R. Losch

Physikalisch-Technische Bundesanstalt, Brunswyck, Lower Saxony, Germany

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Publications (35)52.22 Total impact

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    ABSTRACT: We investigated the local potential distribution on multibridge structures in high magnetic fields made on the basis of GaAs/GaAlAs heterostructures. The samples provide six bridges with different widths ranging from 2 to 100 mu m connected in parallel. The various bridges were found to have different current-voltage characteristics. Negative voltages were detected over some of the bridges in a distinct current range. This is due to non-local edge conduction and can be explained if one considers the inner boundaries between the bridges as regions of macroscopic localized states. These states remain decoupled from the outer boundaries in the quantum Hall regime as long as the longitudinal conductivity sigma xx tends to zero. From the data for the longitudinal resistivity and the Hall resistivity as functions of the magnetic field measured on outer contacts, we calculated the Hall voltage for the single bridges using Ohm's and Kirchhoff's laws. The corresponding experimental curves show a similar shape but smaller plateau ranges in comparison with the dependences calculated in this local approach. Complete agreement between the experimental results on single bridges and the calculations can be obtained if the widths of the current paths for bulk conduction in the various parts of the samples are allowed to deviate from the geometrical widths. The results then indicate an oscillating width of the current path similar to the oscillations in the longitudinal resistivity rho xx.
    Semiconductor Science and Technology 12/1998; 10(4):529. DOI:10.1088/0268-1242/10/4/025 · 2.21 Impact Factor
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    ABSTRACT: The current- and temperature-dependent broadening of the resistivity peaks in the region between adjacent quantum Hall plateaus is measured in two different GaAs/AlxGa1-xAs samples. Power law behaviour is found for temperature and current scaling. Combining both results, a relation for the current-density-dependent effective electron temperature can be extracted. We find Te approximately jalpha with a current heating exponent alpha approximately=0.5.
    Semiconductor Science and Technology 12/1998; 10(7):959. DOI:10.1088/0268-1242/10/7/010 · 2.21 Impact Factor
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    ABSTRACT: We have investigated the injection-locking (IL) technique as a useful tool to substantially reduce the chirp of directly modulated semiconductor lasers in standard fibre transmission systems. Chirpless transmission is presented for Fabry-Perot, bulk DFB and multi quantum well (MQW) DFB lasers directly modulated at 10 to 15 Gbit/s with a current amplitude as high as 40 mA PP. Important aspects for operating conditions of injection-locked directly modulated laser transmitters have been obtained by simulation of standard single mode fibre transmission.
    Pure and Applied Optics Journal of the European Optical Society Part A 12/1998; 4(4):409. DOI:10.1088/0963-9659/4/4/018
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    ABSTRACT: The structure of the longitudinal resistance in the breakdown behaviour of the quantum Hall effect (QHE) of GaAs/AlGaAs heterostructures is studied at a temperature of about 100 mK using samples with a constricted current path. Time-resolved measurements on a sample with a 50 mu m wide constriction show that the current-induced breakdown of the QHE evolves as a switching between at least two states, a non-dissipative one and a resistive one. The two states may coexist in some region of magnetic field and sample current. This shows up in the time behaviour of the longitudinal voltage, which is time-distributed in a statistical manner. Increasing the sample current at fixed magnetic field increases the time the current carriers stay in the resistive state rho dis until at high enough currents all backswitching into the non-dissipative state is suppressed. At filling factors nu <or=2, the resistivity rho dis increases linearly with the difference Delta nu = nu - nu i with nu i=2. The corresponding voltages are found to be less than h(cross) omega c/e at all sample currents, contrary to results of Cage et al. (1990). For filling factors with nu >or=2 the sample shows no similar effect. The results are discussed within the framework of several existing models for the breakdown behaviour of the QHE.
    Semiconductor Science and Technology 12/1998; 8(12):2062. DOI:10.1088/0268-1242/8/12/005 · 2.21 Impact Factor
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    ABSTRACT: The technology and characterization of high-speed partly loss-coupled distributed feedback (DFB) 1.55 μm laser diodes realized on a strain-compensated AlGaInAs/InP MBE/MOVPE grown epitaxial structure is presented. We observe low thresholds, SMSRs of 50 dB, very high -3 dB intensity modulation (IM) bandwidths up to 23 GHz and good large signal behaviour
    Indium Phosphide and Related Materials, 1996. IPRM '96., Eighth International Conference on; 05/1996
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    ABSTRACT: Strain compensated QW samples with compressively strained wells and tensile strained barriers have been grown by MBE and studied by low temperature photoluminescence. The PL linewidths are narrow, decrease with growing well widths, and increase only weakly with rising number of wells
