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  • 1
    Electronic Resource
    Electronic Resource
    Investigation of GaAs/AlAs multilayer systems for optical Bragg reflectors using x-ray double crystal techniques (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2220-2224 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Layer systems for optical Bragg reflectors (grown by molecular beam epitaxy) were studied using double crystal topography and diffractometry. The initiation of stress relaxation was detected topographically in the substrate reflection and in different satellite reflections for undoped and slightly silicon doped layers (up to a doping level of 3×1017 cm−3). Higher silicon doping (1018 cm−3) prevented misfit dislocations. When using Czochralsky substrates it was observed that half-loops were formed by misfit dislocations between pairs of threading dislocations. Misfit dislocations were shown to be situated on different levels of the layer stack. Satellite reflection topography is suggested as a new tool for directly observing strains in multilayer systems since the signal originates in the layer stack. Even for very small layer thicknesses good defect contrasts can be expected for high-quality layers, as shown by simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353125
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    On the preparation and characterization of high-quality GaAs single quantum wells (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 5263-5265 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A regime for growing high-quality GaAs/AlGaAs single-quantum-well structures by molecular-beam epitaxy with interruption only at the AlGaAs-on-GaAs heterointerface is reported. For 15- and 20-nm-wide wells the luminescence linewidth is found to be 0.44 and 0.29 meV, respectively. The data are among the lowest values reported to date. Photoluminescence line splitting due to the formation of extended growth islands was not observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.350588
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  • 3
    Electronic Resource
    Electronic Resource
    Position-dependent exciton-photon mode splitting in a microcavity (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 2214-2216 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Exciton-photon mode interaction of In0.1Ga0.9As quantum wells (QWs) in a λ/2 microcavity has been studied using classical linear dispersion theory. In a system consisting of a GaAs cavity with one QW surrounded by Al0.3Ga0.7As/AlAs and GaAs/AlAs Bragg reflectors grown by molecular beam epitaxy, the interaction is found to be strongest at the ends of the cavity and is vanishing in the center of it. The behavior in the center position is changed by inserting a second QW into the cavity. Reflectance and photoluminescence measurements show modulation effects by QW absorption in accordance with the calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111677
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  • 4
    Electronic Resource
    Electronic Resource
    Raman spectroscopic study on the wirelike incorporation of Si dopant atoms on GaAs(001) vicinal surfaces (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 490-492 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Raman scattering by collective electronic excitations from a δ-doping layer has been used to investigate the ordered incorporation of Si dopant atoms on vicinal GaAs(001) surfaces. In a series of δ-doped samples grown by molecular beam epitaxy (MBE) under specific conditions the Si dopant atoms were found to be incorporated predominantly on Ga sites, even at a doping concentration as high as 1.8×1013 cm−2. A pronounced polarization asymmetry in the Raman scattering intensity of collective intersubband plasmon-phonon modes was observed in a sample grown under conditions established by real-time high-energy electron diffraction to be favorable for wirelike Si incorporation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111139
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  • 5
    Electronic Resource
    Electronic Resource
    Temperature dependence of cathodoluminescence from thin GaAs-AlGaAs multiple quantum wells (1994)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 2382-2384 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The temperature dependence of the cathodoluminescence (CL) intensity originating from GaAs-Al0.3Ga0.7As multiple quantum wells has been measured between 5 and 300 K. The CL intensity drops exponentially by two orders of magnitude above 100 K with an activation energy of 83 meV between 140 and 200 K and 145 meV between 250 and 300 K. These energies are comparable to the effective barrier height of an electron or hole and an exciton, respectively. The decrease of the CL intensity is therefore attributed to thermal re-emission of carriers and excitons out of the quantum well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.111621
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  • 6
    Electronic Resource
    Electronic Resource
    Environmental River Engineering (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Water and environment journal 4 (1990), S. 0 
    Details
    ISSN: 1747-6593
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: In the past, river engineering works have often caused channel instability and adversely affected the river's conservation and amenity value. Recent guidelines have advocated a more natural approach to river engineering practice which retains habitat diversity within the river system. While a more natural approach is desirable, geomorphological guidance is required to ensure that the advocated changes are feasible and sustainable, both in the long and short term. The key requirement for sound environmental river engineering is a basic understanding of the natural processes controlling channel shape and dimensions. Examples are given in the paper to illustrate how such knowledge can be used to (a) stabilize rivers, (b) design environmentally-acceptable and stable flood-alleviation schemes, and (c) restore previously canalized rivers. The basis of the geomorphological input in the assessment and design process is a river survey which determines the factors controlling channel characteristics and how it will respond to planned changes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1747-6593.1990.tb01402.x
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  • 7
    Electronic Resource
    Electronic Resource
