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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A new large area, high power rf source for negative ion beam systems has been developed. The source operates very reliably and has the potential for long pulse operation. Current densities up to 15 mA/cm2 could be extracted with a cold plasma grid at 0.65 Pa source pressure. By pure volume production 9 mA/cm2 current density at the same pressure level has been achieved with the addition of argon to the source operating gas. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 956-958 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Four powerful inductively coupled radio frequency (rf) plasma generators producing 90 A, 55 kV hydrogen beams were manufactured for the second injector of ASDEX Upgrade. The design of the source and the power supply was optimized based on the experience with a prototype source. During conditioning of the first two sources, it was demonstrated that power efficiency, transmission of the beam power and reliability meet all requirements. The start of the neutral beam injection with the second injector is scheduled for October 1997. Recently, also negative ions in rf sources are being investigated within a collaboration with CEA Cadarache. In this development project a positive ion prototype rf source is being utilized with a small extraction area (48 cm2) and a 700 G cm magnetic filter. Without cesium a current density of 4.5 mA/cm2 was achieved so far at elevated pressure (1.6 Pa), the H−-current density being proportional to the rf power. At medium pressure (0.6 Pa) the current density is lower approximately by a factor of 5, but preliminary results with cesium injection show a relative increase by almost the same factor in this pressure range. Probe measurements indicate electron temperatures of 2.5–4.5 eV close to the plasma grid. Attempts to improve the performance by better plasma confinement, different wall materials, and filter strengths are under way. © 1998 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 3730-3732 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report on threefold-stacked vertically aligned InP/GaInP quantum dot injection lasers emitting in the visible part of the spectrum (690–705 nm) with a low threshold current density of jth=172 A/cm2 at 90 K showing a thermally activated increase towards higher temperatures. We derived an activation energy for this behavior, which is found to be just one half of the energetic distance between the dot transition energy and the wetting layer band gap. Thus, we identify thermal evaporation of carriers out of the dots and into the wetting layer states as the process responsible for the increase in the threshold current. The nonradiative carrier lifetime in the wetting layer (τnrWL) is estimated to be approximately 250–400 ps. © 1998 American Institute of Physics.
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  • 4
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Al0.5In0.5P/Ga0.5In0.5P superlattice structures have been investigated as multiquantum barriers (MQB) in 630 nm band laser diodes in order to reduce thermal current losses. By inserting an optimized MQB, we have succeeded in improving both threshold currents and characteristic temperatures of such devices. However, the optimized dimensions of the MQB found experimentally deviated strongly from those predicted theoretically, indicating that the commonly used theoretical description assuming effective mass approximation, electron wave interference, and using transfer matrix calculation is not adequate. © 1999 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 3431-3433 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: X-ray and UV photoelectron spectroscopy measurements of Cu(InGa)Se2 thin films grown on Mo coated soda-lime glass show segregation from the substrate and formation of two different Na species. One of these species is also identified after deliberate deposition of metallic Na. Moreover, the adsorption (or segregation) of this species reduces the native oxide SeO2, while the other, reacted Na species coexists with SeO2. Small amounts of Na (≤0.05 A(ring)) induce a band bending and reduce the surface dipole. These findings reveal a positive influence of segregated Na on the morphology and electrical characteristics of Na-enriched films, improving the overall performance of the solar cells. © 1996 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 72 (1998), S. 1323-1325 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present a systematic analysis of the hole transport over heterobarriers in the InGaAs(P)/InP material system. The experiments have been performed on our recently developed all-optical switching structures [C. Knorr et al., Appl. Phys. Lett. 69, 4212 (1996)], which offer an elegant access to hole transport rates. We have varied barrier thickness, barrier height, bias voltage, and temperature. The time constants vary from 30 μs to 30 ns. Our model calculations, including all heavy and light hole subbands, show that only thermally assisted tunneling can explain both the temperature and electric field dependence of the transport rates. We have extracted the activation energies. The hole capture time is determined as 250±50 fs. © 1998 American Institute of Physics.
