L.M.F. Kaufmann

Publications (14)19.05 Total impact

• Source

  Available from: tue.nl

  Article: Stress-induced effects by the anodic oxide in ridge waveguide laser diodes

  M. Buda, G. Iordache, G.A. Acket, T.G. van der Roer, L.M.F. Kaufmann, B.H. van Roy, E. Smallbrugge, I. Moerman, C. Sys
  [show abstract] [hide abstract]
  ABSTRACT: This paper studies, both theoretically and experimentally, stress-induced effects on the lateral far-field behavior for ridge-type semiconductor laser diodes where anodic oxide is used for the definition of the stripe width. These effects consist of antiguiding under the stripe region, and of two positive waveguiding features near the stripe edges. For low-threshold devices, these effects may be more important than thermal effects, depending on the stress in the oxide. They put a lower limit on the built-in index guiding to be introduced by lateral etch outside the ridge region in order to maintain fundamental mode operation for wider stripes. The magnitude of these effects may be as large as Δn_ef=1×10^-3. An analytical mathematical model is deduced for computing stresses and strains for a certain ridge-shaped interface which bounds the elastic medium
  IEEE Journal of Quantum Electronics 11/2000; · 1.88 Impact Factor
• Article: Final polishing of Ga-polar GaN substrates using reactive ion etching

  F. Karouta, J. L. Weyher, B. Jacobs, G. Nowak, A. Presz, I. Grzegory, L. M. F. Kaufmann
  [show abstract] [hide abstract]
  ABSTRACT: Reactive ion etching of {0001} oriented plate-like GaN single crystals has been investigated using SiCl4:Ar:SF6 chemistry. The reactive ion etching process is highly chemical. Large anisotropy of the etching rate and of the morphology has been established on (000 [`1]\bar 1 ) N-polar and (0001) Ga-polar sides of the GaN crystals, with remarkably higher rate on the N-polar side. Atomic force microscopy measurements have shown smooth surface and good polishing effect obtained on Ga-polar side, while N-polar surface exhibits an increased roughness of a factor of 10 after RIE.
  Journal of Electronic Materials 11/1999; 28(12):1448-1451. · 1.47 Impact Factor
• Conference Proceeding: Continuous wave operation of low confinement asymmetric structure diode lasers

  G. Iordache, M. Buda, G.A. Acket, T.G. Van Der Roer, L.M.F. Kaufmann, F. Karouta, C. Jagadish, H.H. Tan
  [show abstract] [hide abstract]
  ABSTRACT: Summary form only given. High optical power from diode lasers can be obtained using low confinement structures. An asymmetric design makes this requirement easier and avoids the limitations related to the confinement layers thickness. Results of CW measurements on diode lasers having a new low loss, low confinement InGaAs/AlGaAs DQW asymmetric structure with “optical trap layer”, grown by MOVPE are presented
  Lasers and Electro-Optics, 1999. CLEO '99. Summaries of Papers Presented at the Conference on; 06/1999
• Article: High power CW output from low confinement asymmetric structure diode laser

  G. Iordache, M. Buda, G.A. Acket, T.G. Van De Roer, L.M.F. Kaufmann, F. Karouta, C. Jagadish, H.H. Tan
  [show abstract] [hide abstract]
  ABSTRACT: High power continuous wave output from diode lasers using low loss, low confinement, asymmetric structures is demonstrated. An asymmetric structure with an optical trap layer was grown by metal organic vapour phase epitaxy. Gain guided 50 μm wide stripe 1-3 mm long diode lasers were studied. 1.8 W of continuous wave optical power per uncoated facet was obtained at an injection current of 4.7 A (36 mW/μm). The threshold current density is 270-400 A/cm²
  Electronics Letters 02/1999; · 0.96 Impact Factor
• Conference Proceeding: Three terminal double barrier resonant tunneling devices with the base contact to the quantum well

  M.I. Lepsa, Th.G. van de Roer, J.J.M. Kwaspen, E. Smalbrugge, W. van der Vleuten, L.M.F. Kaufmann
  [show abstract] [hide abstract]
  ABSTRACT: Three terminal (3T) Double Barrier Resonant Tunneling (DBRT) devices having the base contact to the quantum well have been fabricated from GaAs/AlAs material system. The technological solution is based on high selective etching processes to reach the plane of the very thin quantum well on which, subsequently, a Schottky base contact is obtained using a nonalloyed Ti/Pt/Au metallization scheme. DC and AC electrical measurements demonstrate that the base contact lies on the quantum well of the DBRT structure and the base voltage modulates the collector current. It is shown that the electrical characteristics of these devices enlarge the possibilities of investigations on DBRT structures and suggest important applications in high-speed electronics
  Semiconductor Conference, 1997. CAS '97 Proceedings., 1997 International; 11/1997
• Conference Proceeding: Microwave analysis of double barrier resonant tunneling diodes