    Pito Rift: How a large-offset rift propagates (1991)
    Springer
    Marine geophysical researches 13 (1991), S. 287-309 
    Details
    ISSN: 1573-0581
    Keywords: SeaMARC II ; side-scan ; bathymetry ; magnetics microplates ; rift propagation ; East Pacific Rise
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The Pito Rift area is the site of actively deforming oceanic lithosphere that has been primarily under extension for at least the past million years, based on kinematic reconstructions. The major morphologic features, Pito Deep and Pito Seamount, are aligned toward the Euler pole for relative motion between the Easter and Nazca plates. SeaMARC II side-scan and bathymetry data indicate that there are two general modes of faulting currently active in the Pito Rift area. One is associated with incipient rifting of old (∼3 Ma) Nazca lithosphere by large NW-SE normal faults, and the other is associated with a broad area of right-lateral transform shear between the Nazca and Easter plates. This transform shear is distributed over a broad region because of the northward growth of the East Rift and parallel tectonic rifting within the Pito Rift area. The majority of the Pito Rift area is composed of preexisting blocks of Nazca plate that are back-tilted away from Pito Deep and strike perpendicular to present and previous relative plate motions. This observation suggests that block-faulting and back-tilting are the primary mechanisms responsible for the distributed lithospheric extension, in agreement with gravity and magnetic analyses (Martinez et al., this issue). The only recent volcanic flows observed in side-scan data are from the Pito Seamount area and to the outside of the outer pseudofault of the East Rift. The significance of the young flows near the outer pseudofault is not understood. We interpret the flows extending northwest from the Pito Seamount as representing a newly formed seafloor spreading axis within the Pito Rift area. Gravity and magnetic analyses (Martinez et al., this issue) together with SeaMARC II bathymetry and side-scan data support this interpretation. Based on the tectonic evolution of the Easter microplate, we propose an evolutionary model for the formation of the Pito Rift area, where new ‘tectonic’ grabens form immediately west of the previous graben and with slightly more counterclockwise orientation. The duration and history of tectonic activity for each graben are not well constrained.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00366280
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  • 8
    Electronic Resource
    Electronic Resource
    ALVIN investigation of an active propagating rift system, Galapagos 95.5° W (1992)
    Springer
    Marine geophysical researches 14 (1992), S. 207-226 
    Details
    ISSN: 1573-0581
    Keywords: plate tectonics ; seafloor spreading ; rift propagation ; rift failure ; lithospheric transfer ; magmatic differentiation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5° W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.−1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01270630
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  • 9
    Electronic Resource
    Electronic Resource
    A SeaMARC II survey of recent submarine volcanism near Easter Island (1990)
    Springer
    Marine geophysical researches 12 (1990), S. 297-315 
    Details
    ISSN: 1573-0581
    Keywords: Submarine volcanism ; hotspot ; Easter Island ; Easter Microplate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract SeaMARC II side-scan sonar data reveal that a large area of seafloor north and west of Easter Island has been disrupted by recent submarine volcanism. A large volcanic area begins approximately 60 km WNW of the island and extends for over 130 km to the west. The volcanic field is characterized by high backscatter intensity in the side-scan sonar records and is elevated 400–1000 m above the N-S seafloor fabric that surrounds it. This field, the Abu Volcanic Field, covers at least 2500 km2 and appears to consist of recent lava flows and small volcanoes. Backscatter intensity of the Abu Volcanic Field is similar to that of the adjacent ridge flank which is less than 0.4 Ma, suggesting a similar age for its formation. Two additional areas of high backscatter immediately north of Easter Island cover a combined area of over 300 km2. The sidescan sonar records show that these features are clearly of volcanic origin and are not debris flows from the nearby island. The flows are nearly 300 m thick and are morphologically similar to subaerial pahoehoe lava shields. Their high backscatter indicates that they are also the products of relatively recent submarine volcanic activity. The presence of these large areas of recent volcanism in the vicinity of Easter Island has important implications for the various models that have been proposed to explain the origin of the Easter Seamount Chain. In addition, the similar ages of Easter Island and the Easter Microplate suggest that the presence of a hotspot near or beneath this fast-spreading portion of the East Pacific Rise about 4.5 m.y. ago may have initiated the large-scale rift propagation that created the microplate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02428200
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  • 10
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    Practical geological comparison of some seafloor survey instruments (1992)
    AGU (American Geophysical Union)
    In:  Geophysical Research Letters, 19 (13). pp. 1407-1410.
    Details
    Publication Date: 2017-07-25
    Description: Seafloor survey instruments are integral to the study of marine geology. Because understanding their resolution and limitations is critical, we compare how different survey systems represent the seafloor. Coincident data collected at the Galapagos propagator (GLORIA, SeaMARC II, Sea Beam, Deep-Tow, camera sled, and Alvin) allow comparisons of how well seafloor features (e.g., faults and volcanoes) observed and characterized in high resolution data are represented in lower resolution, coarser-scale data sets. Our reported values for the minimum sizes of detected and well-represented features show that practical geological resolutions are generally ∼2-10 times lower than theoretical resolutions; care must be taken in evaluating which system to use to address a particular problem.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/92GL01390
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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