    Indium Phosphide and Related Materials, 1996. IPRM '96., Eighth International Conference on; 05/1996
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    ABSTRACT: 10-Gb/s long-haul transmission on standard single-mode fibers at 1.55 /spl mu/m with intensity modulated, low-chirp MQW DFB lasers is investigated. Successful transmission is achieved by operating the laser at increased bias. Thus, the laser chirp acts favorably on the propagation after an initial distance of 30 km in the fiber. A detailed system evaluation with an optical preamplified PIN-receiver, reveals a negligible dispersion power penalty for transmission distances between 35 and 70 km. The power penalty, mainly caused by a lower extinction ratio, must be accepted.< >
    IEEE Photonics Technology Letters 12/1995; DOI:10.1109/68.473497 · 2.18 Impact Factor
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    ABSTRACT: Strain-compensated MQW structures with AlInGaAs barriers and up to 15 AlInGaAs wells were grown by molecular beam epitaxy (MBE) and characterised by photoluminescence and X-ray diffraction. Our structures show very low threshold currents of 4 mA and high AM modulation bandwidths of 21 GHz
    Electronics Letters 09/1995; 31(16-31):1346 - 1348. DOI:10.1049/el:19950902 · 1.07 Impact Factor
  • H. Hillmer, R. Losch, W. Schlapp
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    ABSTRACT: We have grown In x Al z Ga 1-x-z As/InP heterostructure layers of different geometry and composition by molecular beam epitaxy showing very narrow photoluminescence linewidth, small Stokes shift, and high epitaxial quality. For rising z we observe experimental linewidths strongly increasing from 1.45 meV (InGaAs) to 13 meV (InAlAs). The Stokes shift is found to grow with rising z. Several contributions are discussed to explain the measured compositional variation of linewidth and Stokes shift. The possibility of arbitrarily combining different material compositions is demonstrated by the growth and characterization of multiple quantum well structures, including possible combinations of ternary and quaternary materials in the wells and barriers. © 1995 American Institute of Physics.
    Journal of Applied Physics 06/1995; 77(10-77):5440 - 5442. DOI:10.1063/1.359533 · 2.19 Impact Factor
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    ABSTRACT: Buried mushroom multiquantum well DFB laser diodes with compressively strained GaInAs quantum wells and asymmetric confinement layer design are fabricated with a combined MBE/MOCVD technology. Packaged devices exhibit high -3 dB IM bandwidths for very low bias levels and a record bandwidth for this material system of 21 GHz
    Electronics Letters 03/1995; 31(4-31):274 - 275. DOI:10.1049/el:19950212 · 1.07 Impact Factor
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    ABSTRACT: Impact ionization in the channel of InAlAs/InGaAs HEMT's was shown to be a reason for excess gate leakage current. Hot electrons in the high field region of the channel under the gate generate electron-hole pairs. The generated holes can reach the gate (gate leakage) as well as the source, the electrons flow to the drain (kink effect). The number of holes reaching the gate strongly depends on the valence band discontinuity. In order to increase this valence band discontinuity a thin pseudomorphic InAlAs layer with high Al-content was inserted in the spacer of an InAlAs/InGaAs HEMT. The efficiency of this hole barrier was measured by photocurrent and DC measurements, while its influence on transport characteristics was measured by Hall and RF measurements. A reduction of gate leakage by a factor of 200 is demonstrated
    IEEE Transactions on Electron Devices 11/1994; DOI:10.1109/16.324575 · 2.36 Impact Factor
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    ABSTRACT: The refractive indices of layers of the quaternary materials In0.53Al0.21Ga0.26As and In0.53Al0.31Ga0.16As on InP are measured for the first time by spectroscopic ellipsometry in the wavelength range from 280 to 1900 nm. In previous papers we found out that between InP and molecular beam epitaxy (MBE) grown In0.52Al0.48As, In0.53Al0.405-Ga0.065As, In0.53Al0.11Ga0.36As, and In0.53Ga0.47As layers an interface layer exists, owing to the exchange of phosphorus and arsenic atoms during the As4-stabilized oxide desorption procedure before the MBE growth. We investigated two layers (with different thicknesses) of each composition for the determination of the refractive indices and the exact values of the thicknesses.
    Materials Science and Engineering B 11/1993; 21(1-2):174-176. DOI:10.1016/0921-5107(93)90342-K · 2.12 Impact Factor
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    ABSTRACT: In a previous paper, we found that between InP and In0.52Al0.48As grown by molecular beam epitaxy, an interface layer exists, owing to the interaction of arsenic with InP in the preheat phase of the MBE growth. To determine the refractive index and thicknesses of the In0.52Al0.48As and the interface layer in the wavelength range from 300 to 1900 nm, layers 10, 20, 200 and 2000 nm thick of MBE-grown In0.52Al0.48As on InP were investigated using spectroscopic ellipsometry.
    Materials Science and Engineering B 06/1993; 20:180-182. DOI:10.1016/0921-5107(93)90423-K · 2.12 Impact Factor
