Oliver Ambacher

Bio: Oliver Ambacher is an academic researcher from Fraunhofer Society. The author has contributed to research in topics: Amplifier & High-electron-mobility transistor. The author has an hindex of 64, co-authored 848 publications receiving 26256 citations. Previous affiliations of Oliver Ambacher include Osram & Siemens.

Disorder-activated scattering and two-mode behavior in raman spectra of isotopic gan and algan

Abstract: Micro-Raman spectroscopy has been applied to isotopically pure GaiN (i = 14, 15) epitaxial films in which structural disorder was introduced by ion implantation or mechanical damage. A pair of Raman peaks appearing at about 300 (300) and 667 (646) cm—1 is consistent with the anion mass 14 (15) suggesting disorder activation of the silent B1 modes, where the low-frequency mode is related to the Ga vibration (no N isotope shift) and the high-frequency mode to the N vibration (full N isotope shift). Isotopically pure Al0.5Ga0.5iN indicates respective activation of the silent B1 modes due to alloying disorder and confirms a two-mode behavior of the E2 mode. In agreement with recent IR measurements and theoretical predictions, we additionally provide evidence for an E1(TO) two-mode behavior in Raman scattering as obtained from polarisation dependent measurements on AlxGa1—xN epilayers.

High voltage electrochemical exfoliation of graphite for high-yield graphene production

Abstract: We demonstrate a highly efficient, single-step, cathodic exfoliation process of graphite to produce single- to few-layer graphene with a yield of over 70% from natural graphite flakes. By employing boron-doped diamond electrodes high potentials up to −60 V can be applied which was found to greatly increase the yield. The produced graphene flakes are partially hydrogenated during the electrochemical treatment likely aiding in their exfoliation. The resulting flakes have a large lateral size with up to 50 μm diameter. Due to the reversibility of the hydrogenation by thermal treatment the graphene flakes possess a low defect density as judged by the Raman D/G ratio yielding highly conductive films with sheet resistances of 100 to 3200 Ω □−1 at 10 to 70% transparency.

High-Power (>2 W) E-Band PA MMIC Based on High Efficiency GaN-HEMTs with Optimized Buffer

Abstract: The demonstration of a highly-efficient AlGaN/ GaN-HEMT technology for the realization of an E-band power amplifier MMIC with a saturated CW output power of >2 W is presented in this paper. AlGaN/GaN-HEMTs in 100-nm gate-length technology, incorporating AlN-interlayer epitaxy and optimized GaN buffer layer thickness, have been developed to enable multi-Watt power levels at E-band frequencies. The fabricated PA MMIC exhibits >22 dB small-signal gain across 81–86 GHz while delivering peak output power of 33.5 dBm (2.2 W) at 84 GHz with an associated power gain of 17 dB. The maximum achieved PAE at this frequency is 10%. The measurement results endorse the potential of the 100-nm GaN-HEMT technology with enhanced efficiency for use in next generation E-band satellite and fixed wireless communication networks.

Sub-MHz-Linewidth 200-mW Actively Stabilized 2.3-μm Semiconductor Disk Laser

Abstract: We report on the realization of an actively stabilized single-frequency vertical-external-cavity surface-emitting semiconductor laser (VECSEL) at a wavelength of 2.3 μ m. The laser was locked to a high-finesse Fabry-Perot interferometer with a free-spectral range of 1 GHz. From the error signal of the control feedback loop a laser linewidth of 390 kHz could be derived (1-s sampling time). Reducing the sampling time to 100 μs, a much narrower linewidth of 55 kHz was obtained, indicating the dominant effect of low-frequency technical noise. The output power exceeded 200 mW. By rotating the intracavity birefringent filter, a wide tuning range of 62 nm could be achieved. Changing the resonator length with the help of a piezoelectric transducer mounted to a cavity mirror, a modehop-free fine tuning range of 5.5 GHz was achieved.

Prospects and Limitations of Stacked-FET Approaches for Enhanced Output Power in Voltage-Controlled Oscillators

Abstract: This paper reports on the emerging potential of a stacked field-effect transistor (FET) approach with respect to maximum achievable RF output power with special remarks on high electron-mobility transistor (HEMT) based series feedback oscillator monolithic microwave integrated circuits (MMICs). A stacked-FET oscillator can provide benefit in several ways—in the form of an improved RF output power, a high drain efficiency, or a stabilized oscillation behavior, even for changing load impedances. The limitation of the maximum number of transistors is analyzed with reference to the maximum achievable output power of stacked devices. This is done by describing the compromise between an increased voltage swing and a decreased RF output current. The output current is decreased since the current gain of a common gate device is smaller than unity. Based on the proposed theory a W-band stacked-FET voltage-controlled oscillator MMIC with an output power of 15 dBm and a drain efficiency of 23.3% was realized. The MMIC is based on the Fraunhofer IAF 50-nm gate-length metamorphic HEMT process. For an advanced evaluation of the power capability and the optimum target load of the utilized technology, an active W-band on-wafer load–pull system was implemented.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Principles of Neural Science

Abstract: I have developed "tennis elbow" from lugging this book around the past four weeks, but it is worth the pain, the effort, and the aspirin. It is also worth the (relatively speaking) bargain price. Including appendixes, this book contains 894 pages of text. The entire panorama of the neural sciences is surveyed and examined, and it is comprehensive in its scope, from genomes to social behaviors. The editors explicitly state that the book is designed as "an introductory text for students of biology, behavior, and medicine," but it is hard to imagine any audience, interested in any fragment of neuroscience at any level of sophistication, that would not enjoy this book. The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or

Phonons and related crystal properties from density-functional perturbation theory

Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

Band parameters for III–V compound semiconductors and their alloys

Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Conjugated polymer-based organic solar cells

Abstract: The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.