  M.I. Lepsa, J.J.M. Kwaspen, T.G. van de Roer, W. van der Vleuten, L.M.F. Kaufmann
  [show abstract] [hide abstract]
  ABSTRACT: Accurate microwave measurements up to 40 GHz have been carried out on GaAs/AlAs Double Barrier Resonant Tunneling (DBRT) diodes over the whole bias range of its DC characteristic. The resulting small-signal scattering parameters (S-parameters) have been analysed afterwards in terms of equivalent circuits. The analysis demonstrates that the most suitable equivalent circuit of the DBRT structure has to include, apart from the circuit elements used to model the Esaki tunnel diode, an intrinsic inductance which is related to the life time of the quantum well quasi-state involved in the tunneling process. The bias dependency of the equivalent circuit parameters and the maximum oscillation frequency of the DBRT device have been obtained
  Semiconductor Conference, 1997. CAS '97 Proceedings., 1997 International; 11/1997
• Source

  Available from: J.E.M. Haverkort

  Article: Analysis of 6-nm AlGaAs SQW low-confinement laser structures for very high-power operation

  M. Buda, T.G. van de Roer, L.M.F. Kaufmann, Gh. Iordache, D. Cengher, D. Diaconescu, I.B. Petrescu-Prahova, J.E.M. Haverkort, W. van der Vleuten, J.H. Wolter
  [show abstract] [hide abstract]
  ABSTRACT: This paper reports experimental results on single quantum-well separate confinement heterostructures (SQW SCH) with low-confinement factor, designed for very high-power operation. The maximum power output for AR/HR coated 3-mm-long devices, measured in very short pulsed conditions (100 ns/1 kHz), from 10-μm-wide stripes was as high as 6.4 W before catastrophic optical degradation. If scaled to continuous-wave (CW) conditions, this value would be 800-1100 MW, which would mean a factor of 22.7 times more than reported for the best devices with normal design for threshold minimization. The absorption coefficient for the symmetrical structure is as low as 1.1 cm^-1, in spite of the low trapping efficiency of carriers in the quantum well (QW). The maximum differential efficiency is 40% (both faces, uncoated devices) for symmetrical structure and 33% for the asymmetrical one (all measurements in pulsed conditions). Threshold current densities were 800 A/cm² for 5-mm-long devices in the symmetrical case and 2200 A/cm² in the asymmetrical one. The effects of inefficient carrier trapping in the QW on the threshold current densities and differential efficiency are discussed
  IEEE Journal of Selected Topics in Quantum Electronics 05/1997; · 3.78 Impact Factor
• Source

  Available from: tue.nl

  Article: New optoelectronic tip design for ultrafast scanning tunneling microscopy

  R. H. M. Groeneveld, Th. Rasing, L. M. F. Kaufmann, E. Smalbrugge, J. H. Wolter, M. R. Melloch, H. van Kempen
  [show abstract] [hide abstract]
  ABSTRACT: We have developed a scanning tunneling microscope using an optoelectronic switch that gates the tunneling tip current. The switch is fabricated within 30 μm from the tip by photolithography and an accurate cleavage method. We demonstrate this approach by detecting picosecond electrical transients on a coplanar stripline. We have investigated the signal dependence on contact resistance and found significant differences when the tip is brought from low‐ohmic contact into the tunneling regime. In this regime, the THz signal amplitude was found to depend linearly on the tunnel conductance, and disappeared when the tip was retracted. © 1996 American Vacuum Society
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/1996; · 1.34 Impact Factor
• Source

  Available from: alexandria.tue.nl

  Article: 2×2 Mach–Zehnder interferometric switch based on hetero‐n‐i‐p‐i quantum wells

  X. Leijtens, G. W. Yoffe, J. E. M. Haverkort, F. Karouta, J. Brubach, T. Eijkemans, L. M. F. Kaufmann, M. K. Smit, J. A. A. Stegemann, Y. C. Zhu, J. H. Wolter
  [show abstract] [hide abstract]
  ABSTRACT: A 2×2 Mach–Zehnder interferometric switch has been developed employing 3 dB multimode interference (MMI) couplers and laterally contacted hetero‐n‐i‐p‐i quantum wells as the active material providing refractive index changes. Switching over a 4π range between the cross state and the bar state with an on/off ratio of 17:1 has been observed. A record voltage‐length product for switching is observed of less than 1 V mm. By exploiting the quadratic nature of the quantum confined Stark effect, a differential switching voltage as low as 0.2 V mm is observed in the push–pull configuration. © 1995 American Institute of Physics.
  Applied Physics Letters 05/1995; 66(20):2736-2738. · 3.84 Impact Factor
• Article: Low‐voltage hetero‐nipi waveguide modulators with GaAs/AlAs quantum wells