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  • 7
    Publication Date: 2015-09-29
    Description: The formation of transfer material products on coated cutting and forming tools is a major failure mechanism leading to various sorts of wear. To describe the atomistic processes behind the formation of transfer materials, we use ab initio to study the adsorption energy as well as the implantation barrier of Al and Fe atoms for (001)-oriented surfaces of TiN, Ti 0.50 Al 0.50 N, Ti 0.90 Si 0.10 N, CrN, and Cr 0.90 Si 0.10 N. The interactions between additional atoms and nitride-surfaces are described for pure adhesion, considering no additional stresses, and for the implantation barrier. The latter, we simplified to the stress required to implant Al and Fe into sub-surface regions of the nitride material. The adsorption energies exhibit pronounced extrema at high-symmetry positions and are generally highest at nitrogen sites. Here, the binary nitrides are comparable to their ternary counterparts and the average adhesive energy is higher (more negative) on CrN than TiN based systems. Contrary, the implantation barrier for Al and Fe atoms is higher for the ternary systems Ti 0.50 Al 0.50 N, Ti 0.90 Si 0.10 N, and Cr 0.90 Si 0.10 N than for their binary counterparts TiN and CrN. Based on our results, we can conclude that TiN based systems outperform CrN based systems with respect to pure adhesion, while the Si-containing ternaries exhibit higher implantation barriers for Al and Fe atoms. The data obtained are important to understand the atomistic interaction of metal atoms with nitride-based materials, which is valid not just for machining operations but also for any combination such as interfaces between coatings and substrates or multilayer and phase arrangements themselves.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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  • 8
    Publication Date: 2016-01-06
    Description: We demonstrate the growth and single-mode lasing operation of GaAs-AlGaAs core-multishell nanowires (NW) with radial single and multiple GaAs quantum wells (QWs) as active gain media. When subject to optical pumping lasing emission with distinct s -shaped input-output characteristics, linewidth narrowing and emission energies associated with the confined QWs are observed. Comparing the low temperature performance of QW NW laser structures having 7 coaxial QWs with a nominally identical structure having only a single QW shows that the threshold power density reduces several-fold, down to values as low as ∼2.4 kW/cm 2 for the multiple QW NW laser. This confirms that the individual radial QWs are electronically weakly coupled and that epitaxial design can be used to optimize the gain characteristics of the devices. Temperature-dependent investigations show that lasing prevails up to 300 K, opening promising new avenues for efficient III–V semiconductor NW lasers with embedded low-dimensional gain media.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 9
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    American Institute of Physics (AIP)
    In: Chaos
    Publication Date: 2015-12-11
    Description: The description and analysis of spatio-temporal dynamics is a crucial task in many scientific disciplines. In this work, we propose a method which uses the mapogram as a similarity measure between spatially distributed data instances at different time points. The resulting similarity values of the pairwise comparison are used to construct a recurrence plot in order to benefit from established tools of recurrence quantification analysis and recurrence network analysis. In contrast to other recurrence tools for this purpose, the mapogram approach allows the specific focus on different spatial scales that can be used in a multi-scale analysis of spatio-temporal dynamics. We illustrate this approach by application on mixed dynamics, such as traveling parallel wave fronts with additive noise, as well as more complicate examples, pseudo-random numbers and coupled map lattices with a semi-logistic mapping rule. Especially the complicate examples show the usefulness of the multi-scale consideration in order to take spatial pattern of different scales and with different rhythms into account. So, this mapogram approach promises new insights in problems of climatology, ecology, or medicine.
    Print ISSN: 1054-1500
    Electronic ISSN: 1089-7682
    Topics: Physics
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  • 10
    Publication Date: 2015-07-15
    Description: Although many research activities concentrate on transition metal nitrides, due to their excellent properties, only little is known about Mo–N based materials. We investigate in detail the influence of Cr on the structural evolution and mechanical properties of Mo–N coatings prepared at different nitrogen partial pressures. The chemical composition as well as the structural development of coatings prepared with N 2 -to-total pressure ratios ( p N 2 /p T ) of 0.32 and 0.44 can best be described by the quasi-binary Mo 2 N–CrN tie line. Mo 2 N and CrN are face centered cubic (fcc), only that for Mo 2 N half of the N-sublattice is vacant. Consequently, with increasing Cr content, also the N-sublattice becomes less vacant and the chemical composition of fcc single-phase ternaries can be described as Mo 1− x Cr x N 0.5(1+ x ) . These coatings exhibit an excellent agreement between experimentally and ab initio obtained lattice parameters of fcc Mo 1− x Cr x N 0.5(1+ x ) . When increasing the N 2 -to-total pressure ratio to p N2 /p T  = 0.69, the N-sublattice is already fully occupied for Cr-additions of x  ≥ 0.4, as suggested by elastic recoil detection analysis and lattice parameter variations. The latter follows the ab initio obtained lattice parameters along the quasi-binary MoN–CrN tie line for x  ≥ 0.5. The single-phase fcc coating with Cr/(Mo+Cr) of x ∼0.2, prepared with p N 2 /p T = 0.32, exhibits the highest hardness of ∼34 GPa among all coatings studied.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
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