, originating from special loess, had the lowest concentrations of trace metals. Most of the trace metals were associated with the carbonates and residual phases.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We investigated the effects of well thickness on spontaneous and stimulated emission (SE) in GaN/AlGaN separate confinement heterostructures (SCHs), grown by low-pressure metalorganic chemical vapor deposition. The SCH wells are unstrained and lattice-matched to a GaN buffer layer. Our series of SCHs had GaN well thicknesses of 3, 5, 9, and 15 nm. We explain the spontaneous emission peak energy positions of the SCHs in terms of spontaneous and strain-induced piezoelectric polarizations. At 10 K, the carrier lifetime was found to be lowest for a 3 nm well, and the SE threshold was lowest for a 5 nm well. We show that the screening of the piezoelectric field and the electron-hole separation are strongly dependent on the well thickness and have a profound effect on the optical properties of the GaN/AlGaN SCHs. The implications of this study on the development of near- and deep-ultraviolet light emitters are discussed. © 2001 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 2163-2165 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present a technique for evaluating optical confinement in GaN-based lasing structures by studying their spectrally resolved near-field pattern under high optical excitation. Emission spectra were found to be strongly dependent on the position of the collection optics relative to the active region when the sample was excited above the lasing threshold. The spatially resolved spectra contain a modulation signature that can be used to deduce the optical confinement characteristics. We show that the observed index-guided modes result from multiple internal reflections at angles very close to the critical angle for total internal reflection between the semiconductor layers with different refractive indices. This technique was used to evaluate the degree of optical confinement in GaN epilayers and GaN/AlGaN separate confinement heterostructures. The implications of this study on the design of GaN-based laser diodes are discussed. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 1245-1247 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recombination characteristics of n-AlxGa1−xAs/n-In0.5Ga0.5P type-II band line-up heterostructures are investigated using time-integrated and time-resolved photoluminescence (PL) measurements. It is observed that the decay time of the AlxGa1−xAs luminescence depends on whether or not the excitation photon energy,(h-dash-bar)ωe, is larger than the In0.5Ga0.5Pband-gap energy, Eg,InGaP. If (h-dash-bar)ωe〉Eg,InGaP, photoexcited holes in the AlxGa1−xAs and In0.5Ga0.5P layers are found to be in equilibrium within about 0.4 ns. The interface-related below-band-gap (BBG) PL shows a large blueshift as the excitation intensity is increased. The extremely long decay time of the BBG PL is attributed to the somewhat smaller wave function overlap between spatially separated, two-dimensional electrons and holes due mainly to the nonabrupt interfacial nature of the employed samples. The fast transient behavior of the BBG luminescence under high excitation intensity, as well as the peak energy blueshift, are explained by the band filling effect. © 1998 American Institute of Physics.
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  • 5
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optically pumped stimulated emission (SE) from InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic chemical vapor deposition has been systematically studied as a function of excitation length (Lexc). Two distinct SE peaks were observed from these structures: one that originates at 425 nm at 10 K (430 nm at 300 K) and another that originates at 434 nm at 10 K (438 nm at 300 K). The SE threshold for the high-energy peak was observed to always be lower than that of the low-energy peak, but the difference was found to decrease greatly with increasing Lexc. A detailed study of the emission intensity of these two SE peaks as a function of excitation density shows that the two peaks compete for gain in the MQW active region. © 1998 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 1981-1983 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optical absorption measurements were performed on a series of thin GaN epilayers. Sharp spectral features were observed due to the 1s A and B exciton transitions. Using polarization dependent absorption, the C exciton transition was identified. A broad absorption feature was observed at ∼3.6 eV, which is attributed to indirect exciton-phonon absorption. The excitonic structure was found to persist well above room temperature. A fit to the Varshni formula yielded a temperature dependence of E(T)=E(T=0)−11.8×10−4T2(1414+T) eV for the A and B excitons. The exciton absorption linewidth was studied as a function of temperature, indicating that GaN exhibits very large exciton-phonon coupling. © 1997 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 67 (1995), S. 2512-2514 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report the results of time-resolved studies on the exciton radiative decay in single-crystal GaN films grown by metalorganic chemical vapor deposition. Time-resolved photoluminescence (PL) measurements were performed on the samples at various temperatures from 10 to 320 K. The well-resolved near-band-edge luminescence features associated with free excitons and bound excitons in the GaN allow us to unambiguously determine their decay times. We found that the nonradiative recombination processes play an important role and dominate the decay of exciton population. The processes depend on the density of defects and impurities in the GaN samples. © 1995 American Institute of Physics.
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  • 8
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Optically pumped near ultraviolet lasing from single-crystal GaN grown by metalorganic chemical vapor deposition has been achieved over a temperature range from 10 K to over 375 K by using a side-pumping geometry on small barlike samples. The laser emission threshold was measured as a function of temperature and the threshold was found to show weak temperature dependence: ∼500 kW/cm2 at 10 K and ∼800 kW/cm2 at room temperature (295 K) for one particular sample studied. The longitudinal lasing modes were clearly observed. The characteristics of the temperature dependence of the laser emission threshold suggests that GaN is a suitable material for the development of optoelectronic devices required to operate at high temperatures. © 1995 American Institute of Physics.
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  • 9
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Extreme high order nonlinear diffracted signals are found in ZnCdSe/ZnSe quantum wells in two beam, self-diffraction geometry. Diffracted signals of all observed orders are shown to originate predominantly from excitonic resonances. Using four-wave mixing, an indication of hole-longitudinal optical phonon scattering between the heavy and the light hole states is found, and exciton-phonon interaction is studied.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 2267-2269 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of hydrostatic pressure on the quantum confined transition in CdSe/ZnSe single quantum wells grown by molecular beam epitaxy has been studied by low-temperature photoluminescence measurements. Samples with layer thicknesses of CdSe from 1 to 4 monolayers were used. Strong excitonic emissions associated with the lowest Γ-Γ interband transitions were observed in these highly strained quantum well samples. The pressure coefficients of the interband transitions are found to depend on well thickness with the numerical value decreasing as the well width increases. Pronounced sublinear pressure dependence of the excitonic emissions was observed in the samples with 3 and 4 monolayer CdSe wells, indicating the degradation of the samples due to strain relaxation. Our results suggest that the critical thickness for the CdSe layer pseudomorphically grown on ZnSe is less than 4 monolayers.
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