  G. W. Yoffe, J. Brübach, F. Karouta, W. C. van der Vleuten, L. M. F. Kaufmann, J. H. Wolter
  [show abstract] [hide abstract]
  ABSTRACT: We present a waveguide modulator with a GaAs/AlAs multiple‐quantum‐well hetero‐nipi core. Selective lateral ohmic contacts connect all the p‐type layers together and likewise all the n‐type layers. A small applied voltage creates a large change in the field across each thin undoped quantum well region, causing large changes in the absorption coefficient and refractive index. A 300‐μm‐long device gave a 10:1 modulation ratio at 1‐V reverse bias with 4‐dB zero‐bias absorption loss. 400‐ and 900‐μm‐long devices gave a 180° phase shift with negligible absorption loss at 1.5 and 0.9 V, respectively.
  Applied Physics Letters 09/1993; 63(11):1456-1458. · 3.84 Impact Factor
• Article: Far field broadening due to near field phase distortion at contact layers in AlGaAs/GaAs GRIN-MQW lasers

  M.F.C. Schemmann, C.J. van der Poel, H. Ambrosius, G.A. Acket, L.M.F. Kaufmann, Th.G. van de Roer
  [show abstract] [hide abstract]
  ABSTRACT: Experimentally and numerically it is shown that near field phase distortion due to optical losses in the contact regions causes a strong dependence of far field widths on cladding layer thickness for typical MQW lasers.
  Electronics Letters 12/1992; 28(24):2255-2256. · 0.96 Impact Factor
• Article: Microwave noise of DBRT diode over full bias voltage range

  J.I.M. Demarteau, H.C. Heyker, J.J.M. Kwaspen, T.G. Van de Roer, L.M.F. Kaufmann
  [show abstract] [hide abstract]
  ABSTRACT: Experimental results on the noise behaviour of a double barrier resonant tunnelling (DBRT) diode are presented. The measurement method had been developed from a standard noise figure meter calibration procedure. Measurements were performed on a stabilised DBRT diode, mounted in a reflection amplifier configuration. The actual circulator in the setup is represented by an ideal circulator and two error networks. Noise figure and noise measure of a DBRT diode at 1 and 1.5 GHz are reported. The bias voltage range includes two positive-conductance regions as well as the intermediate negative-conductance region. In the active region of a 20 mu m mesa diameter DBRT diode, the lowest noise measure observed was 5.5.
  Electronics Letters 02/1991; · 0.96 Impact Factor
• Source

  Available from: unibo.it

  Article: Dispersion Free Doped and Undoped AlGaN/GaN HEMTs on Sapphire and SiC Substrates

  M.C.J.C.M. Krämer, R.C.P. Hoskens, B. Jacobs, J.J.M. Kwaspen, E.M. Suijker, A.P. de Hek, F. Karouta, L.M.F. Kaufmann
  [show abstract] [hide abstract]
  ABSTRACT: We present dispersion free pulsed current voltage (I-V) and radio frequency (RF) power results of undoped and doped AlGaN/GaN HEMTs on sapphire and SiC substrates. The most significant processing step leading to these results is the application of a reactive ion etching (RIE) argon (Ar) plasma (10sccm, 40mTorr, 20W, t=30s, DC bias = -167V, 20 C) to the AlGaN surface before annealing of the ohmic contacts and surface passivation with plasma enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx). In addition, this process strongly improves the uniformity of RF device performance and is independent of the material source.
• Source

  Available from: Paul R. Hageman

  Article: The processing and scalability of AlGaN/GaN HEMTs

  B. Jacobs, F. Karouta, J.J.M. Kwaspen, P. R. Hageman, L.M.F. Kaufmann, P.K. Larsen
  [show abstract] [hide abstract]
  ABSTRACT: This paper presents a discussion on the processing of AlGaN/GaN HEMTs with different gate widths but all with an optically defined 1 um gate length. Scalability of transistor parameters will be discussed and a small-signal equivalent circuit is extracted which shows the influence of the processing of the Schottky and ohmic contacts.