Publications Search Results

Now showing 1 - 10 of 121
  • Publication
    Carrier dynamics and microwave characteristics of GaAs-based quantum-well lasers
    ( 1999)
    Esquivias, I.
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    Weisser, S.
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    Romero, B.
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    Ralston, J.D.
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    Rosenzweig, J.
    We investigate the effects of carrier capture and re-emission on the electrical impedance, equivalent circuit, and modulation response of quantum- well (QW) laser diodes.The electrical impedance is shown to be a sensitive function of the time constants associated with carrier capture/transport and carrier re-emission. We compare the theoretical results with measured values of the electrical impedance of high-speed InGaAs-GaAs multi pie-quantum-well lasers fabricated using different epilayer structures with a common lateral structure. The experimental results agree well with the theoretical model, allowing us to extract the effective carrier escape time and the effective carrier lifetime in the QW's, and to estimate the effective carrier captureltransport time.
  • Publication
    Uncooled high-temperature (130 deg C) operation of InGaAs-GaAs multiple quantum-well lasers at 20 Gb/s
    ( 1997)
    Czotscher, K.
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    Larkins, E.C.
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    Weisser, S.
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    Benz, W.
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    Daleiden, J.
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    Fleissner, J.
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    Maier, M.
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    Ralston, J.D.
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    Rosenzweig, J.
    Short-haul fiber-optic communication systems require high-speed semiconductor lasers that can operate uncooled over a wide temperature range. In this letter, we describe high-speed short-cavity InGaAs-GaAs multiple-quantum-well lasers operating at 1.1-mu m wavelength. The Fabry-Perot lasers were fabricated in a triple-mesa geometry suitable for on-wafer probing. With 3 x 200 mu m(exp 2) ridge-waveguide lasers, which showed the best compromise between high-temperature and high-speed performance, a 3-dB modulation bandwidth of 14.5 GHz at 130 deg C was achieved. Uncooled 20-Gb/s operation of these lasers is presented over a wide-temperature range from 25 deg C to 130 deg C without automatic power control.
  • Publication
    Lateral carrier profile for mesa-structured InGaAs/GaAs lasers
    ( 1997)
    Torre, M.S.
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    Esquivias, I.
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    Romero, B.
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    Czotscher, K.
    ;
    Weisser, S.
    ;
    Ralston, J.D.
    ;
    Larkins, E.
    ;
    Benz, W.
    ;
    Rosenzweig, J.
    We study the influence of lateral carrier diffusion on the properties of In(0.35)Ga(0.65)As/GaAs multiple quantum well lasers by comparing theoretical and experimental results. A model including the carrier diffusion terms into the rate equations has been used to calculate the dc and small-signal lateral profiles for both unconfined and confined carriers in mesa waveguide devices. The theoretical results were compared with experimental results of the frequency dependence of the subthreshold electrical impedance and small-signal spontaneous emission, and with the measured threshold currents for lasers with different mesa widths. The comparison yielded an estimation for the nonradiative and radiative recombination coefficients, the ambipolar diffusion constant, and the external surface recombination velocity.
  • Publication
    Characterization and modeling of InGaAs/GaAs multi-quantum well lasers by capacitance-voltage measurements
    ( 1997)
    Arias, J.
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    Esquivias, I.
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    Bürkner, S.
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    Chazan, P.
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    Ralston, J.D.
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    Larkins, E.C.
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    Mikulla, M.
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    Weisser, S.
    ;
    Rosenzweig, J.
    We have measured and analyzed the room-temperature capacitance-voltage (C-V) characteristics of In(0.35)Ga(0.65)As/GaAs MQW laser structures with different doping levels in the active region. Average doping densities in the well-barrier regions were directly extracted from the as-measured carrier profiles. A model for the C-V measurements, including the self-consistent solution of Poisson and Schrödinger equations, was developed. The carrier profiles obtained from the simulated C-V characteristics do not correspond to the free carrier profiles since the local charge neutrality hypothesis does not hold for QW structures. Thus, the true carrier distribution can only be determined from a full quantum-mechanical simulation of the laser structure. We have determined, from the comparison between experimental and simulated profiles, a conduction band offset delsta E(c)/delta E(g) of 0.81. We have also applied C-V measurements to samples with interdiffused QWs, and obtained the characteristic interdiffusion length.