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    ABSTRACT: The refractive indices of In<sub>0.53</sub>Al<sub>0.16</sub>Ga<sub>0.31</sub>As and In<sub>0.53 </sub>Al<sub>0.21</sub>Ga<sub>0.26</sub>As/InP were measured for the first time in the wavelength range from 280 to 1900 nm by spectroscopic ellipsometry. The results on InP and GaAs are in good agreement with the measurements in the literature. The interface layer due to the exchange of phosphorus and arsenic atoms during the desorption procedure of the molecular beam epitaxy growth exists and was also found by high resolution X-ray diffraction
    Indium Phosphide and Related Materials, 1993. Conference Proceedings., Fifth International Conference on; 05/1993
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    ABSTRACT: The authors demonstrated the growth and reproducibility of various molecular beam epitaxy InAlGaAs/InGaAs 1.5μm separate confinement heterostructure and graded index separate confinement heterostructure multi quantum well (MQW) laser diodes with lattice matched or compressively strained QWs. Various fundamental material and laser properties were studied for Fabry Perot and distributed feedback lasers. Epitaxial regrowth on Al-containing layers is described
    Indium Phosphide and Related Materials, 1993. Conference Proceedings., Fifth International Conference on; 05/1993
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    ABSTRACT: The refractive index of the quarternary material In0.53Al0.11Ga0.36As/InP is measured for the first time by spectroscopic ellipsometry in the wavelength range from 280 to 1900 nm. In previous papers we found out that between InP and MBE-grown In0.52Al0.48As or In0.53Ga0.47As an interface layer exists, due to the interaction of arsenic with InP in the preheat phase of the MBE growth. Therefore we grew three layers of In0.53Al0.11Ga0.36As on InP with different thicknesses to determine the refractive indices and exact values of the thicknesses.
    Applied Surface Science 05/1993; 69(1-4):355-358. DOI:10.1016/0169-4332(93)90533-H · 2.54 Impact Factor
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    ABSTRACT: The influence of thickness and doping level of the GaInAs cap layer in AlInAs-GaInAs-InP HFET structures on the DC and RF performance is systematically investigated. The authors compare three different approaches, the undoped cap layer, the highly doped thick cap layer, and, as a new approach, the thin doped and therefore surface depleted cap layer. HFET devices with 0.3 mu m gates have been processed. While all devices demonstrate f/sub T/-values around 80 GHz, distinct differences are observed for the f/sub max//f/sub T/ ratios from 1 (highly doped cap) over 1.3 (undoped cap) to 2.7 (surface depleted cap). The best f/sub max/ of 240 GHz is achieved for the new cap layer approach. A systematic investigation of the influence of the g/sub m//g/sub d/ and C/sub gs//C/sub ds/ ratios demonstrates the strong influence of a proper layout of the cap layer at the drain side of the gate region.< >
    IEEE Microwave and Guided Wave Letters 01/1993; DOI:10.1109/75.173398
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    ABSTRACT: The process technology of fully passivated T-shaped 0.18 mu m gate length InAlAs/InGaAs/InP HFETs is described. Using material selective etchants, devices realised with this process yielded gate breakdown voltages in excess of 8 V and drain source breakdown voltages in excess of 5 V. The excellent gate characteristics lead to a noise figure of 0.75 dB at 18 GHz with 13 dB associated gain. The extrapolated maximum frequency of oscillation was determined to be f<sub>max</sub>=290 GHz.
    Electronics Letters 10/1992; DOI:10.1049/el:19921179 · 1.07 Impact Factor
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    ABSTRACT: Using time‐resolved photoluminescence (PL) spectroscopy with high spatial (0.1 μm) and temporal (50 ps) resolution, we have investigated exciton scattering in a set of strained In x Ga 1-x As/GaAs quantum wells as a function of well width L z and In content x. Summarizing the results, a strong correlation between the exciton mobility and the emission linewidth (LW) is found: High mobilities correspond to small luminescence LWs, indicating reduced exciton scattering. From the dependence of the excitonic mobility on temperature and In content x, we conclude, that interface roughness scattering and alloy scattering, respectively, are the dominant processes controlling the mobility as well as the PL LW at low temperatures.
    Applied Physics Letters 09/1992; 61(6-61):702 - 704. DOI:10.1063/1.107827 · 3.52 Impact Factor
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    ABSTRACT: Optical waveguiding in an InGaAs/GaAs strained-layer distributed feedback (DFB) quantum well laser is investigated using the one-dimensional shooting method presented. The numerical approach is used to optimize the waveguide geometry and to calculate the corrugation period and the coupling factor for the integrated Bragg grating. The quantum well DFB structure designed according to the numerical calculations for an emission wavelength of 982 nm was realized for the first time entirely by molecular beam epitaxy (MBE) growth. Thus, side-mode suppression ratios of 49 dB, threshold currents of 7 mA and quantum efficiencies of 0.4 mW/mA were achieved
    Journal of Lightwave Technology 06/1992; DOI:10.1109/50.136097 · 2.86 Impact Factor

Publication Stats

281 Citations
52.22 Total Impact Points

Institutions

  • 1998
    • Physikalisch-Technische Bundesanstalt
      Brunswyck, Lower Saxony, Germany
  • 1996–1998
    • Deutsche Telekom
      Hamburg, Hamburg, Germany
  • 1993–1995
    • Darmstadt University of Applied Sciences
      Darmstadt, Hesse, Germany
  • 1991–1992
    • Telekom Germany GmbH
      Bonn, North Rhine-Westphalia, Germany