  • Publication
    High power tapered InGaAs/GaAs laser diodes with carbon doped cladding layers grown by solid source molecular beam epitaxy
    ( 1996)
    Mikulla, M.
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    Benz, W.
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    Chazan, P.
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    Daleiden, J.
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    Fleissner, J.
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    Kaufel, G.
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    Larkins, E.C.
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    Maier, M.
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    Ralston, J.D.
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    Rosenzweig, J.
    ;
    Wetzel, A.
    High power InGaAs/GaAs tapered laser oscillators with a new type of carbon doping in the p-cladding layers grown by MBE are presented. In these devices carbon partially replaces the common beryllium p-dopant near the core region. SIMS depth profiles show, that the carbon doped layer serves as an efficient diffusion barrier for the beryllium in the p-cladding and contact layers. Thus, the previously observed severe beryllium redistribution from the p-cladding layer into the core region of the laser diodes is completely suppressed. In single quantum well (WQW) lasers this doping profile results in low internal losses of 2.6 cm(exp -1). Tapered laser oscillators show output powers as large as 7.2 watts an a maximum power conversion efficiency of 34 %.
  • Publication
    Carrier escape time in GaAs/AlGaAs and InGaAs/GaAs quantum-well lasers
    ( 1996)
    Esquivias, I.
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    Romero, B.
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    Weisser, S.
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    Czotscher, K.
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    Ralston, J.D.
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    Larkins, E.C.
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    Arias, J.
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    Schönfelder, A.
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    Mikulla, M.
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    Fleissner, J.
    ;
    Rosenzweig, J.
    The transport of carriers along the confinement region, the carrier capture into, and the carrier escape out of the quantum wells (QWs) are limiting processes affecting the high-frequency properties of QW lasers. The influence of these processes on the laser performance depends mainly on the ratio of the effective carrier transport/capture time and the effective escape time. We present experimental results about the escape times for GaAs/A1GaAs and InGaAs/GaAs high-speed QW lasers with varied geometrical dimensions (cavity width and length), number of QWs, In-concentrations, and p-doping levels in the active region, as extracted from electrical impedance measurements in the sub-threshold regime. In addition to the expected increase of the escape time with increasing QW barrier height, we observe an important increase in the escape time for lasers with p-doping. The escape time dependences on the carrier concentration and on the temperature are determined and discussed.
  • Publication
    InGaAsP/InP 1.55-micron lasers with chemically assisted ion beam-etched facets
    ( 1996)
    Daleiden, J.
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    Eisele, K.
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    Keller, R.
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    Vollrath, G.
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    Fiedler, F.
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    Ralston, J.D.
    Chemically assisted ion beam etching (CAIBE) involving an Ar ion beam an a halogen ambient gas (Cl2, IBr3) has been used to etch high-quality laser facets for InGaAsP/InP bulk lasers (1.55 micron). We achieved etch rates of 40.0 - 75.0 nm min(exp -1) at substrate temperatures between -5 and +10 deg C. These low temperatures have allowed us to utilize UV-baked photoresists as well as PMMA as etch masks, facilitating very simple process development. Higher substrate temperatures (50 to 120 deg C) yield still higher etch rates, but at the expense of severely degraded surface morphologies. Angle resolved x-ray photoelectron spectroscopy (XPS) was investigated for observing etched InP surfaces. A disproportioned surface has been detected after etching in the higher temperature range; low temperatures yield stoichiometric surfaces.
  • Publication
    Ultra-high-speed InGaAs/GaAs MQW lasers with C-doped active regions
    ( 1996)
    Czotscher, K.
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    Larkins, E.C.
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    Weisser, S.
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    Benz, W.
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    Daleiden, J.
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    Esquivias, I.
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    Fleissner, J.
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    Maier, M.
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    Ralston, J.D.
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    Romero, B.
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    Schönfelder, A.
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    Rosenzweig, J.
    Short-cavity In(0.35)Ga(0.65)As/GaAs multiple quantum well (MQW) lasers with undoped and p-doped active regions. The epilayer structure consists of four 5.7 nm QWs separated by 20.1 nm barriers in a GaAs core. The cladding layers consist of Al(0.8)Ga(0.2)As. In the case of p-doped devices a 4.5 nm carbon (C)-doped region (2.5x10(exp 9) cm(exp -3)) was inserted above each QW, separated by a 3.1 nm GaAs spacer, resulting in a modulation-doped core region. Using a CAIBE process, short-cavity ridge-waveguide lasers are fabricated in a triple-mesa geometry suitable for on-wafer probing. The best device (6x130 micron(exp 2)) with an undoped active region attained a damping-limited direct modulation bandwitdth exceeding 40 GHz at a CW bias current of 160 mA. In contrast, the p-doped devices, demonstrating a maximum bandwidth of 37 GHz, are still limited by power dissipation. Alpha-factors as low as 1.4 and 1.5 for undoped and p-doped devices, respectively, are extracted from measurements of t he subthreshold gain spectra. In addition, we demonstrate eye diagrams at 25 Gbit/s (limited by the pulse pattern generator) for these laser diodes. a complete characterization of DC and RF properties of these lasers is presented.
  • Publication
    Structural and carrier density dependence of carrier lifetime in InGaAs/GaAs multiple-quantum-well lasers
    ( 1996)
    Czotscher, K.
    ;
    Weisser, S.
    ;
    Larkins, E.C.
    ;
    Fleissner, J.
    ;
    Ralston, J.D.
    ;
    Schönfelder, A.
    ;
    Rosenzweig, J.
    ;
    Esquivias, I.
    The carrier lifetime in undoped and p-doped mesa and ridge waveguide In(0.35)Ga(0.65)As/GaAs multiple-quantum-well lasers is extracted from the frequency response of the spontaneous emission. The radiative recombination coefficient is found to be the same for mesa and ridge waveguide lasers, and is nearly independent of the doping level. For ridge waveguide lasers, a simple method is proposed to obtain the lateral broadening of the active region due to carrier diffusion. When the corrected active region width is considered, the threshold carrier densities for both undoped and p-doped lasers are independent of the lateral structure and cavity width. Further, the surface recombination velocity in mesa lasers is determined.
  • Publication
    Effect of interfacial bonding on the structural and vibrational properties of InAs/GaSb superlattices
    ( 1996)
    Herres, N.
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    Fuchs, F.
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    Schmitz, J.
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    Pavlov, K.M.
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    Wagner, J.
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    Ralston, J.D.
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    Koidl, P.
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    Gadaleta, C.
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    Scamarcio, G.
    We have studied InAs/GaSb superlattices (SLs) grown with either InSb-like or GaAs-like interfaces (IFs) on top of a GaSb buffer layer on (100) GaAs substrates. The InAs layer thickness was varied from 4 to 14 monolayers (ML) while the GaSb layer thickness was kept fixed at 10 ML. The type of IF bonds realized was verified by Raman scattering from mechanical IF modes. High-resolution X-ray diffraction using one- and two-dimensional mapping of symmetric and asymmetric reflections allowed us to determine independently the lattice parameters parallel and perpendicular to the growth direction. The GaSb buffer layer was found to be fully relaxed whereas the SLs with InSb-like IFs were coherently strained to the in-plane lattice parameter of the GaSb buffer for InAs layer thicknesses exceeding 6 ML. The strain distribution within the SLs with GaAs-like IFs was obtained from simulations of the X-ray reflection profiles. The SLs were found to be coherently strained close to the GaSb buffer and showed increasing strain relaxation with increasing distance from the buffer layer. In addition, these simulations provide an accurate determination of the SL periods. Well-resolved Raman spectra of backfolded longitudinal acoustic (LA) phonons were observed showing for SLs with ZnSb-like IFs folded LA phonon lines up to the seventh order. The spectrum of quasiconfined optical SL phonons was examined by Raman spectroscopy and by IR reflection. A detailed analysis of the IR reflection spectra allowed an independent determination of the individual layer widths within the SL stack, including the spatial extent of the GaAs-like IF mode.